Glass transition temperature (Tg) has been identified as a critical factor to predict quality of foodstuffs during processing or storage. Differential Scanning Calorimetry (DSC) was used to determine the Tg of 4 types of berry powders as a function of water content, and equilibrium moisture content was measured over different saturated solutions using the gravimetric method. The Guggenheim-Anderson-deBoer model was used to predict water activity (a w ). Four models were tested for their ability to predict Tg as a function of the solid fraction. The combined effects of Tg and a w were incorporated into a new mathematical expression. The expression requires the determination of Tg (dry solids) of the product and data on equilibrium moisture sorption. The mean percent error of the model predictions is less than 3.6% when compared to experimental data.

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Several quality parameters, such as volumetric shrinkage, internal structure, color changes, and glass transition temperature (Tg) of dry solids, were investigated during freeze-drying of strawberry, apple, and pear, at various combinations of freezing and freeze-drying temperatures. Process conditions had different impacts on the quality parameters. Both mercury intrusion and electron microscopy methods revealed a smaller pore radius and a denser internal structure for pear as compared to the other fruits. The shrinkage phenomena observed in freeze-dried products could be interpreted from the Tg and the internal structure of the materials. Therefore, the determination of Tg of dry products could be used as an indicator for the best choice of products to be freeze-dried.

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