Table of Contents 1 [611262]

Table of Contents 1
TABLE OF CONTENTS
NOMENCLATURE ……………………………………………………………………………………………….. 4
1INTRODUCTION ………………………………………………………………………………………………. 6
2WATER SORPTION ISOTHERMS OF TOMATOES ……………………………………………. 9
2.1 Introduction ………………………………………………………………………………………………… 9
2.2 Literature Review………………………………………………………………………………………. 10
2.2.1 Methods of Determination of Water Sorption Isotherms ……………………….. 10 2.2.2 Desorption Isotherms of Tomato ………………………………………………………… 12
2.2.3 Equations of Sorption Isotherms…………………………………………………………. 13
2.2.4 Heat of Sorption……………………………………………………………………………….. 16
2.3 Materials and Methods ……………………………………………………………………………….. 17
2.3.1 Materials …………………………………………………………………………………………. 17
2.3.2 Methods ………………………………………………………………………………………….. 17 2.3.3 Mathematical Analysis of Sorption Isotherm Data………………………………… 18
2.3.3.1 Calculation of BET Constants …………………………………………………. 19
2.3.3.2 Calculation of GAB Parameters ………………………………………………. 19
2.3.4 Calculation of Heat of Sorption and Binding Energy…………………………….. 20
2.4 Results and Discussion……………………………………………………………………………….. 20
2.4.1 Water Sorption Isotherms of Tomato ………………………………………………….. 20 2.4.2 Effect of Temperature on Water Sorption Isotherms of Tomato……………… 22
2.4.3 Mathematical Modelling of Sorption Data …………………………………………… 23
2.4.3.1 BET Equation ……………………………………………………………………….. 23 2.4.3.2 GAB Model ………………………………………………………………………….. 25
2.4.3.3 Fitting of the Empirical Isotherm Equations ……………………………… 26
2.4.4 Heat of Sorption……………………………………………………………………………….. 31
2.5 Conclusion………………………………………………………………………………………………… 35
3LAYER DRYING BEHAVIOUR OF TOMATOES……………………………………………… 36
3.1 Introduction ………………………………………………………………………………………………. 36 3.2 Materials and Methods ……………………………………………………………………………….. 36
3.2.1 Materials …………………………………………………………………………………………. 36
3.2.1.1 Laboratory dryer ……………………………………………………………………. 36 3.2.1.2 Tomatoes ……………………………………………………………………………… 38
3.2.2 Methods ………………………………………………………………………………………….. 38
3.2.2.1 Drying experiments ……………………………………………………………….. 38 3.2.2.2 Determination of Moisture Content of Tomatoes……………………….. 39
3.2.2.3 Drying Rate…………………………………………………………………………… 39
3.3 Results and Discussion……………………………………………………………………………….. 40
3.3.1 Over-Flow Drying…………………………………………………………………………….. 40

Table of Contents 2
3.3.1.1 Influence of the Drying Air Temperature………………………………….. 40
3.3.1.2 Product Temperature ……………………………………………………………… 42 3.3.1.3 Influence of Air Velocity………………………………………………………… 43
3.3.1.4 Influence of Relative Humidity ……………………………………………….. 45
3.3.1.5 Influence of the Variety on the Drying Behaviour ……………………… 47
3.3.2 Through-Flow Drying……………………………………………………………………….. 50
3.3.2.1 Influence of Air Temperature on the Drying Behaviour ……………… 50
3.3.2.2 Product Temperature ……………………………………………………………… 54 3.3.2.3 Influence of Air Velocity………………………………………………………… 55
3.3.2.4 Influence of Relative Humidity ……………………………………………….. 58
3.4 Conclusions ………………………………………………………………………………………………. 60
4QUALITY CHANGES IN TOMATOES AS AFFECTED BY THE DRYING
PROCESS……………………………………………………………………………………………………………. 62
4.1 Introduction ………………………………………………………………………………………………. 62 4.2 Literature review ……………………………………………………………………………………….. 63
4.3 Materials and Methods ……………………………………………………………………………….. 66
4.3.1 Material…………………………………………………………………………………………… 66 4.3.2 Determination of colour…………………………………………………………………….. 67
4.4 Results and Discussion……………………………………………………………………………….. 68
4.4.1 Colour Changes………………………………………………………………………………… 68
4.4.1.1 Influence of Over-Flow mode …………………………………………………. 68
4.4.1.2 Influence of Over-Flow mode …………………………………………………. 70
4.5 Conclusions ………………………………………………………………………………………………. 72
5SOLAR DRYING OF TOMATOES IN ROMANIA …………………………………………….. 73
5.1 Introduction ………………………………………………………………………………………………. 73
5.2 Literature Review………………………………………………………………………………………. 74 5.3 Romanian Climatic Data …………………………………………………………………………….. 76
5.4 Materials and Methods ……………………………………………………………………………….. 77
5.4.1 Solar Dryer………………………………………………………………………………………. 77
5.4.1.1 Measurement of data ……………………………………………………………… 80
5.4.1.2 Solar radiation……………………………………………………………………….. 80
5.4.1.3 Temperature………………………………………………………………………….. 80 5.4.1.4 Relative Humidity………………………………………………………………….. 81
5.4.1.5 Moisture Content of Tomatoes………………………………………………… 82
5.4.1.6 Quality Analyses of Dried Tomatoes ……………………………………….. 82
5.4.2 Solar Drying Experiments …………………………………………………………………. 83
5.5 Results and Discussions ……………………………………………………………………………… 83
5.5.1 Solar Drying of Tomatoes………………………………………………………………….. 83 5.5.2 Dried Tomatoes Quality…………………………………………………………………….. 92
5.6 Conclusions ………………………………………………………………………………………………. 94

Table of Contents 3
SUMMARY ………………………………………………………………………………………………………… 96
REFERENCES …………………………………………………………………………………………………… 100
.

Nomenclature 4
NOMENCLATURE

va m/s air velocity
Va m3/hr flow rate
M % moisture content (wet base)
Me % equilibrium moisture content
Xo kg/kg initial moisture content (dry base)
X1 kg/kg final moisture content (dry base)
Xm kg/kg monolayer moisture content
Xe kg/kg equilibrium moisture content
m kg mass
m(t) kg total mass
mD kg dry mass
aw water activity
T0C, K temperature
Tdp 0C dew point
t s, h time
C difference
D % relative humidity
db dry base
wb wet base
Qst kJ/mole net isosteric heat of sorption
R kJ/mole K universal gas constant
R2
Ww
mwa
Pws
Pw
g/m3
kg
hPa
hPa
coefficient of determination
weight of water vapour
molecular weight of water vapour
pressure of water vapour saturation
pressure of water vapour

Nomenclature 5
ABBREVIATION

ha hectares
GMP Good Manufactured Practices
I.N.M.H. Romanian National Agency for Meteorology and Hydrology
FAO Food and Agriculture Organisation
FAOSTAT Statistical Database of Food and Agriculture Organisation
M.A.A.P Romanian Ministry for Food, Agriculture and Forestry
STAS Romanian Standards System
EU European Union

Introduction 6
1 INTRODUCTION

Tomatoes (Lycopersicon esculentum, Mill.) is a vegetable plant which belong to
the genus Lycopersicon, the family Solanaceae, order Solanales, Subclass Asterideae,
class Magnoliopsida (Dicotyledonateae), division Magnoliophyta (Angyospermatophyta),
subkingdom Tracheobionta, kingdom Plantae. About a grown way the tomatoes were
divided in two types: Indeterminant and Determinant tomatoes. First type is more common
to the greenhouses culture and second to the open field culture. The fruits of these plants
are international favourites and there are more varieties sold of it than of any other
vegetable. They may be eaten cooked or raw and are a good source of vitamins. The
tomatoes loves sunshine and is grown into warm weather climate. Where summers are too
cool and short it is grown in greenhouses.
Atotal global area of 4,310,669 ha is planted annually with tomatoes and a global
harvest of 113,308,292 t on the 2003 year was obtained, (FAOSTAT, 2004). The main
production was given by Asia (58,041,615 t) followed by Europe (20,744,016 t) and North
America (14,422,944 t). The largest producers of tomatoes are China (28,851,121 t),
United States (10,382,000 t) follow by Turkey, Egypt, Italy and Spain. Romania had in
2003 year a surface area of 49,301 ha and a production of 818,936 t. The crop yield varies
depending on the region and country. The mean yields of tomatoes on the world in 2003
was 262.855 t/ha. The world production of tomatoes has increased over the last decades.
The tomatoes are used both cooked and raw for s wide range of culinary
preparation. Tomatoes production is seasonal and demand is constant during the year,
therefore in cold climate countries a greenhouses technology was developed. But the costs
for greenhouses tomatoes production is quit high since the price of fuels are increased. An alternative long storage technology was developed which permit a long way transport from
low price producing country to Europe and North America countries. Storage facilities
resulted in mass and quality loses, and is also too costly.
Drying is the oldest and the simplest method of preservation of agricultural
products, and is assumed to allow tomatoes available the whole year. Additionally, many
companies manufactured and commercialised prepared food which contain dry tomatoes as
ingredients. The lack of preservation of tomatoes by drying methods were low quality for
sun dried products and a high energy consume for common industrial dryers. Solar dryers
are expected to play an important role in developing of drying technology due to a low cost
operations and a good quality of the dried product.

Introduction 7
However, the demand of dried tomatoes have considerably increased over the last
decades, due to of using of that as colorant in food industry, in a medical diet alimentation,
and instant soups and meal preparation.
It was found that daily consumes of tomatoes may reduce the risk of certain cancers
as prostate, lung, skin and other disease as osteoporosis, atherosclerosis and coronary heart
disease; due to the fact that tomatoes are rich in the carotenoid Lycopene, (Rao, 2002).
The best tomatoes varieties for drying are red uniform colour, high dry mater
content, low sugar content, and high Vitamin C content. Tomatoes are processed in
different ways such as canned, sauce, ketchup, powder, pickled and dried slices. For
Mediterranean countries as Italy and Greece, dried tomatoes are a traditional product.
In Romania, the production of tomatoes is seasonal. An estimated area of 49,000
ha, is annually planted. The first season of harvest of tomatoes is from January to May, as
greenhouses crop, and the second from June to September as open field crop. In first
season the Romanian production is not enough and more than 35,000 tones are imported.
In summer season surpluses of tomatoes are available, the price is to low, and the farmers
are not able to sell their products profitably. In summer season is estimated for tomatoes
post harvest loses up to 10%. The lowest price is registered in July and highest price in
January, the maximum price is about ten times more than lowest price, therefore drying of
tomatoes on farm, storage and selling the dried tomatoes in winter could results in increase
of income of farmers. Recently, there is an increasing interest of dehydrated vegetables for
the local market. In 2002 the amount of dried vegetables increased comparative to 2001 at
1,712 t from 992 t, (FAOSTAT, 2004). About 30% from that value was represented by
dehydrated tomatoes (M.A.A.P., 2004). The quantity of dried tomatoes sold on the local
market is still very small. Using a solar dryer on summer season, when the fresh tomatoes has a small price and solar radiation has a high value could result in a good quality of dried
tomatoes, low operations costs and an higher income to the farmers. The solar dryer has as
advantage no fuels consume and no pollution of environment.
The problems of tomatoes quality changes during the drying process results from
the industrial drying technologies. The major quality problems of tomatoes are the loss of
the characteristic flavour, and colour changes.
The research work was divided in two main parts: laboratory investigations and
field tests. The laboratory investigations were conducted at Hohenheim University,
Germany; and field test were conducted at University of Agriculture and Veterinary
Medicine Bucharest, Romania. The laboratory investigation consists by water sorption and

Introduction 8
thin layer drying experiments. The main objectives of the laboratory investigations were
determination of water sorption isotherms of tomatoes which provides information about
storage characteristics and heat of sorption, and to estimate the energy requirements for
drying. The objectives of thin layer drying experiments were to investigate the influence of
drying parameters on the drying behaviour and quality of dried tomatoes. The objectives of
the field tests were to test the solar tunnel dryer, type Hohenheim, for drying of tomatoes in
Romanian climacteric data; to determine the influence of the weather conditions to
performance of the dryer; and finally evaluation of the quality of solar dried tomatoes.

Water Sorption Isotherms of Tomatoes 9
2 WATER SORPTION ISOTHERMS OF TOMATOES

2.1 Introduction

Water plays a very important and unique role in agricultural products. Being
present in the highest concentration, it influences a wide range of physical, chemical and
biological phenomena, which occur during processing storage. Most importantly, the
concentration of water affects practically all-deteriorative processes that are
microbiological in nature and enzymatic or non-enzymatic in origin. The rate of the
various deteriorative processes depends mainly on water concentration. The potential of
water to take part in the deteriorative processes can be characterized by the water activity
(aw)which is defined, according to the generalized Raoult’s law, as the ratio between the
water vapour pressure of the product at a given temperature and the saturation pressure of
pure water at the same temperature (Wolf et all., 1985). At equilibrium, the water activity
is related to the relative humidity of the surrounding atmosphere (Iglesias and
Chirife, 1982).
The importance of water sorption equations has been stressed by many researchers
including the nutrient retention during dehydration and shelf life of product in a packaging
material (Labuza et al., 1972). They are also needed for evaluating the thermodynamic
functions of the water sorbed in foods (Iglesias and Chirife, 1976), prediction of drying
time (Henderson and Perry, 1976) and simulation of drying systems (Broker et al., 1974).
Such drying simulations could be used to predict drying time, determine the effect of
change in certain parameters on the drying efficiency, or minimize operating costs. It also
constitutes an essential part of the drying theory (King, 1968), and in the measurement of the latent heat of vaporization (Murata, 1988; Tagawa et al., 1993). Currently, sorption
isotherms are gaining in importance as the number of recommendations, official
regulations and product specifications, which use water activity as an evaluation criterion
is growing continuously (Wolf et al., 1985). In general, data on sorption isotherms are
indispensable aid in engineering, food product and process development, and quality
control.
To improve the status of tomato-dried industry, proper drying management should
be introduced to beat the onset of spoilage during period of drying and storage. Data of
water desorption isotherms are very important in that aspect to properly select the final

Water Sorption Isotherms of Tomatoes 10
moisture content which the product is safe for storage and to determine the optimum
storage conditions. High moisture content reduces the product stability, whereas reduction
of the final moisture content below the optimum value increases the drying cost. Water
activity is therefore an important parameter of dried foods.
Due to the complex food composition, theoretical prediction of sorption isotherms
is not possible and experimental measurements are necessary. The objective of that study
was to provide fundamental data on experimental measurements of water sorption
isotherms of tomato at various temperatures, using standardized static method. The
obtained data of sorption characteristic of tomato were fitted by mathematical models. The
experiments results were also used in modeling the storage behavior and quality attributes
of the dried tomato. Finally, the sorption isotherms data were used for calculation of the
heat of sorption of water in tomato, applying the Clausius-Clapeyron equation, which can
be used to estimate the energy requirements for drying.

2.2 Literature Review

2.2.1 Methods of Determination of Water Sorption Isotherms

The measurement of water sorption isotherms in foods has been a subject of
numerous studies. Several methods to determine water sorption isotherms have been
proposed and reported in the literature. Gal (1981) classified these methods into three
categories: gravimetric, manometric and hygrometric. The choice of one method to another
mainly depends on the range, accuracy, precision and speed of measurements required,
(Rizvi, 1995).
The principle of the gravimetric methods is the determination of the weight changes
of samples in equilibrium with different water vapour pressure. These can be determined
either by continuous or discontinuous registration of weight changes, in static or dynamic
systems. Methods of continuous registration of weight changes are done without
interruption of the sorption process using a balance fixed to the whole apparatus. A review
of ultra microbalances proposed for weight measurements in controlled conditions is given
by Fox and Katz (1969). In order to accelerate the diffusion of water molecules from the
reservoir to the sample some devices working with evacuated system have been developed.

Water Sorption Isotherms of Tomatoes 11
However, with this technique the experimental errors of the measurements are relatively
high. Gal (1975) has discussed some factors affecting the accuracy of vacuum
microbalance. The discontinuous registration of weight changes can be done by either
static or dynamic systems. The static systems have been used by numerous laboratories
using samples in desiccators over saturated salt solutions or sulphuric acid, (Wolf et al.,
1973; Iglesias and Chirife, 1982) because of its simplicity and cheapness, (Weisser 1985).
Salt solutions and sulphuric acid that give specific values of equilibrium relative humidity
at various temperatures are reviewed by Greenspan (1977). The system may be evacuated
to accelerate equilibrium. Salts or sulphuric acid solutions for the various relative humidity
are available (Troller and Christians, 1978). A simple and very efficient apparatus for
simultaneous equilibrium of different samples was developed in the framework of an
European collaborative study, (Spiess and Wolf 1987).
In the dynamic systems, an air stream of known relative humidity is forced over the
sample. Based on this principle, several apparatus have been developed (Drexler, 1972;
Multon et al., 1971). Further developments are being done to optimize the performance of
the system.
The manometric methods are using, principally for rapid determination of water
activity. Very few publications on the use of manometers for sorption measurements were
found in the literature, (Labuza, 1972). The manometric methods involve the use of
sensitive manometers. Manometric devices measure the vapour pressure of water in
equilibrium with a product at given moisture content. The device has a very simple
construction and operational requirements, but that has also a low accuracy.
The hygrometric methods measure the equilibrium relative humidity of the air in
contact with a product at given moisture content. Dew point or electric hygrometers have been frequently used for water activity determination of foods. Electric hygrometers
measure the change in physical properties of product, e.g. conductance or capacitance. The
main problems involved in carrying out such measurements include determination of the
exact time necessary to reach the equilibrium states between samples and the sensing
element, proper temperature control to avoid unnecessary fluctuation of the data,
(Guarte, 1996).

