Headspace Volatile Analysis to Evaluate Oxidative and Thermal Stability of Soybean Oil. Effect of Hydrogenation and Additives

Headspace gas chromatographic analysis of heated soybean oil was investigated as a tool to determine what effect hydrogenation and additives have on the formation of total and individual volatile components. Soybean oil was hydrogenated to varying linolenate (Ln) contents with either nickel (Ni) or copper catalysts. Oils were stabilized with citric acid (CA) or a combination of CA with tertiary butyl hydroquinone (TBHQ) and/or methyl silicone (MS). Volatiles were analyzed with a capillary gas chromatograph equipped with a headspace sampler positioned on the injector. Oxidative stability was determined after storage of the oils at 60 C. To study thermal abuse and frying performance of oils, samples were heated for several hours with prolonged bread frying. The deterioration of the oils was accelerated further by static heating in air within the headspace sampler. All hydrogenated oils produced less total volatiles than the unhydrogenated control oil after prolonged heating and bread frying. Static heating at 190 C for one hr showed that the oil hydrogenated with Ni to 0.4% Ln was the most stable. MS decreased the formation of volatiles in all samples and was particularly effective in stabilizing the hydrogenated oils. However, MS had little effect on volatiles in the oil hydrogenated to 0.4% with Ni. Unique volatile compounds identified included 2,4-heptadienal in nonhydrogenated soybean oil and 2-nonenal in most hydrogenated oils. On heating, the amount of 2-heptanal decreased significantly in the Ni hydrogenated oils compared to the control. Hexanal, on the other hand, decreased in all hydrogenated oils compared to the control.