2009_Book_FoodChemistry

196 3 Lipids
Peroxy radicals with isolated β,γ double bonds
are formed as intermediary products after autoxidation
and photooxidation (reaction with singlet
O 2 ) of unsaturated fatty acids having two or
more double bonds.
For this reason the 10- and 12-peroxy radicals
obtained from linoleic acid readily form
hydroperoxy-epidioxides. While such radicals
are only minor products in autoxidation, in photooxidation
they are generated as intermediary
products in yields similar to the 9- and 13-peroxy
radicals, which do not cyclize. Ring formation by
10- and 12-peroxy radicals decreases formation
of the corresponding monohydroperoxides
(Table 3.28; reaction with 1 O 2 ).
Among the peroxy radicals of linolenic acid
which are formed by autoxidation, the isolated
β,γ double bond system exists only for the
12- and 13-isomers, and not for the 9- and
16-isomers. Also, the tendency of the 12- and
13-peroxy radicals of linolenic acid to form
hydroperoxy-epidioxides results in the formation
of less monohydroperoxide of the corresponding
isomers as opposed to the 9- and 16-isomers
(Table 3.28).
Peroxy radicals interact rapidly with antioxidants
which may be present to give monohydroperoxides
(cf. 3.7.3.1). Thus, it is not only the chain reaction
which is inhibited by antioxidants, but also
β-fragmentation and peroxy radical cyclization.
Fragmentation occurs when a hydroperoxideepidioxide
is heated, resulting in formation of
aldehydes and aldehydic acids. For example,
hydroperoxide-epidioxide fragments derived
from the 12-peroxy radical of linoleic acid are
formed as shown in Reaction 3.58.
Peroxy radicals formed from fatty acids with
three or more double bonds can form bicycloendoperoxides
with an epidioxide radical as
intermediate. This is illustrated in Reaction 3.74.
3.7.2.1.4 Initiation of a Radical Chain Reaction
Since autoxidation of unsaturated acyl lipids frequently
results in deterioration of food quality,
an effort is made to at least decrease the rate
of this deterioration process. However, pertinent
measures are only possible when more knowledge
is acquired about the reactions involved during
the induction period of autoxidation and how
they trigger the start of autoxidation.
In recent decades model system studies have revealed
that two fundamentally different groups of
reactions are involved in initiating autoxidation.
The first group is confined to the initiating reactions
which overcome the energy barrier required
for the reaction of molecular oxygen with an unsaturated
fatty acid. The most important is photosensitized
oxidation (photooxidation) which provides
the “first” hydroperoxides. These hydroperoxides
are then converted further into radicals in
the second group of reactions. Heavy metal ions
and heme(in) proteins are involved in this second
reaction group. Some enzymes which generate
the superoxide radical anion can be placed
in between these two delineated reaction groups
sinceatleastH 2 O 2 is necessary as reactant for
the formation of radicals.
The following topics will be discussed here:
• Photooxidation
• Effect of heavy metal ions
• Heme(in) catalysis
• Activated oxygen from enzymatic reactions.
3.7.2.1.5 Photooxidation
(3.58)
In order to understand photooxidation and to differentiate
it from autoxidation, the electronic configuration
of the molecular orbital energy levels
for oxygen should be known. As presented in
Fig. 3.23, the allowed energy levels correspond
to 3 Σ − g, 1 Δ gand 1 Σ + g.
The notation for the molecular orbital of O 2 is
(σ 2s) 2 (σ ∗ 2s) 2 (σ 2p) 2 (π 2p) 4 (π ∗ 2p) 2 .
In the ground state, oxygen is a triplet ( 3 O 2 ). As
seen from the above notation, the term (π ∗ 2p) 2