Water Sorption Isotherms of Tomatoes 12
2.2.2 Desorption Isotherms of Tomato

Studies on sorption isotherms for tomato were cited in the work of Romero-Pena
(2003), Iglesias and Chirife (1982) which included the work of Alcaraz (1977) and Gane
(1950). The studies dealt with one temperature and were conducted using different
methodologies resulting to contradicting results. The difference of results may be due to
use of different methods, range of water activity, accuracy and precision of measurement.
Alcaraz (1977) determined the equilibrium moisture content of tomato at
temperature of 17 and 27 0Cand relative humidity’s in range to 0.10 to 0.80 %. The
relative humidity was maintained by manometric method. Gane, (1950) measured the
equilibrium moisture content of tomato at different relative humidity maintained by
sulphuric acid solution and constant temperature of 10 0C. Romero-Pena made
determination of equilibrium moisture content using gravimetric method recommended by
European Project Cost 90. The analysis was conducted at temperature of 25 0C.
As long as the tomato varieties have a great influence on sorption data, it is
necessary to measure the sorption isotherms under investigation. Desorption isotherms
that is related to the drying process and adsorption isotherms that is relevant for storage
evaluation is considered to have a paramount importance.

0,0 0,2 0,4 0,6 0,8 1 , 00,00,20,40,60,8Moisturecontent,db.kg/kg
Water activity
Figure 2:1 Adsorption isotherms of tomato at different temperatures taken from
literature, at 250C(K)Romero-Pena, 2003, at 170C(L)Alcaraz, 1977.

Water Sorption Isotherms of Tomatoes 13
2.2.3 Equations of Sorption Isotherms

Several attempts have been proposed in literature to describe mathematically the
isotherms of sorption of the food products. Some of these equations have been based on
theories of the mechanism of sorption, others have been purely empirical, or semi
empirical, (Aguerte et al., 1989; Duckworth 1983).
Equations for fitting water sorption isotherms in foods are of special interest for:
prediction of drying times, (King, 1968); prediction of the shelf life of the dried product
(Karel et al., 1973); prediction of equilibrium moisture content after mixing products with
different water activities, (Salwin and Slawson, 1959); and evaluating the thermodynamic
functions of the water sorbed in foods (Iglesias et al., 1976).
For modeling the sorption isotherms on capillary porous materials, there are several
equations available in the literature with varying degrees of fundamental validity. BET
equation (Brunauer, Emmet and Teller) is the most used isotherm model. It is a frequently
used equation for fitting isotherms of food materials in the water activity range from 0.05
to 0.45 (Chirife and Iglesias, 1978; Mazza and LeMaguer 1978; Rizvi 1995; Okos et all.,
1992). The BET equation is generally expressed as:

CXCa
CX Xaa
mw
m ww )1( 1
) 1(
+=
(2.1).

An extension of the BET equation is the GAB equation (Weisser 1985; Maroulis
1988; Chirife and Iglesias 1978). The GAB (Guggenheim-Anderson-de Boer) equation is a
multilayer model, which takes into account different water properties in the multilayer
region. It has been successfully applied to various foods and is the most widely used
isotherm equation, mathematically expressed as:

) 1)( 1(w w ww m
Cka ka kaCkaXX+

= (2.2).

Water Sorption Isotherms of Tomatoes 14
Van den Berg (1985) summarized some major advantages of the GAB model who
concluded that, the model provides the best isotherm equation for the description and
interpretation of practical food isotherms. The equation is able to describe the isotherms of
most foods accurately up to 0.90 water activity with Xm,Cand kbeing parameters with
physical meaning in terms of sorption processes, two of them C and k being functions of
temperature.
The depression of water activity in foods is due to a combination of factors each of
that may be predominant in a given range of the water activity ( Karel, 1973); the sorption
properties may change as a consequence of physical and chemical interaction induced by
heating or pre-treatments (Iglesias and Chirife, 1976); changes of water sorption it usually
undergoes changes of constitution, dimensions and other properties of the product
(McLaren and Rowen, 1951). Water sorption leads to phase transformations of the sugar
contained in the food (Karel, 1973; Iglesias et al., 1975). Due to that, it is no possible of
having a unique mathematical model, either theoretical or empirical for describing
accurately the sorption isotherms in the whole range of water activity and different type of
foods.
The GAB equation was applied by Labuzza et al., (1985) to model moisture
sorption isotherm in the range of 0.1 to 0.9 water activity at various temperatures who
reported an excellent fit to data. Spiess and Wolf (1987) used the GAB model to adsorption
isotherms at 25 0C. Maroulis et al., (1988) used the GAB model to the moisture sorption
isotherms for dried fruits. Weisser (1985) calculated the GAB constants of four different
products.
Van den Berg and Bruin (1981) have compiled and discussed some of empirical
isotherm equations that have been reported in the literature for fitting water sorption isotherms of food. Chirife and Iglesias (1978) made a research of the most of the isotherm
equations for fitting moisture sorption isotherms of foods. The below table shows some of
the equation which are used for fitting water sorption isotherms of food materials.

Water Sorption Isotherms of Tomatoes 15
Table 2.1. Two parameters isotherm equations compiled by Chirife and Iglesias (1978)
used for fitting the sorption isotherms of different food products.
Author
Equation

Caurie (1970)
X=aexp (-b a w)
Chen’s modified (1971) ln (-ln a w)=ln a – b X

Halsey (1948) a w=exp (b / Xa)
Harkins-Jura (1944) a w=bexp (-a / X )
Henderson (1952) 1 – a w=exp (-b Xa)
Kuhn (1972) X = b + a / (ln a w)
Oswin (1946) X = a [aw/(1- a w)]b
Smith (1947) X = a – b ln (1 – a w)
The Caurie equation is valid from 0.00 up to 0.85 water activity of the most foods
(Chirife and Iglesias 1978). The Chen’s modified equation simplified from Chen equation
and reduced to two parameters equation; it is linked to the theory of drying, and is limited
to situations where diffusion is the principal mode of mass transport. Halsey developed an
equation to provide an expression of multilayers at relatively long distance from the
surface. It has been fitted to a variety of foods at water activity values between 0.10 and
0.80. Harkins–Jura developed an equation that is used to describe the sorption isotherms to
regions in which the adsorbed molecules of water form a condensed film. That equation is
used for fitting sorption isotherms at water activity between 0.50 and 0.95. The Henderson
equation is an empirical equation (Rizvi, 1995), and is widely used for fitting sorption isotherms of foods. Kuhn equation was used by Quast and Karel (1972), for correlating
equilibrium moisture content in dried product at water activity above 0.32. The Oswin
equation is a mathematical expansion series for sigmoid–shaped curves. That was applied
by Labuzza et al., (1972) for correlate equilibrium moisture content data in non-fat
products. The Smith equation was developed for describe the sorption isotherms of various
biopolymers, and it was used by Becker and Sallans, (1956) to describe the moisture
desorption isotherms in water activity range of 0.50 and 0.95.

Water Sorption Isotherms of Tomatoes 16
2.2.4 Heat of Sorption

Data of sorption isotherms at different temperatures can be used to calculate the
latent heat of sorption, and provide important information on the state of water in food
products, this is essential for modeling drying of tomato. The energy consumption during
drying is a function of the isosteric heat of sorption and the moisture content, (Rizvi and
Benado, 1984). The level of moisture content of the material at which the net isosteric heat
of sorption reaches the latent heat of vaporization of water is considered as an indicator of
the amount of bound water existing in food (Duckworth, 1975).
The net isosteric heat of sorption can be determined using Eq. (2.3), which is
derived from the Clausius-Clapeyron equation. During the sorption process, the net
isosteric has a positive sign when heat is evolved and negative sign when heat is absorbed,
(Tsami et al., 1990).
RQ
Tdadst w=



)/1(ln (2.3).

The net isosteric heat of sorption ( Qst)represents the difference between the total
heat of sorption and the latent heat of vaporization, of water at the same temperature,
(Tsami, 1991). Integrating Eq. (2.3), and assuming that the net isosteric heat of sorption is
independent of temperature, gives the following equation:

CTRQast
w +
=1ln (2.4).

The value of net isosteric heat of sorption can be calculated from the slope of the
plot between values of ln awand 1/T at constant moisture content, where R is the gas
constant and have value of 8.314 J/moleK.

Water Sorption Isotherms of Tomatoes 17
2.3 Materials and Methods
2.3.1 Materials

Fresh tomatoes, Flandra F1 variety, purchased from a German supermarket chain
were used in that study. The tomatoes were stored in dark at temperature of 4 0Cuntil used.
Representative samples were taken randomly and cut manually into small pieces. The
samples of 5 grams were taken randomly and placed in sorbostats. A small quantity of
thymol was placed in each hygrostat in order to prevent fungal activity, (Wolf et al., 1985).

2.3.2 Methods

The sorption isotherms of tomato were determined using standard gravimetric
method recommended by Spiess and Wolf (1983). Eight salts are selected to give different
relative humidity’s in the range of 0.11 – 0.85. All the salt solutions used in the experiment
were prepared with the reagent grade salts and distilled water, accordingly with Spiess and
Wolf method (1983). Saturated salts solution have the advantage of maintaining a constant
relative humidity of the air as long as the salt present is above saturation level (Karel,
1975). The effect of pressure on adsorption isotherm is negligible at reasonable levels
(Okos et al., 1992). The salts solution used are presented in Table 2.2.
Table 2.2. Equilibrium ofrelative humidity of the saturated salts solutions at six
temperatures used in the experiments (Wolf and Spiess, 1983).
Temperature of saturation salts, 0C Salt
Solutions 20 30 40 50 60 70
LiCl 0.1240 0.1128 0.1121 0.1110 0.1095 0.1075
CH 3COOK 0.2330 0.2161 0.2040 0.1920 0.1800 0.1680
MgCl 2 0.3360 0.3244 0.3160 0.3054 0.2926 0.2777
K2CO 3 0.4400 0.4317 0.4299 0.4265 0.4211 0.4160
Mg(NO 3)2 0.5490 0.5140 0.4842 0.4544 0.4727 –
NaNO 3 0.6530 0.7314 0.7100 0.6904 0.6735 0.6605
NaCl 0.7547 0.7509 0.7468 0.7443 0.7450 0.7506
KCl 0.8511 0.8362 0.8332 0.8120 0.8025 0.7949

Water Sorption Isotherms of Tomatoes 18
The sorption containers were placed in a temperature-controlled cabinet
(HERAEUS type B 5090E, Germany) at 20, 30, 40, 50, 60 and 70 0C. Each sample was
replicated two times. First weighing was done one week from the start of the experiment
using an analytical balance (SARTORIUS type BP 221S of the SARTORIUS AG
Göttingen, Germany) with 0.1 mg accuracy.
Successive weightings were done after every three days. The equilibrium was
reached when the sample weight difference between two successive measurements was
less than the balance accuracy of 0.1 mg. (Saravacos et al., 1986). The time required for
the tomato to reach equilibrium moisture content varied with the relative humidity and the
temperature. The moisture content of the equilibrated samples was determined by drying at
105 0Cusing hot air oven until the moisture content became constant.

2.3.3 Mathematical Analysis of Sorption Isotherm Data

The equilibrium moisture content ( Me)ofthe tomato, expressed in wet basis, was
calculated using Eq. (2.5).
100
eD e
emm mM
= (2.5).

For the analysis and presentation of sorption data. the moisture content obtained
were converted to dry basis expressed in kg water per kg solid (Iglesias and Chirife,1982),
using Eq. (2.6).
ee
eMMX
=100 (2.6).

The data were graphically presented by plotting Xeversus aw,computed using Eq. (2.7).

1000e f
wH
PPa== (2.7).

The data were further analyzed by fitting them to two-parameter isotherm equations
(Table 2.1.). Using regression analysis, each equation was linearized to solve for the

Water Sorption Isotherms of Tomatoes 19
constant and the degree of the linear relationship between Xeand awwas determined for
each temperature by solving the coefficient of correlation R using Eq. (2.8).

[] ()()[]    



=
2 2 2 2) ()() )( (
w w e ew e we
a a n X X na X aXnR (2.8).

To determine the percent of variation of the X e,the coefficient of determination ( R2)
was computed.

2.3.3.1 Calculation of BET Constants

The parameters of the BET equation were determined by plotting aw/X(1-a w)
against aw.From the slope and intercept of the line, the constant Xmand Cwere
determined.
2.3.3.2 Calculation of GAB Parameters

The GAB parameters were calculated from Eq. (1.2), using nonlinear regression.
(Schär and Rüegg, 1985). Rearranging by taking its reciprocal is obtained the following
form:
m w m m w m CX a kXC CkX a CkX X1 1 2 1
)(1 1 1
2 2 2



+



=
 (2.9).
Apolynomial nonlinear regression of ( 1/X)versus ( 1/a w)was carried out in order to
determine the values of the coefficient of the quadratic term U,the linear term coefficient V,
and the constant W ,as shown in Eq.(2.10).
;1
2
 =
CkXm ;2 1
2m m kXC CkX
=
mCX1= (2.10).
Using the regression analysis, the parameters U,V,and Wwere calculated. The GAB
constants C,kand Xmwere determined by solving the equations (2.10)., (Toledo,1991;
Schär and Rüegg, 1985; Weisser, 1985).

Water Sorption Isotherms of Tomatoes 20
2.3.4 Calculation of Heat of Sorption and Binding Energy

The heat of sorption ( Qst)was calculated using the experimental moisture sorption
data at specific moisture content X,and the corresponding water activity determinate at
different temperatures. The regression line of ln awversus 1/T was calculated using
Eq.(1.4). The obtained slope was further used for the calculation of the heat of sorption Qst.
This procedure was repeated for many values of aw,in order to observe the relation
between Qst and moisture content.
The binding energy is defined as the difference between the heat of adsorption of
water and its latent heat of condensation. From Eq.(1.3), at constant moisture content level,
integrating obtains Eq.(2.11).




=
2 1 12 11lnTTRQ
aast
ww(2.11).

Where aw1 and aw2 are the water activities at temperatures T1and T2respectively, at
constant moisture content. The binding energy was been determined from the experimental
measurement data by plotting awversus 1/T at different moisture contents.

2.4 Results and Discussion

2.4.1 Water Sorption Isotherms of Tomato

The results of the experimental measurements of water desorption and adsorption
isotherms of tomato at different temperatures are shown in Figures 2:2 and 2:3
respectively. The experimental points are based on a mean value of two replications.
The obtained sorption isotherms of tomato showed the typical sigmoid shape of type II,
according to the BET classification (Brunauer et al., 1938). Higher equilibrium moisture
contents were found at the lower temperature for the same relative humidity.

Water Sorption Isotherms of Tomatoes 21
0,0 0,2 0,4 0,6 0,8 1 , 00,00,20,40,60,8
700C600C500C400C300C200CMoisturecontent,db.kg/kg
Water activity
Figure 2:2 Water desorption isotherms for tomato at different temperatures.

0,0 0,2 0,4 0,6 0,8 1 , 00,00,20,40,6
700C600C500C400C300C200CMoisturecontent,db.kg/kg
Water activity,
Figure 2:3 Water adsorption isotherms for tomato at different temperatures.

The desorption isotherm is higher than the adsorption isotherm at the same
temperature and relative humidity. The adsorption and desorption isotherms exhibited
hysteresis for the entire range of relative humidity and was consistent to the theory of
physical sorption (Iglesias et al., 1975). Hystereses are found to be higher at lower
temperatures than higher temperatures. It is also observed from the hysteresis curves that
the hysteresis loop decreased with decreasing of relative humidity, ( Figure 2:4).

Water Sorption Isotherms of Tomatoes 22
0,0 0,2 0,4 0,6 0,8 1 , 00,00,20,40,60,8
700C600C500C400C 300C20 0Cadsorption
desorptionMoisturecontent,db.kg/kg
Water activity
Figure 2:4 Water sorption isotherms of tomato at different temperatures.

Romero-Pena and Kieckbusch (2003) presented the similar sorption curve for
Brazilian tomatoes. Figures 2:3 and 2:4 reveal that small amounts of water were retained
by the tomatoes at low water activity levels. As the water activity increase, water begins to
be sorbed by sugar and other low molecular constituents (Mazza and LeMaguer, 1978;
Kapsalis, 1985), and increase the equilibrium moisture content.

