Effect of heating on the characteristics and chemical composition of selected frying oils and fats

Seven commonly used frying oils and fats (beef tallow, canola oil, partially hydrogenated canola oil, corn oil, cottonseed oil, soybean oil, and partially hydrogenated soybean oil) were heated at two different temperatures (190 and 204 degrees C) for 8 h/day until they reached a critical level of polar constituents. Iodine value, color index, and the levels of polar compounds and of dimeric and polymeric triglycerides were monitored daily using AOCS official methods. In general, oils with higher levels of unsaturated fatty acids produced more polar compounds compared to the more saturated oils. Cottonseed oil had the fastest rate of formation of polar material and of polymeric triglycerides of oils heated at 204 degrees C while corn oil had the greatest yield of polar material and polymeric triglycerides at 190 degrees C. For all seven oils and fats, total polar material was highly (r greater than or equal to 0.99) and significantly (P < 0.001) correlated with the dimeric and polymeric triglyceride content and also highly (r > 0.94 and r less than or equal to -0.97) and significantly (P < 0.001 for the majority of cases; P < 0.05 for the poorest correlation) with the color index and iodine value, respectively. Despite its significant correlations with total polar material, color index was not a reliable indicator of oil quality.