Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 22

to yield the phenoxyl radical in competition with electron transfer. Such
information has not been available for oxidation of lipids, but a kinetic study of
methyl linoleate quenching of triplet-riboflavin clearly indicates that lipid
radicals are formed by hydrogen atom transfer (Huvaere et al., 2010). Density
functional calculations confirmed that electron transfer is endergonic, while
hydrogen atom transfer is exergonic.
Iron(II)/iron(III) catalysis of protein oxidation by hydrogen peroxide
becomes site specific through coordination of iron(II) to lysine side chain to
yield protein carbonyls, which are often used as marker of oxidation of meat
proteins (Stadtman, 1990). Protein oxidation is also important for protein
functionality as in bread. The gluten network in wheat bread dough is damaged
by reduction of the disulfide bridges by glutathione, and bromate and other
oxidants have been used for flour improvement (cf. Fig. 1.12). Bromate is now
being replaced by ascorbate. Notably, ascorbate is a reductant, but is oxidized
enzymatically in the dough by oxygen to yield dehydroascorbate, which is the
actual oxidant protecting the gluten disulfide bridges (Grosh and Weiser, 1999).
Other oxidoreductases like laccase, a multicopper enzyme that catalyzes
formation of phenolic radicals in lignin and from tyrosin in proteins may be used
to oxidatively modify protein functionality (Steffensen et al., 2008).
1.6 Antioxidants and antireductants
In relation to food, antioxidants were originally defined as `substrates that in
small quantities are able to prevent or greatly retard the oxidation of easily
oxidizable nutrients such as fats' (Chipault, 1962). Antioxidants can prevent
oxidative damage to food during processing, storage and preparation of meals.
Antioxidants may accordingly provide more healthy food with low levels of
lipid and protein oxidation products. Antioxidants may also have more direct
health effects as part of the diet, but methodological shortcomings have been
identified since both vitamin antioxidants (vitamin E and C) and non-vitamin
antioxidants (polyphenols and carotenoids) are multifunctional in biological
systems and cannot be evaluated by `one-dimensional' methods (Frankel and
Meyer, 2000). A four-step strategy for antioxidant evaluation has been proposed
(Becker et al., 2004). As seen from Fig. 1.13, the final evaluation depends on
storage experiments for antioxidants for food protection, and on human
intervention studies for health effects of antioxidants. Most standard assays for
antioxidant evaluations in use deal with antioxidants as reductants or as
scavengers of radicals (Wolfe and Liu, 2007). Screening of potential
antioxidants for radical scavenging capacity or reducing activity using simple
assays corresponding to step I (quantification), step II (radical scavenging) or
step III (effects in model systems) of the scheme shown in Fig. 1.13 to predict
protective effects on food stability or health effects in humans seems not
scientific justified. Quantification of radical scavenging capacity or reducing
activity alone only provides guidelines for the final evaluation in storage