Chemistry Reference
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increasing prooxidative factors (e.g., light exposure leading to singlet oxygen
production, heat treatments that liberate protein-bound transition metal
catalysts). Finally, the industry trend towards incorporating higher concentra-
tions of unsaturated oils (e.g., oils high in !-3 fatty acids) within processed
foods results in products that are considerably more labile to oxidative
deterioration.
11.1.2 Classification
The generic term antioxidant refers to a broad class of compounds capable of
inhibiting oxidation reactions. From the food scientist's perspective, antioxidants
function either by disrupting the initiation or propagation step of free radical
reactions involving lipids (i.e., primary antioxidants), or by inactivating
prooxidants (i.e., secondary antioxidants). As is the case with either type, the
net result of their activity is a delay in the onset of volatile organic compounds
that are perceived as off-flavors or off-aromas. It should be noted that some
antioxidants are capable of behaving as both primary and secondary antioxidants,
and are referred to as multiple-function antioxidants (Reische et al., 2002). Many
proteins and polyphenols found in food exhibit multiple-function antioxidant
activity, as will be discussed in the following sections.
Primary, or chain-breaking antioxidants interfere with the initiation or
propagation steps of oxidation reactions by readily donating hydrogen atoms
(i.e., hydrogen atom transfer reactions) to lipid peroxyl radicals (Frankel, 1998);
however, some antioxidants in foods exert their activity through single electron
transfer reactions. Hydrogen abstraction from chain-breaking antioxidants yields
a radical antioxidant molecule; however, this radical species is of insufficient
energy to promote further oxidation reactions. The radical antioxidant that
results is a relatively lower energy species, often due to resonance delocalization
of the unpaired electron around a conjugated ring system to form stable reson-
ance hybrids (Reische et al., 2002). Thus, many primary antioxidants comprise
phenolic moieties with bulky alkyl substituents, a classic example being
tocopherols and tocotrienols (Fig. 11.1). It is also important to note that some
chain-breaking antioxidants are prooxidative under certain conditions. For
example, at elevated concentrations and temperatures, -tocopherol can become
prooxidative chain-carriers by regenerating peroxyl radicals (Frankel, 1998).
A second class of antioxidants is referred to as preventative or secondary
antioxidants. Unlike chain-breaking antioxidants, preventative antioxidants do
not inactivate free radical species. In fact, the term preventative antioxidant
denotes a very broad class of antioxidants that all function by slowing the rate of
lipid oxidation reactions, but do so by a variety of mechanisms. Two of the most
common types of preventative antioxidants suitable for food systems are metal
chelators and hydroperoxide destroyers. A brief discussion of these antioxidant
classes follows.
Many redox-active, multivalent transition metals (e.g., Fe, Cu, Mn, Cr, V, Zn,
Al) are capable of
increasing lipid oxidation rates by promoting the
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