Flavor improvement of soybean preparations by genetic removal of lipoxygenase‐2

Abstract

Lipoxygenase-mediated oxidation of fatty acids has long been implicated in the production of off-flavors in soybean products. This assumption was tested by use of preparations from soybean lines nearly isogenic to the cultivar Century that lack the lipoxygenase isozyme or isozyme combinations L1, L~, L3, L1 -}-L3, or L 2 -}-L3. Near-isogenic lines were used to ensure that any effects observed were due to elimination of lipoxygenase isozymes, and not to other unrecognized genetic differences between lines. Full-fat soy flour and unblanched soymilk preparations from the lines were evaluated by a six-member taste panel for eight flavor and/or aroma attributes common to soybeans. By comparison to near-isogenic controls, removal of the L 2 isozyme from soymilk preparations produced significantly lower scores for beany, rancid and oily flavor and aroma attributes, as well as higher scores for dairy and cereal flavor and aroma attributes. Similar trends were noted for soy flour flavor attributes. Thiobarbituric acid (TBA} numbers, a measure of lipid oxidation, were lower in homogenized soy flour suspensions from lines lacking L2. Removal of the L1 and L3 isozymes did not result in improved flavor scores or lower TBA numbers. Total oil content and the fatty acid profile of the near isolines did not vary appreciably. The results indicated that genetic removal of the L 2 isozyme may reduce offflavors in soy products.Soybean oil contains between 7 and 9% linolenic acid (18:3) and about 55% linoleic acid (18:2}. Oxidation of these fatty acids during processing and storage of soybeans poses a significant problem to the food industry. Breakdown products from fatty acids have been associated with grassy-beany and rancid flavors (1,2}. These oxidation products are considered responsible, at least in part, for reduced consumer acceptability of soy products. The flavor problems can occur in protein as well as oil products because the undesirable compounds are reactive and can bind covalently to proteins during processing {3,4}.Considerable indirect evidence has implicated seed lipoxygenase as a major factor contributing to the oxidation of lipids during processing, thereby lowering product quality {3,5}. This enzyme catalyzes the hydroperoxidation of cis-cis pentadienes and is present in relatively large quantities {about 2% of the protein} in soybean seeds (6). The hydroperoxides formed by lipoxygenase activity and their breakdown products are known to exhibit off-flavors {3,7}. Furthermore, lipoxygenase inactivation by heat during processing has been correlated with the improvement of sensory evaluation scores (8}. Heat treatments are widely used 1Cooperative research between USDA/ARS and the Purdue Agricultural Experiment Station.to inactivate lipoxygenase, but they are expensive, can produce undesirable alterations of the proteins and themselves can add to the flavor profile of soy products.Three lipoxygenase isozymes, denoted L1, L2 and L3, are present in soybean seeds (6}. They differ from one another in reaction products, p...

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