Effects of moisture on glass transition and microstructure of glycerol-plasticized soy protein

The glass transition behavior of the glycerol-plasticized soy protein sheets (SL series) at various relative humidity (RH) was investigated by using differential scanning calorimetry with the aluminum pan and O-ring-sealed stainless steel capsule, and the microstructure of these sheets was detected on small-angle X-ray scattering. The results revealed that there were three glass transitions (Tg1, Tg2 and Tg3), corresponding to glycerol-rich, protein-rich and protein-water domains, in the protein-glycerol-water ternary system. The Tg1 values of the SL-series sheets at 75% RH decreased from -49.3 to -83.8 degrees C with an increase of glycerol content from 10 to 50 wt.-%, whereas Tg2 and Tg3 were almost invariable at about 60 degrees C and 3 degrees C, respectively. In addition, the Tg1, Tg2 and Tg3 values of the SL-25 containing 25 wt.-% glycerol at 0%, 35%, 58%, 75% and 98% RH were in the range of -12.7 - -83.2 degrees C, 65.8 - 53.1 degrees C and 3.5 - 1.9 degrees C, respectively. The result from small-angle X-ray scattering indicated that the radii of gyration (Rg) of protein-rich domain were in the range of 60-63 nm; this suggested the existence of protein macromolecules as aggregates in the stable protein-rich and protein-water domains. With an increase of RH, the tensile strength and Tg values of the SL-series sheets decreased, but the elongation at break increased. In view of the results above, the moisture in ambient environment significantly influenced the Tg values and microstructures of the glycerol-plasticized soy protein sheets, leading to the changes of the mechanical and thermal properties.