Food Lipids: Chemistry, Nutrition, and Biotechnology

a free radical scavenger by forming a thiyl radical. GSH also donates electrons for
the reduction of dehydroascorbate to ascorbic acid in the presence of glutathione
dehydrogenase. Ascorbic acid is then returned to its active, free radical scavenging
form. The enzyme has been purified from wheat flour, which has a relatively high
activity [7].
6. Maillard Reaction Products
Maillard reaction products (MRPs) are an excellent example of natural, processinduced
oxidation inhibitors that arise as a result of cooking [35]. MRPs are formed
during the cooking of low-moisture foods at temperatures above 80�C. They are
produced from the reaction of amines and reducing sugars. Lipids, vitamins, and
other food constituents also participate in Maillard reactions. MRPs are presumed to
be safe because they occur naturally as products in cooked foods. They are often
used as bases for flavorants and gravies. The brown pigment associated with MRPs
can be an advantage in cooked products where brown color is desirable. Use of
MRPs as a source of antioxidants in foods has been intensively studied, but our
understanding of the compounds responsible for the antioxidant activity is incomplete
and the mechanisms of action are unknown. Identification of the compounds responsible
for antioxidant activity has proved difficult because of the complexity of
the Maillard reaction, the vast number and variety of MRPs, and the diversity of the
model systems that can be studied. MRPs have been shown to have antioxidant
activity in model systems as well as in some fat-containing foods [36–39]. Lingnert
and Eriksson [39] used processing parameters and the Maillard reaction to prevent
oxidation in cookies, and Sato et al. [40] inhibited warmed-over flavor in cooked
beef with MRPs.
Because of the conflicting views present in the literature, it is difficult to state
conclusively which of the numerous MRPs are actually responsible for antioxidant
activity. It is even more difficult to attempt to describe the mechanism of action of
these suspected antioxidants. Investigators have shown a correlation between colorless,
low molecular weight, intermediate MRPs and antioxidant activity [41–44].
Evans et al. [45] showed a correlation between antioxidant activity and reductone
levels in MRP mixtures. In contrast, Kirigaya et al. [36] showed that antioxidant
activity was proportional to the color intensity of MRP and proposed that nondialyzable,
high molecular weight melanoidins, which inhibit hydroperoxide and carbonyl
compound formation, were responsible for antioxidant activity. By completely
oxidizing the reductones in the MRP solution with DPI butanol, these investigators
also demonstrated that reductones contributed little antioxidant activity in their model
system [36]. Other researchers found a direct relationship between color intensity of
Maillard reaction solutions and antioxidant activity [38]. Yamaguchi et al. [38] also
showed increases in antioxidant activity with increases in melanoidin formation.
Theories on the mechanism of antioxidant activity of MRPs conflict as well.
Kawakishi et al. [46] hypothesized that the protective effects of melanoidins against
autoxidation were likely to depend on their ability to chelate metals. Amadori compounds
may behave like reductones, which inhibit autoxidation. Eichner [44] believes
that MRP intermediates may scavenge oxygen. These conflicting reports likely reflect
the different reaction conditions used in the experiments and the multiple antioxidative
functions exhibited by MRPs with different mechanisms of action. Because
of the overwhelming complexity of even model systems of the Maillard reaction, it
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