Water sorption isotherms and glass transition temperature of spray dried tomato pulp

Adsorption isotherms of tomato pulp spray dried in dehumidified air were determined at six temperatures between 20 and 70 °C using a gravimetric technique. The data obtained were fitted to several models including two-parameter (BET, Halsey, Smith and Oswin), three-parameter (GAB), and four-parameter (Peleg) relationships. A non-linear least square regression analysis was used to evaluate the models constants. The GAB model best fitted the experimental data. The isosteric heat of sorption was determined from the equilibrium adsorption data using the Clausius–Clapeyron equation. Isosteric heats of sorption were found to decrease exponentially with increasing moisture content. The enthalpy–entropy compensation theory was applied to the sorption isotherms and indicated an enthalpy controlled sorption process. Glass transition temperatures (Tg) of spray dried tomato pulp conditioned at various water activities were determined by differential scanning calorimetry. A strong plasticizing effect of water on Tg was found with a large reduction of Tg when the moisture content increased. These data were satisfactory correlated by the Gordon and Taylor model.