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Glass Transitions and State Diagram for Freeze-dried Pineapple

2001; Elsevier BV; Volume: 34; Issue: 4 Linguagem: Inglês

10.1006/fstl.2000.0685

1096-1127

Vânia Regina Nicoletti, Paulo José do Amaral Sobral,

Freezing and Crystallization Processes

Glass transition temperature of freeze-dried pineapple conditioned by adsorption at various water activities at 25 °C was determined by differential scanning calorimetry (DSC). High moisture content samples corresponding to water activities higher than 0.9, obtained by liquid water addition, were also analysed. The DSC traces showed a well-visible shift in baseline at the glass transition temperature (Tg). Besides, no ice formation was observed until water activity was equal to 0.75. For water activities lower than 0.88, the glass transition curve showed that Tgdecreased with increasing moisture content and the experimental data could be well-correlated by the Gordon-Taylor equation. For higher water activities, this curve exhibited a discontinuity, with suddenly increasing glass transition temperatures approaching a constant value that corresponds to the Tgof the maximally freeze-concentrated amorphous matrix. The unfreezable water content was determined through melting enthalpy dependence on the sample moisture content.