2.4.2 Effect of Temperature on Water Sorption Isotherms of Tomato

Temperature has a significant effect on sorption isotherms. Its knowledge is
essential for drying and storage. In general the equilibrium moisture content of tomato at a
given water activity decreased as the temperature increased. An increase of temperature
causes a decrease of amount of sorbed water, as is shown in Figure 2:5 for desorption and
Figure 2:6 for adsorption, respectively. At low water activity levels, the temperature has
less effect on the isotherms as predicted by the theory of physical absorption, the amount
of absorbed water increase with the decrease of temperature. This result agrees with the
behaviour of the various isotherms reported by Unadi (1998), Romero-Pena (2003) and
Iglesias (1982).
The temperature shifts have an important practical effect on chemical and
microbiological reactions related to quality. An increase of temperature causes an increase
of water activity for same moisture content, which increases the rates of reactions leading
to quality deterioration.

Water Sorption Isotherms of Tomatoes 23
0 2 04 06 08 00,00,20,40,6
0,10,20,30,4
0,50,6=0,7Moisturecontent,kg/kg
Temperature, 0C
Figure 2:5 Influence of temperature on water desorption at moisture equilibrium content
of tomatoes.

0 2 04 06 08 00,00,20,40,60,8
0,10,20,30,4
0,50,6=0,7Moisturecontent,kg/kg
Temperature, 0C
Figure 2:6 Influence of temperature on water adsorption at moisture equilibrium
content of tomatoes.

2.4.3 Mathematical Modelling of Sorption Data

2.4.3.1 BET Equation

The parameters of the BET were determined by plot of a w/X(1-a w)against a w.
Alinear relationship was found to exist in the range of a w=0.11 to 0.44. From the slope

Water Sorption Isotherms of Tomatoes 24
and the intercept of the line, the constants X mand C were determined at different
temperatures and presented in Table 2.3.
Table 2.3. BET equation parameters and goodness of fit of absorption of tomato at
different temperatures.
Temperature. 0C Xm C R2
20 0.0931 21.1305 0.9636
30 0.0857 8.0228 0.9815
40 0.0705 6.7581 0.9848
50 0.0632 3.2063 0.9970
60 0.0483 1.9913 0.9950
70 0.0437 0.9043 0.9975
The values X mand C decreased with increasing temperature. These observation
have been reported by a number of investigators (Iglesias and Chirife, 1976; Mazza and
LeMaguer, 1978; Aguerre et al., 1989) who also stated that the decrease in the BET
monolayer with increasing temperature may be due to a reduction in the total number of
active sites for water binding as a result of physical and chemical changes induced by
temperature.

Table 2.4. BET equation parameters and goodness of fit of desorption of tomato at
different temperatures.
Temperature. 0C Xm C R2
20 0.1192 39.8205 0.9496
30 0.1053 13.6488 0.9847
40 0.0877 9.6443 0.9839
50 0.0753 4.5278 0.9965
60 0.0555 2.7118 0.9978
70 0.0474 1.6953 0.9978

Water Sorption Isotherms of Tomatoes 25
2.4.3.2 GAB Model

The results of the non-linear regression analysis of experimental data of tomatoes
are shown in Table 2.5 and 2.6respectively.

Table 2.5. GAB parameters and goodness of fit of adsorption of tomato at different
temperatures.
Temperature. 0C Xm C kR2
20 0.1619 3.0738 0.8758 0.9978
30 0.1507 2.0346 0.8776 0.9957
40 0.1174 2.0250 0.8916 0.9957
50 0.0717 2.4312 0.9747 0.9975
60 0.0418 4.1288 1.0550 0.9983
70 0.0369 1.1832 1.0647 0.9976
The regression analysis resulted in Xmvalues ranging between 0.1619 and 0.0369.
indicating a decrease of Xmvalue with increasing of temperature. The value of C is
independent from evolution of the temperature, while the value of ( k)increased also slowly
with temperature, which is in agreement with results of Maroulis et al., (1988); Manuel and
Sereno (1993). The calculated GAB constants were used to predict the sorption isotherms
of tomato at various temperatures.

Water Sorption Isotherms of Tomatoes 26
Table 2.6. GAB parameters and goodness of fit of desorption of tomato at different
temperatures.
Temperature. 0C Xm C kR2
20 0.2075 3.9161 0.8668 0.9983
30 0.1511 4.1439 0.9143 0.9986
40 0.1348 2.9373 0.9015 0.9971
50 0.0946 2.6899 0.9494 0.9987
60 0.0492 3.6654 1.0240 0.9985
70 0.0381 2.6726 1.0398 0.9990
2.4.3.3 Fitting of the Empirical Isotherm Equations

Data on sorption isotherms of tomato were fitted to different equations presented in
Table1. The suitability of each equation at various temperatures and water activities is
indicated by value of the coefficient of determination (R2)shown in the Table 2.7 for
adsorption and Table 2.8 for desorption, respectively.
Table 2.7. Coefficient of determination (R2)for fitting isotherm equations to adsorption
isotherm data of tomato at different temperatures and water activity values.
Temperatures. 0C Isotherm
Equations 20 30 40 50 60 70
Caurie 0.9990 0.9995 0.9993 0.9972 0.9870 0.9953
Chen 0.9630 0.9526 0.9471 0.9304 0.9131 0.9029
Halsey 0.9182 0.9690 0.9758 0.9964 0.9919 0.9717
Harkins-Jura 0.9963 0.9934 0.9935 0.9965 0.9985 0.9917
Henderson 0.9953 0.9910 0.9886 0.9740 0.9538 0.9322
Kuhn 0.9636 0.9778 0.9812 0.9969 0.9969 0.9958
Oswin 0.9960 0.9939 0.9946 0.9979 0.9939 0.9974
Smith 0.9963 0.9913 0.9895 0.9799 0.9610 0.9566

Water Sorption Isotherms of Tomatoes 27
Table 2.8. Coefficient of determination (R2)for fitting isotherm equations to desorption
isotherm data of tomato at different temperatures and water activity values.
Temperatures. 0C Isotherm
Equations 20 30 40 50 60 70
Caurie 0.9987 0.9980 0.9991 0.9983 0.9917 0.9929
Chen 0.9713 0.9620 0.9571 0.9477 0.9154 0.9167
Halsey 0.8818 0.9581 0.9661 0.9936 0.9969 0.9895
Harkins-Jura 0.9973 0.9987 0.9961 0.9969 0.9981 0.9975
Henderson 0.9915 0.9934 0.9933 0.9869 0.9618 0.9551
Kuhn 0.9560 0.9813 0.9796 0.9940 0.9988 0.9987
Oswin 0.9961 0.9985 0.9965 0.9990 0.9966 0.9973
Smith 0.9979 0.9948 0.9934 0.9888 0.9679 0.9652
Among the equations considered, the Oswin equation described best the isotherm
data for all temperature used (R2=0.9939 – 0.9990). This was followed very closely by
the Harkins-Jura equation (R2=0.9934 – 0.9987), Caurie equation (R2=0.9870 – 0.9995)
and the Smith equation (R2=0.9566-0.9979). The other equations gave some merits in
describing the equilibrium moisture content of tomatoes.
It is worthwhile to note that the very good fit of Caurie equation gave the highest
value of R2for adsorption (R2=0.9990-0.9995) at the low temperatures (20-400C); and also
for desorption (R2=0.9917-0.9991). However, at temperatures of 40-700C, its suitability
declined. On the other hand, GAB equation gave the highest R2value which is in
agreement with recommendations of European Project Group COST 90 (Wolf, Spiess and
Jung, 1985). The values of constants of the respective isotherm equations are shown in
Table 2.9 for desorption and respectively Table 2.10 for adsorption. The best fit of
Oswin’s equation is shown in Figure 2:5. It should be noted that, the goodness of fit of
any sorption model to the experimental data shows only a mathematical quality and not the
nature of the sorption process (Rizvi, 1995).

Water Sorption Isotherms of Tomatoes 28
Table 2.9. Experimentally derived constants a. b of the different isotherm models applied
for fitting the desorption data of tomato at different temperatures and water activities.
Isotherm
Equations Caurie Chen Halsey Harkins-Jura
a 0.0651 2.3532 2.2865 0.2260 200C
b -2.8152 3.9791 0.3082 1.1600
a 0.0471 2.2928 2.4060 0.1676 300C
b -3.0028 4.6946 0.2835 1.1058
a 0.0350 2.2128 2.5865 0.1313 400C
b -3.1366 5.6417 0.2517 1.0881
a 0.0225 2.0959 2.7765 0.0897 500C
b -3.4246 6.9742 0.2238 1.0307
a 0.0112 2.0550 3.1707 0.0529 600C
b -3.8681 10.1714 0.1793 0.9824
a 0.0066 2.0353 3.4552 0.0372 700C
b -4.2589 12.7289 0.1552 0.9528
Isotherm
Equations Henderson Kuhn Oswin Smith
a 1.6141 -0.1111 0.2724 0.0286 200C
b 1.6141 0.0864 0.5702 0.3601
a 1.7645 -0.1051 0.2162 0.0116 300C
b 1.7645 0.0438 0.6270 0.3116
a 1.9496 -0.0891 0.1722 0.0020 400C
b 1.9496 0.0274 0.6535 0.2613
a 2.1919 -0.0794 0.1273 -0.0087 500C
b 2.1919 0.0036 0.7341 0.2158
a 2.6566 -0.0586 0.0805 -0.0111 600C
b 2.6536 -0.0059 0.8259 0.1519
a 2.9819 -0.0486 0.0589 -0.0113 700C
b 2.9819 -0.0106 0.9040 0.1225

Water Sorption Isotherms of Tomatoes 29
0,0 0,2 0,4 0,6 0,8 1 , 00,00,20,40,60,8
700C400C200COs w i n
GA B
experimentalMoisturecontent,db.kg/kg
Water activity
Figure 2:7 Desorption isotherms of tomato at different temperatures predicted by GAB
and Oswin’s equation compared to the experimental results.

0,0 0,2 0,4 0,6 0,8 1 , 00,00,20,40,6
700C400C200COs w i n
GA B
experimentalMoisturecontent,db.kg/kg
Water activity
Figure 2:8 Adsorption isotherms of tomato at different temperatures predicted by GAB
and Oswin’s equation compared to the experimental results.

Water Sorption Isotherms of Tomatoes 30
Table 2.10. Experimentally derived constants a. b of the different isotherm models
applied for fitting the adsorption data of tomato at different temperatures and water activities.
Isotherm
Equations Caurie Chen Halsey Harkins-Jura
a 0.0437 2.2029 2.5179 0.1616
200C
b -3.0055 4.9740 0.2631 1.1259
a 0.0312 2.1250 2.6185 0.1240
300C
b -3.2544 5.6474 0.2466 1.0775
a 0.0239 2.1050 2.8245 0.0971
400C
b -3.3342 6.9523 0.2175 1.0648
a 0.0150 2.0093 3.0005 0.0665
500C
b -3.6883 8.3978 0.1966 1.0040
a 0.0075 2.0467 3.3939 0.0393
600C
b -4.1490 11.8799 0.1599 0.9566
a 0.0033 1.8818 3.7575 0.0265
700C
b -4.8902 15.2201 0.1347 0.9300
Isotherm
Equations Henderson Kuhn Oswin Smith
a 1.8367 -0.0896 0.2019 0.0088
200C
b 1.8367 0.0543 0.6065 0.2891
a 1.9755 -0.0877 0.1636 -0.0044
300C
b 1.9755 0.0224 0.6730 0.2598
a 2.1942 -0.0727 0.1303 -0.0060
400C
b 2.1942 0.0142 0.6913 0.2127
a 2.4346 -0.0667 0.0976 -0.0129
500C
b 2.4346 -0.0033 0.7884 0.1801
a 2.8838 -0.0499 0.0629 -0.0105
600C
b 2.8838 -0.0085 0.0884 0.1292
a 3.3408 -0.0409 0.0416 -0.0148
700C
3.3408 -0.0145 1.0204 0.1027

Water Sorption Isotherms of Tomatoes 31
2.4.4 Heat of Sorption

The net isosteric heats of sorption of tomato calculated from the sorption data are
given in Figures 2:9 and 2:10 for desorption and adsorption isotherms respectively. The
net isosteric heat of sorption was determinate by plotting the natural logarithm of the water
activity (ln a w)and the reciprocal of temperature (1/T). The slope of the lines decrease as
the moisture increased. This could be attributed to a reduction in the binding energy. The
data of tomato isosteres at constant moisture content were fitted to exponential, logarithmic
and linear regression. The exponential regression model gave the best fit for desorption
isosteres, with coefficient of determination (R2)between 0.9555 and 0.9941, then followed
by the logarithmic as shown in Table 2.11.
Table 2.11. Regression models and coefficient of determination (R2)oftomato desorption
isosteres.
Regression
model Exponential Logarithmic Linear
X a b R2a b R2a b R2
0.08 5456.0 -3122.6 0.9941 -3.0201 -17.067 0.9602 -952.56 3.3344 0.9516
0.10 2312.2 -2789.0 0.9940 -3.1901 -17.987 0.9734 -1007.4 3.5668 0.9669
0.15 333.89 -2075.9 0.9905 -3.1293 -17.518 0.9898 -990.41 3.6322 0.9877
0.20 77.332 -1553.4 0.9848 -2.7755 -15.392 0.9923 -879.81 3.3711 0.9933
0.30 13.1 -926.45 0.9724 -2.0114 -10.873 0.9817 -638.97 2.7292 0.9868
0.40 5.0431 -590.97 0.9555 -1.4232 -7.4122 0.9606 -452.88 2.2144 0.9690

Water Sorption Isotherms of Tomatoes 32
0,0028 0,0030 0,0032 0,0034 0,00360,11
0,40
0,30
0,20
0,1 5
0,1 0
X=0,08Wateractivity
1/ T , K-1

Figure 2:9 Desorption isosteres of tomatoes

The data of tomato adsorption isosteres at constant moisture content is presented in
Figure 2:10 .The exponential regression model gave also the best fit, with coefficient of
determination (R2)between 0.9437 and 0.9951, and then followed by the linear as shown
in Table 2.12.
Table 2.12. Regression models and coefficient of determination (R2)oftomato adsorption
isosteres.
Regression
model Exponential Logarithmic Linear
X a b R2a b R2a b R2
0.08 676.58 -2369.4 0.9951 -2.9420 -16.532 0.9795 -929.42 3.3471 0.9737
0.10 291.85 -2053.3 0.9935 -2.9333 -16.420 0.9874 -927.70 3.4026 0.9836
0.15 55.753 -1451.7 0.9907 -2.6000 -14.388 0.9929 -823.64 3.1875 0.9924
0.20 18.42 -1055.9 0.9881 -2.1631 -11.791 0.9936 -686.08 2.8336 0.9956
0.30 4.9896 -593.86 0.9783 -1.4133 -7.3709 0.9823 -449.00 2.1867 0.9874
0.40 2.4742 -346.47 0.9437 -0.8919 -4.307 0.9433 -283.88 1.7264 0.9517

Water Sorption Isotherms of Tomatoes 33
0,0028 0,0030 0,0032 0,0034 0,00360,11
0,40
0,30
0,20
0,1 5
0,1 0
X=0,08Wateractivity
1/ T , K-1

Figure 2:10 Adsorption isosteres of tomatoes

The net isosteric heat of sorption at each moisture content was computed from the
slope using Eq.1.4., and it is presented in Figure 2:11 .The values of isosteric heat of
desorption were found to be higher than those for adsorption, which means that during the
desorption process the energy of binding of water in tomato is higher, (Benato and Rizvi,
1985) or could be attributed to the swelling process (Mazza and LeMaguer, 1978). At low
moisture content the heat of desorption it is much higher than the heat of adsorption. The
differences between values of desorption and adsorption heat decreases with increasing of
moisture content. At a moisture content of 0.08 kg/kg the difference between heat of
desorption and adsorption have value of 39.74 kJ/mole and at a moisture content of 0.40
kg/kg the difference decrease to value of 0.03 kJ/mole. This was found to be similar with
results of Iglesias and Chirife (1976), Mazza and LeMaguer (1978), Bashir (1998).
Tsami (1991) gave a physical explanation of the steep increase of the heat of
sorption at low moisture content and this is due to the existence of highly active polar sites
on the surface of tomato, which are covered with water molecules forming monomolecular
layer. As the moisture content is increased, the difference is decreased.
The empirical equation was developed as a power non-linear regression to predict
the net isosteric heats of desorption of tomato as a function of moisture content (Eq.2.12).

4049,4* 0007,0
=e st X Q (2.12).

The coefficient of determination (R2)was found to have value of 0.9997 for desorption.

Water Sorption Isotherms of Tomatoes 34
An other equation (Eq.2.13) was developed for describe the relation between
moisture content (X e)and the net heat of adsorption.

5777,3* 0006,0
=e st X Q (2.13).

The coefficient of determination (R2)has value 0.9986 for adsorption, that indicate
avery good feet with experimental data.

0,0 0,1 0,2 0,3 0,41020304050HeatofsorptionkJ/moleofwater
Moisture content, db. kg/ kg
Figure 2:11 The net isosteric heats of adsorption and desorption of tomato at different
moisture content.

