Soybean Components Affect Physicochemical Properties of Soymilk, Coagulation Reactivity and Tofu Texture

When a coagulant is added to soymilk, oil globules combine with protein particles to form tofu curd. Since physicochemical properties of soymilk, such as the number and size of the protein particles, and their behavior are key factors for processing properties for tofu-making, the effect of soybean components on the physicochemical properties of soymilk was studied. Soymilk was fractionated to precipitate, soluble and floating fractions by centrifugation to study the amount of the protein particles and their interaction with lipid. The amount of precipitate increased and more triglyceride (TAG) was incorporated into the precipitate as the 11S/7S ratio increased. A similar event was observed as MgCl 2 concentration in soymilk increased. The stain intensity of bands after sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated that the ratio of oleosin, a membrane protein of oil body, increased in the precipitate as the MgCl 2 concentration or the 11S/7S ratio increased. These results indicated that protein composition affects not only the number and size of the protein particles but also their behavior. Variation of the 11S/7S ratio among different soybean cultivars affected coagulation reactivity of soymilk probably due to a change of physicochemical properties of soymilk. Soymilk with more calcium, polysaccharides, and 7S basic globulin also showed more protein particulate content. Extracts of okara (a by-product of soybean processing for soymilk) mainly contain minerals, polysaccharides and 7S basic globulin. Increase in the 11S/7S ratio and addition of calcium or dialyzed extracts from okara resulted in increase in breaking stress of tofu.