Chemistry Reference
In-Depth Information
7.4.
Factors that Influence Maillard Reactions
Both the overall rate and product profile of the Maillard reaction in foods are
highly dependent on a number of parameters, the most important of which
are reactants, pH and temperature. The impact of moisture content and water
activity is mentioned briefly below but the physical state of the food system is
beyond the scope of the present review. Readers are referred to the papers by
Lievonen and Roos (2002), Miao and Roos (2004) and Thomas et al. (2004)
for further information.
7.4.1.
Reactants
Both the nature and the molar ratio of the reacting species have a consider-
able influence on the rate and mechanism of the Maillard reaction. In general, low
molecular weight reactants tend to react more readily than high molecular weight
reactants, partially as a result of steric hindrance in the latter. Thus, glucose is
more reactive than lactose and contributes to the increased rate of browning in
lactose-hydrolysed milks (Lea, 1948; Evangelisti et al., 1994). Colour intensity
produced in model systems may be ranked according to the reacting sugars
as follows: xylose
>
arabinose
>
fructose
>
glucose
>
maltose
>
lactose (Yang and
Shin, 1980). Although glucose, mannose (C2 epimer of glucose) and galac-
tose (C4 epimer of glucose) all resulted in a similar loss of free amino groups
on reaction with ovalbumin, the browning that developed in the galactose-
containing system was two- to threefold higher than with the other sugars
(Kato et al., 1986). In addition, ovalbumin stored in the presence of galac-
tose showed a dramatic decrease in solubility (
40% that in the presence of
glucose or mannose). Comparisons with talose, which has the same config-
uration as galactose at C3 and C4, suggested that the configuration con-
tributed to the high rate of advanced reactions preceding browning in the
galactose system. It was suggested that the configurations of galactose and
talose contribute to a higher browning rate by stabilizing the cyclic chair
conformations of the Amadori products by hydrogen bonding (Kato et al.,
1986). It is not known whether sugars are released from glycoproteins in
heated milk and participate in Maillard reactions. However, even if such
reactions occur, their quantitative significance would be negligible relative
to the huge excess of lactose in milk.
Steric effects play an important role in determining the reactivity of
amino groups on proteins. Thus, it appears that some internal amino groups
on globular proteins, such as
-lactoglobulin, may be inaccessible to sugars
and unavailable for Maillard reactions. In addition, the pK
a
value of amino
groups and acid-base catalysis by adjacent proton donor/acceptor groups
contribute to the reactivity of individual amino groups. Although all the
