Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 4 Excitation and deactivation of photosensitizer for the formation of singlet oxygen
[13].
involves the generation of singlet oxygen via triplet sensitizer–triplet oxygen annihilation.
Upon collision of the excited triplet sensitizer and triplet oxygen, energy is
transferred from the sensitizer to the oxygen, resulting in the generation of singlet
oxygen and singlet sensitizer. The alternative interaction between triplet sensitizer
and triplet oxygen occurs by electron transfer from the excited triplet sensitizer to
triplet oxygen, resulting in the formation of superoxide ion. Less than 1% of the
interactions of triplet sensitizer and triplet oxygen result in the formation of superoxide
ion [27]. The rate of type 2 pathway is mostly dependent on the solubility and
concentration of oxygen in the food system. Oxygen is more soluble in nonpolar
lipids than it is in water [28]. Therefore, if a sensitizer such as chlorophyll were
present in soybean oil, the type 2 pathway would be expected to be active. In contrast,
water-based food systems such as milk may tend toward the type 1 pathway
due to the reduced availability of oxygen. The shift from type 1 to type 2 or vice
versa is dependent on the concentration of oxygen and the types and concentration
of substrate.
Types 1 and 2 reactions will enhance oxidation by either the formation of
reactive substrate species or the production of singlet oxygen. The competition be-
Figure 5 Formation of excited triplet sensitizer and its reaction with substrate (type I) and
triplet oxygen (type II) [13].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.