2012 Shimamura and Ukeda, licensee InTech. This is an open access chapter distributed under the terms [618477]

Chapter 5

© 2012 Shimamura and Ukeda, licensee InTech. This is an open access chapter distributed under the terms
of the Creative Commons A ttribution License (http://creat ivecommons.org/licenses/ by/3.0), which permits
unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Maillard Reaction in Milk –
Effect of Heat Treatment
Tomoko Shimamura and Hiroyuki Ukeda
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/50079
1. Introduction
Milk is usually subjected to heat treatment to ensure microbiological safety before retail and
consumption. There are three types of heat trea tment; (1) low temperature long time (LTLT)
pasteurization, (2) high temperature short time (HTST) pasteurization, and (3) ultra-high
temperature (UHT) treatment. In all types of he at treatment, the Maillard reaction occurs in
milk.
The Maillard reaction (nonenzymatic glycation) is a chemical reaction between amino group
and carbonyl group; it is the extremely complex reaction that usually takes place during
food processing or storage. In the case of m ilk, lactose reacts with the free amino acid side
chains of milk proteins (mainly ε- a m i n o g r o u p o f l y s i n e r e s i d u e ) t o p r o c e e d t o e a r l y ,
intermediate, and advanced stages of Maillard reaction and forms enormous kinds of Maillard reaction products. The reactions of lact ose and milk proteins have been frequently
investigated and the formations of various Maillard reaction products in milk during heat
treatment have been demonstrated [1]. In th e general Maillard reacti on, firstly an Amadori
product is generated, and it progresses to the 3-deoxyosone or 1-deoxyosone route
depending on the reaction pH. In the case of the Maillard reaction of disaccharides such as
lactose, there is a third reaction route. It is the 4-deoxyosone route. A main carbohydrate in
milk is lactose. Thus, the Maillard reaction in milk progresses via th e above described three
routes. Finally, the Maillard reaction results in the formation of melanoidins (browning
compounds).
2. Effect of the Maillard reaction on milk proteins
The Maillard reaction shows various effects on milk proteins such as bioavailability,
solubility, forming property, em ulsifying property, and heating stability [1-4]. In addition,

Milk Protein 148
the formation of flavor compounds and browning compounds is caused as the consequences
of the Maillard reaction between lactose and milk proteins [1, 5].
As for the effect of the Maillard reaction on the bioavailability of milk proteins, various
studies were performed. Generally, in the Mailla rd reaction in milk, lactose mainly reacts
with ε-amino group of lysine residue of milk proteins. Thus, the lysine loss by the Maillard
reaction increases with a severity of heat trea tment. The modified lysine cannot be available
as a nutrient any more. For example, steam inje ction process (direct heating) generated 3.6%
(120°C for 400 sec) and 6.8% (130°C for 290 sec) of the blocked lysine in whole milk. The
indirect heating at 115°C for 10 to 40 min increa sed the modified lysine from 11.0 to 13.0%
[ 6 ] . I n a d d i t i o n , i t w a s r e v e a l e d t h a t t h e l y s i n e r e s i d u e s i n s k i m m i l k p o w d e r w e r e m o r e susceptible to heating than those in skim milk [7].
Le et al. [3] recently suggested that the Maillard reaction was responsible for the solubility
loss in milk protein concentrate powder. It was also reported that the glycated β-
lactoglobulin was more stable at acidic pH and more stable against heat ing. The glycation of
β-lactoglobulin, moreover, could improve its fo rming and emulsifying properties [4]. These
results suggested the usefulness of the Maillard reaction for enabling milk proteins to have
different properties.
3. Monitoring of the Maillard reaction of milk using XTT assay
The Maillard reaction has a lot of effects on the function of milk proteins and sensory
property of milk and dairy products as descri bed above. Particularly, in the manufacturing
of milk, the excess progress of Maillard reac tion and the formation of melanoidins are
undesired, because a commercial value of milk is drastically decrease d by them. Therefore,
the detection of the Maillard reaction products is important for the quality control of milk. So far, several heat-induced markers have b een proposed to control and check the heat
treatment given to milk and dairy products. For example, furosine, hydroxymethylfurfural
(HMF), and lactulose concentrations have been recognized to be the most promising indicators, since these concentrations increase wi th the heat treatment [1]. In Japan, protein
reducing substance value (PRS) obtained by a ferricyanide assay is also widely-used conventional indicator [8]. It is based on the detection of reducing substances such as sulfhydryl group which are generated by heatin g in the fraction of acid-precipitated milk
protein. These method, however, are generally time-consuming and complicated.
We proposed an assay method for determin ing the ability of milk to reduce 3’-[1-
[(phenylamino)-carbonyl]-3,4-tetrazolium]-bis (4-methoxy-6-nitro)benzensulfonic acid
hydrate (XTT: Figure 1) as a method of evaluati ng the extent of the Maillard reaction [8-11].
The tetrazolium salt XTT is reduced to water- soluble formazan which is suitable for the
spectrophotometric measurement. Taking account into an economical view and a rapidity of
assay, we tried to develop a microplate assay. The assay conditions were as follows: XTT
concentration, 0.5 mM; reaction pH, 7.0; menadi one concentration, saturation level (ca. 0.55
mM); reaction temperatur e, room temperature; volume of XTT solution, 60 μL; volume of
sample solution, 40 μL; detection wavelength, 492 nm; reference wavelength, 600 nm;

