Effect of plasticizers on glass transition behavior and functional properties of vital wheat gluten and gluten

The objective of this work was to examine the relationship between water mobility and the glass transition behavior of vital and chemically-treated wheat gluten. Dynamic Mechanical Analyzer (DMA) and Differential Scanning Calorimetry (DSC) was used to study thermal transition behavior and water mobility was evaluated by $\sp2$H and $\sp{17}$O Nuclear Magnetic Resonance (NMR). $\sp2$H NMR percent detected intensity reflected the changes occurring in the sample through the glass-rubbery transition (T$\sb{\rm g}$). With the addition of deuterated water, below 0.10 g D$\sb2$O/g total, in the glassy region, detected NMR intensity was almost zero and above 0.26 g D$\sb2$O/g total, in the rubbery region, the detected signal was the maximum. Freezable water occurred at $\u3e$0.18 g water/g total, close to the midpoint of the T$\sb{\rm g}$ region. Fermi\u27s equation was used to develop empirical models relating moisture and temperature with relative stiffness for gluten. The critical moisture content and steepness (a) from the relative stiffness data correlated well with the $\sp2$H NMR detected intensity. Potassium bromate and ascorbic acid-treated gluten showed lower detected signal and higher water sorption than the control. The oxidant-added gluten showed a broader TB (approx. 150$\sp\circ$C) at moisture content $\u3e$0.10 g water/g total as compared against the control (100$\sp\circ$C). The T$\sb{\rm g}$, mechanical, and water vapor barrier properties of wheat gluten films containing glycerin, sucrose, glycerin-sucrose and glycerin-sorbitol were also studied. Glycerin and sucrose showed limited miscibility with each other and displayed two separate transitions. The low-T tan $\delta$ peak height (due to glycerin-rich phase) was found to influence the tensile strength and elongation linearly and the water vapor permeability (WVP) curvilinearly. However, the T$\sb{\rm g}$ did not change with composition and exhibited low correlation with barrier and mechanical properties. Initial addition of glycerin increased the WVP dramatically. The presence of sucrose decreased the WVP (only slightly), but resulted in a rigid and fragile film. Sorbitol resulted in a single effective T at $-$42$\sp\circ$C and intermediate values for tensile strength, percent elongation, and WVP (between the 15:6:0 and 15:4:2 gluten:glycerin:sucrose films). Both low-T$\sb{\rm g}$ and high-T$\sb{\rm g}$ transitions showed strong moisture dependence. Sorbitol-added film, showed a stronger moisture effect as compared to glycerin- and sucrose-added film. The sucrose ester and glycerin-added film showed two transitions similar to the earlier case. The magnitude of the low-T transition was affected by the concentration of the sucrose ester used