Food Biochemistry and Food Processing (2 edition)

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4 Browning Reactions 65

0.10

0.09

0.08

0.07

02040

0.05 M

Buffer concentration

Glycine concentration (M)

Time (d)

0.2 M

0.5 M

60 80 100 120 140 160 180

Figure 4.6.Effect of phosphate buffer concentration on the loss of

glycine in 0.1 M glucose/glycine solutions at pH 7 and 25◦C. (Bell

1997).

Participation of amino acids in the Maillard reaction is vari-

able; thus lysine was the most reactive amino acid (Figure 4.7) in

the heated model system of glucose and lysine, threonine, and

methionine in phosphate buffer at different pH values (4–12)

(Ajandouz and Puigserver 1999). The influence of type of amino

acid and sugar in the Maillard reaction development has also

been studied (Carabasa-Giribert and Ibarz-Ribas 2000, Mundt

and Wedzicha 2003). The reaction between free amino acids

and carbonyl compounds has been studied extensively; how-

ever, only a minor part of the Maillard reaction studies focused

on peptides and proteins. Van Lancker et al. (2010) have stud-

ied the formation of flavor compounds from model systems of

lysine-containing peptides and glucose, methylglyoxal, and gly-

oxal. The main compounds found were pyrazines, which con-

tribute significantly to the roasted aroma of many heated food

products.

Studies on the effect of time and temperature of treatment on

the Maillard reaction development have also been conducted in

different model systems, and it has been shown that an increase

in temperature increases the rate of Maillard browning (Martins

and van Boekel 2003, Ryu et al. 2003).

Concentration and ratio of reducing sugar to amino acid have a

significant impact on the reaction. Browning reaction increased

with increasing glycine to glucose ratios in the range 0.1:1–5:1

in a model orange juice system at 65◦C (Wolfrom et al. 1974). In

a model system of intermediate moisture (aw=0.52), Warmbier

et al. (1976) observed an increase of browning reaction rate when

the molar ratio of glucose:lysine increased from 0.5:1 to 3.0:1.

awis another important factor influencing the Maillard reac-

tion development; thus, this reaction occurs less readily in foods

with highawvalue due to the reactants are diluted. However,

at lowawvalues, the mobility of reactants is limited despite

their presence at increased concentrations (Ames 1990). Nu-

merous studies have demonstrated a browning rate maximum

atawvalue from 0.5 to 0.8 in dried and intermediate-moisture

foods (Warmbier et al. 1976, Tsai et al. 1991, Buera and Karel

3

2

1

050

Time (min)

Absorbance (420 nm)

100

0

Glucose + Lys

+ Trp

+ Phe

+ le

+ Val

+ Leu

+ Met

+ Thr

Glucose

Figure 4.7.Brown color development in aqueous solutions containing glucose alone or in the presence of an essential amino acid when

heated to 100◦C at pH 7.5 as a function of time (Ajandouz and Puigserver 1999).