2009_Book_FoodChemistry

206 3 Lipids
Fig. 3.27. Proton-catalyzed cleavage of linoleic acid
13-hydroperoxide (according to Ohloff, 1973)
Fig. 3.28. Reaction rate of an autoxidation process
(according to Lillard and Day, 1964).
–▽–▽– Linolenic acid methyl ester, –◦–◦– linoleic
acid methyl ester, ×–×– 2-nonenal, ––
2,4-heptadienal, –•–•– nonanal
fatty acids (Fig. 3.28). In addition, the autoxidation
of 2,4-decadienal yields hexanal and other
volatiles which coincide with those obtained from
linoleic acid. Since saturated aldehydes oxidize
slowly, as demonstrated by nonanal (Fig. 3.28),
they will enrich the oxidation products and become
predominant.
Also the delayed appearance of hexanal
during the storage of linoleic acid containing
fats and oils compared to pentane and
2,4-decadienal, supports the hypothesis that
hexanal is not directly formed by a β-scission of
the 13-hydroperoxide. It is mainly produced in
a tertiary reaction, e. g., during the autoxidation
of 2,4-decadienal.
Other studies to elucidate the multitude of aldehydes
which arise suggest that the decomposition
of minor hydroperoxides formed by autoxidation
of linoleic acid (cf. Table 3.28) contribute
to the profile of aldehydes. This suggestion is
supported by pentanal, which originates from the
14-hydroperoxide.
The occurrence of 2,4-heptadienal (from
the 12-hydroperoxide isomer) and of 2,4,7-
decatrienal (from the 9-hydroperoxide isomer) as
oxidation products is, thereby, readily explained
by accepting the fragmentation mechanism
outlined above (option “B” in Fig. 3.26) for
the autoxidation of α-linolenic acid. The formation
of other volatile carbonyls can then
follow by autoxidation of these two aldehydes
or from the further oxidation of labile
monohydroperoxides.
Malonic Aldehyde. This dialdehyde is preferentially
formed by autoxidation of fatty
acids with three or more double bonds. The
compound is odorless. In food it may be
bound to proteins by a double condensation,
crosslinking the proteins (cf. 3.7.2.4.3). Malonic
aldehyde is formed from α-linolenic acid by
a modified reaction pathway, as outlined under
the formation of hydroperoxide-epidioxide
(cf. 3.7.2.1.3). However, a bicyclic compound
is formed here as an intermediary
product that readily fragments to malonic
aldehyde: