Water sorption isotherms of starch powders. Part 2: Thermodynamic characteristics

A thermodynamic approach was used to interpret the experimental adsorption and desorption isotherm data for potato, highly amylopectin and highly amylose powders starch. Calculation of the thermodynamic properties (differential enthalpy, integral enthalpy, differential entropy and integral entropy) provides an understanding of the properties of water and energy requirements associated with the sorption behaviour. The isosteric heat of sorption (differential enthalpy) was determined from the equilibrium adsorption and desorption data, using the Clausius-Clapeyron equation in the temperature range 30-60 °C. The differential and integral enthalpy decreased with increasing moisture content. The integral entropy was found to be negative in magnitude and increased with moisture content, reaching the entropy of free water at a moisture content of approximately 1.20 kgkg^-1 dry basis. The exponential trend of the differential entropy with moisture content was found to be similar to that of differential enthalpy. The spreading pressure increased with increasing water activity, and decreased with increasing temperature. The entropy production during the adsorption and desorption process clearly showed that the sorption process in starch materials is irreversible.