Maillard Reaction in Milk – Effect of Heat Treatment 149
reaction time, 20 min. The proc edure of the XTT assay was as follows: the XTT solution was
added into each well in a microplate. Afterward, the sample solution was added to the well.
After mixing on a microplate shaker at 500 rp m for 15 sec, a difference in the absorbance
between 492 and 600 nm was read on a microplate reader as the absorbance at 0 min. After
20 min, the absorbance difference was read agai n. An increase in the absorbance for 20 min
was recorded as the ability of the sample to reduce XTT (XTT reducibility).

Figure 1. Structure of tetrazolium salt XTT.
Using the XTT assay described above, the XTT re ducibility of milk was examined (Figure 2).
In this test, an LTLT milk (Milk A: 65°C for 30 min) and two kinds of UHT milk (Milk B:
130°C for 2 sec, Milk C: 140°C for 3 sec) were used. When the milk was mixed with XTT
solution, intense orange color which was de rived from the XTT formazan was recognized.
A s s h o w n i n F i g u r e 2 , M i l k C s h o w e d t h e h i ghest XTT reducibility and the order of XTT
reducibility was Milk C, B, and A. At the sa me time, the HMF content in Milk B and C was
also determined as the conventional indicator of the Maillard reaction, because it was
reported that the HMF content clearly increased with the severity of th e heating treatment of
milk [12, 13]. As a result, the content of HMF in Milk C (12.6 μM) was higher than that of
Milk B (8.95 μM). From these results, it was revealed that the XTT assay could estimate the
degree of thermal stress delivered to the milk as well as the HMF value.
In addition, the changes in the XTT reducibilit y during storage of UHT milks (Milk B and C)
were investigated. For purpose of comparison, the PRS of Milk B and C were also examined
by the ferricyanide assay [8]. The UHT milks we re stored at 4°C and 37°C for about 4 weeks
(Figure 3, 4). The XTT reducibility of Milk B and C gradually decreased depending on the
storage period and the rate of decrease in th e XTT reducibility was clearly larger at higher
storage temperature (Figure 3). This result strongly suggested that, when the XTT assay is
applied to the milk heated under a given conditio n, the result can serve to estimate not only
the heating condition but also the storage pe riod after heat treatment if the storage
temperature is known or if storage period is known.
On the other hand, the PRS of Milk B and C were almost constant at all storage conditions
(Figure 4). This result showed that the ferri cyanide-reducing substances might be stable
unlike the XTT-reducing substanc e. In addition, we found that the HMF value of Milk C did
not significantly change during its storage for 60 days at 5°C [11]. It was in accordance with

Milk Protein 150
the result of Fink and Kessler about HMF [ 13]. They also reported that the lactulose
concentration of UHT milk was constant throug hout the storage period for 70 days at room
temperature [13]. It could be concluded, theref ore, that the XTT assay is applicable for the
estimation of storage conditions which was impossible by the conventional method.

Figure 2. Comparison of XTT reducibility of LTLT m ilk (Milk A) and UHT milks (Milk B and C).