From knowledge of Q st,using (Eq.2.11), the sorption isotherms at different
temperatures can be predicted, (Okos et al.1992).

Water Sorption Isotherms of Tomatoes 35
2.5 Conclusion

The water sorption isotherms of tomato were determined using the gravimetric
method based on the recommendation of the European Cooperative Project COST 90.
Results of the experimental measurement of sorption isotherms, at temperature range
between 20 and 70 0C, showed a typical sigmoid shape of type II according to the BET
classification. The temperature had the expected effect predicted by theory of physical
adsorption, the quantity of sorbed water at a given water activity increased as the
temperature decreased. The amount of sorbed water depends on the equilibrium
temperature. The increase of the temperature has as result the increase of the water activity
for the same moisture content. Hysteresis was found for the entire range of relative
humidity, for both adsorption and desorption isotherms. Hysteresis loops decreased with an
increase of temperature. These indicate the irreversibility of sorption process and chemical
and microbiological deterioration during adsorption or desorption of tomato.
Ten sorption isotherm equations were used to fit to the experimental adsorption and
desorption data for tomatoes. From the models which were analysed, the GAB and Oswin
models fitted very well the experimental data at different temperatures in the range of
water activity of 0.11 to 0.85. The Oswin equation is recommended for prediction of the
adsorption and desorption isotherms, and GAB equation for prediction of monolayer
moisture content of tomato.
Knowledge of the water sorption isotherm of tomato provides information about
moisture changes, which may occur during storage and predicting shelf life stability, with
respect to physical, biochemical and microbial stability.
The net heat of sorption of water in tomato was calculated from the sorption
isotherms, applying the Clausius-Clapeyron equation. The values of the net heat of
desorption were higher than the values of the net heat of adsorption. Knowledge of the net
heat of sorption provides an indication of the binding energy of water molecules and
provides information on the energy requirement of drying operation. The net isosteric heats
of sorption were found as a function of moisture content, and decreased exponentially as
the moisture content increased.

Layer Drying Behaviour of Tomatoes 36
3 LAYER DRYING BEHAVIOUR OF TOMATOES

3.1 Introduction

Drying is a complex thermophysical and biochemical process comprising heat and
mass exchange between product and surrounding media, and a transfer of heat and
moisture within the substance. The transfer of moisture from the interior of the product to
its surface depends of the structure and the properties of the material.
In the frame of this research, thin layer drying experiments of tomatoes were
conducted under controlled laboratory conditions using a dryer developed at the Institute
for Agricultural Engineering in the Tropics and Subtropics. The dried tomatoes are fully
exposed to air forced over the product. As drying parameters were selected air temperature,
air velocity and relative humidity of the air. Were used for experiments four different
varieties of tomatoes.

3.2 Materials and Methods

3.2.1 Materials

3.2.1.1 Laboratory dryer

The laboratory dryer developed at the Institute for Agricultural Engineering in the
Tropics and Subtropics, University of Hohenheim to be used in systematic research of
drying of vegetables, fruits medicinal herbs and other agricultural products consisted of four basic section: air flow control, humidifier section with inter-cooler, heating control
section with primary and secondary heating elements, and two drying compartments,
Figure 3:1 .
The process is automatically controlled by a computer and can be regulated with
high accuracy to maintain the required drying conditions in terms of the drying
temperature, humidity and air velocity. The entering air is heated by a primary heating
system and channelled to the drying chamber. In the drying chamber, the incoming air was
exposed further to secondary heating system in order to obtain uniform temperature
distribution within the product bed with accuracy of ± 0,5
0K. Thermocouples Ni-Cr-Ni

Layer Drying Behaviour of Tomatoes 37
were used to measure the air temperature, and the temperature of tomatoes during the
drying process.
The drying temperature can be varied in the range of 30 to 100 0C. The air velocity
could be kept constant during drying experiments at any desired level ranging from 0,1 up
to 1,47 m/s. This was achieved by the airflow control assembly which regulated the
amount of fresh air entering in the drying compartment with an accuracy of ± 0,05 m/s. In
addition to these, the humidity of the drying air could be precisely controlled and desired
initial dew point temperature of the drying air was perfectly adjusted with an accuracy of
±0,5 K

a. weight plate e. bypass i. airflow controler
b.air flow regulator f. weight plate j. airflow controler
c. fine temperature controler g. bypass k. blower
d.temperaturecontroler h. refrigerator l. entrance gate valve
Figure 3:1 Schematic diagram of the thin layer dryer and instrumentation, [Lutz, 1989].

In that experience it was used both the over-flow and through flow mode. The
weight loss during the drying process can be measured either continuously or
discontinuously. The continuous measurement could be done by recording the measured
values while the drying air passed over the drying product. In this case, the measured
weight was less than the real value because of airflow forces. The airflow forces have an
effect on the tray and the tomatoes. However, the forces caused by the air flow over the

Layer Drying Behaviour of Tomatoes 38
sample was not constant over the whole drying time since the volume of tomatoes changes
during drying due to shrinkage. In order to prevent that effect during weight measurement,
the discontinuous method is used during the drying experiments. In that case, prior to the
weight measurement a bypass is opened automatically for few seconds to divert the drying
air from passing over the tomatoes. The tomatoes temperature was measured during the
drying experiments using thermocouples Ni-Cr-Ni, located at two different positions.
Values of the measured temperature were taken with an accuracy of ± 0,5 K. The time
interval for taking the measurements (weight, temperature, air velocity) could be
programmed in the computer. During the drying experiments, ten and twenty minutes
intervals were used.

3.2.1.2 Tomatoes

In those experiments, were used fresh tomatoes, which were supplied by a German
supermarket chain. It was used variety Flandra F1 of bush tomatoes. Tomatoes were
manually sorted, washed and cut in the quarters. The ripening level was tested by
refractometer and has value of 4,6 Brix. Four different varieties of tomato: Orco, Fleisch,
Flandra and Cronos were tested during drying experiments.

3.2.2 Methods

3.2.2.1 Drying experiments

The variables taken into consideration were the air temperature, velocity and air
humidity. The drying experiments were carried out at different temperatures ranging from
40 to 90 0C, with intervals of 10 K. The effect of temperature, drying rate and drying time
were conducted at constant air velocity of 1 m/s, with a dew point temperature of 12 0C,
characteristic for Romanian climatic and normal atmospheric pressure. Six levels of drying
air velocities ranging from 0.25 to 1.45 m/s were used. Four different varieties of tomatoes
were used for drying at 60 0Cand 1m/s air velocity and relative humidity of air of 7 %. The
experiments on the effects of drying air velocity were conducted at temperature of 60 0Cat
constant dew point temperature of 12 0C. Different relative humidity of the drying air in
the range of 7 to 45 % was applied. The effect of relative humidity was conducted at 60 0C

Layer Drying Behaviour of Tomatoes 39
and constant air velocity of 1 m/s. Drying air temperatures, drying air velocity, weights,
and atmospheric conditions were recorded in 10 or 20 minutes intervals.

3.2.2.2 Determination of Moisture Content of Tomatoes

For determination of moisture content of tomatoes was used Karl – Fischer method.
After drying of product, that was stored 24 hours in refrigeration in hermetic containers for
homogenization. Regardless of the method used, there are possibilities of measurement
errors in determining the moisture content. To reduce the possibilities of errors, two
replicates were used. The wet base moisture content of tomatoes was calculated using
formula:
100*)()()(



=tmmtmtMD (3.1).
The dry base moisture content is computed using the following mathematical
formula based on the instantaneous weight changes of tomatoes during the drying process.

DD
mmtmtX
=)()( (3.2).

3.2.2.3 Drying Rate

Drying rate was computed using Eq. (3.3) expressed as follows:

tt tXtX
dtdX
+
=) ( )((3.3).

The drying rate was analysed by plotting the dry base moisture content against the
drying time.

Layer Drying Behaviour of Tomatoes 40
3.3 Results and Discussion

3.3.1 Over-Flow Drying

3.3.1.1 Influence of the Drying Air Temperature

The drying characteristics of tomatoes to reach the final moisture content is
significantly influenced by the drying temperature such that the drying is faster with
increasing temperature, as illustrated in Figure 3:2. The increase of temperature to
accelerate the drying process is limited by the heat sensitivity of the product. In that way,
the temperature above 60 0Chas to be avoided in order to prevent colour changes induced
by high temperature, (W. Bieg, 1985).
There were significant differences in the drying time as a function of air
temperatures between 40 and 70 0C. These differences were found to less in the
temperature range above 70 0C, as indicated in Figure 3:3.
01 5 3 0 4 5 6 005101520
900C800C
700C600C
500C400C400C
500C
600C
700C
800C
900C
h.kg/kgMoisturecontent
Drying time
Figure 3:2 Thin layer drying behaviour of quarter’s tomatoes at different drying
temperatures, (T dp=12 0C, V a=1m/s, variety Flandra F1).

Increasing drying temperature from 40 to 60 0Cresulted in a reduction of the drying
time of 56 hours, whereas increasing the temperature from 60 to 90 0Cresulted in a
reduction of the drying time of only 9 hours. Drying of tomatoes at a temperature of 90 0C

Layer Drying Behaviour of Tomatoes 41
compared to the dried at 40 0Cresulted in the reduction of the drying time by 87.2 %.
Recommendation of optimal drying temperature must be according with the colour
changes and quality characteristic of dried tomatoes.

4 05 06 07 08 09 0020406080Dryingtime,h
Drying temperature, 0C
Figure 3:3 Influence of drying temperature on the drying time of quarter’s tomatoes,
(Tdp=12 0C, V a=1m/s, X 1=0,09 kg/kg, variety Flandra F1).

0 20 40 60 80 1 00012345
400C900C
800C
700C
600C
500CDryingrate,kg/kg*h
Drying time, h
Figure 3:4 Drying rate of quarter’s tomatoes at different drying temperatures versus
drying time, (T dp=12 0C, V a=1m/s, variety Flandra F1).

When the other variables remained constant, the drying rate of tomatoes is
increased with increasing of drying temperature. Figures 3:4 and 3:5 show the drying rate
of tomatoes at different temperatures with respect to drying time and moisture content
respectively.

Layer Drying Behaviour of Tomatoes 42
0 5 10 15 20012345900C
700C800C
600C
500C
400C
kg/kgDryingrate,kg/kg*h
Moisture content, db.

Figure 3:5 Drying rate of quarter’s tomatoes at different drying temperatures versus
moisture content, (T dp=12 0C, V a=1m/s, variety Flandra F1).

3.3.1.2 Product Temperature

Reactions between food components are accelerated during drying leading to
significant reduction in quality and nutritional value. The reaction rates are strongly
affected by the temperature and moisture content of food during drying (Labuza 1972,
Adam 1998, Bieg 1985). The most important component of the colours of tomatoes,
Lycopen is destroyed at 50 0C(Bieg 1985, Cole and Kapur 1957) or at 70 0C(Nguyen,
Francis and Schwartz, 2001). For this reason knowledge of the product’s temperature
distribution during the drying process is needed in order to evaluate possible nutrient losses and quality changes.
The tomatoes’ temperature during the drying process is given in Figure 3:6. The
higher the drying air temperature the faster the increase of the product temperature. At the
beginning of the drying process, the product temperature was lower than the temperature of
drying air due to the effect of evaporative cooling. As the drying process proceeded, the
moisture content decreased and the internal resistance to water transport increased. At the
end of the drying process, the temperature curves levelled off indicating equilibrium of
heat transfer between the tomatoes and the drying air. The results indicated a difference
between the drying air temperatures and tomatoes for all temperatures of drying air used.

Layer Drying Behaviour of Tomatoes 43
That difference between the drying air temperature and product temperature increased with
increasing of drying temperature from 2 K at 40 0Cto 13 K at 90 0C. Increasing drying
temperature from 40 to 90 0Cresulted in a increasing of final temperature of product from
38 to 77 0C. This difference can be attributed to adiabatic cooling gave by water
evaporation.

Figure 3:6 Temperature profile of tomatoes during drying process at different drying
temperatures, (T dp=12 0C, V a=1m/s, variety Flandra F1).

3.3.1.3 Influence of Air Velocity

The effect of air velocity on drying behaviour of tomatoes at 60 0Cand dew point
temperature of 12 0Cis given in Figure 3:7. Air velocity below than 0.5 m/s had a
significant influence on the drying time. Increasing of air velocity up to 0.75 m/s had as
results decreasing of drying time. These results revealed that, the air velocity must be
higher than that required for moisture transport. The higher air velocity the higher the
drying rate at any product moisture level, Figure 3:8.0 1 53 04 56 07 59 0020406080
900C 800C
700C
600C
500C 400C0C
hTemperature
Drying time

Layer Drying Behaviour of Tomatoes 44
0 1 0 20 30 40 50 6002040608010 0
1,45 m/ s1,25 m/ s
1m/ s
0,75 m/ s
0,50 m/ s0,25 m/ s
9%%Moisturecontent,wb.
Drying time, h
Figure 3:7 Influence of the air velocity on the layer drying behaviour of quarter’s
tomatoes (T dp=12 0C, T= 60 0C, variety Flandra F1).

05 1 0 1 5 2 00,01, 53,04,5
1,45 m/ s
1,25m/ s
1m / s
0,75m/ s
0,50m/ s
0,25m/ s
kg/ kgDryingrate,kg/kg*h
Moisture content,db.
Figure 3:8 Drying rate of quarter’s tomatoes at different air velocity, (T dp=12 0C,
T= 60 0C, variety Flandra F1).

At the beginning of the drying process, the air velocity has significant influence on
the drying rate, but at low level of moisture the effect of the air velocity is insignificant.
The drying time to rich the final moisture content at different air velocities is showed in
Figure 3:9. Increasing of air velocity from 0.25 m/s to 1 m/s resulted in decreasing time of
drying with 50 %. Further increase of velocity has insignificant influence on the drying
time. The increase of air velocity from 1 to 1.45 m/s has as result a decrease of drying time
by only 10 %.

Layer Drying Behaviour of Tomatoes 45
0,0 0,5 1 ,0 1 ,5020406080Dryingtime,h
Air velocity, m/ s
Figure 3:9 Influence of air velocity on the drying time of quarter’s tomatoes, (T dp=12 0C,
T= 60 0C, X 1=0,09 kg/kg. variety Flandra F1).

3.3.1.4 Influence of Relative Humidity

The driving force for drying is provided by gradient of water vapour pressure
between the drying air and the evaporating surface. As the moisture is progressively
removed from the tomatoes, the coupled heat and mass transfer between tomatoes and the
drying air undergoes several changes. The effect of air relative humidity on the drying
behaviour of quarters of tomatoes when all drying parameters were kept constant is show
in Figure 3:10 .During the drying process, the relative humidity of the drying air increases
whereas the temperature at the exhaust reduces. The knowledge of the influence of
different humidities on the drying process is especially of the interest in whether condition
in the summer season in South of Romania area, where variation in the air humidity during
day and night is highly significant. Therefore, the experimental drying investigation of
various air humidity is assumed of important consideration regarding the solar drying in
South of Romania regions. The drying time to reach the desired final moisture content of
dried tomatoes is increased gradually with the humidity of the air. Drying with air
humidity about 45 % resulted in an increase the drying time exponentially, as indicated in
Figure 3:11.

Layer Drying Behaviour of Tomatoes 46
01 5 3 0 4 5025507510 0
7%
15 %
25%
35%
45%%Moisturecontent,w.b.
D rying time, hr.
Figure 3:10 Influence of relative humidity of air on the drying time of quarter’s tomatoes,
(va=1 m/s, T= 60 0C, variety Flandra F1).

01 5 3 0 4 501020304050
6%
9%
12 %hDryingtime,h
Relative humidity, %
Figure 3:11 Drying time at different relative humidity of air to reach the final moisture
content at 6, 9 and 12 % (v a=1 m/s, T= 60 0C, variety Flandra F1).

Drying with air humidity of 45 %, at a temperature of 60 0C, the time is increased
with 18h, comparative with drying at relative humidity of air of 7 %, to reach the final
moisture content of 9 %. At the product moisture content of 12 %, the relative humidity of
the drying air has just a small effect on the drying time. At this moisture level, increasing
the relative humidity of the drying air from 7 to 45 % is resulted in increasing the drying
time with 32 %. Where the same increase in the humidity to reach the moisture of 9 % is resulted in increasing the drying time with 58 %. Drying at a temperature of 60
0C, with

Layer Drying Behaviour of Tomatoes 47
relative humidity of air higher than 15 %, is not possible to achieve a final moisture
content of product of 6 %. Therefore, it is recommended to avoid drying with high relative
humidity.

3.3.1.5 Influence of the Variety on the Drying Behaviour

Four different varieties were used for analysis, as is shown in Table 3.1. It was
observed differences in the initial moisture content of the fresh tomatoes, which had as
result a variation in the drying time. The shape of fruit which is characteristic to every
variety influenced the drying time also.

