Air-dried soybeans absorb water rapidly for the first 2 hr followed by a slower rate of uptake. The beans take up an equal weight of water (100% hydration) after approximately 2.5 hr at 37°C to 5.5 hr at 20°C and reach complete hydration (140% hydration) after 6 hr at 37°C and 16 hr at 20°C. Soluble solids are leached out of the beans at a fairly steacly rate throughout the hydration, and the amount is greater with higher temperatures. Temperature is the most important factor in determining the rate of water absorption and of solids losses. Of the total solids lost, 7-16% is protein. The proportion of protein loss increases as the soaking time and temperature increase. About 30-50% of soluble sugars, including fructose, sucrose, raffinose and stachyose, are removed from the beans after overnight soaking at 25°C. On the other hand, the amounts of trypsin inhibitor and hemagglutinin found in the soybean soak are relatively small as compared to that present in the air-dried beans. Hydrating soybeans to 100% prior to cooking reduces the cooking time, increases the tenderness and weight of the cooked beans, and improves their appearance. Complete hydration results in no further improvement in cooking rate or cooking quality of the beans.

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Growth rates of 8 Lactobacillus acidophilus strains and four Lactobacillus bulgaricus strains were compared in soybean milk and soybean milk enriched with glucose, lactose, and sucrose. Four L. acidophilus strains grew well in soybean milk; the remainder grew better in soybean milk supplemented with glucose or lactose. In general, soybean milk was not an adequate media for strains of L. bulgaricus. Almost all these cultures, however, could adapt themselves to the environments of the media tested. A soybean milk drink fermented by L. acidophilus NRRL B-1910 was prepared and evaluated by a taste panel. The drink had a refreshing sweet-sour taste, and the beany flavor of soybeans was masked by the fermentation process.

The trypsin-inhibitory activity observed in cooked soybeans fermented by Rhizopus oligosporus (fungus used in tempeh fermentation) has been examined. The active compounds have now been isolated by ethanol extraction and thin-layer chromatography and have been identified as free fatty acids by infrared spectroscopy and gas-liquid chromatography. Oleic, lineoleic, and linolenic acids are primarily responsible for the increased trypsin-inhibiting activity of cooked soybeans after fermentation. The free fatty acids are liberated from oil in the soybeans by fungal lipase, and they differ from other reported soybean trypsin inhibitors that are protein in nature. Free fatty acids have been previously reported to inhibit various enzymes, such as glycolytic, glyconeogenic, lipogenic, and also proteolytic. Their effect appears to be a nonspecific type of inhibition. Further studies are required to determine their physiological relevance, if any.