Chemistry Reference
In-Depth Information
reactive free amino acid is erroneous and that tryptophan is
30 times more
reactive. In a comparative study on the reactivity of amino acids in total
parenteral nutrition solutions, Labuza and Massaro (1990) reported that lysine
browned the slowest whereas cysteine had the highest rate of browning (
20
times that of lysine). However, paradoxically, in a mixed system containing
lysine, tryptophan and cysteine, there was evidence that cysteine acted as an
inhibitor of browning (Labuza and Massaro, 1990). Cysteine, N-acetyl-
L
-
cysteine and reduced glutathione have been reported to inhibit non-enzymatic
and enzymatic browning in both model and real food systems (Friedman and
Molnar-Perl, 1990; Molner-Perl and Friedman, 1990). The strong nucleophili-
city of such sulphur compounds probably contributes to their activity as brown-
ing inhibitors. Among the possible mechanisms for the activity of sulphur
compounds, the suppression of free radical formation and the trapping and
inactivation of intermediates of non-enzymatic browning, thus preventing
further reactions, have been proposed (Friedman and Molnar-Perl, 1990).
Clearly, many unrelated factors influence the reactivity of amino acids
in the Maillard reaction. For example, on the basis of pK, arginine might be
expected to be more reactive than lysine (the pK of the guanidino group is
12 compared with 9.4-10.6 for the
"
-NH
2
of lysine). However, the guani-
dino group has been reported not to undergo the Maillard reaction (Ledl and
Schleicher, 1990). Nevertheless, protein-bound arginine undergoes extensive
modification during the Maillard reaction by virtue of reactions involving
- and
-dicarbonyl compounds and lysine resulting in the formation of the
imidazopyridinium crosslink, pentosidine.
It is generally agreed that an excess of reducing sugar over the amino
compound promotes the rate of Maillard browning (O'Brien and Morrissey,
1989). Quantitatively, increasing sugar concentration at a constant amine
concentration has a greater effect on the rate of browning than an increase
in amine concentration at a constant sugar concentration (Labuza, 1994).
Presumably, the latter results in a mass action inhibitory effect on the deami-
nation steps of the enolization pathways. In addition, because of the recycling
of amines in the reaction, the concentration of sugar is more likely to be rate
limiting than that of the amino compound. Thus, increasing the lactose-
protein ratio in an infant formula was shown to increase lysine blockage
due to Maillard reaction (Evangelisti et al., 1993).
There are significant quantitative and qualitative differences in the
Maillard reactions of disaccharides compared with those of monosacchar-
ides. Degradation of the 1-deoxyglycodiuloses of disaccharides shows
major differences from those of monosaccharides, in part because the
second sugar residue is a poor leaving group in the subsequent elimination
reactions, forcing the selection of an alternative elimination. Thus, whereas
the major products of heated monosaccharides (fructose or glucose) are
