Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 1 Molecular orbital of triplet oxygen [1].
Interaction among light, sensitizer, and oxygen is mainly responsible for singlet
oxygen formation in foods [13,24]. The chemical mechanism for the formation of
singlet oxygen in the presence of sensitizer, light, and triplet oxygen is shown in
Figure 4. The ability of photosensitizers to absorb the energy from light and then
pass it to triplet oxygen is a convenient method for transferring energy from a light
source to form singlet oxygen. The photosensitizer can absorb light very rapidly, in
picoseconds, and becomes an unstable, excited, and singlet state molecule ( 1 Sen*).
The excited singlet photosensitizer will immediately seek to return to ground state
in one of three ways: internal conversion; emission of light; or intersystem crossing
(ISC), as shown in Figure 4.
Internal conversion involves the transformation from one excited state to another
of the same spin state, resulting in the loss of energy as heat. The sensitizer
may return to ground state by the emission of fluorescence. The excited sensitizer
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.