Leche Maillard Actividad de Agua

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Maillard reaction kinetics in milk powder: Effect of water activityat mild temperatures

A.S. Pereyra Gonzales, G.B. Naranjo, G.E. Leiva, L.S. Malec*

Departamento de Qumica Organica, Area Qumica y Microbiologa de Alimentos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina

a r t i c l e i n f o

Article history:

Received 20 April 2009

Received in revised form

9 July 2009

Accepted 20 July 2009

a b s t r a c t

The kinetics of the Maillard reaction in skim milk powder was studied at a wide range of water activities

(0.310.98) under storage at mildly elevated temperature conditions (37, 50 and 60 C). The reaction rate

was determined by the loss of available lysine content using the o-phthaldialdehyde method. Water

activity affected the rate of loss of lysine above the glass transition temperature only at high water

activity values. However, the mobility of the reactants seemed to play a role in damage to lysine, as the

rate constant decreased considerably at temperatures close to the glass transition. Crystallization of

lactose did not affect the rate constant values. Water activity did not influence the temperature-

dependence of the reaction rate, although a significant increase in activation energy was observed in the

vicinity of the glass transition temperature.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Maillard reactions frequently occur between proteins and

reducing sugars in thermally-processed foods. Lysine is the main

amino acid involved in the initial states, producing an Amadori

compound in which lysine is unavailable, thereby affecting nutri-

tional value. Therefore, control of the reaction rate has been given

much attention in order to optimize processing and storage

conditions of foods prone to nonenzymatic browning (NEB) (Hur-

rell, 1990; Mauron, 1990).

In milk, NEB begins by condensation of lactose with proteins,

forming lactulosyl-lysine, a stable protein-bound Amadori product.

The high concentration of lactose and lysine-rich proteins make

milk products very sensitive to this reaction. The occurrence of

lactosylationmay not cause browning,but can lessen the nutritional

quality of milk due to the blockage of lysine, which is no longer

available for digestion (Finot, 1990; van Boekel, 1998). This reaction

is mainly induced by heat treatment. Nevertheless, some authorshave demonstrated that lactosylation also occurs during storage of

milk powder (De Block, Merchiers, & Van Renterghem, 1998;

Guyomarch, Warin, Muir, & Leaver, 2000; Kessler & Fink, 1986).

The rate of Maillard reaction is influenced by many factors, such

as temperature, water activity (aw), pH, reactant source and

concentration and the type and ratio of reducing sugar to lysine

(Labuza & Baisier, 1992). In particular, the rate of deteriorative

reactions and storage stability are related to water activity of foods.

The maximal rate of NEB reaction was reported to occur in the awrange of 0.50.75 (Kaanane & Labuza, 1989; Labuza & Saltmarch,

1981).

Glass transition has also been found to affect the kinetics of

diffusion-controlled chemical reactions such as Maillard reaction.

The high viscosity of a system below their glass transition

temperature (Tg) was associated with limited molecular mobility

and the consequent retarding of the reaction rate (Bell, Touma,

White, & Chen, 1998; Karmas, Buera, & Karel, 1992; Lievonen,

Laaksonen, & Roos, 1998). Water plasticization increases molecular

mobility, which may result in the conversion of lactose from the

amorphous state to the crystalline state (Roos & Karel, 1992). In

closed systems, lactose crystallization will induce an increase inawdue to the release of water from amorphous lactose and may also

accelerate deteriorative changes, such as NEB (Jouppila & Roos,

1994; Vuataz, 2002). Crystallization increases with increasing

relative humidity (RH) and heat treatment (Jouppila & Roos, 1994;Vuataz, 1988).

Most of the kinetic analysis of the Maillard reaction at different

aw values and temperatures has been accomplished using simple

model systems with reducing sugars and amino acids (Bell, 1996;

Buera & Karel, 1995; Karmas et al., 1992; Lievonen & Roos, 2002;

Miao & Roos, 2004) or proteins (Desrosiers, Bergeron, & Savoie,

1989; Desrosiers & Savoie, 1991; Malec, Pereyra Gonzales, Naranjo,

& Vigo, 2002). A few studies have reported browning rates during

heating of milk at different water activities, typically at tempera-

tures above 60 C (Acevedo, Schebor, & Buera, 2006; Franzen, Singh,

& Okos, 1990; Vuataz, 1988). So far, no information regarding the* Corresponding author. Tel./fax: 54 11 4576 3346.

E-mail address: [email protected](L.S. Malec).

