Food Lipids: Chemistry, Nutrition, and Biotechnology

natural antioxidants. Other sources of natural antioxidants include carotenoids, flavonoids,
amino acids, proteins, protein hydrolysates, Maillard reaction products,
phospholipids, and sterols. Numerous naturally occurring phenolic antioxidants have
been identified in plant sources and vegetable extracts. Enzymes also play important
roles as antioxidants. Processing of foods can induce the formation of antioxidants.
Maillard reaction products, protein hydrolysates, fermentation products, and nitrosyl
compounds from curing have been reported to possess antioxidant activity. Natural
antioxidants allow food processors to produce stable products with ‘‘clean’’ labels
that tout all-natural ingredients. However, these products can have several drawbacks,
including high usage levels, undesirable flavor and/or color contributions, and lack
of stability due to low antioxidant efficiency.
The safety of natural antioxidants should not be taken for granted. Cautions
must be heeded, since numerous natural products are potential carcinogens, mutagens,
or teratogens, and the safety of many natural compounds with antioxidant
activity has not been established. A case in point is nordihydroguaiaretic acid
(NDGA), which was used extensively as an antioxidant earlier in this century. NDGA
is a natural constituent of the creosote bush, which was removed from GRAS status
when unfavorable toxicological results were reported. Regardless of the politics surrounding
the issue of safety and ‘‘natural’’ additives, natural antioxidant products are
commercially important and desired by the consumer. The main advantage of substances
naturally present in foods is that the burden of proof of safety may be less
rigorous than that required for synthetic products. No safety testing is required if the
antioxidant is a natural constituent of GRAS ingredients. In addition, some natural
antioxidants derived from spices, herbs, and Maillard reactions can be listed as flavorants
rather than antioxidants, a technical distinction that serves to exempt the
substances from safety testing requirements.
1. Tocopherols and Tocotrienols
Tocopherols and tocotrienols (Fig. 3) comprise the group of chromanol homologs
that possess vitamin E activity in the diet. They are natural monophenolic compounds
with varying antioxidant activities. Eight naturally occurring homologs are included
in the vitamin E family [12]. They are fat-soluble 6-hydroxychroman compounds.
The �-, �-, �-, and �-tocopherols are characterized by a saturated side chain consisting
of three isoprenoid units. The corresponding tocotrienols (�, �-, �-, and �-)
have double bonds at the 3�, 7�, and 11� positions of the isoprenoid side chain. Only
RRR isomers are found naturally. Synthetic �-tocopherol (all-rac-�-tocopherol) consists
of eight stereoisomers found in equal amounts in the synthetic mixture. Biologically,
RRR-�-tocopherol is the most active vitamin E homolog. Tocopherols and
tocotrienols are widely distributed in the plant kingdom, with vegetable oils providing
the most concentrated source of vitamin E. Tocotrienols are less common but
are present in palm oil, rice bran oil, cereals, and legumes. Palm oil has a unique
vitamin E profile, providing tocotrienols in higher concentrations than other food
sources. Tocopherols and tocotrienols are retained throughout the edible oil refining
process, although there is some loss during the deodorization step. Worldwide, the
main commercial source of natural tocopherols is in the soybean oil refining industry.
Commercial natural antioxidant preparations prepared from soybean oil typically
consist of greater than 80% �- and �-tocopherol [2]. Synthetic tocopherols are commercially
available and vary in isomeric form. Tocopherols provide a useful natural
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.