EFFECT OF TRIACYLGLYCEROL COMPOSITION AND STRUCTURES ON OXIDATIVE STABILITY OF OILS FROM SELECTED SOYBEAN GERMPLASM

The oxidative stability of soybean oil triacylglycerols was studied with respect to composition and structure. Crude soybean oils of various fatty acid and triacylglycerol composition, hexane-extracted from ground beans, were chromatographed to remove non-triacylglycerol components. Purified triacylglycerols were oxidized at 60-degrees-C, in air, in the dark. The oxidative stability or resistance of the substrate to reaction with oxygen was measured by determination of peroxide value and headspace analysis of volatiles of the oxidized triacylglycerols (at less than 1% oxidation). The correlation coefficients (r) for rates of peroxide formation (r = 0.85) and total headspace volatiles (r = 0.87) were related positively to oxidizability. Rate of peroxide formation showed a positive correlation with average number of double bonds (r = 0.81), linoleic acid (r = 0.63), linolenic acid (r = 0.85). Rate of peroxide formation also showed a positive correlation with linoleic acid (r = 0.72) at the 2-position of the glycerol moiety. A negative correlation was observed between rate of peroxide formation and oleic acid (r = -0.82). Resistance of soybean triacylglycerols to reaction with oxygen was decreased by linolenic (r = 0.87) and increased by oleic acid (r = -0.76)-containing triacylglycerols. Volatile formation was increased by increased concentration of linolenic acid at exterior glycerol carbons 1,3 and by linoleic acid at the interior carbon 2. Headspace analysis of volatiles and high-performance liquid chromatography of hydroperoxides indicated that as oxidation proceeded there was a slight decrease in the linolenic acid-derived hydroperoxides and an increase in the linoleic acid-derived hydroperoxides. The oxidative stability of soybean oil triacylglycerols with respect to composition and structure is of interest to the development of soybean varieties with oils of improved odor and flavor stability.