Contents lists available atScienceDirect

International Dairy Journal

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i d a i r y j

0958-6946/$ see front matter 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.idairyj.2009.07.007

International Dairy Journal 20 (2010) 4045
mailto:[email protected]://www.sciencedirect.com/science/journal/09586946http://www.elsevier.com/locate/idairyjhttp://www.elsevier.com/locate/idairyjhttp://www.sciencedirect.com/science/journal/09586946mailto:[email protected]


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influence of aw and temperature on Maillard reaction during

storage of milk powder at mild temperatures has been reported.

The purpose of the present work wasto evaluatethe influence of

aw on Maillard reaction kinetics in milk powder under storage at

mildly elevated temperatures. The effect of lactose crystallization

on the reaction was also analyzed. A kinetic analysis of the Maillard

reaction is difficult because it is a complicated reaction with many

consecutive and competitive steps. Here, to study the first stage of

this reaction, the loss of available lysine content was determined.

Three different temperatures were chosen to estimate activation

energies and to relate the influence of temperature and water

activity to the extent of lysine damage. Kinetic data were compared

with those of a previous study carried out in this laboratory with

a milk-like model system (Malec et al., 2002) in an attempt to

evaluate if the results obtained with the model system could be

applied to a real food.

2. Materials and methods

2.1. Materials

Commercial skim milk powder (51.7%, w/w, lactose and 34.2%,

w/w, protein, dry basis) was purchased at the local market.o-Phthaldialdehyde (OPA), N-acetyl-L-cysteine (NAC), MgCl2, K2CO3,

Mg(NO3)2, KI, KCl and K2SO4were purchased from Sigma Chemical

Co. (St. Louis, MO, USA) and potassium sorbate and sodium dode-

cylsulphate were purchased from Mallinckrodt (Hazelwood, MO,

USA).

2.2. Preparation of samples

A dispersion of milk powder (20%, w/w) with 0.06% (w/w)

potassium sorbate added as an antimicrobial agent was freeze-

dried. A Stokes freeze-dryer, model 21 (F.J. Stokes Company,

Equipment Div., Pennsalt Chem. Corp., Philadelphia, PA, USA) was

used, which operated at a40 C condenser plate temperature and

a chamber pressure of less than 100 mm Hgfor48 h.Portions of10 gof the freeze-dried sample were equilibrated at 25 C until weight

was constant in vacuum desiccators over saturated salt solutions at

the following water activities: 0.33 (MgCl2), 0.43 (K2CO3), 0.52

(Mg(NO3)2), 0.69 (KI), 0.85 (KCl) and 0.98 (K2SO4).

2.3. Kinetic study

Experiments were performed under mildly elevated storage

conditions at three temperatures: 37, 50 and 60 C. The system

equilibrated ataw0.33 was also analyzed at 30, 40 and 45C. After

the systems were equilibrated to the desired water activity, several

200 mg samples were sealed immediately into glass flasks to avoid

adsorption or desorption of water from the ambient, weighted and

stored at the different temperatures; the awwas controlled beforeheat treatment of the powder and no changes were detected due to

the transfer. Two flasks were removed from the oven at regular

periods of time and the extent of available lysine loss was deter-

mined. Control samples of the systems after equilibration at each

aw valuewithout thermal treatment were analyzed in triplicate and

designated as time zero.

Zero and first-order kinetic rate constants for loss of lysine (k)

and their standard error were calculated by linear regression

analysis of either (i) Lt L0 kt or (ii)Lt L0ekt for zero and first-

order, respectively, where Lt available lysine concentration at

time tand L0 available lysine concentration at t 0. The confi-

dence intervals were estimated for a significance level of 95% by

means of the Student t-test. Analysis of variance (ANOVA) was

performed to check if the rate constants were significantly different

from zero. The validity of the linear equations was tested by F-test

for lack of fit. Activation energies with 95% confidence limits were

estimated by the point by point analysis suggested by Labuza

(1984).

2.4. Measurement of water activity and available lysine

The water activity of the milk systems were measured using an

AquaLab Water Activity Meter Series 3TE with internal temperature

control (Decagon Devices, Inc., Pullman, WA, USA). The awat each

storage temperature was recorded. As aw may change during

storage due to crystallization and/or Maillard reaction, it was

reported as initialaw.

