2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 205
(Z)-4-heptenal), which occurs in beef and mutton
and often in butter (odor threshold in Table 3.32).
Also, the processing of oil and fat can provide an
altered fatty acid profile. These can then provide
new precursors for a new set of carbonyls. For example,
(E)-6-nonenal, the precursor of which is
octadeca-(Z,E)-9,15-dienoic acid, is a product of
the partial hydrogenation of linolenic acid. This
aldehyde can be formed during storage of partially
hardened soya and linseed oils. The aldehyde,
together with other compounds, is responsible
for an off-flavor denoted as “hardened flavor”.
Several reaction mechanisms have been suggested
to explain the formation of volatile
carbonyl compounds. The most probable mechanism
is the β-scission of monohydroperoxides
with formation of an intermediary short-lived
alkoxy radical (Fig. 3.26). Such β-scission is
catalyzed by heavy metal ions or heme(in)
compounds (cf. 3.7.2.1.7).
There are two possibilities for β-scission of each
hydroperoxide fatty acid (Fig. 3.26). Option “B”,
i. e. the cleavage of the C–C bond located further
away from the double bond position, is the
energetically preferred one since it leads to
resonance-stabilized “oxoene” or “oxo-diene”
compounds. Applying this β-scission mechanism
(“B”) to both major monohydroperoxide
isomers of linoleic acid gives the products shown
in Formula 3.72 and 3.73.
From the volatile autoxidation products which
contain the methyl end of the linoleic acid
molecule, the formation of 2,4-decadienal and
pentane can be explained by reaction 3.72.
The formation of hexanal among the main
volatile compounds derived from linoleic acid
(cf. Table 3.31) is still an open question. The
preferential formation of hexanal in aqueous
(3.72)
Fig. 3.26. β-Scission of monohydroperoxides (according
to Badings, 1970)
(3.73)
systems can be explained with an ionic mechanism.
As shown in Fig. 3.27, the heterolytic
cleavage is initiated by the protonation of the hydroperoxide
group. After elimination of a water
molecule, the oxo-cation formed is subjected
to an insertion reaction exclusively on the C–
C linkage adjacent to the double bond. The carbonium
ion then splits into an oxo-acid and hexanal.
The fact that linoleic acid 9-hydroperoxide
gives rise to 2-nonenal is in agreement with this
outline.
However, in the water-free fat or oil phase of
food, the homolytic cleavage of hydroperoxides
presented above is the predominant reaction
mechanism. Since option “A” of the cleavage
reaction is excluded (Fig. 3.26), some other reactions
should be assumed to occur to account for
formation of hexanal and other aldehydes from
linoleic acid. The further oxidation reactions of
monohydroperoxides and carbonyl compounds
are among the possibilities.
The above assumption is supported by the finding
that 2-alkenals and 2,4-alkadienals are oxidized
substantially faster than the unsaturated