Phase diagram for freeze-dried persimmon

Phase transitions of freeze-dried persimmon in a large range of moisture co ntent 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 (a(w) = 0.11-0.90) at 25 degreesC. For the high moisture content region, samples w ere obtained by water addition. At a(w) less than or equal to 0.75 two glas s transitions were visible, with T-g decreasing with increasing water activ ity due to water plasticizing effect. The first T-g is due to the matrix fo rmed by sugars and water, The second one, less visible and less plasticized by water, is probably due to macromolecules of the fruit pulp. At a(w) bet ween 0.80 and 0.90 a devitrification peak appeared after T-g and before T-m . At this moisture content range, the Gordon-Taylor model represented satis factorily the matrix glass transition curve. At the higher moisture content range (a(w) > 0.90), the more visible phenomenon was the ice melting. T-g appeared less visible because the enthalpy change involved in glass transit ion is practically negligible in comparison with the latent heat of melting . In the high moisture content domain T-g remained practically constant aro und T-g' (-56.6 degreesC). (C) 2001 Elsevier Science B.V. All rights reserv ed.