Evaluation of the effects of glycerol and sorbitol concentration and water activity on the water barrier properties of cassava starch films through a solubility approach

The effects of glycerol and sorbitol on the water sorption isotherms and water vapor permeability (K-w) of cassava starch films prepared by casting were investigated. K-w values were determined in three ranges of relative humidity, RH, (2-33%, 33-64% and 6490%) and the GAB model was used to fit experimental water sorption isotherms. These data were used to determine the relative influence of the diffusion coefficient of water (D-w) and the average solubility coefficient ((beta) over bar) of water in the films on the k(w) value. In all cases, an increase in K-w values were observed with increasing plasticizer concentration and RH. The D-w, (beta) over bar and K-w values of films prepared with glycerol were greater than those of films prepared with sorbitol. For high RH, the (beta) over bar values increased 6-fold for films with glycerol and 7-fold for films with sorbitol, while D-w values did not change significantly. These results indicate that K-w values are dependent on the solubility coefficient ((beta) over bar) of water in the film, which is not consistent with the proposal that an opening of polymer chains promoted by plasticizers leads to an increase in D-w and K-w. (c) 2007 Elsevier Ltd. All rights reserved.