Carotene and Chlorophyll Bleaching by Soybeans with and without Seed Lipoxygenase-1

Carotene and chlorophyll bleaching activities of whole mature seed extracts of two soybean genotypes that lack lipoxygenase-1 (L-1) activity were compared with two soybean genotypes with normal L-1 activity. Assays were conducted with three substrates: (1) methyl linoleate, pH 7.0; (2) linoleic acid, pH 7.0; (3) linoleic acid, pH 9.0. No carotene or chlorophyll bleaching occurred with substrate 1. The two soybean genotypes without L-1 activity showed no conjugated diene formation with substrate 2 or 3 although carotene and chlorophyll were bleached with substrate 2. The presence of L-1 in seed extracts stimulated carotene and chlorophyll bleaching some at pH 7.0 and much at pH 9.0. The presence of L-1 stimulated chlorophyll cooxidation more than carotene cooxidation. L-1 purified from soybean seeds by ammonium sulfate fractionation and ion-exchange chromatography bleached both carotene and chlorophyll. However, the cooxidation of carotene and chlorophyll relative to the peroxidation of linoleic acid (cooxidation potential) is lower for purified L-1 than for whole mature seed extracts. The relevance of this information to food processing is discussed.