Table 3.1 Different tomatoes varieties
Initial moisture content Variety
% kg/kg Total sugar
kg/kg dry mater
Fleisch 95,15 19,63 0,78
Orco 92,97 13,22 0,79
Flandra 93,90 15,39 0,80
Cronos 93,81 15,92 0,83
The drying time of halves tomatoes was less than double time of the drying of
quarter’s tomato to any variety used in experiments, as is shows in Figure 3:12. However,
efficient energy utilisation of the drying air was achieved for drying of halves tomatoes,
but in the other hand, drying time will be increase and that can have as result modification
of quality of product.
Drying characteristics of four varieties of tomatoes are presented in Figure 3:13 .

Layer Drying Behaviour of Tomatoes 48
Cronos Fleisch Flandra Orco020406080Dryingtime,h
Varietyhalves
quarters

Figure 3:12 Influence of variety and shapes on the drying time of tomatoes (v a=1 m/s,
T= 60 0C, X 1=0,09 kg/kg, RH= 7%).

05101520
02 0 4 0 6 0 8 002040608010 0
%kg/ kgMoisturecontent,db,kg/kg Moisturecontent,wb.%
h
Drying time, h
Figure 3:13 Drying characteristic of different tomatoes varieties, (v a=1 m/s, T= 60 0C,
RH= 7%).

Layer Drying Behaviour of Tomatoes 49
Variety have an influence on the loading capacity, every variety have a
characteristic weight for fruit. For varieties which have large fruits was registered a long
time for drying comparative with varieties with small fruits. The drying time is found to be
influenced significantly by variety used; that could be a result of reduction of heat and
mass transfer in the case of the variety which has a large fruit.

0 1 53 04 56 001234
Cronos
Fl eish
Flandra
Or c okg/ kg* h
Dryingrate,kg/kg*h
Drying time, h
Figure 3:14 Drying rate of quarter’s tomatoes for different variety versus drying time
(va=1 m/s, T= 60 0C, RH= 7%).

0 20 40 60 800,00,51,01,5
Fleishtomatoes
Or c o
Cronos
FlandraDryingratekg/kg*h
Drying time, h
Figure 3:15 Drying rate of halves tomatoes for different variety versus drying time rate
(va=1 m/s, T= 60 0C, RH= 7%).

Drying rate for quarters and halves tomatoes for four varieties of tomatoes, are
presented in Figures 3:14 and 3:15, respectively. For all varieties, drying rate have a

Layer Drying Behaviour of Tomatoes 50
bigger value for quarters than for halves; that could be as a result of a better diffusivity of
moisture transfer from the product and a better heat transfer into the product.
Considering the solar drying of tomatoes, it might be desirable to use quarters of
tomatoes in orders to achieve a good quality of product and to prevent spoilage of product
which can appear during the night, at intermediate moisture content of product.

3.3.2 Through-Flow Drying

3.3.2.1 Influence of Air Temperature on the Drying Behaviour

Using the through flow mode, the increasing of air temperature has a significant
influence on the drying behaviour of tomatoes, as illustrated in Figure 3:16 .At higher air
temperature, the drying process is shorter. There were significant differences in the drying
time as function of drying temperature between 40 and 60 0C. These differences were
found to be smaller between temperature 60 and 80 0C.

0 20 40 60 80025507510 0
800C
700C600C5 00C 400C
h%Moisturecontent,wb.
Drying time, h
Figure 3:16 Influence of airflow mode on the drying behaviour of quarter’s tomato at
different drying temperatures (T dp=12 0C, V a=1m/s, variety Flandra F1).

In Figure 3:17 is shown the influence of the airflow mode on the drying
characteristic of tomatoes. The drying characteristics were kept constant. Using the
through flow mode, the drying was faster compared to the over-flow mode. This can be

Layer Drying Behaviour of Tomatoes 51
attributed to a relatively good heat exchange establish between the tomatoes and drying air,
and to the higher heat and mass transfer in case of through flow mode. However, the
resistance to the through airflow is assumed to be higher.

05101520
01 5 3 0 4 5 6 005101520800C700C600C
500C400C
kg/ kgkg/ kgMoisturecontent,db.
800C700C
600C5 00C400CMoisturecontent,db.
h
Drying time, h
Figure 3:17 Influence of airflow mode on the drying behaviour of quarter’s tomato at
different drying temperature (T dp=12 0C, V a=1m/s, variety Flandra F1).

Figure 3.18 shows the influence of drying air temperature on the drying time of
tomato by through and over flow modes. Increasing the drying temperature from 40 to
60 0Chas as result reducing a drying time with 64.8 %, and increasing up to 80 0C, has as
result reducing of drying time with 83 %, in through flow mode. At the temperature of
60 0C, the through flow mode drying is resulted in 25 % reduction of the drying time
compared with over flow mode, at the same drying conditions. It can be seen that the
difference between drying time in over flow and through flow mode it was higher at low
temperature (22 hours at 40 0C) and smaller at high temperature (2.5 hours at 80 0C).

Layer Drying Behaviour of Tomatoes 52
30 45 60 75 9002040608010 0over- flow
through- flowDryingtime,h
Drying temperature, 0C
Figure 3:18 Influence of airflow mode on the drying time of quarter’s tomato at different
drying temperature (T dp=120C, V a=1m/s, X 1=0,09 kg/kg, variety Flandra F1).

When other variables remained constant, the drying rate of tomatoes, using through
flow air method, is increased with increasing of drying temperature. Figure 3:19 show the
drying rate of tomatoes at different temperatures with respect to moisture content.

0 5 10 15 2 00246
400C500C600C70oC80oCkg/ kg* h
kg/ kgDryingrate,kg/kg*h
Moisture content, db.
Figure 3:19 Drying rate of quarter’s tomatoes at different drying temperatures versus
moisture content, (T dp=12 0C, V a=1m/s, variety Flandra F1).

The drying rate was affected by the airflow drying method. The graphics of the
drying rate of tomato, in through and overflow air mode, at different drying temperature

Layer Drying Behaviour of Tomatoes 53
are presented in Figure 3:20. The drying rate has a lower value when the tomato is dried
by overflow drying method. Because of low drying rate, the drying time of tomato on over
flow mode is longer compared with the drying on through flow air mode.

0246
02 0 4 0 6 0 8 00246800C
700C
600C
500C
400CDryingrate,kg/kg*h
800C
700C
600C
500C
400CDryingrate,kg/kg*h
h
Drying time, h
Figure 3:20 Influence of airflow mode on the drying rate of quarter’s tomato at different
drying temperature (T dp=12 0C, V a=1m/s, variety Flandra F1).

Layer Drying Behaviour of Tomatoes 54
3.3.2.2 Product Temperature

The tomatoes’ temperature during the drying process is given in Figure 3:21. The
results indicated a difference between the drying air and tomatoes temperatures for all
values of temperatures of air used, both on over and through flow mode. That difference
between final temperature of tomatoes the drying air temperature was higher in over flow
than the through flow mode. This could be attributed to the equilibrium of heat transfer
between the tomatoes and the drying air.

020406080
0 20 40 60 800204060800C
0CProducttemperature Producttemperature
h
Drying time

Figure 3:21 Influence of airflow mode on the temperature profile of tomatoes during
drying process at different drying temperatures, (T dp=12 0C, V a=1m/s,
variety Flandra F1).

Layer Drying Behaviour of Tomatoes 55
3.3.2.3 Influence of Air Velocity

Reducing the air velocity resulted in increasing the drying time in through flow air
mode. Figure 3:22 shows the drying behaviour of tomato using different air velocities. By
using the through-flow method, increasing the air velocity from 0.25 to 1 m/s has as result
reducing of the drying time from 32.17 h to 22.5 h. Further increase of the air velocity to
1.25 m/s has as result reducing to 19.5 h the drying time. The characteristic curves of the
through flow drying at the air velocity of 0.75 and 1.0 m/s are very close to each other
which indicated that it is no significant differences on the drying behaviour of tomatoes.

0 1 0 20 30 4002040608010 0
1,251, 0 0
0,75
0,500,25 m/ s
h%Moisturecontent,wb.
Drying time
Figure 3:22 Influence of the air velocity on the layer drying behaviour of quarter’s
tomatoes (T= 60 0C, T dp=12 0C, variety Flandra F1, through flow mode).

The characteristic curves of the over and through-flow methods for drying tomatoes
at 60 0Cand relative humidity of 7 % are showed in Figure 3:23. Using the through flow
mode, the drying was faster compared to the over-flow mode for all air velocities used.

Layer Drying Behaviour of Tomatoes 56
05101520
0 15 3 0 4 5 6 0051015201, 2 5 m / s1,00m/ s
0,75m/ s
0,50m/ s 0,25m/ s
kg/ kgkg/ kg
Moisturecontent,db.
1, 2 5 m / s1, 0 0 m / s
0,75m/ s
0,50m/ s0,25m/ sMoisturecontent,db.
h
Drying time, h
Figure 3:23 Influence of airflow mode on the drying behaviour of quarter’s tomato at
different drying temperature (T=60 0C, T dp=12 0C, variety Flandra F1).

The drying time to rich the final moisture content of 0.09 kg/kg at different air
velocities in over and through mode is showed in Figure 3:24. Between through and over
flow drying method were registered differences for all the investigated air velocities. The
highest difference was registered for air velocity of 1.25 m/s, and has value of 28 h. The
method of drying and also the air velocity of drying air have influence on the drying
behaviour of tomato.

Layer Drying Behaviour of Tomatoes 57
0,0 0,5 1 ,0 1 ,5020406080
over- flow
through-flowDryingtime,h
Air velocity, m/ s
Figure 3:24 Influence of airflow mode on the drying time of quarter’s tomato at different
drying temperature (T=60 0C, T dp=12 0C, X 1=0,09 kg/kg, variety Flandra F1).

In a through flow mode, increasing the air velocity has as effect increasing the
drying rate at any product moisture level. Figure 3:25 reveals that there where no constant
rate periods and the drying took place during the falling rate periods, for all air velocities.

05 1 0 1 5 2 001234
0,25m/ s0,50m/ s0,75m/ s1,00m/ s1,25m/ s
kg/ kgkg/ kg* h
Dryingrate,kg/kg*h
Moisture content, db.
Figure 3:25 Drying rate of quarter’s tomatoes at different drying temperatures versus
drying time, in through flow mode (T=60 0C, T dp=12 0C, variety Flandra F1).

Layer Drying Behaviour of Tomatoes 58
3.3.2.4 Influence of Relative Humidity

08 1 6 2 4 3 20481216
7%15 %
25%
35%=4 5%
hkg/ kg Moisturecontent,db.
Drying time, h
Figure 3:26 Influence of the relative humidity of the air on the layer drying behaviour of
quarter’s tomatoes (T= 60 0C, V a=1m/s, variety Flandra F1).

Drying in a through flow mode with air humidity of 45 %, the time is increased
with 8 h, comparative with drying at relative humidity of air of 7 %, to reach the final
moisture content of X 1=0.09 kg/kg. The knowledge of the influence of relative humidity of
the drying air is especially of the interest in quality changes of tomatoes during the drying.

0 1 0 20 30 40 5001020304050
through- flow
over- flowhDryingtime,h
Relative humidity, %
Figure 3:27 Influence of airflow mode on the drying time of quarter’s tomato at different
relative humidity of air (T=600C,V a=1m/s, X 1=0,09kg/kg, variety Flandra F1)

Layer Drying Behaviour of Tomatoes 59
Where the same increase in the relative humidity of drying air from 7 to 45% to
reach the moisture of X 1=0.09 kg/kg is resulted in increasing the drying time with 58 % in
over flow mode and with 35 % in through-flow mode, as is shows in Figure 3:28. The
value of relative humidity of drying air has less influence on a drying time using throw
flow mode than using over flow mode.

025507510 0
01 0 2 0 3 0 4 0025507510 07%15 %
35%25%
=4 5%
kg/ kgkg/ kgMoisturecontent,db.
7%15 %
25%
35%
=4 5%Moisturecontent,db.
h
Dr ying time, h
Figure 3:28 Influence of airflow mode on the temperature profile of tomatoes during
drying process at different drying temperatures, (T= 600C, V a=1m/s, variety
Flandra F1).

In a through flow mode, increasing the relative humidity of the air has as effect
decreasing of the drying rate at any product moisture level.

Layer Drying Behaviour of Tomatoes 60
05 1 0 1 5 2 00123
7%
15 %
25%
35%
=4 5%
kg/ kgDryingrate,kg/kg*h
Moisture content, db
Figure 3:29 Drying rate of quarter’s tomatoes at different relative humidity of air, in
through flow mode (T= 60 0C, V a=1m/s, variety Flandra F1).

3.4 Conclusions

Drying experiments were carried out using a laboratory dryer which offers the
possibility of drying in over- flow mode, which the aim to study the influence on the
drying behaviour. Some series were dried at different temperatures ranging from 40 to
90 0C, with intervals of 10 K. The drying time of tomatoes to reach the desired final
moisture content of 9 % was significantly affected by the temperature of air of drying.
Preliminary quality evaluation, on effect of drying temperature on the quality deterioration,
revealed that higher drying temperature affects significantly the colour of the dried
tomatoes. The temperature of 60 0Cwas founded to be the optimum drying temperature,
from thermodynamic point of view; this argument is supported by Bieg (1985).
The moisture removal rate was founded to be fast at the beginning, due to the
higher level of moisture content in tomatoes, and drying is accelerated by increasing the air
velocity rate. Increasing the air velocity to a certain limit is resulted in significantly
decreasing of the drying time. The increase of the velocity from 0.25 to 0.50 m/s is resulted
in 32 % decrease in the drying time, while the increase from 0.5 to 1.0 m/s is resulted in
decreasing the drying time with 32 %, and increase from 1.0 to 1.45 m/s is resulted in
decreasing the drying time by only 19.2 %. Higher drying rates were achieved by higher

Layer Drying Behaviour of Tomatoes 61
air velocity of drying air. Therefore, during solar drying it might be necessarily to increase
the velocity of the drying air. From drying technical point of view, during field application
of solar drying of tomatoes, the drying time could be decreased by increasing the air
velocity.
In the Romanian condition climatic, in the summer season, the variation in the air
humidity during day and night is quite significant. This fluctuation is assumed to be of
main important consideration regarding the sun drying in these regions, especially on
beginning of drying.
The size of tomatoes used for drying was found to be of important consideration
relative to internal mass transport. Results revealed that, a different between quarters and
halves affect significantly the drying time, because increasing of evaporative surface and
decreasing of diffusion path. When other variables remained constant, the fastest drying
rates was achieved at variety of tomatoes with small fruits, because reducing the diameter
is resulted in reducing the diffusion path and increasing heat and mass transfer between the
surface and the surrounding area.
The airflow mode has also a paramount importance in behaviour of drying
tomatoes. Higher drying rates were achieved by through-flow air mode for all experiments
comparative by over flow mode. The drying rate using over flow mode was relatively low
and the drying time was long. However, for the same air velocity, in the through flow
mode the pressure drop was higher than to the over flow mode. Therefore, from technical
point of view, during solar drying of tomatoes the over flow mode is recommended.
Apossibility to reduce the drying time during solar drying in over flow mode could
be increasing the air velocity since the air velocity affect significantly the drying time.
Another possibility could be using of another variety of tomatoes.
In both, over flow and through flow mode, the required time for drying of tomato
depends on parameters as: tomato variety (the soluble solid contents –Brix- of the fresh
tomato, the size of tomato) the air temperature and velocity and the air humidity.
.

Quality Changes in Tomatoes as Affected by the Drying Process 62
4 QUALITY CHANGES IN TOMATOES AS AFFECTED BY THE DRYING
PROCESS

4.1 Introduction

Quality is the sum of characteristics of product concerning their suitability to fulfil
defined and underlying parameters as: form, colour, aroma, taste, mouth feeling, texture,
consistence, concentration of Lycopene, type and concentration of carbohydrates, fats,
protein, vitamins, minerals, technical suitability for storage, type and concentration of
toxins.
Dried tomato products are made in several forms: quarters, halves, sliced and
powdered. There is an increasing demand for high quality dried tomato which is used as
colorant and flavour additive in food industry. Different physics and chemical process
occur during drying process and have as results changing of value of different compounds
which can lead to various type of quality deterioration.
In literature are presented some of quality change of tomatoes during drying
process. Colour and flavour are considered the most important criteria for evaluation of
dried tomatoes. Colour of tomatoes is given by Lycopene and V-carotene content.
Contribution of those to the colour formation of tomatoes is gave by Lycopene 92-93 %
and 7-8 % by V -carotene, (Bieg, 1985).
Figure 4:1 Chemical formula of V -carotene (up) and Lycopene (down).

Changes of quality which occur during drying process of tomatoes are affected
mainly by drying air temperature and have as results brown enzymatic and non-enzymatic
reaction. Brown enzymatic reactions take place mainly during manipulation, slicing or
during drying, (Labuza and Schmidt, 1986).

