Quality deterioration of freeze-dried foods as explained by their glass transition temperature and internal structure

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.