Abstract: Effects of lipoxygenase I-B (LOX) catalyzing oxidation of linoleic acid on soybean proteins were evaluated by electron paramagnetic resonance (EPR) spectroscopy in different model systems in the presence or absence of antioxidants. A strong central singlet signal was detected by EPR spectroscopy and identified as the carbon radical (g value range 2.0041 to 2.0054). A downfield shoulder attributed to the sulphur radical (g value 2.019 to 2.028) was also observed. Natural antioxidants such as ascorbic acid and α-tocopherol as well as synthetic antioxidants butyl hydroxytoluene (BHT) inhibited the development of the free radical signal when added to soybean proteins prior to incubation with linoleic acid and lipoxygenase I-B; the central singlet signal attributed to the carbon radical was reduced by 35%～65%.

Key words: soybean protein, lipoxygenase, linoleic acid, free radicals, EPR