Available lysine was measured by the o-phthaldialdehyde/N-

acetyl-L-cysteine (OPA/NAC) spectrophotometric method (Medina

Hernandez & Garca Alvarez-Coque, 1992). For analysis, samples

were dissolved in 5% (w/v) sodium dodecylsulphate solution. To

10 mL of OPANAC reagent (25 mL of a 0.05 M ethanolic OPA

solution, 25 mL of a 0.05 M aqueous NAC solution and 200 mL of

0.02 M boric acid-borate buffer solution, pH 9.5, in 1 L of water),

2.0 mL of the sample solution were added and diluted to 25 mL

with water. Absorbance was measured at 335 nm with a Hewlett

Packard spectrophotometer HP 8453 (Hewlett Packard, Palo Alto,CA, USA). Three replicates of each sample were analyzed and, as

two flasks were taken at each time, the average of six measure-

ments is reported. The coefficient of variation for this assay was


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The first-order rate constants for available lysine loss at 37, 50

and 60 C are shown in Table 1; the extent of loss of lysine was quite

different at each temperature studied. At 50 and 60 C, significant

losses of lysine were detected only at prolonged times of heating,

too much longer than those generally used during processing of

milk products. Thus, heat treatment at mild temperatures would

not significantly affect the lysine content of milk proteins. At 37 C,

extensive losses were noticed after a few days of storage at mostawvalues analyzed. For example, when milk powder was stored at an

initial aw of 0.47, there was a 20% loss of lysine after 7 days.

Considering that, in some places, ambient temperatures are closeto

37 C, it can be concluded that long-term storage conditions of milk

powder products should be carefully controlled to maintain

nutritional quality, particularly when packages have been opened.

3.2. Effect ofawon rate constants

To analyze the effect ofawon rate constants at 37, 50 and 60C,

ln(k) values were plotted against the initial aw for the three

temperatures (Fig. 2; full lines). The curves at 50 and 60 C showed

similar trends: the rate constants of samples stored in the initialawrange 0.310.71 were not significantly different (P>0.05). There-

fore, at 50 and 60 C, no maximum values were observed at inter-

mediateaw. At an initial aw of 0.85, the rate constants decreased

slightly, and at aw of 0.97 their values were approximately four-foldlower than at the initial awof 0.70. This decrease at high awmay be

explained by the dilution of reactants and the inhibitory effect of

water, a product of the reaction (Eichner & Karel, 1972; van Boekel,

2001). Corresponding results have been reported by other authors

(Acevedo et al., 2006; Franzen et al., 1990) who found a wide awrange, 0.440.85, in which nonenzymatic browning was at

a maximum for milk powder heated at temperatures from 70 to

130 C.

At 37 C, the behaviour at intermediate and high aw was the

same. However, at the initial aw of 0.32, the Maillard reactionoccurred at a much slower rate than at aw of 0.470.70. This

substantial reduction in the rate constant at 37 C seems not to be

caused only by the decline inaw, since at 50 and 60 C no lessening

was observed at aw of 0.32. It should be noted that 37 C is very

close to the Tg reported byJouppila and Roos (1994)for skim milk

stored at aw0.33 (33 C). The effect of glass transition on Maillard

reaction was reported by many authors, since the physical state of

the matrixmay affect the rate of diffusion-controlled reactions such

as NEB. The rate of the reaction is extremely low in the vicinity of

the glass transition temperature, because of limited molecular

mobility as due to of the high viscosity of the system (Craig, Parker,

Rigby, Cairns, & Ring, 2001; Karmas et al., 1992; Lievonen et al.,

1998). At aw 0.43,Jouppila and Roos (1994) reported a Tg of 9 C,

much lower than 37

C. Hence, at this temperature, the increase inthe mobility of the reactants at awvalues above 0.33, as shown by

the dramatic rise in reaction rate, could be mainly attributed to the

plasticizing effect of water, which lessened the glass transition

temperature, rather than to the influence of water activity. Similar

results were obtained by Bell (1996), who compared the effect of

glass transition and aw on browning rate and concluded that the

rate of pigment formation was more significantly influenced by the

Tg of a material than by its aw. Unlike at 50 and 60 C, the lowest

reaction rate at 37 C occurred at the initial aw of 0.32 (Fig. 2).

Consequently, the physical state of the system would have a more

rate-limiting effect on the reaction than the dilution of the

reactants.

