Changes in the structure of soybean triacylglycerols due to heat

Commercially refined soybean oil was heated at 180 C for 50, 70 or 100 hr with aeration. The triacylglycerol fractions separated from the oils by column chromatography on silica gel were fractionated further by silicic acid thin layer chromatography (TLC), and species compositions were determined by argentation TLC and lipase hydrolysis. There was a decrease in the absolute amounts of the triacylglycerols with longer heating periods, such as 21% for 50 hr and 42% for 100 hr. Relatively large changes occurred in the proportion of the molecular species of the triacylglycerols during heating; there was an increase in the more saturated species, 1-4 double bonds, and a decrease in species containing 5-7 double bonds. Although these changes occurred in the percentage of each triacylglycerol species, the positional distribution of the fatty acids in the 2-position remained virtually unchanged throughout heating. Oleic and linoleic acids were commonly found in the 2-position of the acylglycerol moiety, whereas most of the palmitic and stearic acids favored esterification in the 1- and 3-positions. The results indicate that unsaturated fatty acids located in the 2-position are protected significantly from thermal oxidative decomposition.