Pyrazine contents and oxidative stabilities of roasted soybean oils

Soybeans were roasted with constant stirring for 15 min at 130 °C, 150 °C and 170 °C, and oils were extracted from the roasted soybeans using an expeller. Color, conjugated diene, conjugated triene, peroxide values, tocopherols and fatty acid compositions in the oils were determined. Pyrazines in the oils were extracted by both simultaneous distillation-extraction and solvent extraction. Isolation and identification of pyrazines were carried out by gas chromatography and gas chromatography/mass spectrometry. Oxidative stabilities of the roasted soybean oils were studied during storage for 60 days at 65 °C. As roasting temperature increased, the oil became darker; the red and yellow colors increased. Conjugated diene and conjugated triene increased with increasing roasting temperature. Peroxide values of all the oils were 0 meq kg−1 oil. The contents of total tocopherols in the oils decreased markedly with increasing roasting temperatures. The total tocopherol contents in the oils from soybeans roasted at 130 °C, 150 °C and 170 °C were 70.0, 8.3 and 0.0 mg per 100 g oil, respectively. Fatty acid composition varied slightly with roasting temperatures. Nine alkyl pyrazines were identified: pyrazine, 2-methylpyrazine, 2,5-dimethylpyrazine, 2,6-dimethylpyrazine, 2,3-dimethylpyrazine, 2-ethyl-6-methylpyrazine, 2-ethyl-5-methylpyrazine, 2,3,5-trimethylpyrazine and tetramethylpyrazine. Total pyrazine contents greatly increased with increasing roasting temperature, showing 3.07, 9.46 and 19.7 mg total pyrazines per 100 g oils extracted from soybeans roasted at 130 °C, 150 °C and 170 °C, respectively. The oxidative stability tests showed that, as the roasting temperature increased, the oxidative stability of the oil increased greatly.