O O

Scheme 11.

Quinone methine Scheme 12.

Ascorbic Acid

Vitamin C is one of the least stable vitamins mainly due to oxidative degradations (54). Ascorbic acid can be oxidized by metal ions such as Fe(III) and Cu(II) in a two-sequential one-electron transfer to yield dehydroascorbic acid. Alternatively, metal ions can also catalyze the oxidation of ascorbic acid via the formation of an ascorbate-metal-dioxygen complex immediate (Scheme 11). In this reaction, the two-electron transfer occurs between the ascorbic and the dioxygen leading to the formation of dehydroascorbic acid and hydrogen peroxide (55).

Dehydroascorbic acid undergoes further degradation in acid medium (56), or reacts with amino acids via Strecker degradation. The latter reaction leads to the formation of scorbamic acid and, subsequently, polymeric compounds (57,58).

Tocopherol

Vitamin E also undergoes oxidative degradation via a free-radical pathway that accounts for its high efficiency as a chain-breaking antioxidant. The first step of the free-radical reaction involves the abstraction of two hydrogen atoms, forming the quinone methine that undergoes successive rearrangements to the stable quinone (Scheme 12). In the reaction, the tocopherol molecule consumes two per-oxy radicals (LOO-) to form a nonradical product (59,60).

From the subject matter covered in the discussion, it is obvious that oxidation is a widespread reaction affecting every constituent in food and food-related systems. The chemistry involves free radical, singlet oxygen, coupled reduction-oxidation, oxidizing chemical, metal ion, enzyme catalysis, cofactor, and so on. Product interaction often leads to new degradation reactions, cross-linking, polymerization, and subsequently alteration in physical and chemical properties as well as functionality of the constituents. These reactions and interactions are complex and certainly should deserve more attention from food scientists and researchers.

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