Moisture adsorption isotherms and glass transition temperature of xanthan gum

The moisture adsorption isotherms of the xanthan gum were determined at 30 - 70 degrees C over water activity ranging from 0.11 to 0.94. The moisture adsorption isotherms revealed that the equilibrium moisture content increased with water activity. Increased temperature, in general, resulted in decreased equilibrium moisture content. However, in some cases equilibrium moisture content values increased with temperature. Selected sorption models were tested to describe the adsorption isotherms. Parameters of each sorption model were determined by nonlinear regression analysis. The modified Oswin model gave the best fit for xanthan gum. The isosteric heat of sorption decreased with increase in moisture content and varied between 1.66 and 7.61 kJ/ mol. The glass transition temperature decreased with increase in moisture content for xanthan gum.