Chemistry Reference
In-Depth Information
1.3 times faster than the latter at 1108C and 1.8 times faster at 1408C. Glucose
has one of the lowest equilibrium concentrations of acyclic form of the
monosaccharides and one of the lowest rates in Maillard reactions among
monosaccharides. Thus, it has been suggested that glucose may have been
selected as the universal metabolic fuel during evolution because of its stabi-
lity (Kaanane and Labuza, 1989). For example, fructose reacted
7 times
faster than glucose with haemoglobin at 378C (Kaanane and Labuza, 1989)
Little is known of the influence of factors other than pH, temperature
and a
w
on mutarotation rate. Amino acids have been reported to be weak
catalysts for the mutarotation of reducing sugars (Shallenberger, 1984).
Depending on the presence of complex-forming configurations, the presence
of ions such as Ca
2+
,Mg
2+
,Na
+
and K
+
can influence the tautomeric
equilibrium of a sugar. It has been reported that Ca
2+
shifts the tautomeric
equilibrium of
D
-glucose towards the
-anomer. Addition of 1% CaCl
2
at a
constant initial pH to hydrolysed concentrated whey (50% solids), caused a
slight decrease in browning rate (Buera et al., 1990). The authors suggested
that a complexation reaction between Ca
2+
and the sugar resulted in a shift in
anomeric equilibrium. The effect was greater in the absence of protein. In
contrast, Kaanane and Labuza (1989) reported that FeCl
2
, CaCl
2
, CoCl
2
,
KCl or NaCl had no significant effect on the mutarotation of glucose or
fructose. However, there was a highly significant difference between systems
studied in D
2
OorH
2
O, illustrating that solvent has an important influence on
the kinetics of mutarotation. Addition of KCl, CaCl
2
or MgCl
2
had no effect
on the formation of 5-HMF from glucose, fructose or sucrose in the presence
of citric acid (Lee and Nagy, 1990). NaCl and NH
4
Cl have been shown to
have a significant inhibitory effect on the rate of browning in model
food systems, including casein-glucose, when added to a concentration of
0.5% (w/w) (Pham and Cheftel, 1990).
7.4.4. Moisture Content and a
w
Both the moisture content and the a
w
of a food system exert a major
influence on the Maillard reaction. Water may influence the rate of reactions
by controlling the viscosity of the liquid phase and by dissolution, concentra-
tion or dilution of reactants (Warmbier et al., 1976; Labuza, 1980). At very
low a
w
values, the proportion of total reactants in solution is negligible and,
therefore, the reaction rate is minimal. As the a
w
increases, the concentration
of reactants remains constant provided excess solute is available to maintain a
saturated solution. However, the total volume in which the reaction takes
place increases. Eventually, an a
w
value is reached when the solution of
reactants is diluted and the reaction rate decreases again. Since water is a
product of the Maillard reaction, the a
w
may increase with the development of