It must noted that, during storage at mild temperatures, lactose

could crystallize, affecting the rate of Maillard reaction by

Table 1

First-orderrate constants (k) and activationenergies (Ea) with 95% confidence limits

for available lysine loss from skim milk powder stored at 37, 50 and 60 C and

equilibrated at various water activities (aw).

aw k 103 (hs1) Ea (kJ mol1)

37 C 50 C 60 C

0.33 0.0198 0.0020 7.14 0.51 17.46 0.93

0.43 1.115 0.075 7.24 0.78 27.4 1.9 121.1 2.7

0.52 0.762 0.095 6.35 0.32 26.0 1.3 129.2 3.7

0.69 0.802 0.046 7.08 0.53 27.2 1.9 131.4 3.1

0.85 0.538 0.051 4.56 0.34 20.8 1.4 135.4 2.9

0.98 0.245 0.020 1.89 0.13 7.54 0.58 128.5 4.1

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

aw

lnk

Fig. 2. Effect of water activity on rate constants of loss of lysine (ln k) in skim milk

(closed symbols) and in a lactose-casein system (open symbols; Malec et al., 2002)

stored at 37 C (C, B), 50 C (:, 6) and 60 C (-, ,).

10

100

100500 150 200 250 300 350

Time (days)

Remainingavailablelys

ine(%)

Fig. 1. Loss of available lysine in skim milk stored at 37 C at initial water activities of

0.32 (:), 0.47 (C), 0.56 (6), 0.70 (,), 0.85 (A) and 0.98 (A) as a function of time.

A.S. Pereyra Gonzales et al. / International Dairy Journal 20 (2010) 404542


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increasing theawdue to the release of water from the amorphous

lactose (Jouppila & Roos, 1994; Vuataz, 2002). To analyze crystalli-

zation behaviourof lactose in systems stored at the initial aw of 0.32

and 0.43, loss of adsorbed water was recorded. When milk powder

was exposed to RH 32% and 50 C, the system did not show

decreasing water content over 200 h of storage (Fig. 3). Thus, no

crystallization of lactose occurred during the experiment. At 60 C,

a loss of adsorbed water was detected after a delay of 40 h; hence,

under these conditions, crystallization of lactose did not affect the

reaction rate, as it occurred when more than 50% lysine was lost.

Instead, at RH 43% and 37 C, adsorbed water was lost because of

lactose crystallization in a rather short time (within 23 h).

Although, at 50 and 60 C and at the same RH, faster crystallization

could be expected, there were no significant differences between

the rate constant values of samples equilibrated ataw0.33 and 0.43

at 50 and 60 C. Apparently, in this study crystallization of lactose

had no effect on the reactivity of lysine. Similar results were

reported by other authors (Morgan, Appolonia Nouzille, Baechler,

Vuataz, & Raemy, 2005; Vuataz, 1988) who suggested that, despite

the fact that an increase ofawcould accelerate NEB, crystallization

immobilizes lactose and thus less dissolved lactose is available for

reaction with proteins.

It should be noted that aw could also rise during storage, aswater is a product of Maillard reaction. However, in this study the

early stage of this reaction, which only produces a small amount of

water, was predominant, and its contribution to the overall increase

ofawwas not relevant (Vuataz, 2002).

Data reported in our previous study (Malec et al., 2002) for

a lactose-casein system stored at the same temperatures, equili-

brated to similar water activities and with the same reactant

concentrations, are included for convenient comparison in Fig. 2

(dotted lines). The kinetic constants obtained in this study were

slightly different from those corresponding to the model systems at

similar conditions. Most of the rate constants were lower (P< 0.05)

in milk powder than in the model system. The differences could be

mostly attributed to the use of phosphate buffer in the model

system; this avoids the decrease in pH due to Maillard reaction, andthus the reduction of the reaction rate throughout the experiment.

In certain conditions, such as elevated temperatures or dilute

solutions, phosphate buffers have been shown to increase the rate

of nonenzymatic browning (Bell, 1997; Potman & van Wijk, 1989;

Saunders & Jervis, 1966). Nevertheless, in low-moisture solid

systems, no enhancing effect was observed on the extent of loss of

amino acids (Bell, White, & Chen, 1998). In addition, the rate

constant values in milk powder could also be affected by the pres-

ence of salts and whey proteins. The greatest difference between

the rate constants of both systems was observed at the initial awof

0.32 and 37 C. Unlike in milk, no glass transition was detected in

the model system around 37 C and this was probably the reason

why the decrease of the reaction rate was much less. Another

notable difference was that, in themodelsystem at 37 and50 C,the

largest rate constant value was observed at an initial aw of 0.52, and

at 60 C the rate was maximal at a wider range ofaw. Instead, no

maximum was observed in milk powder stored at intermediateaw.

Therefore, the effect ofaw at intermediate values was less significant

in milk powder than in the model system, despite the similar

composition and conditions of storage of both systems.

