Chemistry Reference
In-Depth Information
turn resulted in the formation of carboxymethyl lysine from lysine via an
intramolecular Cannizaro reaction. It has been reported that oxidized ascor-
bic acid reacts rapidly with casein to produce a red coloration (Namiki et al.,
1986; Gopalan et al., 1994). Namiki et al. (1986) showed that incubation of
ascorbic acid in the presence of an air stream was necessary to produce red
coloration in the presence of casein. Thus, it appears that the reactivity of
ascorbic acid per se in Maillard reactions is negligible compared with that of
dehydroascorbic acid. It is recognized that volatile aldehyde products of lipid
peroxidation react readily with amino acids and proteins in Maillard-type
reactions (Okitani et al., 1986; Shibamoto and Yeo, 1992). Hexanal, a major
volatile product of lipid peroxidation, reacts with proteins resulting in block-
ing of lysine and tryptophan residues, and polymerization (Okitani et al.,
1986). Aliphatic aldehydes produced on UHT processing of milk have long
been known to decrease on storage, presumably due to such reactions
(Earley and Hansen, 1982). The major lipid peroxidation product of butter-
fat, 4,5-epoxy-2-heptenal, has been shown to develop a brown colour and
fluorescence on reaction with lysine (Hidalgo and Zamora, 1993). Interest-
ingly, the activation energies for browning and fluorescence development on
reaction of 4,5-epoxy-2-heptenal with lysine were comparatively low
(66.5 and 50 kJ mol
-1
, respectively). Products of the Maillard reaction have
been shown to have a pro-oxidant effect in food systems (Miyazawa et al.,
2005) (see also Section 7.8.1).
Maillard reactions are also responsible for the formation of a host of
powerful antioxidant compounds, many of which have not been character-
ized (Manzocco et al., 2001; Morales and Jimenez-Perez, 2001; Cejpek et al.,
2004). The formation of such compounds is now increasingly being exploited
industrially to improve the oxidative stability of foods. The inhibitory effect
of pasteurization on the oxidation of milk lipids has been attributed to the
release of sulphydryl groups during heating (Calvo and de la Hoz, 1992).
However, it is possible that products of the Maillard reaction may also have a
positive role in this respect. For example, maltol, an important flavour
molecule in heated milk, is a powerful antioxidant.
The influence of oxygen on the formation of maltol and other pro-
ducts in a model system was studied by Yaylayan and Mandeville (1994).
Although the decomposition of glucose and fructose did not appear to be
sensitive to the presence of oxygen, the exclusion of oxygen seemed to
promote the formation of maltol and HMF from maltose. In contrast,
exclusion of oxygen from a tagatose system shifted the profile of products
in favour of HMF, minimizing the formation of 2,3-dihydro-3,5-dihydroxy-
6-methyl-4H-pyran-4-one. Oxygen is unlikely to have an important
influence on lactose degradation during the heating of milk because of
decreasing O
2
solubility at high temperatures. However, the presence of
