3.7 Changes in Acyl Lipids of Food 195Fig. 3.22. Autoxidation of linoleic acid methyl
ester. Analysis of primary products (after reduc-
tion of the hydroperoxy group) by HPLC (accord-
ing toChanandLevett, 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-octa-
decadienoic acid methyl ester
Table 3.28.Monohydroperoxides formed by autoxi-
dation (^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
910 2348
10 8 23 52
11 9 27
Linoleic acid 8 9,12 1. 5
9 10,12 46. 532
10 8,12 0. 517
12 9,13 0. 517
13 9,11 49. 534
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
conditions. Cis-hydroperoxides are the main
products at temperatures < 40 ◦C. Competi-
tion exists between conversion of the peroxy
radical to monohydroperoxide and reactions
involvingβ-fragmentation and cyclization. Allyl
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:(3.55)However, hydroperoxides can also be isomer-
ized by such a reaction pathway. When they
interact with free radicals (H-abstraction from
–OOH group) or with heavy metal ions (cf. Reac-
tion 3.64), they are again transformed into peroxy
radicals. Thus, the 13-hydroperoxide of linoleic
acid isomerizes into the 9-isomer and vice versa:(3.56)3.7.2.1.3 Hydroperoxide-Epidioxides...............................
Peroxy radicals which contain isolatedβ,γdou-
ble bonds are prone to cyclization reactions in
competition with reactions leading to mono-
hydroperoxides. A hydroperoxide-epidioxide
results through attachment of a second oxygen
molecule and abstraction of a hydrogen atom:(3.57)