Effect of Microwave Freeze-Drying at Different Heating Rates on the Quality and Nutrient Content of Strawberries

The aim of this study is to dry whole strawberries, which are a sensitive and high value-added food, with different heating rate approach in different drying stages developed to overcome the problems encountered in microwave freeze-drying (MFD). To do this, a specially designed microwave (2450 MHz) freeze-drying system developed in our laboratory was used. Internal temperatures at two different whole strawberries which are one at the center and one at the edge of the basket were recorded during drying experiments by using a signal conditioner and fiber optic probes. Drying experiments were carried out until a moisture content of below 10% by weight was attained. Heating rates determined by preliminary trials of 0.035 °C/min and 0.2 °C/min were applied to whole strawberries in the primary and secondary drying stages, respectively. The quality parameters of microwave freeze-drying and freeze-dried whole strawberries were compared. In this context, dried strawberries were subjected to some physical (moisture content, water activity, texture, and color) and chemical (vitamin C, total phenolic content, and antioxidant capacity) analyses. Microwave freeze-drying (MDF) was able to preserve the vitamin C content of all strawberries by approximately 55% while yielding a final product of substantially similar quality to freeze-drying (FD).