Abstract :
Phase transitions of freeze-dried persimmon in a large range of moisture content were determined by differential scanning calorimetry (DSC). In order to study this transitions at low and intermediate moisture content domains, samples were conditioned by adsorption at various water activities (aw=0.11–0.90) at 25°C. For the high moisture content region, samples were obtained by water addition. At aw≤0.75 two glass transitions were visible, with Tg decreasing with increasing water activity due to water plasticizing effect. The first Tg is due to the matrix formed by sugars and water. The second one, less visible and less plasticized by water, is probably due to macromolecules of the fruit pulp. At aw between 0.80 and 0.90 a devitrification peak appeared after Tg and before Tm. At this moisture content range, the Gordon–Taylor model represented satisfactorily the matrix glass transition curve. At the higher moisture content range (aw>0.90), the more visible phenomenon was the ice melting. Tg appeared less visible because the enthalpy change involved in glass transition is practically negligible in comparison with the latent heat of melting. In the high moisture content domain Tg remained practically constant around Tg′ (−56.6°C).
