Role of Lipoxygenases in Soybean Seed Protein Quality

Soybean seeds contain three lipoxygenase isozymes: lipoxygenase 1 (L-l), lipoxygenase 2 (L-2), and lipoxygenase 3 (L-3). Use was made of recessive mutants lacking L-l that have been found and the more rapid heat inactivation of L-2 and-3 relative to L-l to determine the roles of these lipoxygenase isozymes in the generation of volatile carbonyl compounds associated with poor flavor of soybean protein. All lipoxygenase isozymes increased the level of carbonyl compounds and thiobarbituric acid reacting substances in aqueous extracts of whole soybean seeds with the effect of L-2 and-3 being greater than that of L-l. A disproportionate level of free fatty acid hydroperoxides was detected in water extracts of soybeans, indicating that lipase action precedes lipoxygenase-catalyzed lipid oxidation. The evidence indicates that the use of the L-1-deficient mutants of soybeans can reduce the level of volatile carbonyl compounds and TBA-reacting substances with a minimum of heating and protein loss.