Glass transitions and state diagram for freeze-dried pineapple

Glass transition temperature of freeze-dried pineapple conditioned by adsor ption at various water activities at 25 degreesC was determined by differen tial scanning calorimetry (DSC). High moisture content samples correspondin g to water activities higher than 0.9, obtained by liquid water addition, w ere also analysed. The DSC traces showed a well-visible shift in baseline a t the glass transition temperature (T-g). Besides, no ice formation was obs erved until water activity was equal to 0.75. For water activities lower th an 0.88, the glass transition curve showed that T-g decreased with increasi ng moisture content and the experimental data could be well-correlated by t he Gordon-Taylor equation. For higher water activities, this curve exhibite d a discontinuity, with suddenly increasing glass transition temperatures a pproaching a constant value that corresponds to the T-g of the maximally fr eeze-concentrated amorphous matrix. The unfreezable water content was deter mined through melting enthalpy dependence on the sample moisture content.