Food Lipids: Chemistry, Nutrition, and Biotechnology

II. ANTIOXIDANTS
In foods containing lipids, antioxidants delay the onset of oxidation or slow the rate
at which it proceeds. These substances can occur as natural constituents of foods,
but they also can be intentionally added to products or formed during processing.
Their role is not to enhance of improve the quality of foods, but they do maintain
food quality and extend shelf life. Antioxidants for use in food processing must be
inexpensive, nontoxic, effective at low concentrations, stable, and capable of surviving
processing (carry-through effect); color, flavor, and odor must be minimal. The
choice of which antioxidant to use depends on product compatibility and regulatory
guidelines [2].
Antioxidants not only extend product shelf life but reduce raw material waste,
reduce nutritional losses, and widen the range of fats that can be used in specific
products [3]. By extending keeping quality and increasing the number of oils that
can be used in food products, antioxidants allow processors to use more available
and/or less costly oils for product formulation.
A. Classification
Antioxidants can be broadly classified by mechanism of action as primary antioxidants
and secondary antioxidants. Some antioxidants exhibit more than one mechanism
of activity and are often referred to as multiple-function antioxidants. Chemical
modes of action vary greatly, for these substances are able to function at all stages
of the free radical reaction.
1. Primary Antioxidants
Primary, type 1, or chain-breaking antioxidants are free radical acceptors that delay
or inhibit the initiation step or interrupt the propagation step of autoxidation. Initiation
of autoxidation occurs when an �-methylenic hydrogen molecules is abstracted
from an unsaturated lipid to form a lipid (alkyl) radical (R�) [Eq. (1)].
RH → R� �H� (1)
This highly reactive lipid radical can then react with oxygen to form a peroxy radical
(ROO�) in a propagation reaction [Eq. (2)].
R� �O2 → ROO� �H� (2)
During propagation, peroxy radicals react with lipid to form a hydroperoxide and a
new unstable lipid radical [Eq. (3)]. This lipid radical will then react with oxygen
to produce another peroxy radical, resulting in a cyclical, self-catalyzing oxidative
mechanism [Eq. (4)].
ROO� �RH → ROOH � R� (3)
R� �O2 → ROO� �H� (4)
Hydroperoxides are unstable and can degrade to produce radicals that further accelerate
propagation reactions. These reactions are typically referred to as branching
steps [Eqs. (5) and (6)�.
ROOH → RO� �OH� (5)
RO� �RH → ROH � R� (6)
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.