Antioxidants and Shelf Life of Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 174

and free radicals (Equations 6.2 and 6.3). This process initiates free-radical reactions
in which lipid radicals are formed and autoxidation started. 10
Sources of singlet oxygen have been discussed by Bradley. 10 Oxidation of unsat-
urated fatty acids by singlet oxygen can be inhibited by compounds that react faster
with this initiator such as quenchers which deactivate singlet oxygen to the ground
state form. 8 The most efficient natural quenchers are tocopherol and -carotene while

others include amino acids, proteins, sulfides, phenols, and metal chelators. 11

β

ANTIOXIDANTS

Halliwell et al. 12 defined an antioxidant as “any substance that, when present at low
concentrations compared to those of an oxidizable substrate, significantly delays or
prevents oxidation of that substrate.” While antioxidants are associated primarily with
inhibition of lipid peroxidation, free radicals can also damage other components, so
that“oxidizable substrate” includes almost everything found in foods and in living
tissues such as proteins, lipids, carbohydrates, and DNA. This chapter, however, will
focus on antioxidant activity against lipid peroxidation. In general, antioxidants do
not prevent oxidation, but rather extend or retard the induction period.

M ECHANISMOF A CTION

Antioxidants can act at different steps in the oxidation sequence, depending on their
mode of action ( Figure 6.2 ). The primary or chain-breaking antioxidant reacts with
lipid radicals to yield more stable products and such antioxidants are known as free-
radical interceptors. The secondary or preventative antioxidants reduce the rate of
chain initiation by a variety of mechanisms which include metal inactivators, hydro-
peroxide decomposers, oxygen scavengers, and synergists.
A primary antioxidant rapidly donates a hydrogen atom to a lipid radical, or is
converted to other stable products. 13 Free-radical interceptors inhibit two important
steps in the free-radical chain sequence of lipid oxidation. They react with peroxyl
radicals (LOO*) to stop chain propagation, thus inhibiting formation of peroxides
(Eq. 6.5), and with alkoxyl radicals (LO*) to decrease the decomposition of hydro-
peroxides to harmful degradation products (Eq. 6.6). 14

LOO* + AH → LOOH + A* (6.5)

LO* + AH → LOH + A* (6.6)

Metal inactivators or chelating agents act as preventative antioxidants by remov-
ing or deactivating metal ions, which act as initiators as well as catalyze the decom-
position of hydroperoxides. Decomposers of hydroperoxides transfer them into
stable hydroxy compounds by reduction, while oxygen scavengers react with oxygen
to deplete the supply of oxygen needed for autoxidation. 14 Synergism can be expected
between substances with differing modes of action, so that multicomponent antiox-
idant systems exhibit much greater antioxidant activity beyond that expected from
the additive effects of individual antioxidants. 15 Synergists generally extend the life