192 3 Lipids
Table 3.26.Induction period and relative rate of oxida-
tion for fatty acids at 25◦C
Fatty acid Number Induction Oxidation
of allyl period rate
groups (h) (relative)
18:0 0 1
18:1 (9) 1 82 100
18:2 (9, 12) 2 19 1,200
18:3 (9, 12, 15) 3 1. 34 2500
idant concentration is high in some foods. In these
cases, illustrated in Fig. 3.18-2, the induction per-
iod may be nonexistent.
3.7.2.1.1 Fundamental Steps of Autoxidation
The length of the induction period and the rate
of oxidation depend, among other things, on the
fatty acid composition of the lipid (Table 3.26);
the more allyl groups present, the shorter the in-
duction period and the higher the oxidation rate.
Both phenomena, the induction period and
the rise in reaction rate in the series, oleic,
linoleic and linolenic acid can be explained as
follows: Oxidation proceeds by a sequential free
radical chain-reaction mechanism. Relatively
stable radicals that can abstract H-atoms from
the activated methylene groups in an olefinic
compound are formed. On the basis of this
assumption and, in addition, on the fact that the
oxidation rate is exponential,Farmeret al. (1942)
andBolland(1949) proposed an autoxidation
mechanism for olefinic compounds and, thus,
also for unsaturated fatty acids. This mechanism
has several fundamental steps. As shown in
Fig. 3.19, the oxidation process is essentially
a radical-induced chain reaction divided into
initiation (start), propagation, branching and
termination steps. Autoxidation is initiated by
free radicals of frequently unknown origin.
Measured and calculated reaction rate constants
for the different steps of the radical chain reaction
show that due to the stability of the peroxy free
radicals (ROO•), the whole process is limited
by the conversion of these free radicals into
monohydroperoxide molecules (ROOH). This
reaction is achieved by abstraction of an H-atom
from a fatty acid molecule [reaction step 2 (RS-2
in Fig. 3.19)]. The H-abstraction is the slowest
and, hence, the rate limiting step in radical (R•)
formation. Peroxidation of unsaturated fatty
acids is accelerated autocatalytically by radicals
generated from the degradation of hydroperox-
ides by a monomolecular reaction mechanism
(RS-4 in Fig. 3.19). This reaction is promoted
by heavy metal ions or heme(in)-containing
molecules (cf. 3.7.2.1.7). Also, degradation of
hydro-peroxides is considered to be a starting
point for the formation of volatile reaction
products (cf. 3.7.2.1.9).
After a while, the hydroperoxide concentration
reaches a level at which it begins to generate free
radicals by a bimolecular degradation mechanism
(RS-5 in Fig. 3.19). Reaction RS-5 is exothermic,
unlike the endothermic monomolecular decom-
position of hydroperoxides (RS-4 in Fig. 3.19)
which needs approx. 150 kJ/mol. However, in
most foods, RS-5 is of no relevance since fat (oil)
oxidation makes a food unpalatable well before
reaching the necessary hydroperoxide level for
the RS-5 reaction step to occur. RS-4 and RS-5
(Fig. 3.19) are the branching reactions of the free
radical chain.Fig. 3.19.Basic steps in the autoxidation of olefins