Glass transition of soy globulins using differential scanning calorimetry and mechanical spectrometry

The glass transition of soy 7S and 11S globulin fractions, isolated from defatted soy flour by the method of Thanh and Shibasaki (J. Agric. Food Chem. 1976, 24, 6), was studied as a function of moisture content, using differential scanning calorimetry (DSC) and mechanical spectrometry. Both proteins showed to be plasticizable by water, suggesting that their hydrophilic domains play an important role in determining their calorimetric and rheological properties. Due to the presence of a contaminating 7S fraction in the 11S fraction, and vice versa, the DSC and mechanical spectrometry scans from both fractions showed two different glass transitions which correspond to the 7S and 11S globulins, respectively. The glass transition temperature (T-g) of the 7S fraction ranged from 114 to -67 degrees C for moisture contents from 0 to 35%, while the T-g of the 11S fraction ranged from 160 to -17 degrees C for moisture contents from 0 to 40%. Results from both methods superimposed each other very well in the overlapping moisture range. Similar results have been found in other amorphous polymers.