Changes in the structure of soybean triacylglycerols due to heat

Abstract

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.

Effects of microwave treatment on the oxidative stability of peanut (Arachis hypogaea) oils and the molecular species of their triacylglycerols

“…As the number of double bonds increases, the rate of oxidation increases, the ratios of the rates of oxidation of oleate to linoleate to linolenate are reported to be 1:10:20 [27]. The obtained results are in agreement with those obtained by Raghuveer and Hammond [28], by Wada and Koizumi [29], and by Yoshida and Alexander [30] for FA thermal oxidative degradation. These authors reported that the TAGs, with unsaturated FA linked at the sn-2-position of glycerol, are more stable toward thermal oxidation than those with the same acid at the sn-1-or sn-3-position.…”

Effect of stirring on the thermooxidative stability of refined rice bran oil

“…Raghuveer and Hammond [1] suggested that the TAG having an unsaturated FA linked at the sn-2 position of glycerol were more stable towards autoxidation than those linked at the sn-1 or sn-3 position. These results were in agreement with those of Wada and Koizumi [2] and Yoshida [3]. In contrast, Park and coworkers [4] found no difference in autoxidative activity between randomized and native oil TAG.…”

Effect of randomization on the oxidative stability of corn oil

“…It is likely that we did a more complete purification removing all the prooxidants. Although many articles have suggested that the positional change of unsaturated fatty acid in TAG may have some effect on its oxidation (Hoffmann et al, 1973;Wada and Koizumi, 1983;Yoshida and Alexander, 1984;Miyashita and Takagi, 1988;Neff et al, 1992), no difference was shown between natural and randomized com oil TAG in our study. This maybe due to the polyunstaturated nature of com oil TAG, which makes such an effect not significant.…”

“…They also proposed that the concentration of unsaturated fatty acid at the sn-2 position of glycerol should stabilize a fat toward oxidation. Later studies confirmed the effect of randomization in stabilizing mixtures of trisaturated and unsaturated TAG towards oxidation at 50°C (Wada and Koizumi, 1983) and also the protection of unsaturated fatty acids at the sn-2 position in oxidizing refined soybean oil under high temperature (180°C) treatment (Yoshida and Alexander 1984). However, others showed that there was no preferential oxidation between the src-l(3) and sn-2 positions in trilinolein (Neff et al, 1990).…”

“…In a study to examine the changes in the structure of soybean TAGs during heat treatment (180°C for 50-100 h) with aeration, Yoshida and Alexander (1984) found that the percentage of the fatty acids remaining in the 2-position of the residual TAGs remained nearly unchanged throughout heating. Their results indicate that unsaturated fatty acids located at sn-2 position of TAG are protected significantly from thermal oxidative decomposition.…”