Chemistry Reference
In-Depth Information
mechanism of the Maillard reaction by determining the type of enolization
favoured (1,2- or 2,3-enolization) and hence the pattern of Amadori com-
pound degradation. Consequently, the influence of other factors on reaction
rate is frequently pH dependent. For example, the addition of
D
-alanine or
L
-lysine to an
L
-ascorbic acid model system had a small or negligible effect on
browning rate at pH 5 or 7, whereas an increase in browning rate occurred
when the amino acids were added at pH 8.0 (L oscher et al., 1991). In contrast,
addition of glycine resulted in an increase in browning at all three pH values.
The authors attributed this to a reaction of glycine with furfural produced on
degradation of ascorbic acid with a resultant increase in brown pigment
formation. By contrast with brown colour formation, the development of
fluorescence in an epoxyaldehyde-lysine system appeared to be independent
of pH above 6.0; brown colour development showed a maximum at pH 9.0
(Hidalgo and Zamora, 1993). At shorter incubation times, development of
fluorescence was linear with increasing pH. Increasing pH may have a major,
albeit indirect, effect on the Maillard reaction by increasing the rate of
mutarotation of both the parent sugar and of hemiacetal and hemiketal
forms of intermediates formed in the reaction. Since the pH of a system
decreases during the course of Maillard browning (due to the disappearance
of basic amino groups and the formation of formic, saccharinic and other
acids), the buffering capacity of the system has an important effect on the rate
of reaction. The inhibitory effect of supercritical carbon dioxide treatment on
Maillard reactions appears to be due to a reduction in pH due to both high
pressure and CO
2
(Casal et al., 2006).
7.4.3.
Mutarotation
Since it is generally accepted that sugars can react only as their acyclic
form (at least at low temperatures), the rate of mutarotation is likely to be a
significant rate-limiting step because the amount of sugar in the open-chain
form is normally limited to a few percent of total sugar. Vogel et al. (1988)
reported that the rate of glucose mutarotation was minimal at pH 3. As pH is
increased, the rate of mutarotation increased rapidly; the rate of mutarota-
tion of glucose at pH 7 was reported to be approximately 8 times that at pH 3
(Vogel et al., 1988). The rate of mutarotation also increases with increasing a
w
and temperature (Angyal, 1984; Vogel et al., 1988). The significance of
mutarotation rate and the concentration of acyclic forms were studied by
Yaylayan and Forage (1992) who reacted tryptophan with glucose or man-
nose; since they are C2 epimers, both sugars produce the same Amadori
product which enables differences in reaction rate due to mutarotation to
be determined. Mannose, which has a higher rate of mutarotation and a
higher equilibrium concentration of acyclic form than glucose, reacted
