Biomedical Engineering Reference
In-Depth Information
Even in a simple Maillard model system consisting of a single reducing sugar
and amino acid, there are thousands of reaction products which may contribute to
color and flavor. Control and optimization of the Maillard reaction are major research
themes within the food manufacturing and food ingredients industries but the com-
plexity of the reaction and the many ill-defined products, places considerable limi-
tations on the ability to study the kinetics of the rate-determining steps. For this
reason little real progress has been made to identify the factors that determine
browning in specific foods or to devise food flavor formulations from first principles.
An important way forward is illustrated by the painstaking analytical work of van
Boekel's group, 65 who have measured the concentrations of known intermediates in
the early stages of the browning of sugars with casein and have attempted to relate
these to an overall kinetic model for the reaction.
Understanding of the kinetics of the reactions of S(IV) during its inhibition of
Maillard browning now offers a new and focused approach to understand better the
mechanism of the Maillard reaction. 61,62,66,67 The approach is based on the premise
that since S(IV) reacts quantitatively and irreversibly with a precursor of color and
flavor, the rate at which S(IV) reacts in this way is a measure of the rate at which
reducing sugars are converted to Maillard intermediates up to the point where S(IV)
exerts its inhibitory effect. Thus, measurement of the rate of loss of S(IV) allows
the rate constants k 1 and k 2 to be obtained in relation to variables of interest in
Maillard browning, e.g., concentration, pH, water activity, and the presence of
different amino acids. 57,59,61,67,68 It has been shown that k 1 can have an overall con-
trolling effect on the rate of browning, 60 and a combination of k 1 and k 2 , with a third
rate constant for color formation, can be used to model accurately the rate of
browning of glucose-glycine mixtures. 61,62
The inhibition of the browning of fructose by S(IV) also results in the formation
of DSH. The kinetics of the fructose-amino acid-S(IV) reaction are simpler than
those of the Maillard reaction, and may be described by
()
k
slow
SIV
fast
fructose
→→
INT
DS
the major difference being the absence of an early amino acid-dependent step. 69 This
point is adequately demonstrated by the fact that the rate of reaction of S(IV) in the
absence of amino acid ([fructose] = 1.0 mol l -1 ; [S(IV)] = 0.05 mol l -1 ; pH 5.5, 55°C)
is 53 µmol l -1 h -1 and 53, 55, and 50 µmol l -1 h -1 in the presence of 0.5 mol l -1
glycine, glutamic acid, and arginine, respectively. The reaction is, however, catalyzed
by the components of buffer systems, e.g., acetate ion, 70 and these results all provide
insight into the reported discrepancies between the rates of browning of fructose
and glucose in buffered and unbuffered media.
The browning of ascorbic acid is a form of nonenzymic browning reaction,
which is related to the Maillard reaction. In the absence of oxygen, the reaction is
inhibited by S(IV) and so leads to the formation of 3,4-dideoxy-4-sulfopentosulose
(DSP), the 5-carbon analog of DSH. 71,72 In this respect, the reaction shares a common
step in the S(IV)-inhibited Maillard reaction of pentoses, i.e.,
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