Food Lipids: Chemistry, Nutrition, and Biotechnology

etween the oxygen and the oxygen of the R-O-O-H, i.e., R-O-O-H R-O � O-H
instead of R-O-O-H R-O-O H. Hiatt and others [39] reported that the activation
energy of the cleavage of R-O-O-H was 44 kcal/mol, compared with the cleavage
between the oxygen and hydrogen of R-O-O-H, which has high activation energy
(90 kcal/mol). Therefore, the hydroperoxide groups are cleaved by homolysis to yield
an alkoxy and a hydroxy radical, as shown in Figure 12.
The alkoxy radical formed from hydroperoxide is cleaved by the hemolytic �
scission of a carbon–carbon bond to produce oxo compounds and an alkyl or alkenyl
radical. The hemolytic � scission is an important free radical reaction that produces
volatile compounds in edible oils during oxidation. The unsaturated alkoxy radical
can be cleaved by � scission in two mechanisms of cleavage (A and B of Fig. 12).
Scission of the carbon–carbon bond on the side of the oxygen-bearing carbon atom
will result in formation of unsaturated oxo compounds and an alkyl radical, while
scission of the carbon–carbon bond between the double bond and the carbon atom
bearing the oxygen will produce a 1-olefin radical and an alkyl oxo compound. The
alkyl radical can combine with hydroxy radical to produce an alcohol, and the
1-olefin radical can be converted to a 1-enol. The 1-enol will produce the corresponding
oxo compound by tautomerization. An alternative reaction of the radicals
eliminated by � scission of the alkoxy radical is hydrogen abstraction from a compound
RH. The 8-hydroperoxy methyl oleate can produce 2-undecenal, methyl
7-hydroxyheptanoate, methyl heptanoate, methyl 8-oxo-octanoate, decanal, and 1decene.
The 13-hydroperoxy methyl linoleate can produce methyl 13-oxo-9,11-tridecadienoate,
1-pentanol, pentane, hexanal, methyl 12-oxo-10-dodecenoate, and
methyl 9,11-dodecadienoate. Frankel [10] reported the volatile compounds formed
from methyl oleate hydroperoxides formed by free radical oxidation and photosensitized
oxidation (Table 4).
Frankel [10] reported that the amounts of 2-decenal, 9-oxodecanoate from triplet
oxygen oxidation of hydroperoxides differed from those formed in photosensitized
singlet oxygen oxidation, as shown in Table 4. Frankel [10] demonstrated that 9and
10-hydroperoxides formed in the photosensitized oxidation of oleic acid are
isomerized into a mixture of 8-, 9-, 10-, and 11-hydroperoxides at 210�C, as shown
in Table 4. Chan and others [40] reported that the relative concentrations of hexanal,
methyl octanoate, 2,4-decadienal, and methyl 9-oxononanoate from 9-monohydroperoxide
of methyl linoloeate were 1%, 37%, 51%, and 12%, respectively. However,
the relative concentrations of hexanal, methyl octanoate, 2,4-decadienal, and methyl
9-oxononanoate formed from 13-monohydroperoxide of methyl linoleate were 28%,
24%, 33%, and 16%, respectively. There was a significant difference in the composition
of volatile compounds formed from 9-monohydroperoxide and 13-monohydroperoxide
of methyl linoleate. Different amounts of pentane are formed from
linoleic acid by triplet oxygen and singlet oxygen.
X. FLAVOR PROPERTIES OF VOLATILE COMPOUNDS FROM
OIL OXIDATION
The types of volatile compounds produced from the oxidation of edible oils are
influenced by the composition of the hydroperoxides and the types of oxidative
cleavage of double bonds in the fatty acids [41]. A variety of compounds, such as
hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds, are formed
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