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Molecular Dynamics of Soy-Protein Isolate Films Plasticized by Water and Glycerol

films were affected by moisture content. Cho and Rhee (2002) found that the effect of RH on mechanical properties of soy protein isolate (SPI) films varied with the type and concentra tion of plasticizers. The mechanical and rheological properties of foods and biological materials depend on the molecular dynamics of polymers, water, and other constituents. Nuclear magnetic resonance (NMR) is a powerful technique for determining molecular dynamics and physical structure of foods and biological materials through the analysis of spin magnetization relaxation. Relaxation of spins occurs due to the interactions between individual spins and between a spin and its environment. Whereas the signal intensity in an NMR experiment is proportional to the number of nuclei present, the decay of spin magnetization is related to the mobility of such nuclei. The relaxation behavior is commonly characterized by the transverse relaxation time (T 2 ) and longitudinal relaxation time (T 1 ). Kou and others (2000), investigating the mobility of waxy cornstarch using wide-line 1 H NMR, reported that proton relaxation times (that is, T 2 *, T 2 , and T 1 ) can be indicators of glass transitions in starch samples. Using pulsed 1 H NMR techniques, Choi and Kerr (2003) identified and quantified the mo lecular mobility of both starch and water molecules in starchwater suspensions at low moisture level and at different a w levels. The goals of this research were to investigate the effects of various combinations of water and glycerol on the molecular mobility of SPI-based edible films, and to study the interac tions between the water and glycerol molecules in the films. This will help to more accurately characterize the roles of glycerol and water in the formulation of protein-based edible films and to better understand changes in functional properties of the films under different relative humidity conditions.