2009_Book_FoodChemistry

198 3 Lipids
agreement with the above reaction mechanism.
The reaction is illustrated in Fig. 3.24 for the
oxidation of linoleic acid. In addition to the
two hydroperoxides with a conjugated diene
system already mentioned (Fig. 3.21), two
hydroperoxides are obtained with isolated double
bonds.
The mechanism reveals that double bonds of unsaturated
fatty acids containing more than one
double bond behave as isolated C=C units rather
than 1,4-diene systems. Consequently, the difference
in the reaction rates of linoleic acid and
oleic acid with singlet oxygen is relatively small.
In model reactions linoleic acid was photooxidized
only 2 to 3 times faster. This is in contrast
to autoxidation where linoleic acid is oxidized
at least 12 times faster (cf. 3.7.2.1.1). However,
furan fatty acids (cf. 3.2.1.3) react much faster
with 1 O 2 than linoleic or linolenic acid. An endo
peroxide is the main reaction product. In addition,
diacetyl, 3-methyl-2,4-nonandione (MND)
and 2,3-octandione are formed. MND contributes
to the aroma of tea (cf. 21.2.5.8) and, with diacetyl,
to the aroma changes in stored soybean
oil (cf. 14.3.2.2.5). The formation of MND is
explained by the secondary reactions shown in
Fig. 3.25. First, the allyl group of C-9 to C-11 is
oxidized to an 11-hydroperoxide. The following
β-cleavage of hydroperoxide results in the formation
of a carbonyl group at C-11 and a hydroxyl
radical which combines with C-13 after homolysis
of the furan ring. The furan fatty acid thus is
split into MND and a fragment of unknown struc-
ture. The odor threshold of MND in water is fairly
low (Table 3.32), even in comparison with that of
octandione.
Formation of 1-singlet oxygen ( 1 O 2 ) is inhibited
by carotenoids (car):
(3.62)
The quenching effect of carotenoids
(transition of 1 O 2 to 3 O 2 ) is very fast
(k = 3 ×10 10 1mole −1 s −1 ). They also prevent
energy transfer from excited-state chlorophyll
to 3 O 2 . Therefore, carotenoids are particularly
suitable for protecting fat (oil)-containing food
from Type II photooxidation.
3.7.2.1.6 Heavy Metal Ions
These ions are involved in the second group of
initiation reactions, namely, in the decomposition
of initially-formed hydroperoxides into radicals
Fig. 3.24. Hydroperoxides derived from linoleic acid by type-2 photooxidation