Properties of Extracts from Soy Sauce Cake Using Subcritical Water Treatment

Abstract

Soy sauce, Shoyu, is a seasoning commonly used in Japan. In the production of Shoyu, ca. 25% of its raw materials becomes a by-product, soy sauce cake. Although part of the by-product is utilized as animal feed, most is discarded as waste [1]. Because soy bean which is the raw material for the production of Shoyu is rich in protein, isoflavones and so on, useful materials are still included in the by-product. Water which maintains its liquid state at high temperatures below 374℃ under pressurized conditions is called subcritical water, pressurized hot water, compressed hot water or superheated water. In this study, we use the term subcritical water. The water has two distinct features compared with ambient water; one is a lower relative dielectric constant, and the other is a higher ion product. Subcritical water treatment is an emerging technique for extracting useful materials from agricultural by-products or food wastes or for degrading environmental pollutants [2-4]. We also used this method to obtain functional substances having emulsifying or antioxidative ability from defatted rice bran [5-7] and wheat bran [8]. In this study, the subcritical water extraction was applied to soy sauce cake to produce useful materials from it. The properties of the extract were compared with those of the extracts from defatted rice bran and wheat bran. Although hydrolysis and degradation reactions would occur during the treatment, the term of extraction is used for the purpose of the treatment. Because the soy sauce cake contained sodium chloride at ca. 10% (w/w) [9], corrosion of the apparatus during subcritical water treatment was a concern. Therefore, the soy sauce cake was used after demineralization.