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 un-
saturated fatty acids containing more than one
double bond behave as isolated C=C units rather
than 1,4-diene systems. Consequently, the differ-
ence in the reaction rates of linoleic acid and
oleic acid with singlet oxygen is relatively small.
In model reactions linoleic acid was photooxi-
dized 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 addi-
tion, 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 di-
acetyl, 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 forma-
tion of a carbonyl group at C-11 and a hydroxyl
radical which combines with C-13 after homoly-
sis of the furan ring. The furan fatty acid thus is
split into MND and a fragment of unknown struc-
Fig. 3.24.Hydroperoxides derived from linoleic acid by type-2 photooxidation
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 × 1010 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 HeavyMetalIons
These ions are involved in the second group of
initiation reactions, namely, in the decomposition
of initially-formed hydroperoxides into radicals