3.3. Loss of lysine and temperature-dependence

The rate constants of the systems equilibrated at the six

different aw values increased with increasing temperature. Thetemperature-dependence of the kinetic constants was analyzed

using the Arrhenius equation. Linear relationships were obtained

when ln k wasplotted against T1 for the systems equilibrated at aw0.430.98. The activation energies (Ea) of the Maillard reaction

calculated for systems equilibrated at aw 0.430.98 are shown in

Table 1, together with their 95% confidence limits. The values varied

from 121 to 135 kJ mol1 and were within the range found by other

authors for loss of lysine in milk-related systems (Kessler & Fink,

1986; Malec et al., 2002; Morales, Romero, & Jimenez-Perez, 1995;

Naranjo et al., 1998). Some authors (Malec et al., 2002; Miao & Roos,

2004) found a higher temperature-dependence of Maillard reaction

with decreasing moisture content. However, in this study, the water

activity did not affect the temperature-dependence of the rate of

loss of lysine, as Ea values for the systems equilibrated at awvaluesof 0.430.98 were not significantly different (P> 0.05).

Regarding the Arrhenius plot of the system equilibrated at aw0.33, no linear relationship was observed. In order to analyze the

temperature-dependence of kinetics of lysine loss at this aw, value

other temperatures near Tg were studied (30, 40 and 45 C). The

equation was fitted to the experimental data in three parts, with

different slopes (Fig. 4). The shape of the figure was similar to those

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100

Time (h)

Waterconten

t(g100-1g

solids)

Fig. 3. Water sorption of skim milk powder stored at relative humidities of 32% (C),

31% (-) and 43% (:) at 50 C, 60 C and 37 C, respectively as a function of storage

time.

-12

-10

-8

-6

-4

-2

0

0.00295 0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335

1/T (K-1

)

lnk

Tg = 33 C

Fig. 4. Arrhenius plot for loss of lysine in skim milk equilibrated at aw0.33.

A.S. Pereyra Gonzales et al. / International Dairy Journal 20 (2010) 4045 43


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reported previously for browning rate in different model systems

(Karmas et al., 1992; Lievonen & Roos, 2002). Three zones were

observed and the three straight lines corresponded to temperature

ranges of 3040 C, 4050 C and 5060 C. The activation energies

for lysine loss were calculated separately for each temperature

range and were 65.2, 498.0 and 79.9 kJ mol1, respectively. Ea in

the intermediate region (4050 C) was considerably higher than

the two others, and also much higher than typical values for the

Maillard reaction. This higher temperature-dependence of the

reaction rates was explained by the abrupt change of the viscosity

of the system above the glass transition temperature. In the present

study, the break was observed at 7 C above Tg, which was in

agreement with previous studies ofKarmas et al. (1992), Lievonen

and Roos (2002) and Roos, Jouppila, and Zielasko (1996), who

reported that a large increase in NEB occurred at a range of

2 C40 C above Tg. At 30 C, where the system was in the glassy

state, the Maillard reaction still occurred, although at a very low

rate, and the Ea corresponding to the temperature range in the

vicinity of the Tg (3040 C) was lower than the usual values

reported for the loss of lysine (85166 kJ mol1)(Kessler & Fink,

1986; Labuza & Saltmarch, 1981; Malec et al., 2002; Naranjo et al.,

1998). The second break occurred at 13 C above the Tg and the Ea

at the higher temperature zone (5060 C) was closer to the rangeof rate constant values for the loss of lysine informed above, indi-

cating that at temperatures well above Tg, the reaction became

diffusion-controlled and followed Arrhenius kinetics.

4. Conclusions

In contrast to the results obtained in our previous study with

a milk-like model system, the effect ofaw above the glass transition

on the rate of loss of lysine by mildheattreatment orduring storage

of milk powder was only significant at high aw values, and the

temperature-dependence of the reaction rate was not affected by

aw. However, the mobility of the reactants seemed to play a role in

lysine damage. Although the Maillard reaction did not stop in the

glassy state, the rate constant decreased considerably at tempera-

tures close toTg. The influence ofaw on Maillard reaction kinetics in

milk powder can be interpreted as a consequence of concentration

and diffusion of reactants. A dilution effect was observed ataw over

0.7, which was independent of the temperature of the system, at

least up to mild temperatures. At low aw, loss of lysine decreased

only at temperature conditions where the high viscosity affected

molecular diffusion. For this reason, as milk powder is commonly

exposed to extended periods of storage, it is critical to keep the

system at temperature andaw conditions below the glass transition

in order to minimize its nutritional damage.

Acknowledgements

This work was supported by grants UBACYT X021 and UBACYT

X062 from Secretara de Ciencia y Tecnica de la Universidad de

Buenos Aires.

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