Studies of lipid peroxide formation in irradiated synthetic diets and the effects of storage after irradiation.

The effect of irradiation doses of gamma-rays or electrons within the range 100--2000 krad has been studied on lipid peroxide formation in artificial food mixtures containing lard, corn oil or herring oil mixed with starch, casein or other proteins. Lipid peroxide formation after irradiation was very low in lard: starch or corn oil: starch mixtures but very large concentrations of peroxide were formed in irradiated herring oil: starch mixtures. This was mainly a result of the occurrence of the highly unsaturated C20:4, C20:5 and C22:6 fatty acids in the herring oil which readily form lipid peroxide. Lipid peroxide formation immediately after irradiation was much lower after doses of electrons given at a high dose-rate than after similar doses of gamma-rays given at low dose-rates but it increased rapidly in the samples irradiated with electrons during the period immediately following irradiation. Lipid peroxide formation changed rapidly during post-irradiation storage and was dependent on the type of radiation, the dose, the dose-rate, the time after irradiation and the temperatures of the post-irradiation storage. The concentration of peroxide in mixtures irradiated with gamma-rays formed after 2--3 days post-irradiation storage was dose-rate dependent and much greater after irradiation with gamma-rays given at a very low dose-rate than with gamma-rays given at a high dose-rate. Lipid peroxide increased after irradiation much more rapidly at 37 degrees C than at 4 degrees C or 21 degrees C but all irradiated samples a maximum concentration of peroxide was eventually formed, the time delay being temperature dependent. After reaching a maximum, the concentrations of both hydroperoxides and aledhyde breakdown products in all irradiated samples steadily fell to relatively low values. Peroxide yields were greater if the fat was dispersed in an inert medium such as starch than when irradiated in the pure form and were also dependent on the presence of water in the dispersant medium. Proteins such as casein or ovalbumin inhibited lipid peroxide formation in unsaturated fats.