Milks were stored at 4°C ( ●, Milk B; ș, Milk C) and 37°C ( ○, Milk B; □, Milk C).
Figure 3. Changes in XTT reducibility during storage. 00.10.20.30.40.5
Milk A
65°C
30 minMilk B
130°C
2 secMilk C
140°C
3 secXTT reducibility
00.10.20.30.40.5
0 5 10 15 20 25 30XTT reducibility
Storage period (days)

Maillard Reaction in Milk – Effect of Heat Treatment 151

Milks were stored at 4°C ( ●, Milk B; ș, Milk C) and 37°C ( ○, Milk B; □, Milk C).
Figure 4. Changes in PRS during storage.
4. Demonstration of the presence of aminoreductone formed during the
Maillard reaction in milk
4.1. In the model system of lactose and butylamine [9]
In order to clarify the XTT-reducing substance th at is formed during the Maillard reaction in
milk, we firstly used a model system consisting of lactose and butylamine, and then
performed the spectrophotometric analysis of the heated model solution. The model
solution of lactose-butylamine heated at 80- 100°C for 0-30 min showed a characteristic UV
absorption maximum at 320 nm. During the he ating at 80-100°C for 30 min, the changes in
the absorbance at 320 nm (Figure 5) and the X TT reducibility (Figure 6) were investigated.
As a result, both indices increased gradually in accordance with the rise in temperature and
heating time. This result indicated that the compound with the absorption maximum at 320
nm was formed by heating. Moreover, the behavi or of increase in absorbance at 320 nm was
similar to that of the XTT reducibility. Since the similar trend was recognized in the time
course of both indices, the XTT reducibility wa s plotted against the absorbance at 320 nm. In
consequence, a significant relationship betw een them was recognized with a correlation
coefficient of 0.967 (n = 19, p < 0.001). From these results it was found that the compound
with the absorption maximum at 320 nm might be responsible for the reduction of XTT.
The 13C- and 1H-NMR analyses of the compound with the absorption maximum at 320 nm
which was extracted from lact ose-butylamine model solution heated at 100°C for 15 min
were performed. The signals of the 13C- and 1H-NMR could be assigned the compound as
the aminoreductone, 1-(butylamino)-1,2-dehyd ro-1,4-dideoxy 3-hexulose (Figure 7). This
compound was reported as the Maillard reaction product formed in the 4-deoxyosone route. 05101520
0 5 10 15 20 25 30Protein reducing substance value
(PRS)
Storage period (days)

Milk Protein 152
It was also reported as the characteristic comp ound in the Maillard reaction of disaccharides
[14]. In addition, we demonstrate a linear rela tionship between the X TT reducibility and the
amount of aminoreductone which was determined more specifically by HPLC [10]. These results strongly indicated that the aminoreductone formed during Maillard reaction of lactose was mainly responsible for the reduction of XTT.

Lactose (262 mM) and butylamine (1.16 M) in 1.28 M phosphate buffer (pH 7.0) were heated at 80°C ( ●), 90°C (○), and
100°C (ș).
Figure 5. Effect of heating temperature and time on the absorbance at 320 nm.

Lactose (262 mM) and butylamine (1.16 M) in 1.28 M phosphate buffer (pH 7.0) were heated at 80°C ( ●), 90°C (○), and
100°C (ș).
Figure 6. Effect of heating temperature and time on the XTT reducibility. 0500100015002000
0 5 10 15 20 25 30Absorbance at 320 nm
Heating time (min)
0102030405060
0 5 10 15 20 25 30XTT reducibility
Heating time (min)

