Cereal Chem 56:84 - 89.  |  VIEW ARTICLE
Lipoxygenase and Lutein Bleaching Activity of Durum Wheat Semolina.

C. E. McDonald. Copyright 1979 by the American Association of Cereal Chemists, Inc. 

Durum semolina contains lipoxygenase and lutein bleaching activities but no linoleic acid hydroperoxide isomerase activity. Lipoxygenase activity was determined on linoleic acid by light absorption at 234 nm and bleaching activity on purified lutein by light absorption at 452 nm. The activity optimum for lipoxygenase was at pH 4.8, but lower peaks of activity occurred at pH 6.0 and 7.0. The optimum for bleaching was at pH 9.0 with a lower peak of activity at pH 6.0. Heating at 69 C for 1 min inactivated most but not all of both activities. Some activators and inhibitors also were tested. Results with a sulfhydryl blocking reagent and cysteine did not indicate an essential sulfhydryl group in either the bleaching or lipoxygenase enzymes. Ascorbic acid was much more effective as an inhibitor of bleaching activity than of lipoxygenase activity. Calcium chloride had little effect except at the highest pH levels of the assay where both bleaching and lipoxygenase activities were inhibited. Both bleaching and lipoxygenase activities varied widely between different varieties of durum, and bleaching activity was highly correlated with lipoxygenase activity. Compared with the potential for linoleic acid, the lutein oxidizing potential was low but evidently still high enough to cause marked color loss in spaghetti made from poor varieties of durum.