Page 70 - Natural Antioxidants, Applications in Foods of Animal Origin

Natural Antioxidants: Occurrence and Their Role in Food Preservation 49
VetBooks.ir is site for tocol biosynthesis and the aromatic amino acid tyrosine is consid-

ered to be its precursor. The mechanism of biosynthesis of tocols involves
coupling of phytyl diphosphate with homogenestic acid (2,5-dihydroxyphen-
ylacetic acid), followed by cyclization and methylation reactions. Vitamin E
compounds include the tocopherols and tocotrienols. Tocotrienols have a
conjugated triene double bond system in the phytyl side chain, while tocoph-
erols do not. Methyl substitution affects the bioactivity of vitamin E, as
well as its in vitro antioxidant activity. Tocopherols or vitamin E have eight
known homologues, that is, α-, β-, γ-, δ- tocopherols and α-, β-, γ-, δ- toco-
trienols. Tocopherols are fat-soluble antioxidants that function as scaven-
gers of lipid peroxyl radicals. Tocopherols’ content is found to be related to
RSA and antioxidant activity of oils. Tocopherols’ content decreases during
processing of oils. There have been many reports on the protective effect of
tocopherols as food antioxidants (Dougherty, 1988). Tocopherols protect PL
and cholesterol against oxidation (Faustman et al., 1989; Li et al., 1996).
Total tocopherol content and major tocopherol homologues differ from one
oil to another (Table 2.1 and Fig. 2.1). The level of tocopherols decreases
with time of storage and heating of oils (Li et al., 1996).

R'
· a/p/7a-tocopherol -CH3 -CH3

beta-tocopherol -CH3 -H
gamma-tocopherol -H -CH3
delta-tocopherol -H -H

CH3
R"

R' tocomeools

FIGURE 2.1 Different tocopherol homologues of vegetable oils.