Sorption isotherm, glass transitions and state diagram for freeze-dried plum skin and pulp

Differential scanning calorimetry (DSC) was used to determine phase transitions of freeze-dried plums. Samples at low and intermediate moisture contents, were conditioned by adsorption at various water activities (0.11 <= a(w) <= 0.90) at 25 degrees C, whereas in the high moisture content region (a(w) > 0.90) samples were obtained by direct water addition, with the resulting sorption isotherm being well described by the Gualgenheim-Anderson-deBoer (GAB) model. Freeze-dried samples of separated plum skin and pulp were also analysed. At a(w) <= 0.75, two glass transitions were visible, with the glass transition temperature (T.) decreasing with increasing aw due to the water plasticising effect. The first T, was attributed to the matrix formed by sugars and water. The second one, less visible and less plasticised by water, was probably due to macromolecules of the fruit pulp. The Gordon-Taylor model represented satisfactorily the matrix glass transition curve for aw:5 0.90. In the higher moisture content range T, remained practically constant around T-g' (-57.5 degrees C). Analysis of the glass transition curve and the sorption isotherm indicated that stability at a temperature of 25 degrees C, would be attained by freeze dried plum at a water activity of 0.04, corresponding to a moisture content of 12.9% (dry basis).