Moisture sorption and water vapor permeability of soy protein isolate/poly(vinyl alcohol)/glycerol blend films

The aim of the present work was to investigate the moisture sensitivity of films of soy protein isolate (SPI) blended with poly(vinyl alcohol) (PVA) and plasticized with glycerol. Water vapor permeability (WVP) and moisture sorption were measured based on the PVA and glycerol content of the films. WVP values of SPI/PVA films with and without glycerol were in the range 8.25–10.9 g mm (m2 h kPa)−1, and were significantly affected by both PVA and glycerol. WVP values decreased with increasing PVA content. XRD analysis confirmed that the glycerol can enter the macromolecular blend structure and destroy the crystallinity of the blends. Crosslinking between glycerol molecules and SPI reduced the interstitial spaces in the protein matrix, thus decreasing the rate of diffusion of water molecules through the films. Moisture adsorption and sorption isotherm data of SPI/PVA/glycerol were mathematically fitted to the GAB model. With increasing PVA content, the equilibrium moisture content (EMC) of film samples clearly decreased. That trend indicates that the addition of PVA decreased the ability of SPI protein molecules to absorb and accept water. EMC data and contact angle measurements showed that the addition of plasticizer not only loosened the microstructure of blended films, but also increased the hydrophilicity by exposing their hydroxyl groups.