Hydration of Whole Soybeans Affects Solids Losses and Cooking Quality

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 steady 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.