Extrusion heating was applied to SPI in order to investigate the interrelationship between the physicochemical properties and the enzymatic digestibility of SPI containing 70% water. Extrudates were prepared at the dough temperature of 60, 89, 126, 139, 154℃ corresponding to the die temperature of 80, 100, 125, 148, and 160℃ respectively. Conventionally heated samples were prepared by heating at 100℃ for 2hr. Melting point of SPI was measured by using a laboratory scale twin-screw extruder (Laboruder TEX-L Japan Steel Works, Ltd.). The measured melting point was about 135℃. Properties of extrudates were investigated by protein determination, SDS-PAGE, amino acid analysis and enzymatic hydrolysis. Over melting point, the solubility of extrudates increased and the digestibility was extremely enhanced. From the result of SDS-PAGE of water soluble fraction, major bands of all water soluble fraction were acid subunit of glynicin and Gly m Bd 34 K. In amino acid analysis, the amount of hydrophiric amino acids gradually increased, with rising die temperature.
Key words: Soy protein; extrusiom heating; trypsin digestion.