Maillard Reaction in Milk – Effect of Heat Treatment 153

Figure 7. Structure of aminoreductone generated by the Maillard reaction of lactose and butylamine.
(R = butyl group)
From these results, we presumed that the aminoreductone is formed by the Maillard
reaction between lactose and ε-amino groups of milk proteins, and then it is responsible for
the reduction of XTT. However, at that time, there was no report to prove the presence of
aminoreductone in milk. Thus, we tried to demo nstrate it using model system consisting of
lactose and milk proteins and UHT milk.
4.2. In the model system of lactose and milk proteins
As a model system of milk, the solution cons isting of lactose (4.6%) and casein (2.6%), α-
lactalbumin (0.12%), or β-lactoglobulin (0.32%) was used and heated at 130°C for 15 min.
After heating, the characteristic absorp tion maximum or shoulder at 320 nm was
recognized. In addition, the ch anges of the absorbance at 320 nm (Figure 8) and the XTT
reducibility (Figure 9) were investigated. In all model systems, the increases in the
absorbance at 320 nm and the XTT reducibility depended on the heating time. Because
similar tendencies were observed between two in dices in all model systems, correlations were
examined. Consequently, there were significant lin earities as follows: casein (r = 0.993, n = 6, p
< 0.001), α-lactalbumin (r = 0.996, n = 6, p < 0.001), and β-lactoglobulin (r = 0.975, n = 6, p <
0.001). From these results, it was suggested that aminoreductone is generated in the Maillard
reaction between lactose and milk proteins and it is responsible for the reduction of XTT.
4.3. In milk [15]
As described above, a possibility of the formation of aminoreductone on the milk proteins
during the Maillard reaction with lactose was clearly shown. However, direct
demonstration of the presence of aminoreduc tone in milk had not been accomplished
because of a difficulty in isolation of an in tact aminoreductone fr om milk proteins. For
instance, aminoreductone is labile and hence not suitable for enzyme hydrolysis and
multiple extraction steps. To achieve the pr actical application of the XTT assay in food
industries including dairy products, it was essential to demonstrate the presence of
aminoreductone in milk. Because of this background, we attempted to isolate
aminoreductone from milk proteins using 2,4-dinitrophenylhydrazine (DNP), a common
labeling reagent for the carbonyl group, and Cu2+ [16]. A mechanism of derivatization of

Milk Protein 154
aminoreductone in milk is shown in Figure 10. In this derivatization step, Cu2+ plays as an
oxidizing agent against aminor eductone, and the oxidized aminoreductone (OAR) has two
or three carbonyl groups. Finally, it was assume d that two or three ca rbonyl groups in OAR
are derivatized by DNP (OAR-DNP).

Lactose (4.6%) and casein ( ș: 2.6%), α-lactalbumin ( ●: 0.12%), or β-lactoglobulin ( ▲: 0.32%) in 20 mM phosphate buffer
(pH 6.7) were heated at 130°C.
Figure 8. Effect of heating time on the absorbance at 320 nm.

Lactose (4.6%) and casein ( ș: 2.6%), α-lactalbumin ( ●: 0.12%), or β-lactoglobulin ( ▲: 0.32%) in 20 mM phosphate buffer
(pH 6.7) were heated at 130°C.
Figure 9. Effect of heating time on the XTT reducibility. 0246810
0369 1 2 1 5Absorbance at 320 nm
Heating time (min)
00.150.30.45
0369 1 2 1 5XTT reducibility
Heating time (min)

Maillard Reaction in Milk – Effect of Heat Treatment 155

Figure 10. Derivatization mechanism of aminoreductone in milk using DNP and Cu2+.
The derivatization using DNP and Cu2+ was applied to aminoreductone in UHT milk
(140°C, 3 sec). In this study, the UHT milk was reheated at 130°C for 15 min in order to
increase the content of aminoreductone, becaus e the original content of aminoreductone in
the commercially available UHT milk was not so high. As a result, the reheating of UHT
milk could increase the content of DNP deriva tive by 40 times. The DNP derivative which
was thought to be corresponding to OAR-DNP in the reheated UHT milk was purified by
preparative normal-phase HPLC and prepar ative reversed-phase HPLC. Finally, the
purified compound (4.2 mg) was obtained from 980 mL of UHT milk and analyzed by 13C-
and 1H-NMR. The NMR signals of the DNP derivati ve from UHT milk could be assigned to
the structure of OAR-DNP shown in Figure 10. In addition, the NMR signals of DNP
derivative from UHT milk were nearly the same as those of the OAR-DNP from lactose-
butylamine model system. These results demons trated that aminoreductone was formed by
the Maillard reaction on the milk proteins and present in milk.
Therefore, considering the above, the principle of the present XTT assay can be concluded as
follows (Figure 11): (1) Lactose and ε-amino groups of lysine residue in milk proteins
react non-enzymatically to form the Amadori product by the heating process. (2)
Aminoreductone structure is formed on the milk proteins after elimination of galactose
moiety from lactose through 4-deoxyosone pa thway. (3) Aminoreductone is oxidized by
XTT, whereas XTT is simultaneously reduced to the corresponding water-soluble
formazan.
It would be thought that the above mentioned steps (1) and (2) progresses depending on the
time and temperature of heating process in milk production, so the XTT assay can
differentiate the extent of heat treatment of milk. Based on the study using model system of
lactose and butylamine, the relationship be tween aminoreductone concentration and XTT
reducibility was examined. As a result, there was a good linearity was recognized (r = 0.98)
and a regression equation was y = 0.606 x + 0.046, in which x and y represented the
concentration of aminoreductone (mM) and the XTT reducibility [17]. Based on this
equation, the concentration of aminoreductone in UHT milk could be estimated as 0.44 mM.

