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

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 degrees 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%; P<0.05). During frying cycles, the final acid value (AV) was significantly higher in PSBO (0.84 +/- 0.02mg KOH/g fat) than in TG-SESBO (0.59 +/- 0.01mg KOH/g fat) and FG-SESBO (0.56 +/- 0.03 (mgKOH/g oil; P<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.