Studies on Decreasing Energy Consumption for a Freeze-Drying Process of Apple Slices

The shape, color, flavor, and rehydration capacity of freeze-dried (FD) products are all better than other dried products. However, the energy consumption during FD is very high, which limits the application of this drying method for common materials. In this article, microwave vacuum drying (MWVD) was applied before or after FD to decrease the energy consumption during FD. Moreover, energy consumption was divided into two parts: valid and invalid energy consumption. Apple slices were used as an example to calculate the saving percentage of invalid energy consumption by comparing combination drying with FD. Apple slices freeze dried for 8.28 h first and then dried by MWVD have the best appearance, with a savings of 39.20% in invalid energy consumption. But apple slices freeze dried for 6 h first followed by MWVD have the highest savings of invalid energy consumption, 54.02%, while still maintaining an acceptable appearance.