Milk Protein 156
CO H
CO
CH2
CHOH
CH2OHHC NHNHCO
Aminoreductone bound
to milk proteinsCO
HN(CH 2)4CHLactose + Milk protein (casein etc.)
Amadori product(1)
(2)
– galactose
Oxidized form of aminoreductoneXTT
XTT formazan(3)

Figure 11. Principle of XTT assay.
As described above, during the course of el ucidation of the XTT-reducing substance, Cu2+
was used as the oxidizing agent (Figure 10), beca use we empirically knew that it was easily
decomposed by the addition of Cu2+ [18]. In fact, the aminored uctone extracted from the
heated model solution of lactose and butylami ne was rapidly oxidized by the addition of
Cu2+, and simultaneously the XTT reducibility wa s also lost. On the other hand, in our
previous work, it was revealed that the commercially available UHT milk contains Cu2+ at
the concentration of 30 μg/L [18]. Thus, it was easily pr esumed that the aminoreductone
formed by the heating process was gr adually oxidized by endogenous Cu2+ in milk during
storage period. This was the reason why that the XTT reducibility decreased depending on
the storage period (Figure 3). The detailed investigation about the relationship between the
aminoreductone concentration, Cu2+ concentration, and storage stability of milk is now in
progress.
5. Functionality of aminoreductone
The functionalities of aminoreductone have attrac ted interest and, so far, some studies were
performed. Trang et al. [17] reported a protective effect of aminoreductone against riboflavin
(vitamin B 2) photolysis. It is well known that the milk is important source of riboflavin and
its content is 1.5 mg/L. It is stable to heat and oxidation, but is rapidly photo-degraded. In
experimental condition at 7000 lux light intens ity, the riboflavin (1.5 mg/L) was almost
completely degraded for 150 min. On the othe r hand, the addition of aminoreductone (0.22
mM: half concentration in UHT milk) could extend the half-life period of riboflavin. The protective effect of aminoreductone against ri boflavin photolysis was higher than that of
ascorbic acid which was famous antioxidant. In a ddition, the antioxid ative activity of

Maillard Reaction in Milk – Effect of Heat Treatment 157
aminoreductone was reported [19]. From these results, it was suggested that the
aminoreductone formed by the Maillard reaction would contribute to keep the nutritional value and sensory quality of milk.
Furthermore, aminoreductone showed antimicrobial activity against Helicobacter pylori (H.
pylori ). In vitro it effectively inhibited the growth of 24 kinds of H. pylori strains including
antibiotic-resistant strains and had a bacteric idal activity [20, 21]. The Killing ability was
observed even in acidic condition. In additi on, aminoreductone also had the antimicrobial
activity against me thicillin-resistant Staphylococcus aureus (MRSA) [21, 22]. These results
indicated that foods containing aminoreductone, such as milk and dairy products, have a
potential health benefits in medical practice.
6. Conclusion
In this chapter, the demonstration of the presence of aminoreductone formed by the
Maillard reaction in milk and the specific as say method for aminoreductone were focused
and introduced. Since the novel functionality of aminoreductone have come out one after
another, the information of aminoreductone obtained by the XTT assay would gain
importance in the quality control in milk and dairy products.
Author details
Tomoko Shimamura* and Hiroyuki Ukeda
Faculty of Agriculture, Kochi University, Nankoku, Japan
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