Chemistry Reference
In-Depth Information
forms of the parent sugars may also be possible. It is envisaged that such
dehydrations occur via the formation of pyrylium or furylium ions, which
may then react with each other to form polymers (Yaylayan, 1990; Yaylayan
and Lachambre, 1990). Amadori products may decompose by dehydration
reactions or by thermally induced non-hydrolytic scissions (C-C and C-N
bond cleavages). The latter become more important at elevated temperatures.
Under acidic conditions, the nitrogen atom of the Amadori compound is
protonated, and 1,2-enolization (designated the 1,2-E pathway) is promoted
by the withdrawal of electrons from C1 of the sugar residue by the positively
charged nitrogen atom. As the pH increases, the deprotonation of the nitro-
gen atom increases the electron density at C1 of the sugar moiety which, in
turn, discourages 1,2-enolization. Thus, as pH increases, 2,3-enolization
(designated the 2,3-E pathway) becomes more favourable. This effect of pH
on the electron density across C1 and C2 is more pronounced with Amadori
products derived from basic amino acids. The 2,3-E pathway is a particularly
important source of flavour volatiles in food systems, yielding furanones,
pyranones and Strecker degradation precursors. The mechanistic similarity
between Maillard reactions and caramelization was emphasized by Feather
(1981) who used the term 'amine-assisted sugar dehydration reactions'. How-
ever, in contrast to caramelization-type reactions, Amadori products can
undergo enolization more readily and under milder conditions than the
corresponding sugars (as a result of the stabilizing influence of the amino
moiety). Thus, the formation of Amadori and Heyns rearrangement products
in Maillard reactions can be regarded as low-energy pathways for the decom-
position of sugars compared with caramelization.
The hydrolysis of the imminium ion has been reported as the possible
rate-limiting step in the decomposition of the Amadori compound to form
5-hydroxymethyl-2-furaldehyde (Yaylayan and Forage, 1991). Similarly,
cleavage of the C-N bond in the 2,3-E pathway may be a rate- determining
step (Yaylayan and Forage, 1991). There is evidence that amino acids may
actually inhibit the decomposition of Amadori products to form brown
pigments, possibly due to inhibition of the deamination steps of the 1,2-
and 2,3-E pathways or because of reaction with the carbonyl groups of the
Amadori compound forming less reactive derivatives (Nursten, 2005). The
presence and nature of amino acids have a strong influence on the pathway of
reaction of Amadori products and the products formed. Yaylayan and Man-
deville (1994) showed a marked increase in the formation of 2,3-dihydro-3,5-
dihydroxy-6-methyl-4H-pyran-4-one, hydroxymaltol and maltol from the
Amadori product, fructosyl proline, when an equal weight of alanine was
added. Addition of proline to the fructosyl proline led to significant, though
less marked, increases in the yield of 2,3-dihydro-3,5-dihydroxy-6-methyl-
4H-pyran-4-one and hydroxymaltol. Simple methods for following the 1,2-E
