Effects of the molecular weight on the properties of thermoplastics prepared from soy protein isolate

Commercial soy protein isolate (SPI) was fractionated into four fractions b y an acidifying method from pH 5.7 to 4.5 with 2M HCL A mixture of SPI with glycerin (50 g/100 g of dry SPI) was compression-molded to obtain thermopl astic sheets. The weight-average molecular weight (M.) of the fractions, th e structure, and the mechanical properties of the thermoplastic SPI sheets were investigated with light scattering, IR spectroscopy, wide X-ray diffra ction patterns, differential scanning calorimetry, ultraviolet spectroscopy , scanning electron microscopy, and tensile testing. After heating compress ion, the SPI sheets were transparent and exhibited a smooth and homogeneous structure. Moreover, the crystallinity degree of the thermoplastic SPI was obviously higher than that of the pre mix before compression because of th e formation of intermolecular hydrogen bonding. The M-omega's of the fracti ons were 1.17 X 10(5) to 3.21 X 10(5), and they increased with increasing p H value in fractionation. The mechanical properties and water resistance (R ) of the SPI sheets increased with increasing M-omega of the SPI fractions. The tensile strength and breaking elongation of the SPI sheets with an M-o mega value of 3.21 X 10(5) were 5.7 MPa and 135%, respectively, and the R v alue was 0.54 after immersion in water for 15 days. (C) 2001 John Wiley & S ons, Inc.