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reacting carbonyl compound has a major influence on the mechanism of
the Maillard reaction because the Amadori and Heyns compounds differ
considerably in their reactivity (Pilkova et al., 1990). Heyns products are
unstable compared with their corresponding Amadori products, especially
in the presence of amino acids. They readily react further to produce Amadori
products via an LA-type transformation. However, in spite of their inherent
instability, the rate of browning of Heyns products has been reported to be
slower than that of corresponding Amadori products.
Although the degradation of the sugar moiety during the early stages of
the Maillard reaction is irreversible, some of the amino acids may be recov-
ered by acid hydrolysis of the Amadori product. While Amadori and Heyns
products would be expected to brown more readily than mixtures of the
corresponding amino acids and sugars (Westphal et al., 1988), such products
are relatively stable in food systems and, in many products, are the major or
only products of the Maillard reaction.
Glycosylamines may also decompose via a free radical mechanism
involving the formation of N,N
0
-dialkylpyrazine cation radicals (Hayashi
and Namiki, 1981). Such radicals are formed prior to the Amadori rearrange-
ment in a process believed to involve fragmentation of the glycosylamine in a
reverse aldol reaction to give 2-carbon enaminols (see review by Rizzi, 2003).
7.3.2.
Reactions of Amadori and Hynes Products
The Amadori product of amino compounds is a secondary amine and,
as such, may react with a second molecule of sugar to form a diketosyl amine,
although this would be expected to be a relatively minor pathway due to both
electronic and steric effects. However, once diketosyl Amadori products are
formed they are highly reactive and brown more readily than monoketosyl
Amadori products. The addition of sugars tends to promote the browning of
Amadori products whereas amino acids tend to inhibit browning (Nursten,
2005). The former may be associated with the formation of reactive diketosyl
Amadori rearrangement products. The latter is probably due to inhibition of
the deamination step (see below).
The extent to which Amadori product degradation contributes to
advanced products of the Maillard reaction (including browning) in food
and biological systems is contentious and somewhat difficult to quantify.
There is evidence that non-Amadori pathways become more significant with
increasing pH (
>
8). Based on observations on the kinetics of Maillard reac-
tions at high and low temperatures, and studies on the fragmentation of
sugars by electron impact and Amadori products, Yaylayan (1990) suggested
that the direct dehydration of the cyclic forms of Amadori compounds may
occur at high temperatures. Similarly, the direct dehydration of the cyclic