Quality Changes in Tomatoes as Affected by the Drying Process 63
4.2 Literature review

The quality changes which occur after harvest and during drying process of
tomatoes had been studied by several researchers. Petrescu et al., (1992) presented a
chemical composition of tomatoes fruits and nutritional importance of them, ( Table 4.1).
The non-enzymatic reactions (Maillard) are considerate to affect the change of
colour during drying process. Maillard reaction is gave by reaction between aldehydes,
ketones, and reducible sugar with amino- compounds from amino-acids and proteins, as is
presented in Figure 4:2 ,(Hodge, 1953).
Tomatoes variety type Roma, contain a large quantity of dry mater and therefore
should be more affected by Maillard reaction during drying process. However, the
tomatoes varieties which has a green colour ring near calyx, should have after drying a
different colour and therefore are non recommended for drying, (Bieg, 1985).
Legault et al., (1951) made research about Maillard reaction during drying of
vegetables and conducted to increasing of brown colour with increasing of temperature and
relative humidity of drying air. The Maillard reaction activity is increase with increase of
water activity value. The brown rate could be decrease by drying of products al low
temperature.
Vitamin C is soluble in water, and has a higher stability at low water activity value,
(Haralampu and Karel, 1983). At high level of moisture content of product, Vitamin C,
became very sensitive at high temperature, (Okos et al., 1992). Vitamin C acts as a
conserving of colour during drying process, and has a paramount influence on final colour.
Drying characteristics of tomatoes have been studied by some researchers and
reported in the literature. Romero-Pena and Kieckbusch (2003) conducted a laboratory study on thin layer drying of tomatoes at 60, 80 and 100
0C. They recommended drying in
two-step: first at high temperature (pre-drying) and second at 60 0Cto have faster drying
rate without reducing the red colour of tomatoes. Bieg found the optimum temperature
from 50 to 70 0Cand optimum air velocity of 0.5 m/s for tomatoes drying, in through flow
mode. Andritsos (2003) reported that tomatoes should be dried at a temperature of
50 to 57 0Cfor a good retaining of the colour and the aroma.

Quality Changes in Tomatoes as Affected by the Drying Process 64
Table 4.1 .Chemical composition of tomatoes fruits – mean values, (Petrescu, 1992)
Fresh fruits Substance UM
(per 100 g)
Total sugar g 4.32
Glucoze g 2.02
Fructoze g 2.30
Water mg 94.20
Carotene mg 0.82
Vitamin B1 mg 0.057
Vitamin B2 mg 0.035
Vitamin B6 mg 0.10
Vitamin C mg 24.20
Vitamin K mg 0.65
Biotine mg 4.00
Acid nicotinic-starch mg 0.53
Iron mg 0.95
Magnezium mg 20.00
Natrium mg 6.30
Kalium mg 279.00
Phosfor mg 26.00
Fluor mg 60.00
Leucine g 0.039
Izoleucine g 0.020
Valine g 0.026
Metionine g 0.006
Fenilalanine g 0.027
Tirozyne g 0.014
Treonyne g 0.031
Triptofanum g 0.008
Lizyne g 0.040
Histidine g 0.014
Arginine g 0.028

Quality Changes in Tomatoes as Affected by the Drying Process 65
Figure 4:2 Maillard reaction, (Hodge, 1953).

Quality Changes in Tomatoes as Affected by the Drying Process 66
Drying of tomato causes oxidative heat damage to the product, shown by loss of
Lycopene, ascorbic acid and increase of Amadori compounds. All those are reflected in a
reducing of alimentary value of dried tomato and changing of colour. However, the change
in colour of tomatoes is only caused by the deterioration of Lycopene (Unadi et all, 2001).
The browning of product during drying at high temperature is reported by some
researchers (Cornwell and Wrolstand, 1981; Lozano and Ibarez, 1996) could also take
place in tomatoes.
The target of experiments about changes of quality during the drying is
optimisation of tomato drying in terms to maximising of drying rate and minimising the
oxidative heat damage and isomerisation of Lycopene. A systematic study on the
influences of temperature, relative humidity and air velocity on the quality changes during
the drying of tomatoes is not reported in the literature.

4.3 Materials and Methods

4.3.1 Material
In that experience, were used fresh tomatoes, which was supplied by a German
supermarket chain. It was used variety Flandra F1 of bush tomatoes. Tomatoes were
manually sorted, washed and cut in the quarters. The drying experiments were conducted
using a laboratory dryer as described previously in chapter one. The dried tomatoes were
packed in hermetic container of glass and stored in dark at a temperature of 4 0Cfor 24
hours for homogenizing. The colour measurements were conducted at the Institute for
Agricultural Engineering in the Tropics and Subtropics, University of Hohenheim.

Figure 4:3 Initial colour of tomatoes use in drying experience (variety Flandra F1)

Quality Changes in Tomatoes as Affected by the Drying Process 67
4.3.2 Determination of colour
For colour measurements was used Minolta Chroma Meter CR-100/CR-110. That
consists of two main parts, measurement and optical units. The measurement unit converts
the signal received from the optical unit into colour specifications represented by L*
lightness, a* redness and b* yellowness, which are displayed on the digital readout, stored,
and printed. During colour measurements of dried tomatoes, average of seventeen
successive measurements were taken. The L* parameter which is vertical axis represented
white at the top and black at the bottom. The a* parameter represented a wide range of
colours from red (+a*) to green (-a*). The b* parameter indicated also a huge range of
colours from yellow, indicated by +b*, to blue, indicated by –b*. The colour saturation C*
of the material is defined by horizontal position of a* and b* and the inclination angle U.
The theory relates the L*, a* and b* values to three-dimensional coordinate system for
colours (Hunter and Gibson, 1969), which is illustrated in Figure 4:4 .The angle Uis
calculated using the following equation:
**
abarctg= (4.1).
When Uis 00indicated that the colour is red, when the angle is 900the colour is
yellow, and when the angle is 1800the colour is green. Among these coordinates, a mixture
of these colours is represented. The colour saturation C* is calculated from Equation 4.2.
2 2* *b a C += (4.2).

Figure 4:4 Three dimensional coordinate system for colour measuring using method
L*,a*,b*, (Hunter and Gibson, 1969)

Quality Changes in Tomatoes as Affected by the Drying Process 68
4.4 Results and Discussion

4.4.1 Colour Changes

4.4.1.1 Influence of Over-Flow mode

The conducted colour measurement experiments were concerned primarily with
evaluation of colour changes resulted from different drying conditions of tomatoes. The
results obtained from an extensive evaluation made, proved that the drying temperatures of
air exert a pronounced effect on the colour changes to brown of the tomatoes, Figure 4:5 .
Figure 4:5 Colour changing during drying at different temperature of air in over-flow
mode (T dp=12 0C, V a=1m/s, variety Flandra F1).

Increase the drying temperatures show significant and uniform changes of colour
changes on the tomatoes, Figure 4:6. Increase the drying temperature from 40 to 50 0C
show an exponential change of colour saturation on the tomatoes, the drying temperature
above 50 0Chave a linear influence on the value of saturation on the tomatoes. The change
in the colour could be attributed to the browning reactions that took place during the drying
process. The results revealed that the rate of browning reactions increased with the increase
of temperature. Browning is also more severe near the end of the drying period when the
moisture level is low and less evaporative cooling is taking place, which causes the product
temperature to increase. However, there are several way to reduce browning during drying,
in which they all emphasized that unnecessary heat to be avoided especially at low

Quality Changes in Tomatoes as Affected by the Drying Process 69
moisture levels. The data found for L*, a* and b* at 900C, for drying in over flow mode
agree with the data reported by Shi et al., (1999).

4 05 06 07 08 09 001020304050
10203040
0CLightness,L*
Temperature, 0CC*L*
Saturation,C*
Figure 4:6 Colour changes of dried tomatoes at different temperature of drying air
(Tdp=12 0C, V a=1m/s, variety Flandra F1).

The rate of the browning reactions is also increased at higher relative humidity of
the drying air as indicated in Figure 4:7.
0 1 0 20 30 40 5020304050
20304050L*
C*Lightness,L*
Relative humidity, %%
Saturation,C*
Figure 4:7 Influence of relative humidity on colour changes of dried tomatoes (T=60 0C,
Va=1m/s, variety Flandra F1).

When the tomatoes was dried at 60 0Cand a humidity of 45 %, the lightness was
reduced by 5.1 % in the pulp and 4.3 % in the peel and saturation was increased with
13.1 % in the peel and by 26.5 % in the pulp. This could be attributed to the fact that at

Quality Changes in Tomatoes as Affected by the Drying Process 70
higher humidity the drying time is increased with 58 % as indicated in Figure 3.11 .In the
other hand at high humidity could be destroyed Lycopene and V-carotene during the drying
process. Anyway, drying at extreme relative humidity has to be avoided in order to
prevent colour changes.
When all other drying parameters are kept constant, the velocity of the drying air
has a significant influence on the colour changes of the dried tomatoes as illustrated in
Figure 4:8. For value of air velocity, less than 0,5 m/s when the drying time was increased
tremendously might be a high level of destroyed of Lycopene and V-carotene, and a colour
have a yellow tend. Increasing the air velocity above 0.75 m/s did not affect significantly
the drying time and hence the colour of the dried tomatoes. Therefore, the airflow rate
must be higher than that is required for moisture transport in order to reduce drying time.

0,0 0,4 0,8 1 , 21 , 61020304050
1020304050L*
C*
m/ sLightness,L*
Air velocity, m/ s
Saturation,C*
Figure 4:8 Influence of air velocity on colour changes of dried tomatoes (T dp=12 0C,
T= 60 0C,variety Flandra F1).

4.4.1.2 Influence of Over-Flow mode

The increase of temperature to accelerate the drying process is limited by the heat
sensitivity of tomatoes. The effect of drying air temperature on colour of tomatoes is
shown in Figure 4:9. It is revealed from the figure that an increase of drying air
temperature from 40 to 60 0C, the lightness value remains almost constant. Above the
drying air temperature of 60 0C, the lightness value decreases. Drying temperatures above
60 0Chave to be avoided in order to prevent colour changes induced by high temperature.

Quality Changes in Tomatoes as Affected by the Drying Process 71
4 05 06 07 08 0102030405060
102030405060
C*L*
0CLightness,L*
Temperature, 0C
Saturation,C*
Figure 4:9 Colour changes of dried tomatoes at different temperature of drying air
(Tdp=12 0C, V a=1m/s, variety Flandra F1).

Relative humidity is the next most important factor affecting the quality of the dried
product, (Guarte, 1996; Bashir, 1998). The effect of relative humidity on the colour of dried tomatoes is presented in Figure 4:10.
0 1 0 20 30 40 50102030405060
102030405060
%Lightness,L*
Relative humidityC*L*
Saturation,C*
Figure 4:10 Influence of relative humidity on colour changes of dried tomatoes (T=60 0C,
Va=1m/s, variety Flandra F1).

Quality Changes in Tomatoes as Affected by the Drying Process 72
For the increase in relative humidity from 7 to 25 %, no significant change was
found in the saturation value. Increasing the relative humidity above that 25 % has as result adecreasing of saturation value. The value of lightness is decreasing with increasing of
relative humidity.
There is no significant effect of air velocity on colour of dried tomatoes, ( Figure 4:11).

0,0 0,2 0,4 0,6 0,8 1 , 01 , 21 , 42030405060
1020304050
C*L*Lightness,L*
Air velocity, m/ s
Saturation,C*
Figure 4:11 Influence of air velocity on colour changes of dried tomatoes (T dp=12 0C,
T= 60 0C,variety Flandra F1).

4.5 Conclusions

Colour is one of the most important factors of quality evaluation of agricultural
product. The results obtained from investigation on colour measurement proved that higher
drying temperatures have a higher influence on colour changes of tomato, which can be
explained by the browning reaction. Drying temperature above 60 0Chas significant
influence on the colour of tomatoes, therefore should be avoid. The investigation revealed
that the relative humidity of the drying air has less influence on the drying behaviour
compared to the drying temperature, but have a high influence in colour changes. The
optimum air humidity was found to be less than 15 %. The velocity of the drying air has
insignificant influence in colour changes, but could have an influence on drying behaviour,
therefore it is recommended to be used air velocities higher than 1 m/s.

Solar Drying of Tomatoes in Romanian 73
5 SOLAR DRYING OF TOMATOES IN ROMANIA

5.1 Introduction

In Romania a mean area of 46,382 ha, with a multiannually average yield of
144.826 t/ha, is planted annually with tomatoes. In 2003 year the surface area was 49,301
ha with a yield of 166.109 t/ha. Primary crop, in greenhouses are made during winter and
spring season and are destination to fresh consumption. During the summer, the crop of
tomatoes is oriented to the fresh consumption but mainly to the industrialization due to the
high production and small price on the market. The varieties grown at present are of local
origin and from import, mainly from Holland, Bulgaria and Turkey. The most of these
cultivars are suitable for dehydration because they have a uniform red colour and high dry
matter content. In the summer time when surpluses of production are available and the
farmers are not able to sell their tomatoes profitably, the post harvest loses of tomatoes are
about 30 %. Preservation of tomatoes as juice, ketchup, teens and cold storage are costly
and required use of high investition in building, machinery, and use of conservants. The
introduction of solar drying technique to use the surplus of tomatoes production can lead to
raise the income of the farmers and reduce the import of tomatoes during the cold season.
At that moment in Romania the common way to dry tomatoes is drying at high
temperature with electrical power supply dryer. The drying technology required using of
high amount of conservants witch make the dried product to be not accepted on the market
accordingly with international quality and microbiological safety standards. However, the
plants are old and the consume of energy is assumed to be high and the drying to be costly.
It is an increasing interest in dehydrated tomatoes for local market and to export
especially in Union European Market. The quantity of local dried tomatoes sold on local
market is still to small and this could be attributed to the low quality and high price. The
demand for dried vegetables is increasing on Romanian market, and imports of dried
vegetables has increased from 992 t in 2002 year to 1,712 t in 2003, and in the while time
the exports has decreased from 152 t to 37 t respectively (FAOSTAT Database, 2004).
About 35 – 40 % from those values was represented by dried tomatoes (MAAP, 2004).
As an alternative to conventional drying, solar drying is an alternative and it is
expected to play more important role in Romanian’s agriculture due to high level of solar
radiation during the harvest season of tomatoes, the increasing demand of dried vegetables

Solar Drying of Tomatoes in Romanian 74
and especially of dried tomatoes on the market, low cost of solar drying and storage of
dried product and due to a high quality of solar dried tomatoes.
Based on the results of the laboratory experiments, field test were conducted in
Romania in summer 2004 using a solar tunnel dryer developed at the Institute for
Agricultural Engineering in the Tropics and Subtropics, University of Hohenheim,
Stuttgart, Germany. The dryer was installed in Bucharest at Department of Agricultural
Engineering, University of Agricultural Sciences and Veterinary Medicine Bucharest,
where several drying test with different tomato varieties were carried out. The objectives of
the field study were to investigate solar drying of tomatoes under Romanian’s weather
conditions, to find the better varieties of tomato, which could be dry, and to evaluate the
quality attributes of the dried product.

5.2 Literature Review

Many types of solar dryers with different design and varying capacities from small
household size have been developed, but most of them are not suitable for Romanian
climatic condition or for drying of tomato.
Based on heating designs, the solar dryers can be classified into several categories
depending upon the mode of heating into direct, indirect and mixed mode, (Szulmayer,
1973; El Fadil, 1998). In direct solar dryers, the products are directly exposed to the solar
beam. In indirect flow, dryers the products are dried with air heated by solar collector, and
are protected from the solar beam. Such a design is recommended when controlled
conditions are required for the product to be dried, or some compounds of products are too
sensitive and can be easily destroyed by solar radiation. In mixed solar dryers, drying take place by combined action of heat from solar beam radiation on the surface of products and
the preheated air from the solar collector.
The solar dryer can be also classified by the way of air movement into natural and
forced convection dryers. In the natural convection dryers, the movement of air is made
due to the temperature and relative humidity gradients, between inlet and outlet points. The
forced convection dryers use a source of power to blow air into the dryer.
Furthermore, these dryers are classified by the way of airflow inside the dryers in
the over-flow dryers and through-flow dryers; and by the way of feed of the products into
the plant in continuous and discontinuous dryers.

Solar Drying of Tomatoes in Romanian 75
Only very few designs of solar dryers of tomato are described in literature. The
tomatoes is manually sliced in halves and put in dryer.
Jinasena et al., (1996) presented the tent dryer, which has a shape, as a Canadian tent is
the simplest type of these solar dryers. The substructure is made of wood covered with at
transparent plastic film. The floor is covered by a layer of dark stones or a black plastic film.
Drying the products in the tent dryer is done by spreading them on a wattle made of wood or
plastic mats. The waddle is fixed to the wooden substructure in the center of the tent. The
cover gives additional heating and prevents remoistening of the product by dew during the
night. The humid air is removed by natural convection. The capacity of the tent dryer is
limited to a maximum of about 20 kg of fresh product.
Sodha et al., (1987) presented the box dryer, which consists of a box with a sloping
transparent cover and blackened interior surface. Drying is effected by spreading the product
in thin layers on a tray which is exposed to the sun. Air flaps in the base and the upper parts of
the sidewalls maintain natural convection. The design of the dryer is simple and the costs are
low but the main disadvantage is its very small capacity of less than 10 kg of fresh tomatoes.
In 1990 FAO have initialized a developing program for pour area of Africa.
Preservation of vegetables by solar drying was part of this program, and small size dryer
was developed.
Bansal et al., (1993) presented an indirect solar cabinet dryer. The drying capacity this
type of dryer was about 50 kg; increased by connecting a solar air heater to a drying chamber.
Instead of using only the crop as absorber, solar radiation is further converted into thermal
energy in the solar air heater. The sloping air heater has to be mounted facing the sun and
tilted at an optimum angle depending on the region and the particular season. The drying air,
heated up in the solar air heater, enters at the base of the drying chamber, ascends and passes through the crop being spread in thin layers on vertically stacked trays. The airflow can be
increased either by wind or by a chimney.
Despite of worldwide efforts on the development of numerous types of solar cabinet
dryers, its use is still limited to demonstration units. This is because the investment is
relatively high, related to the capacity, (Bux, 2003). All the solar dryers described in the
literature are of low performance of technical, energetically and product quality.

