Ascorbic acid (vitamin C; Fig. 2) acts as a water-soluble
free radical scavenger in both plant and animal tissues.
Like phenolics, ascorbate (E°´= 282 mV) has a reduction
potential below peroxyl radicals (E°´=1000 mV) and thus
can inactivate peroxyl radicals. In addition, ascorbate’s reduction
potential is lower than the α-tocopherol radical
(E°´= 500 mV), meaning that ascorbate may have an additional
role in the regeneration of oxidized α-tocopherol.
Interactions between ascorbate and free radicals result in
the formation of numerous oxidation products. Although
ascorbate seems to primarily play an antioxidant role in
living tissues, this is not always true in food systems.
Ascorbate is a strong reducing agent especially at low pH.
When transition metals are reduced, they become very active
prooxidants that can decompose hydrogen and lipid
peroxides into free radicals. Ascorbate also causes the release
of protein-bound iron (e.g., ferritin), thus promoting
oxidation. Therefore, ascorbate can potentially exhibit
prooxidative activity in the presence of free transition metals
or iron-binding proteins. This does not typically occur
in living tissues due to the tight control of free metals by
systems that prevent metal reduction and reactivity. However,
in foods the typical control of metals can be lost
by processing operations that cause protein denaturation.
Thus in some foods, ascorbate my act as a prooxidant and
accelerate oxidative reactions.
Ascorbate is found in numerous plant foods including
green vegetables, citrus fruits, tomatoes, berries, and potatoes.
Ascorbate can be lost in foods due to heat processing
and prolonged storage. Transition metals and exposure to
air will also cause the degradation of ascorbic acid.
|FIGURE 2 Chemical structures of miscellaneous natural antioxidants.