Abstract
Summary: 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 (T~g1~, T~g2~ and T~g3~), corresponding to glycerol‐rich, protein‐rich and protein‐water domains, in the protein‐glycerol‐water ternary system. The T~g1~ values of the SL‐series sheets at 75% RH decreased from −49.3 to −83.8 °C with an increase of glycerol content from 10 to 50 wt.‐%, whereas T~g2~ and T~g3~ were almost invariable at about 60 °C and 3 °C, respectively. In addition, the T~g1~, T~g2~ and T~g3~ 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 °C, 65.8–53.1 °C and 3.5–1.9 °C, respectively. The result from small‐angle X‐ray scattering indicated that the radii of gyration (R~g~) 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 T~g~ 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 T~g~ values and microstructures of the glycerol‐plasticized soy protein sheets, leading to the changes of the mechanical and thermal properties.
Three glass transitions corresponding to glycerol‐rich, protein‐water and protein‐rich domains in glycerol‐plasticized soy protein sheets at a relative humidity of 75%.
magnified imageThree glass transitions corresponding to glycerol‐rich, protein‐water and protein‐rich domains in glycerol‐plasticized soy protein sheets at a relative humidity of 75%.