Food Biochemistry and Food Processing

4 Browning Reactions 81

also has been studied more recently (Carabasa-

Giribet and Ibarz-Ribas 2000, Mundt and Wedzicha

2003).

Studies on the effect of time and temperature of

treatment on Maillard reaction development have

been also conducted in different model systems, and

it has been shown that an increase in temperature

increases the rate of Maillard browning (Ryu et al.

2003, Martins and van Boekel 2003).

Concentration and ratio of reducing sugar to

amino acid have a significant impact on the reaction.

Browning reaction increased with increasing gly-

cine:glucose ratios in the range 0.1:1 to 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 in-

crease of browning reaction rate when the molar

ratio of glucose to lysine increased from 0.5:1 to

3.0:1.

Water activity (aw) is another important factor

influencing Maillard reaction development; thus,

this reaction occurs less readily in foods with high

awvalues. At high awvalues, reactants are diluted,

while at low awvalues the mobility of reactants is

Figure 4.7.Brown color development in aqueous solutions containing glucose alone or in the presence of an essen-
tial amino acid when heated to 100°C at pH 7.5 as a function of time (Ajandouz and Puigserver 1999).

Bell (1997) studied the effect of buffer type and con-
centration on initial degradation of amino acids and
formation of brown pigments in model systems of
glycine and glucose stored for long periods at 25°C.
The loss of glycine was faster at high phosphate
buffer concentrations (Fig. 4.6), showing the catalyt-
ic effect of the phosphate buffer concentration on the
Maillard reaction.
The type of reducing sugar has a great influence
on Maillard reaction development. Pentoses (e.g.,
ribose) react more readily than hexoses (e.g., glu-
cose), which, in turn, are more reactive than dis-
accharides (e.g., lactose) (Ames 1990). A study on
brown development (absorbance 420 nm) in a heat-
ed model of fructose and lysine showed that brown-
ing was higher than in model systems with glucose
(Ajandouz et al. 2001).
Participation of amino acids in the Maillard reac-
tion is variable; lysine was the most reactive amino
acid (Fig. 4.7) in the heated model system of glu-
cose and lysine, threonine, and methionine in buffer
phosphate at different pH values (4–12) (Ajandouz
and Puigserver 1999). The influence of type of amino
acid and sugar in the Maillard reaction development