Food Lipids: Chemistry, Nutrition, and Biotechnology

Hydroperoxide degradation leads to the undesirable odors and flavors associated with
rancidity in later stages of oxidation.
Primary antioxidants react with lipid and peroxy radicals and convert them to
more stable, nonradical products. Primary antioxidants donate hydrogen atoms to the
lipid radicals and produce lipid derivatives and antioxidant radicals (A�) that are
more stable and less readily available to further promote autoxidation. As hydrogen
donors, primary antioxidants have higher affinities for peroxy radicals than lipids
[4]. Therefore, peroxy and oxy free radicals formed during the propagation [Eqs. (2)
and (4)] and branching [Eqs. (5) and (6)] steps of autoxidation are scavenged by
primary antioxidants [Eqs. (7) and (8)]. Antioxidants may also interact directly with
lipid radicals [Eq. (9)].
ROO� �AH → ROOH � A� (7)
RO� �AH → ROH � A� (8)
R� �AH → RH � A� (9)
The antioxidant radical produced by hydrogen donation has a very low reactivity
with lipids. This low reactivity reduces the rate of propagation, since reaction
of the antioxidant radical with oxygen or lipids is very slow. The antioxidant radical
is stabilized by delocalization of the unpaired electron around a phenol ring to form
stable resonance hybrids. Antioxidant radicals are capable of participating in termination
reactions with peroxy [Eq. (10)], oxy [Eq. (11)], and other antioxidant radicals
[Eq. (12)]. The formation of antioxidant dimers (dimerization) is prominent in fats
and oils and indicates that phenolic antioxidant radicals readily undergo termination
reactions. This effectively stops the autocatalytic free radical chain mechanism as
long as the antioxidant is present in its nonradical form.
ROO� �A� → ROOA (10)
RO� �A� → ROA (11)
A� �A� → AA (12)
Prior to initiation of autoxidation, there must be an induction period in which
antioxidants are consumed and free radicals are generated. Therefore, primary antioxidants
are most effective if they are added during the induction and initiation stages
of oxidation when the cyclical propagation steps have not occurred. Addition of
antioxidants to fats that already contain substantial amounts of peroxides will quickly
result in loss of antioxidant function [5]. In addition to radical scavenging, primary
antioxidants can reduce hydroperoxides to hydroxy compounds. However, the main
antioxidative mechanism of primary antioxidants is radical scavenging.
Primary antioxidants are mono- or polyhydroxy phenols with various ring substitutions.
Substitution with electron-donating groups ortho and para to the hydroxyl
group of phenol increases the antioxidant activity of the compound by an inductive
effect. These hindered phenolic antioxidants decrease the reactivity of the hydroxyl
group by increasing its electron density. Substitution with butyl or ethyl groups para
to the hydroxyl enhances the antioxidant activity. Because of steric hindrance, however,
the presence of longer chain or branched alkyl groups at the para positions can
decrease antioxidant effectiveness [6]. Substitutions of branched alkyl groups at ortho
positions enhance the phenolic antioxidant’s ability to form stable resonance struc-
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