Water sorption isotherms and phase transitions in kiwifruit

Adsorption and desorption isotherms were determined in entire and homogenized kiwifruit tissue. Fresh samples (desorption process) and freeze-dried samples (adsorption process), were conditioned at various water activities (0–0.675) at 30 °C and, at equilibrium, had attained different water contents. In each sample, glass transition was analysed by differential scanning calorimetry (DSC). BET and GAB models were fitted to sorption data and the Gordon and Taylor equation was used to model the water plasticization effect. Results showed that the different pretreatments applied did not imply differences in those relationships. The mean values for the parameters of the fitted models were: w0 = 0.057 g water/g dry product and C = 7.9 (BET model); w0 = 0.046 g water/g dry product, C = 10.6 and K = 1.20 (GAB model); k = 4.88 and Tg(as) = 40.3 °C (Gordon and Taylor model). The state diagram of the kiwifruit liquid phase was obtained including the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) Tg′=-52.0±0.4 °C, the melting temperature of ice crystals surrounding the m.c.m. Tm′=-40.4±0.4 °C, and the amount of non-freezable water content Wg′=0.186 g water/g m.c.m.