Studies on the Deterioration of Frying Oils in Continuous Water-Spraying and Heating System. IX. Frying Quality of Oils Used for Frying in Japan

Physical and chemical changes were compared with soybean oil, corn oil, cottonseed oil , rice oil, rapeseed oil, lard, palm oil and coconut oil in continuous water spraying and heating system (Table -1 , Fig,-1•`4). Thermal oxidative changes such as viscosity increasing ratio, carbonyl and iodine value , showed close relation with the initial iodine value of these oils. However, in coconut oil in spite of the low iodine value, the carbonyl value increased very rapidly. This fact suggests the oxidation of saturated fatty acid in coconut glyceride. On the other hand, hydrolytic changes observed in acid value was great with rapeseed oil , lard and palm oil and was very small with coconut oil and cottonseed oil. In . the case of coconut oil, this result may depend on the high volatility of free fatty acid produced in the oil. In the case of cottonseed oil, the high stability to hydrolysis may depend on the cyclopropenoid fatty acid contained about 0.3% in the oil, similarly to that of kapok seed oil. The lowering of fat stability to autoxidation was observed as the deterioration proceeds . The rate of lowering was different by the kind of oils. Cottonseed oil and rice oil showed the smallest rate. In the case of lard, the fat stability was very low from the initial stage of test . This may depend on the low content of r-tocopherol in lard. From these results, the author believes that the fatty acid composition, the behavior of antioxidant and hydrolytic property in the frying oils are the important factors affecting to the frying quatity.