Solar Drying of Tomatoes in Romanian 76
5.3 Romanian Climatic Data
Bucharest is located at 44050’- North Latitude and 26012’ East Longitude, in central
part of Romanian Plane, at 86 m average altitude. Bucharest area has a temperate
continental climate. The annual mean temperature is 10, 5 0C, and the mean of warmest
month (July) of 22.1 0C. During summer season the average temperatures are above 20 0C,
and during the day could reach 44 0C. In the summer season (June, July, August), the mean
relative humidity of air is less than 60 %, ( Figure 5:1 )and during afternoon could decrease
up to 30 %.
JF M A MJJAS O N D406080100
-100102030Relative humidity °C%Relativehumidity,%
MonthTemperature
Temperature,0C
Figure 5:1 Annual temperature and relative humidity at Bucharest, (I.N.M.H. Baneasa).

The annual mean of rainfall is 556.1 mm/year, Figure 5:2. Rainfall along the year
is not uniform, with a rainy season during summer and a drought season during the winter.
JF M A MJJAS O N D020406080100mmRainfall,mm
Month

Figure 5:2 Annual rainfall at Bucharest, (I.N.M.H. Baneasa).

Solar Drying of Tomatoes in Romanian 77
Figure 5:3 shows the monthly solar shinning time measured at Bucharest. The
drying season starts in second part of May when the solar shinning reached 250 hours per
month and ended in first part of September when the solar shinning time is steel longer
than 250 hours per month. The drying season has a same time as tomatoes harvest season,
therefor tomatoes could be dried directly and is not need a storage devices and expenses.
JFMAMJ JASOND0100200300400hSunshintime,h
Month

Figure 5:3 Monthly solar shinning time at Bucharest, (I.N.M.H. Baneasa).

5.4 Materials and Methods

5.4.1 Solar Dryer

The experiments was conducted using a solar dryer typ tunnel model Hohenheim
which was developed at the Institute of Agriculture Engineering in the Tropics and
Subtropics, Hohenheim University, Figure 5:4 .The version of the solar tunnel dryer used in
that experiment is 2.0 m width, and 19 m length. The solar air heater is 8 m in length and the
tunnel dryer is 10 m in length. The collector area is 16 m² and drying area is 20 m².
The capacity of the dryer is mainly influenced by the size, shape and moisture content
of the fruit to be dried. The loading capacity ranges from 100 kg for medicinal plants to 300
kg for figs, apricots or coffee.
The solar tunnel dryer consists basically out of a plastic foil-covered flat plate solar air
heater, a drying tunnel and two small axial flow fans, Figure 5:4 .To simplify the construc-
tion and to reduce the production costs, the solar air heater is connected directly to the drying

Solar Drying of Tomatoes in Romanian 78
tunnel without additional air ducts. Both, the air heater and the drying tunnel are installed
steel pipe feet at 1 m high to ease loading and unloading of the dryer. Due to the modular
design, the dryer can be enlarged in length up to 20 m for drying products in arid regions.

1. air inlet 5. metal frame 9. support for the polyethylene cover
2.fan 6. outlet of the collector 10. iron pipe feet support of the dryer
3.solar module 7. metal net support 11. rolling bar
4.solar collector 8. plastic net 12. outlet of the drying tunnel
Figure 5:4 Solar tunnel dryer, model Hohenheim, (Esper, 1996).

The solar tunnel dryer is operated with one photovoltaic module type BP that
is direct coupled to the DC-motor of fan. The solar generator is installed at the inlet of the
solar air heater. This enables cooling of the solar generator by forcing ambient air underneath
the backside of the module, Figure 5:5.
The floor of the solar tunnel dryer consists of polyurethane foam sandwiched between
two metal sheets with groove and tongue system. The insulator sheets are connected by a
corrugated metal frame, which also enables easy fixing and replacement of the transparent
plastic cover foil by using reinforced plastic clamps, Figure 5:6 .

Solar Drying of Tomatoes in Romanian 79
1. air inlet 4. side metal frame
2. fan 5. photovoltaic module BP type
3. back side insulator 6. transparent polyethylene cover

Figure 5:5 Collector section of the solar tunnel dryer model Hohenheim, (Esper, 1996).

Solar air heater and dryer are covered with a transparent UV-stabilized PE plastic foil,
0.2 mm in thickness, with a transmissivity of about 90 % for visible radiation. In general, the
transparent PE foil can be used for 1 – 2 years before UV radiation and mechanical stress
causes damage. On one length side of the dryer, the plastic foil is fixed to the metal frame and
on the other side, to a metal tube that allows rolling of the foil up and down for loading and
unloading the dryer, Figure 5:6 .To convert solar radiation into heat, the top surface of the
solar air heater is painted black showing an absorptivity of about 90 %. The covering sheet is
tilted like a roof, which prevents water entering or even flooding the device.

Figure 5:6 Drying section of the solar tunnel dryer model Hohenheim, (Esper, 1996).

Solar Drying of Tomatoes in Romanian 80
Two axial fans are incorporated into the sandwich substructure at the backside of the
air inlet of the solar air heater to suck ambient air into the air heater. Due to the extremely low
pressure drop of 20 Pa, maximum 50 Watt electrical energy are required to force 400 to 1200
m3/hr of air between floor and cover foil, which is sufficient to ensure drying to safe storage
conditions before growth of microorganism or enzymatic reactions will cause spoilage. The
crop spread out in a thin layer in the tunnel dryer itself acts as an absorber enabling the drying
air to gain additional heat, when passing through the dryer. The heat losses caused by
evaporation of the moisture during drying are compensated by this additional energy gain,
resulting in an almost uniform drying.
5.4.1.1 Measurement of data

To analyze the drying parameters of tunnel dryer and drying process of tomatoes
measurement of temperature, relative humidity and solar radiation. The data were collected
to a PC by a Datalogger RS 2232 of Wettercom Company at every 10 minutes. The
moisture content of product was measured every hour and handy registered.

5.4.1.2 Solar radiation

The solar radiation was measured using a µMETOS MCR – 300 special design
photocell M002 type, with beam absorbance between 400 and 1100 nm and an accuracy of
±5%, part of Weather station, which was fixed just near dryer plant . The solar radiation
was computed as global radiation which are composed by solar beam and solar diffuse
radiation.
5.4.1.3 Temperature

The inlet air temperature value was measured with a thermometer type M7080C
that was included in µMETOS MCR – 300 weather station produced by PESSL-
Instruments GmbH Austria, with an accuracy of 0,1
0C. The measurement was made
continuously and the computed data was unloaded once at every ten minutes. At the end of
collector, the temperature was measured with a digital thermometer TM-902 C, using a
temperature sensor type K (NiCr-Ni thermocouple, with an accuracy of ±0.15%). The data
were collected every ten minutes. Along the dryer section the temperature values were

Solar Drying of Tomatoes in Romanian 81
measured continuous using a digital multimeter Multi-Function Environmental Meter
model 2232 with an accuracy of ± 0,1 0C.

5.4.1.4 Relative Humidity

The relative humidity of the air was measured continuously using an hygrometer
cell type M7080Cincluded in µMETOS MCR – 300, weather station with an accuracy of ±
0,5% from the inlet point and multimeter Multi-Function Environmental Meter model 2232
along the dryer. The value of relative humidity of air at the end of collector was computed
using h-x Mollier diagram, (Deutscher Wetterdienst).
Air is a mixture different gases. One this gas is the water vapour. The quantity of
water vapour, which can be contained in air, is however limited. The more warmly air is,
the more water vapour can be contained in it. The relative air humidity indicates, how
much percent of the maximum steam content contains air at the moment. Since the
maximum steam content with rising temperature rises, the relative air humidity with rising
temperature falls (and in reverse).
The dew point temperature is defined as the temperature, at which the current steam
content in air the maximum (100 % relative air humidity) is. The dew point temperature is
thereby a variable independent of the current temperature. From temperature and relative
humidity and temperature of dew point, also the absolute humidity content of air in gram
water vapour per cubic meter can be calculated.
The basis of the computations is the real steam pressure for the saturation steam
pressure (Eq. 5.1).




+
=TbTa
Pws*
10* 1078.6 (5.1).
The relative air humidity is defined as the relationship from the present steam
pressure to the saturation steam pressure (Equation 5.2).
Pws Pw *100= (5.2).
At the dew point temperature the saturation pressure of the steam is by definition
equal to the current steam pressure. From these two definitions results Equation 5.3.

Solar Drying of Tomatoes in Romanian 82
100*PwsPw= (5.3).

The formula follows for the computation of the relative air humidity from the dew
point temperature.
vbvaTdp+=*,with 1078.6lgPwv= (5.4).
From the general gas equation results the Equation 5.5:
TPw
RmWwwa* *105= (5.5).

Designations:
a=7,5,b=237,3 for T > = 0
a=7,6,b=240,7 for T < 0
R = 8314,3 J/(kmol*K) (universal gas constant)
mwa = 18,016 kg (molecular weight of the water vapour)

5.4.1.5 Moisture Content of Tomatoes
The moisture content of tomatoes during drying was computed using gravimetric
method (Eq.3.1 and 3.2). The weight changes of tomatoes were measured using an
electronic balance Sartorius MC1, type laboratory LC 2200, produced by SARTORIUS
AG Göttingen, Germany, with an accuracy of ± 0.01 g. For measurement, the samples
were taking out of the dryer every hour, weighing and replaced.
5.4.1.6 Quality Analyses of Dried Tomatoes

The quality determinations of dried tomatoes were made accordingly with
Romanian Standard (STAS) for dried vegetables. It was measured Ascorbic Acid,
Carbohydrates content, Total Acidity, and Indices of Acidity before and after drying
Vitamin C was determinate using Iodometric Method (Galben et al., 1984). Total
carbohydrates and glucose content was measured using Schoorle Method (Galben et al.,
1984). Total acidity was determinate in Oxalic Acid extract and titration with Natrium
hydroxide 0.1 % solution and Indices of Acidity was calculated by titration of free fat

Solar Drying of Tomatoes in Romanian 83
acids with Potassium hydroxide in presence of ethylic alcohol and phenolphthalein
(Galben et al., 1984).

5.4.2 Solar Drying Experiments

Two different varieties of tomatoes, named Marissa F1 and Cristal F1, were used in
the drying experiments. These varieties are characterized by uniform red colour, with
different dry matter content and are among the most cultivated varieties in South part of
Romania. The field study is coincided with tomato harvesting season. The fresh tomatoes
were supplied by farmers and use to drying experiments next day after harvesting.
The tomatoes were washed, sorted and then sliced manually in quarters using a
knife with stainless steel blade. As long as tomatoes are uniformly largest the slice are
uniformly. The preparation of tomatoes was started early in the morning, usually at six
o’clock. Three persons were needed for slicing and loading the dryer in the way to start the
drying experiments at eight o’clock.
In order to monitor the product weight loss during the drying process, specific
samples were taken from fixed point in the dryer and weight during drying process every
hour. The fans of dryer were driven by the solar module; the flow rate is dependent on
solar radiation.

5.5 Results and Discussions

5.5.1 Solar Drying of Tomatoes

Several drying experiments were conducted in the Romania in the summer of 2004
to test the suitability of solar tunnel dryer, model Hohenheim, for drying of tomatoes in
Romanian climatic condition. Two different varieties were selected in this study. The
selection was based on the high yield production of the cultivars (about 180 t/ha), amount
of surface area that was cultivated with those varieties, and strong flavour.
The temperature profile along the dryer is presented in Figure 5:7. The temperature
is increased continuously from the inlet towards outlet when the dryer is empty. Due to the
moisture evaporation from the product, the temperature profile along of dryer is changed
when it is loaded with tomatoes. At high product moisture content, the cooling effect is
quite significant as a result of evaporative cooling.

Solar Drying of Tomatoes in Romanian 84
0 5 10 15 200204060801008:am
12:am
4:pm
8:pm
m0CTemperature,0C
Lenght, m

Figure 5:7 Temperature profile along the empty dryer at different time of a typical day
of the summer season, (Bucharest 2.07.2004).

Comparative with unloaded dryer, at a moisture content of tomatoes about 90%, at
beginning of drying, the temperature drop at the outlet was about 25 K; while at a moisture
content around 15% the temperature drop was about 5 K and at 50 % moisture content the
temperature drop was about 8 K, Figure 5:8 This could be attributed to the fact that, at low
tomato moisture content, the surface of the product acts as a collector for solar energy. The
additional heat gained combined with low relative humidity of air has as effect to maintain
almost constant the air temperature over the length of the dryer.

0 5 10 15 20020406080100dryer collector
moisture content 90%
moisture content 50%
moisture content 15%
m0CTemperature,0C
Length, m

Figure 5:8 Temperature profile along the dryer at various tomato moisture content
(variety Marissa F1, Bucharest 9-11.07.2004).

Solar Drying of Tomatoes in Romanian 85
The airflow rate varied with the levels of solar radiation. At high incident solar
radiation, more energy is received by the collector absorber, which is resulted in increasing
the drying air temperature. In the same time, increasing incident solar radiation has as
result increasing of the airflow rate inside the tunnel, which conducts to reduce increasing
of temperature of air into the dryer. The temperature rise inside the dryer is proportional to
the incident solar radiation on the collector surface, as is presented in Figure 5:9.
0 300 600 900 1200010203040
W/m2KTemperaturerise,K
Global radiation, W/m2
Figure 5:9 Temperature rise inside the collector at various incident radiation levels,
(Bucharest 3-7.07.2004).

The relative humidity of the drying air is measured at the collector inlet and dryer
outlet. The relative humidity is calculated at the collector outlet from measured values of
temperature as is shown in Chapter 5.4.1.4. The relative humidity of the air is decreased
along the collector as result of increasing the temperature, while along the dryer section the
moisture content of product has a major influence in evolution of relative humidity of
drying air. In Figure 5:10 is presented values of relative humidity along the dryer plant at
different moisture content of tomatoes.
At the initial stage of drying, when the moisture content of tomatoes was high
(90 %) the relative humidity of drying air increased due to evaporation of much amount of
moisture (free water) at surface of tomatoes. At this stage, the relative humidity at the
outlet of collector and outlet of dryer were 34.7 % and 34.1 %, respectively. When the
moisture content of tomatoes became about 50 %, the relative humidity of the drying air
significantly reduced due to evaporation of less amount of moisture from the tomatoes. At
this stage, the relative humidity at the outlet of collector and outlet of dryer were 54.6 %
and 44.6 %, respectively. At this stage of drying, the relative humidity of the air exhausted

Solar Drying of Tomatoes in Romanian 86
from the dryer has sufficient drying potential. At the final stage of drying when the
moisture content of tomatoes was about 15 %, the relative humidity of drying air at the
outlet of collector and outlet of dryer were 24.7 % and 9.7 %, respectively. The different
values at the outlet of collector were registered due to of different of relative humidity of
air during the drying time.
0 5 10 15 20020406080100dryer collector
moisture content 90%
moisture content 50%
moisture content 15%
m%Relativehumidity,%
Length, m

Figure 5:10 Relative humidity along the dryer at various tomato moisture content (variety
Cristal F1, Bucharest 12-16.07.2004).

The average initial moisture content of the varieties Marissa F1 and Cristal F1 were
93.4 % and 92.7 % respectively. The final moisture content of the dried tomatoes was
about 11 %. Drying of tomatoes, sliced in quarters, in the solar tunnel dryer was achieved
in three-four days depending on the variety, loading capacity, incident solar radiation, air
temperature and relative humidity of the air.

The moisture reduction of tomatoes on a typical experiment run conducted in the
July 2004 for solar drying of Marissa F1 variety is shown in Figure 5:11 .The moisture
content of samples of tomatoes dried in the solar dryer reduced from 93.40 % to 11.07 %
(wb) in 56 hours. From those 24 hours were during the night, and therefore could be
considered the active drying time of 32 hours. The loading capacity of the dryer was about
7kg/m2.The drying rapport was found to be 15:1.

Solar Drying of Tomatoes in Romanian 87
0 15 30 45 60051015
h.kg/kg Moisturecontent,db.
Drying time, h.

