2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 195
peroxy radicals can undergo β-fragmentation
which results, after a new oxygen molecule is
attached, in a peroxy radical positional isomer,
e. g. rearrangement of an oleic acid peroxy
radical:
Fig. 3.22. Autoxidation of linoleic acid methyl
ester. Analysis of primary products (after reduction
of the hydroperoxy group) by HPLC (according
to Chan and Levett, 1977). 1 13-Hydroxy-cis-
9, trans-11-octadecadienoic acid methyl ester, 2 13-
hydroxytrans-9,trans-11-octadecadienoic acid methyl
ester, 3 9-hydroxy-trans-10, cis-12-octadecadienoic
acid methyl ester, 4 9-hydroxy-trans-10, trans-12-octadecadienoic
acid methyl ester
Table 3.28. Monohydroperoxides formed by autoxidation
( 3 O 2 ) and photooxidation ( 1 O 2 ) of unsaturated
fatty acids
Fatty acid
Monohydroperoxide
Position of
Proportion
(%)
HOO- double 3 O 2 1 O 2
group bond
Oleic acid 8 9 27
9 10 23 48
10 8 23 52
11 9 27
Linoleic acid 8 9,12 1.5
9 10,12 46.5 32
10 8,12 0.5 17
12 9,13 0.5 17
13 9,11 49.5 34
14 9,12 1.5
Linolenic acid 9 10,12,15 31 23
10 8,12,15 13
12 9,13,15 11 12
13 9,11,15 12 14
15 9,12,16 13
16 9,12,14 46 25
(3.55)
However, hydroperoxides can also be isomerized
by such a reaction pathway. When they
interact with free radicals (H-abstraction from
–OOH group) or with heavy metal ions (cf. Reaction
3.64), they are again transformed into peroxy
radicals. Thus, the 13-hydroperoxide of linoleic
acid isomerizes into the 9-isomer and vice versa:
3.7.2.1.3 Hydroperoxide-Epidioxides
(3.56)
Peroxy radicals which contain isolated β,γ double
bonds are prone to cyclization reactions in
competition with reactions leading to monohydroperoxides.
A hydroperoxide-epidioxide
results through attachment of a second oxygen
molecule and abstraction of a hydrogen atom:
conditions. Cis-hydroperoxides are the main
products at temperatures <40 ◦ C. Competition
exists between conversion of the peroxy
radical to monohydroperoxide and reactions
involving β-fragmentation and cyclization. Allyl
(3.57)