Relations between the Behavior of Oils and the Flavor of Soy Milk

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(1) Purpose. Enzymatic lipid peroxidation reaction have influence on the flavor and taste of soy milk. In this study, the behavior of lipid peroxidation products, which were the precursors of soy flavor, n-hexanal, was investigated. The chemical reaction of soybean proteins with carbonyl compounds was also investigated.(2) Methods. Various substances were made reaction with soy milk and the degree of lipid peroxidation and n-hexanal contents were determined. The change of molecular weight of soybean protein was determined by SDS-PAGE.(3) Results. By warming to 37ﾟC, TBA value of soy milk rose up in the first 10 min, and the value reached to plateau at 60 min. The TBA value at 0 min-warming was fairly high. By the addition of lipoxigenase inhibitors, SnCl_2 or quercetin, during preparation of soy milk, TBA value at 0 min-warming wa remarkably lowered compared with no addition soy milk, indicating the increase of TBA value caused by enzymatic action during homogenization. Addition of ethanol and SDS increased the TBA values, but DTAC lowered it, while Tween 20 and lecithin caused no change. The amount of hexanal produced increased by the addition of DTAC, but lowered with SDS, while no change with ethanol. Addition of linoleic acid lowered the TBA value. Decreasing of solubilized oxygen was correlative with TBA value. Electro-chemical determination in HPLC showed a decrease of triacylglycerol hydroperoxide in ethanol or SDS containing system compared with the system of no addition. It was presumed from the results mentioned above that triacylglycerol were peroxidized enzymatically, and the products were decomposed to secondary oxydized products via a process which differed from the decompose pathway of the peroxidized products of free fatty acid. By the action with n-hexanal, Lys, Met and Trp residues in soybean protein were damaged, and polymerization of proteins and addition of carbonyl compounds to protein were observed.