Effects of frying on polar material and free fatty acids in soybean oils


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The purpose of this study was to investigate the quality deterioration of pressed soybean oil (PSBO), first grade solvent extracted soybean oil (FG-SESBO) and third grade solvent extracted soybean oil (TG-SESBO) with respect to time at 180 °C. It was found that there was a significant increase in total polar material (PM; P < 0.01) and free fatty acids (FFA; P < 0.05) in PSBO, FG-SESBO and TG-SESBO with increased time of frying. After forty-five frying cycles, the final concentration of total PM was significantly higher in TG-PSBO (30.39%) than in PSBO (21.10%) and FG-SESBO (25.93%; < 0.05). During frying cycles, the final acid value (AV) was significantly higher in PSBO (0.84 ± 0.02 mg KOH/g fat) than in TG-SESBO (0.59 ± 0.01 mg KOH/g fat) and FG-SESBO (0.56 ± 0.03 (mgKOH/g oil; < 0.05). The content of saturated fatty acids in the three types of frying oils was significantly increased after forty-five cycles of frying. However, even larger changes were observed in the content of unsaturated fatty acids, with decreases in C18:2, 9 c 12 c and C18:3, 9c 12c 15c and increase in C18:1, 9 c. The highest increasing slopes of PM and AV were observed in the TG-SESBO.