Sorption isotherm and state diagram in evaluating storage stability for sultana raisins

To investigate the optimum storage condition, the interrelation between water activity (aw) and glass transition temperature (Tg) of sultana raisins were evaluated. Water adsorption isotherms of sultana raisins were measured at 15, 25 and 35 °C using an isopiestic method. The measured data were fitted into BET and GAB models. Thermal transitions of equilibrated raisins and separated crystals of raisins were analyzed using differential scanning calorimetry (DSC). The plasticizing influence of water on glass transition was modeled by Gordon-Taylor, Khalloufi and an empirical model. The state diagram was constructed by adsorption isotherm and the glass line to investigate relation between the two distinct criteria of the raisins stability. The water sorption data provided the monolayer moisture content values of 0.0789 and 0.082 g water/g dry product at 25 °C in the BET and GAB models, respectively. The glass transition temperature of raisins decreased with increasing water contents. According to the state diagram, the predictions of critical water content (Cwc) for stability of raisins at 25 °C was underestimated in comparison with the sorption isotherm. Results showed that the state diagram and water adsorption data may be used to predict the stability of sultana raisins and both Tg and aw should be considered to control sugaring phenomenon in raisins.