2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 197
accounts for two unpaired electrons in the
oxygen molecule. These are the two antibonding
π orbitals available: π ∗ 2p y and π ∗ 2p z .Thetwo
electrons occupy these orbitals alone. The net
angular momentum of the unpaired electrons
has three components, hence the term “triplet”.
When the electrons are paired, the angular
momentum can not be split into components
and this represents a singlet state. In the triplet
state, oxygen reacts preferentially with radicals,
i. e. molecules having one unpaired electron. In
contrast, direct reactions of tripletstate oxygen
with molecules which have all electrons paired,
as in the case of fatty acids, are prevented by spin
barriers. For this reason the activation energy of
the reaction
RH+ 3 O 2 −→ ROOH (3.59)
is so high (146–273 kJ/mole) that it does not occur
without some assistance.
Oxygen goes from the ground state to the
short-lived 1-singlet-state ( 1 O 2 ) by the uptake of
92 kJ/mole of energy (Fig. 3.23). The previously
unpaired single electrons are now paired on
the π ∗ 2p y antibonding orbital. The reactivity
of this molecule resembles ethylenic or general
olefinic π electron pair reactions, but it is more
electrophilic. Hence, in the reaction with oleic
acid, the 1-singlet-state oxygen attacks the 9−10
double bond, generating two monohydroperoxides,
the 9- and 10-isomers (cf. Table 3.28).
The second singlet-state of oxygen ( 1 Σ + g) has
a much shorter life than the 1-singlet-state and
plays no role in the oxidation of fats or oils.
For a long time it has been recognized that the
stability of stored fat (oil) drops in the presence
of light. Light triggers lipid autoxidation. Low
amounts of some compounds participate as sensitizers.
According to Schenk and Koch (1960), there are
two types of sensitizers. Type I sensitizers are
those which, once activated by light (sen ∗ ), react
directly with substrate, generating substrate radicals.
These then trigger the autoxidation process.
Type II sensitizers are those which activate the
ground state of oxygen to the 1 O 2 singlet state.
Type I and II photooxidation compete with each
other. Which reaction will prevail depends on the
structure of the sensitizer but also on the concentration
and the structure of the substrate available
for oxidation.
Table 3.28 shows that the composition of hydroperoxide
isomers derived from an unsaturated
acid by autoxidation ( 3 O 2 ) differs from that obtained
in the reaction with 1 O 2 . The isomers can
be separated by analysis of hydroperoxides using
high performance liquid chromatography and,
thus, one can distinguish Type I from Type II
photooxidation. Such studies have revealed
that sensitizers, such as chlorophylls a and b,
pheophytins a and b and riboflavin, present in
food, promote the Type II oxidation of oleic and
linoleic acids.
As already stated, the Type II sensitizer, once activated,
does not react with the substrate but with
ground state triplet oxygen, transforming it with
an input of energy into 1-singlet-state oxygen:
(3.60)
The singlet 1 O 2 formed now reacts directly with
the unsaturated fatty acid by a mechanism of
“cyclo-addition”:
Fig. 3.23. Configuration of electrons in an oxygen
molecule
a Electrons in 2p x and 2p y orbitals
b Dependent on solvent, e. g. 2µs in water, 20µs in D 2 O
and 7µs in methanol
(3.61)
The fact that the number of hydroperoxides
formed are double the number of isolated double
bonds present in the fatty acid molecule is in