Sorption Behavior of Extruded Rice Starch in the Presence of Glycerol

The modeling approach to predict the sorption behavior of nonelectrolytic mixtures has been related to moisture content at a specifi c equilibrium relative humidity (ERH) and to the component weight fraction in the system. The work presented here attempts to identify deviations of the predicted sorption profi le of a model system based on rice starch and glycerol. Accurate sorption isotherms (25 ° C) were obtained by using dynamic vapor sorption (DVS) for the ERH range of 0% - 90%. The glass transition temperature ( T g ) of the samples was obtained by differential scanning calorimetry (DSC). Sorption studies show a reduction in moisture content for an ERH of 60% in the presence of glycerol. This behavior was represented by a reduction in Guggenheim - Anderson - de Boer (GAB) modeling parameters, the monolayer value (from 8.9% to 6.6%); a reduction in the constant related to the net heat of sorption (from 6.4 to 3.5); and an increase in the constant related to the heat of sorption at the multilayer (from 0.72 to 0.94). The sorption profi les of the starch - glycerol mixtures were also modeled using sorption isotherms and moisture contents from each component. The estimated interaction factor of 1 indicated a reduction in water uptake specifi cally for an ERH of 60%. The 10% and 20% glycerol systems showed a signifi cant increase in independent interaction factors (IIFs) from 0.6 to 0.9, suggesting a relationship between water uptake and the matrix - polyol interaction. This behavior was related to the molecular mobility of the system, as indicated by plotting IIF versus T T g ( T = 25 ° C).