Process Parameters as Tools to Intensify the Freeze-Drying Process and Modify the Sorption Properties of the Obtained Freeze-Dried Products

This study aimed to investigate the effect of the application of different freeze-drying conditions on the process’s kinetics and the sorption properties of dried apples. Slices of apples were frozen and subjected to a freezing-drying process with different combinations of shelf temperature (−20, 10, 20, and 30 °C) and pressure (37, 63, 103, and 165 Pa). During the freeze-drying, the temperature in the centre of the material was recorded. The moisture content in the dried material and changes in the water content in dried apples stored at a humidity of 75.3% were obtained. The Midilli et al. model was used to describe the drying kinetics of the freeze-drying with a good fit. Drying time increased from 660 (variant with a constant shelf temperature of 30 °C, pressure 63 Pa) to 1305 min (variants with temperatures −20:10:20:30 °C, pressure 63 Pa). For this reason, the most favourable experimental parameters were a temperature of 30 °C and a pressure of 63 Pa. However, applying these parameters caused higher absorption of water vapour during storage. Therefore, the selection of freeze-drying parameters should also be related to the expected properties of the final product.