Light-induced oxidative changes in a model dairy spread. wavelength dependence of quantum yields

Light-induced formation of lipid peroxides in a water-in-oil emulsion based on purified rape-seed oil (80%) was found to increase with decreasing wavelength and to have the (apparent) quantum yields (1.1 +/- 0.1) x 10(-3) for 436 nm, (2.6 +/- 0.1) x 10(-3) for 405 nm, and (4.5 +/- 0.4) x 10(-3) for 366 nm irradiation, as determined after 12 h of exposure to monochromatic light of an approximate intensity of 10(18) quanta.min(-1) ml(-1) and related to total light absorption. Riboflavin (0.8 ppm) had no effect on lipid peroxidation, but photodegraded with a quantum yield ((1.5 +/- 0.3) x 10(-5) for 436 nm, (1.7 +/- 0.2) x 10(-5) for 405 nm and (1.39 +/- 0.09) x 10(-5) for 366 nm irradiation) independent of irradiation wavelength. beta-Carotene was only photodegraded to a minor extent, but protected riboflavin against photodegradation and the lipids against peroxidation for 436 and 405 nm irradiation (reduction in quantum yield three times for 4.5 ppm beta-carotene for lipid oxidation and more for riboflavin degradation), but not for 366 nm irradiation, where beta-carotene has an absorption minimum.