Qve

The singlet-state oxygen molecule is joined onto one end of a double bond simultaneously as it abstracts an allylic proton, and a new double bond is formed between the allylic position and the other end of the original C = C bond (7). A mixture of conjugated and nonconjugated hydroperoxides is formed by the reaction of singlet oxygen with linoleic acid. Because the flavor stability and quality of vegetable oils can be decreased by singlet oxygen oxidation, attempts have been made to inhibit singlet oxygen oxidation. Antioxidants such as /¡-carotene, lycopene, and tocopherols, which have been found to be able to deactivate singlet-state oxygen back to its low-energy triplet-state (6,8,9), are effective at minimization of singlet oxygen oxidation.

Although free-radical autoxidation is the primary mechanism for the formation of volatile flavor compounds from fats and oils, photosensitized oxidation initiated by sensitizers such as chlorophyll in the presence of light should be included as oxidation mechanisms that contribute to the formation of off-flavor volatile compounds (2,5). The reaction of fatty acids with oxygen to produce hydroperoxides requires a change in their electron spin states because the unsaturated fatty acid and hydroperoxide are in singlet-states, whereas oxygen is in a triplet-state. The reaction between singlet-state fatty acids and triplet-state oxygen to form singlet-state hydroperoxides can only occur if enough energy is present to overcome the spin barrier. Electrophilic singlet-state oxygen can directly react with the electron-rich, singlet-state unsaturated fatty acids to form singlet-state hydroperoxides. Naturally occurring pigments such as chlorophyll in vegetable oils can absorb ultraviolet and visible light and act as photosensitizers (sens) (6). The sensitizer transfers energy to triplet-state oxygen to form singlet-state oxygen, which reacts with unsaturated fatty acids to form singlet-state hydroperoxides:

sens 1sens* 3sens* 3sens* + ®02 -> x02 + xsens x02 + RH -» ROOH

Singlet-state oxygen reacts directly with singlet-state double bonds of unsaturated fatty acids by an "ene" reaction to form hydroperoxides:

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