Chemistry Reference
In-Depth Information
important feature of Maillard and caramelization reactions in milk systems
compared with other foods. In the presence of amino compounds, maltol may
be formed from disaccharides or monosaccharides. However, in the absence
of amino compounds, monosaccharides do not result in maltol formation.
The formation of maltol is promoted under alkaline conditions. Although
maltol may be formed via the 2,3-E pathway, Yaylayan and Mandeville
(1994) reported that the favoured pathway for its formation is via ortho-
elimination of the Amadori product. The addition of amino acids to the
Amadori product, fructosyl proline, resulted in dramatic increases in the yields
of 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one, hydroxymaltol and
maltol (Yaylayan and Mandeville, 1994).
There is evidence that carbonyl intermediates of the Maillard reaction
may react with sulphur compounds released in heated milk, leading to a
decrease in the latter as the intensity of heat treatment is increased (Jaddou
et al., 1978). It appears that oxygen may also play a role in this process (Calvo
and de la Hoz, 1992). The reactions of sulphydryl compounds formed in milk
with Maillard intermediates have not been investigated in detail. However,
many sulphur compounds are inhibitors of the Maillard reaction and such
interactions may have an inhibitory effect on the reaction in milk systems by
making carbonyl compounds unavailable for further reactions and browning.
The reactions are also likely to be of organoleptic significance because of the
powerful odour of low molecular weight S-compounds.
Strecker degradation reactions involving oxidative degradation of
-amino acids by dicarbonyl intermediates of the Maillard reaction are
among the most important reactions in food systems. Such reactions result
in the formation of unstable Schiff's bases which easily decarboxylate to form
enamines and CO
2
. The CO
2
is derived from the amino acid. The enamines
subsequently undergo hydrolysis to form an aldehyde from the amino acid
(Strecker aldehyde) and an
-aminoketone from the dicarbonyl compound.
Dicarbonyl compounds are formed in both the 1,2-E and 2,3-E pathways.
The key Maillard intermediate, 3-deoxyglucosulose, has been shown to react
with
L
-phenylalanine in a Strecker degradation (Ghiron et al., 1988). Strecker
degradations have long been known to occur during the processing and
storage of dairy products. Patton (1955) considered 3-methylbutanal to be a
possible Strecker aldehyde of potential flavour significance in milk. Based on
studies using
14
C-lactose, Dutra et al. (1958) suggested Strecker degradation
as the major source of CO
2
produced during the sterilization of milk; only
4% of total CO
2
was attributable to lactose caramelization. Strecker
aldehydes have been detected in pasteurized milk (3-methylbutanal,
2-methylbutanal), UHT milk (2-methylbutanal, isobutanal), sterilized milk
(3-methylbutanal, 2-methylbutanal) and milk powder (2-methylbutanal, iso-
butanal) (Calvo and de la Hoz, 1992). In addition, Calvo and de la Hoz (1992)
