The Moisture Migration Behavior of Plasticized Starch Biopolymer

Abstract

Using plasticized starch pellets as a precursor for making thermoformed products is a commercially viable and profitable idea. However, drying of plasticized starch is quite complex in nature, partly due to the synergistic interactions between starch and plasticizers in the presence of water. The migration of water from starch pellets plasticized by two components, glycerol and xylitol, at three different temperatures was investigated in the present work. Evidence for synergistic interaction between plasticizers and water within starch is shown by the reduced effective moisture diffusivities and moisture migration fluxes at different overall plasticizer concentrations. In addition, the effective moisture diffusivities showed stronger dependence on moisture concentration and the plasticizer molecular weight even though the moisture flux was comparable. The drying process was characterized by two effective diffusion coefficients (D ₁, D ₂) and, interestingly, the coefficients were an order of significance apart. The Peleg model was investigated for predicting the drying behavior and it is shown that the Peleg constants k ₁ and k ₂ increase with temperature. k ₂, Which is related to material structure and morphology, showed comparable modification by addition of plasticizers, indicating that plasticizers were able to modify the fundamental structure, and xylitol showed greater average k ₂ values than glycerol. Further, because k ₁ is related with moisture diffusivity, the effect of temperature on diffusivity was interpreted using the Arrhenius relationship. The activation energy values confirm that plasticizers can lock in water within the new structure. Overall, the larger structure of xylitol showed better stability in controlling moisture diffusivities and migration fluxes. These findings can provide better insights in designing and controlling the vapor barrier properties of starch-based packaging materials.

Keywords:

• Biopolymer
• Diffusion
• Drying
• Extrusion
• Starch

Notes

^a Standard deviation for all values is less than 0.5%.

^a R ² values (confidence limit for E _a ) are also shown. Standard deviation for percentage weight change is <2%. Percentage weight change is reported as average values across the temperature range.