Effect of soaking conditions on the formation of lipid derived free radicals in soymilk

Lipid derived free radical in soymilks were studied by combining 5,5-dimethyl-pyrroline-l-oxide (DMPO) spin trap, chloroform-methanol extraction and electron spin resonance (ESR) spectroscopy. Five lipid derived free radical adducts: DMPO-X%, DMPO-L center dot, DMPO-R center dot, DMPO-LOO center dot, and DMPO-RO center dot were presented in soymilks. The total amounts of spins increased as the soaking temperature increased from 4 degrees C to 50 degrees C and the soaking pH increased from 3 to 9 and in paralleled with the diffusion of soybean exudates to soaking water. Prolonged soaking of soybean at 50 degrees C resulted in a higher signal intensity of DMPO-R center dot than that of DMPO-LOO center dot. Soybean lipoxygenases (LOXs) were responsible for the formation of lipid derived free radicals in soymilks. Soybean exudates affected the total amounts of lipid radicals in linoleic acid (LA) - LOX model system. The relative signal intensities of DMPO-R center dot and DMPO-LOO center dot were depended on the contents of soybean exudates in the system.