Hydration of soybeans is the first step in soy milk manufacture. Small, hard beans imbibe less water, which results in a significantly reduced yield of soy milk. The initial moisture content of beans also influences the hydration rate of soybean (Smith et al., 1960).

The rate of water absorption in soybean is regulated by the calcium content in the seed coat, the surface area and the structure of the micropyle (Saio, 1976). It has been found that soaking soybeans for 10 hours at ambient temperature, followed by blanching in 0.5% sodium bicarbonate for 30 min, resulted in the loss of 11% total bean solids and 5% protein (Nelson et al., 1976). At 100°C, soybeans absorb water equal to their weight in approximately 15 min and reach a peak within 2 hours; in comparison, at 21°C soybeans absorb an equal weight of water after approximately 4 hours of steeping. Steeping soybean in 0.5% sodium bicarbonate at 40°C reduces the water uptake process (Johnson and Snyder, 1978). Therefore, temperature and time significantly affect water uptake. Soaking seeds in an alkaline medium aids in the processing of soy milk and improves its quality as it helps to reduce the undesirable beany flavour (Muhammad and Khan, 2000). Wong et al. (1983) reported that the bulk of solids lost during soaking comprises low molecular weight water-soluble oligo-saccharides such as raffinose and stachyose, which are considered responsible for causing flatulence. The remaining quantities of carbohydrates leach out during soaking.

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