Figure 5:11 Moisture content evolution during solar drying of tomatoes (quarters, variety
Marissa F1, Bucharest 3-5.07.2004).

Drying rate of Marissa F1 tomatoes has a maximum value at 14.00 hours, when was
registered also the maximum value of the incident solar radiation which conduct to the
highest air temperature and airflow rate. The maximum value of drying rate was 1.04 kg
water per kg of dry mater, Figure 5:12.
0 20 40 600.00.51.01.5
h.kg/kg*hDryingrate,kg/kg*h
Drying time, h.

Figure 5:12 Drying rate evolution during solar drying of tomatoes (quarters, variety
Marissa F1, Bucharest 3-5.07.2004.

For the same time of the day, the drying rate has a higher rate when moisture
content of tomatoes is higher, Figure 5:13.

Solar Drying of Tomatoes in Romanian 88
0 5 10 150.00.51.01.5
kg/kgkg/kg*hDryingrate,kg/kg*h
Moisture content, db.

Figure 5:13 Drying rate vs. moisture content dry base, during solar drying of tomatoes
(quarters, variety Marissa F1, Bucharest 3-5.07.2004).

Variations of moisture contents of Cristal F1 variety with drying time for a typical
experimental run are shown in Figure 5:14. The moisture content of samples of tomatoes
dried in the solar dryer reduced from 92.70 % to 11.37 % (wb) in 70 hours. From those 36
hours were during the night, and therefore could be considered the active drying time of 34
hours. The loading capacity of the dryer was also about 7 kg/m2.The final drying raport
was found to be 14:1. That might be due to a high sugar content of the fruit of tomatoes,
which can keep the moisture as a link water and as water of hydrolyses. The energy
required for vaporisation of 1 kg of water seams to be higher for Cristal F1 variety than to
the Marissa F1 variety. Similar results were founded in laboratory experiments.

Solar Drying of Tomatoes in Romanian 89
0 20 40 60 80051015
h.kg/kg Moisturecontent,db.
Drying time, h.

Figure 5:14 Moisture content evolution during solar drying of tomatoes (quarters, variety
Cristal F1, Bucharest 5-8.07.2004).

The drying rate Cristal F1 tomatoes was found higher than that of Marissa F1
variety at beginning and lower at the end. The reason is that equilibrium moisture content
is a function of drying air temperature and of chemical composition content of material,
(Labuzza, 1972). The maximum value of drying rate was 1.41 kg water per kg of dry
mater, Figure 5:15 .
0 20 40 60 800.00.51.01.5
h.kg/kg*hDryingrate,kg/kg*h
Drying time, h.

Figure 5:15 Drying rate evolution during solar drying of tomatoes (quarters, variety
Cristal F1, Bucharest 5-8.07.2004).

Solar Drying of Tomatoes in Romanian 90
The drying rate of versus moisture content, for Cristal F1 variety is shown in
Figure 5:16. The value of drying rate is relatively to the moisture content of product and
also to the energy transferated to the product into the dryer. Therefore, at different time,
could be found same values of the drying rate for different moisture content of product.
0 5 10 150.00.51.01.5
kg/kgkg/kg*hDryingrate,kg/kg*h
Moisture content, db.

Figure 5:16 Drying rate vs. moisture content dry base, during solar drying of tomatoes
(quarters, variety Cristal F1, Bucharest, 5-8.07.2004).

Among the factors which has an influence on the drying time could be count also
the loading capacity. When the tomatoes were dried in thin layer of loading capacity of
7kg/m2,the drying process is achieved in 32 hours for Marissa F1 and 34 hours for Cristal
F1 variety. The increase of loading capacity to 10 kg/m2is resulted in longer drying time,
for both variety to 34 hours for Marissa F1 and 37 hours for Cristal F1, as is shown in
Figure 5:17. For drying in controlled condition that should be not so significant. But using
asolar tunnel dryer, the drying have to be finished late in the afternoon better than drying
further a few hours next day due to rewetting of product that is occurred during the night.
Therefore a longer effective drying time with two hours could means a longer of drying
time with fourteen to eighteen hours. In the other hand the relative humidity of the air
exhausted from the dryer has sufficient drying potential to increase the loading capacity to
10 kg/m2,and also to increase the length of the dryer. A good planning and an economical
calculation should be made before start of drying.

Solar Drying of Tomatoes in Romanian 91
Marissa Cristal0204060Loading capacity
7kg/m2
10 kg/m2h.Dryingtime,h.
Variety

Figure 5:17 Drying time at different loading capacity of two varieties of tomatoes
(quarters, Bucharest, July.2004).

The drying behaviour of the investigated varieties is presented in Figure 5:18 .
When they were dried in similar condition of atmospheric temperature, solar radiation and
relative humidity of air, the effective drying time of Cristal F1 was longer with two hours
then drying time of Marissa F1.
0 10 20 30 40051015
Marissa F1Cristal F1
h.kg/kg Moisturecontent,db.
Drying time, h.

Figure 5:18 Drying behaviour of two different varieties of tomatoes (quarters, Bucharest,
July.2004).

Solar Drying of Tomatoes in Romanian 92
5.5.2 Dried Tomatoes Quality

Vitamin C content in tomatoes is considered among the nutritive quality criteria of
dried tomatoes. In the fresh tomatoes, The Vitamin C was found to 19,7 mg/100 g dry
mater. However, the content varied with variety of tomatoes. It was found that is greatly
affected by drying process as is indicated in Figure 5:19.
Cristal Marissa050100150200250Fresh
DriedVitaminC,mg/100gdrymater
Variety

Figure 5:19 Vitamin C degradation as effect of solar drying process on two varieties of
tomatoes (quarters, Bucharest, July.2004).

Different varieties of tomatoes have different amount of sugar content as shown in
Figure 5:20. Drying of tomatoes in solar tunnel dryer have a large influence in decreased
of sugar content. After drying was registered a loses of glucose about 49 % for Cristal F1
and 37 % for Marissa F1 and for total sugar 47% and 56% for Cristal and F1 and Marissa
F1 respectively. Those losses are attributed to Maillard reaction and caramelization during
drying process, (Wilford et al., 1997; Bieg, 1995)

Solar Drying of Tomatoes in Romanian 93
Glucoze Total sugar0.000.050.100.150.20
Cristal Fresh
Cristal Dried
Marissa Fresh
Marissa Driedkg/kgSugar,kg/kgdrymater
Figure 5:20 Sugar content in two different varieties of tomatoes before and after drying
(quarters, Bucharest, July.2004).

The total acidity content is considered as indicator of flavour intensity of tomatoes.
The largest acidity compound in tomatoes is given by oxalic acid, (Galben, 1984). The
total acidity content depends mainly on the type of tomatoes variety, as is illustrating in
Figure 5:21. The highest values after drying process were obtained by the Marissa variety.
However, the content of oxalic acid in product is higher in dried tomatoes than in fresh
product. That could explane the stronger flavour of dried tomatoes.

Cristal Marissa05101520Fresh
DriedOxalicacid,mg/100gdrymater
Variety

Figure 5:21 Total acidity of two different varieties of tomatoes before and after drying
(quarters, Bucharest, July.2004).

Solar Drying of Tomatoes in Romanian 94
The index of acidity gave information about quality of product, mainly for
degradation of vegetal fat; and is represented in mg of acid per 1g of fat. Acidity of oil and
fat is due to free fat acids which results by hydrolyzes, by enzymatic and bacterial activity.
Indices of acidity could give information also about storage conditions. The values of
Indices of acidity that were found for Marissa and Cristal varieties, before and after drying
are presented in Figure 5:22.
Cristal Marissa0246
Fresh
DriedIndicesofacidy,mg/1gfat
Variety

Figure 5:22 Index of acidity for two varieties of tomatoes before and after drying
(quarters, Bucharest, July.2004).

5.6 Conclusions

The solar dryer was tested under field conditions of Bucharest, Romania, and a
good quality product was obtained. The drying experiments revealed that the drying
process could be achieved in 3-4 days in typical summer day and 5-6 days in colder days
(e.g. autumn days). The drying time depends mainly to the solar radiation and air
temperature values. The drying time depends also, but in a small range to the variety of
tomatoes and loading capacity of the dryer.
Loading and unloading of the dryer is found to be easy and low labours (two people
for one hour loading and one person for one hour unloading). The maintenance of the dryer
plant can be made by the farmers without high technical knowledge. The dryer can be
easily constructed with only two men in one day.
The performance of the solar collector to heat the drying air is assumed quite good.
It could raise the ambient temperature with about 30 K, depending of the solar radiation

Solar Drying of Tomatoes in Romanian 95
values, mainly. With a temperature about of 60 0Cat the inlet of dryer section could be
considerate adequate for tomatoes drying.
The quality flavour, expressed as total acidity is increased significantly and also the
shelf life of the product. Quality loses of content of sugar and Vitamin C during drying is
found to be acceptable. The dried tomatoes product achieves the quality parameters of
Romanian standards for dried vegetables. The drying technology respect the Romanian and
EU rules about protect of contamination of product during drying with dust, insect, fungi
and bacteria; and GMP, respectively. In the same time, drying of tomatoes in solar tunnel
dryer is no conventional energy consumer and is environmentally protected technology.
The method of driving fans by solar cell module is optimal to control the increase
of drying air temperature by increasing the airflow rate since the output power of the solar
module increases with increase of the incident solar energy.
At the beginning of the drying, when the moisture content of the tomatoes is high,
the air temperature decreases along the length of the drying tunnel. Due to the evaporative
cooling on the surface of tomatoes, the air temperature in the dryer falls. When the amount
of energy required for evaporation of moisture from tomatoes becomes less than the
received energy then the air temperature in the dryer increases. After completion of initial
stage of drying, drying air temperature in the dryer is found almost constant along the
length of the dryer. This indicates uniform thermal stress throughout the dryer and the
constant air temperature inside the dryer provides uniform drying throughout the length of
the dryer. At the final stage of drying when the moisture content of tomatoes became
constant (12 %), the relative humidity of drying air along the length of drier becomes
almost constant.
The shape of tomatoes sliced in quarters has a large area of contact between product and drying air and a large evaporative surface. The shape of quarters of tomatoes is easily
for an fast slicing and to achieve the drying of product without spoiling with pathogens
agents. The optimum loading capacity was found to be about 10 kg/m
2.
Hence, solar tunnel dryer was found to be technically suitable for drying of
tomatoes. The solar tunnel dryer may be recommended for drying tomatoes as well as
vegetables and fruits Romania, in the way to increase the income of the farmers and to
reduce the imports of dry vegetables. In the other hand, the storage time of dried tomatoes
is longer than fresh tomatoes, and furthermore the storage cost for dried tomatoes is
extremely low comparative with fresh tomatoes.

Summary 96
SUMMARY

Tomatoes (Lycopersicon esculentum L.) are more widely grown as any cultivated
crop and are considered to be one of the most popular vegetables used in many kinds of
food preparation. The demand for tomatoes is world –wide and its use is not limited to any
climate or country.
Sun drying of tomatoes is a common activity for Mediterranean countries, but not
used in any others country of Europe. For European countries, the wide used method of
drying is drying at high temperature in huge dryer plant using electrical energy. Since the
sun drying has a low quality of the product and industrially drying is too costly, solar
drying technology was developed. The dried vegetables products are used more and more
due to the good storage skills and low price of storage and transport. However, the dried
vegetables are used to food industry as row material, especially for producing instant
soups, spices, food colorants and some semi products.
Basic experiments to determine water sorption isotherms of tomatoes at various
temperature and relative humidity were conducted using the gravimetric method European
Cooperative Project COST 90. Knowledge of the water sorption isotherm of tomato
provides information about moisture changes, which may occur during storage and
predicting shelf life stability. The experimental results were fitted to ten different
mathematical models. Results of the experimental measurement of sorption isotherms, at
temperature range between 20 and 70 0C, showed a typical sigmoid shape of type II
according to the BET classification. The increase of the temperature has as result the
increase of the water activity for the same moisture content. Hysteresis was found for the
entire range of relative humidity, for both adsorption and desorption isotherms. Hysteresis loops decreased with an increase of temperature. These indicate the irreversibility of
sorption process and chemical and microbiological deterioration during adsorption or
desorption of tomatoes. From the models which were analysed, the GAB and Oswin
models fitted very well the experimental data at different temperatures in the range of
water activity of 0.11 to 0.85. The Oswin equation is recommended for prediction of the
adsorption and desorption isotherms, and GAB equation for prediction of monolayer
moisture content of tomatoes.
The net heat of sorption of water in tomatoes was calculated from the sorption
isotherms, applying the Claussius-Clapeyron equation. The net isosteric heats of sorption

Summary 97
were found as a function of moisture content, and decreased exponentially as the moisture
content increased.
The main tasks of this study were to elaborate optimal drying conditions applicable
to the processing of tomatoes. The thin layer drying experiments of sliced quarters and
halves tomatoes were conducted under controlled conditions, using a laboratory dryer
developed at the Institute of Agricultural Engineering in the Tropics and Subtropics,
Hohenheim University, Germany. The drying experiments were conducted using over-flow
and through-flow mode. The influence of drying air temperature, air velocity, humidity
slice shape, loading capacity and airflow mode on the drying behaviour of different
varieties of tomatoes were investigated.
The drying time of tomatoes to reach the desired final moisture content of 9 % was
significantly affected by the temperature of air of drying. Preliminary quality evaluation,
on effect of drying temperature on the quality deterioration, revealed that higher drying
temperature affects significantly the colour of the dried tomatoes. The temperature of 60 o
Cwas founded to be the optimum drying temperature, from thermodynamic point of view.
The moisture removal rate was founded to be fast at the beginning, due to the
higher level of moisture content in tomatoes. Increasing the air velocity to a certain limit is
resulted in significantly decreasing of the drying time. Higher drying rates were achieved
by higher air velocity of drying air. From drying technical point of view, during field
application of solar drying of tomatoes, the drying time could be decreased by increasing
the air velocity.
In the Romanian climatic conditions, in the summer season, the variation in the air
humidity during day and night is quite significant. This fluctuation is assumed to be of
main important consideration regarding the solar drying in these regions, especially on beginning of drying.
The size of tomatoes used for drying was found to be of important consideration
relative to internal mass transport. Results revealed that, a different between quarters and
halves affect significantly the drying time, because increasing of evaporative surface and
decreasing of diffusion path. When other variables were remained constant, the fastest
drying rates were achieved by varieties of tomatoes with small fruits, because reducing the
diameter is resulted in reducing the diffusion path and increasing heat and mass transfer
between the surface and the drying air.
The airflow mode has also a paramount importance in behaviour of drying
tomatoes. Higher drying rates were achieved by through-flow air mode for all experiments

Summary 98
comparative by over flow mode. However, for the same air velocity, in the through flow
mode the pressure drop was higher than to the over flow mode. Therefore, from technical
point of view, during solar drying of tomatoes the over flow mode is recommended.
The required time for drying of tomato depends on parameters as: tomato variety
the drying air temperature, air velocity and the relative humidity.
Colour is one of the most important factors of quality evaluation of agricultural
product. Drying temperature above 60 0Chas significant influence on the colour of
tomatoes, therefore should be avoid. The optimum relative humidity of drying air was
found to be less than 15% for obtaining a good colour quality.
Based on the results from the laboratory experiments, field test were conducted at
Bucharest, Romania in summer 2004. The drying experiments revealed that the drying
process can be achieved in 3-4 days in typical summer day and 5-6 days in colder days
(e.g. autumn days). The drying time depends mainly to the solar radiation and air
temperature values. The drying time depends also, but in a small range to the variety of
tomatoes and loading capacity of the dryer.
The performance of the solar collector to heat the drying air was found to be quite
good. It could raise the ambient temperature with about 30 K, depending of the solar
radiation values, mainly. With a temperature about of 60 0Cat the inlet of drying section
could be considerate adequate for tomatoes drying.
The quality flavour of tomatoes dried in solar tunnel dryer, expressed as total
acidity is increased significantly and also the shelf life of the product. The dried tomatoes
product achieves the quality parameters of Romanian standards for dried vegetables. The
drying technology respect the Romanian and EU rules about protection of contamination of
product during drying with dust, insect, fungi and bacteria; and GMP, respectively. In the same time, drying of tomatoes in solar tunnel dryer is no conventional energy consumer
and is environmentally protected technology.
The method of driving fans by solar cell module is optimal to control the increase
of drying air temperature by increasing the air flow rate since the output power of the solar
module increases with increase of the incident solar energy.
The shape of tomatoes sliced in quarters has a large area of contact between product
and drying air and a large evaporative surface. The optimum loading capacity was found to
be about 10 kg/m
2.
Hence, solar tunnel dryer was found to be technically suitable for drying of
tomatoes. The solar tunnel dryer may be recommended for drying tomatoes as well as

Summary 99
vegetables and fruits Romania, in the way to increase the income of the farmers and to
reduce the imports of dry vegetables. In the other hand, the storage time of dried tomatoes
is longer than fresh tomatoes, and furthermore the storage cost for dried tomatoes is
extremely low comparative with fresh tomatoes.

.

References 100
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