Isoflavones present in soybean (Glycine max) have been credited with performing several health-promoting functions, such as, prevention of cardiovascular diseases, cancers, and menopausal symptoms. In this study, the effect of the processing of soybean on the total content of isoflavones (including aglycones and glucosides) and the relative concentrations of 12 isoflavone compounds during the preparation of soy beverage and tofu were investigated. The mean recoveries of isoflavones in soy beverage and tofu in relation to their initial concentration in the raw soybeans were 54 and 36%, respectively. The estimated percentage of total isoflavones lost in the water used to soak raw soybeans, the okara (waste from heat-treated slurry), and whey were 4, 31, and 18%, respectively. The isoflavone profile of raw soybeans was altered as a result of processing. During processing, the detectable levels of aglycones, glucosides, and acetyl glycoside groups increased, whilst the corresponding malonyl glucosides decreased. The loss of isoflavones through the by-products, such as, okara and whey, was considerable. Appropriate techniques should be developed to recover and utilize these functional constituents from soybean by-products. In addition, processing techniques have to be optimized, so that the final products contain the nutrient and nutraceutical content of the starting material as much as possible. (C) 2002 Elsevier Science Ltd. All rights reserved.