Studies on Deterioration of Heated Soybean Oil during Storage at Low Temperatures

Choice of storage temperature, at room or low temperatures, for once heated oils and for fatcontaining processed foods at home has not drawn so much attention. Therefore, the conventional methods employed for fat analyses have been investigated if they can be used for detection of deterioration of heated soybean oil stored at low temperatures. Soybean oil heated for 30 min, 2 and 5 hr at 185°C was stored at 25, -5 and -25°C up to 150 days and each sample was analyzed for oxygen absorption (at 50°C), acid value, carbonyl value and fatty acid composion by gas liquid chromatography. The acid and carbonyl values of the samples stored in all the temperatures under consideration increased up to 3060 days, followed by decrease, indicating decomposition of acids and carbonyl compounds during the course of storage. Similarly, peroxide value of the samples stored at low temperatures increased up to 15 days, followed by decrease, whereas that of the samples stored at 25°C increased steadily. No significant differences in the proportions of fatty acids were observed between the samples before and after the storage when examined by gas liquid chromatography. However, the oxygen absorption curves obtained showed an absorption maxima or one or more plateaus when examined at a bath temperature of 50°C for 56 hr, and this may be attributed to the effects of some volatile compounds formed by heating and or autoxidation during storage. A white precipitate formed in the samples stored at -5°C was confirmed as triglycerides comprising relatively larger amounts of saturated fatty acids than those of soybeanoil. The decomposition of the oxygenated compounds as reflected by changes in acid, peroxide and carbonyl values in the samples stored at low temperatures suggests that these methods of analyses are not necessarily appropriate for detection of deterioration of oils stored at low temperatures, and therefore, further studies are necessary.