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Control of Lipid Oxidation in Muscle Food 355
VetBooks.ir 9.3.1 NATURAL ANTIOXIDANTS

Due to the damage caused by oxidations in live tissues, animals and vege-
tables accumulate antioxidant molecules as a mechanism of defense against
these undesirable changes. Most of these antioxidants are supplied by the
feed in the animals.
Several natural antioxidants are frequent in animal and vegetable tissues
as such or as precursors. We will shortly review the most representative:

9.3.1.1 PHENOLIC COMPOUNDS

Phenolic compounds are natural antioxidants widely distributed in vegetable
tissues. Its characteristic common chemical structure consist in a benzene
ring having an alcohol (hydroxyl) group bonded to a carbon atom (phenol).
Phenol itself has not antioxidant activity, but substitution of the hydrogen
atoms placed in the ortho- and para-positions with alkyl groups enhances its
reactivity toward free lipid radicals (Shahidi et al., 1992). Phenolics are clas-
sified as simple phenols or polyphenols, these having more than one phenol
unit in their molecules. Most of them are soluble and the smaller molecules
are usually volatiles.
Several polyphenols have antioxidant activity due to scavenging activity
on free radicals by donating a hydrogen atom or an electron to the free
radical and stabilizing it. They can also act as singlet oxygen quenchers and
also through the regulation of some concrete chelation reactions.
Briefly, three main groups of phenolic compound have a high-recognized
antioxidant activity in foods: tocopherols, flavonoids, and phenolic acids.
Tocopherols are a family of compounds naturally found in vegetable oils,
fish, nuts, and leafy green vegetables, which also have vitamin E activity.
Tocopherols derive from a common alcohol matrix named tocol (2-methyl-
2(4’, 8’, 12’-trimethyltridecyl)chroman-6-ol), and differ according to the
number and position of the methyl groups placed in the ring structure (chro-
manol ring), giving rise to different forms, called α, β, γ, and δ tocopherol.
The antioxidant activity of tocopherols increases from α to δ, while the
vitamin E activity and the reactivity with the peroxyl radicals decrease from
α to δ forms. Despite its low reactivity with the free radicals, the higher anti-
oxidant efficiency of the γ-tocopherol when compared to the α-tocopherol
is a consequence of the high stability of the γ-tocopherol and of the differ-
ence in the products formed in both cases during the antioxidative reactions
(Belitz et al., 2009). All the tocopherol forms have a higher rate of reaction