Abstract
Extrusion cooking of whole soybeans was performed under high-temperature and low-moisture conditions with a twin-screw extruder (Model: Alpharizer 100, Suehiro Iron Works, Yokkaichi, Japan). Extrusion, proceeded under the following conditions: screw diameter, 89mm; L/D=12; screw rotatioa speed, 120rpm; temperature at die plate, 239, 216 or 175°C, die diameter (4holes), 4mm each; water addition rate, 10l/h (to make the feed moisture content 20%); feed rate, 125kg/h. Chemical changes in the components throughout the extrusion cooking were investigated and nutritional evaluation was carried out on raw soybeans and their extrudates. With regard to chemical and physicochemical properties, no change in amino acid or available lysine content was observed throughout the extrusion process. Trypsin inhibitor, lectin and urease activities were almost completely lost. Furthermore, it was found that the proteins in raw soybeans became insoluble. However, it was observed that the proteins decomposed into smaller components on SDS-PAGE. As for the nutritional properties, the proteins in the extrudates were more digestible by pepsin and trypsin than those in raw soybeans. Moreover, the nutritional characteristics (PER, BV and NPU) of the proteins were significantly improved by the process. These results indicate that the extrusion cooking of whole soybeans under the conditions employed in this study does not have any adverse effects on the nutritional value of the beans.
