Conclusions
Microwave drying of apples at any constant temperature cannot balance the time, energy, and quality requirements. A high temperature consumed short time and less energy, but resulted in poor product quality. A low temperature resulted in good product quality, but consumed long time and more energy. Although drying apples at a middle drying temperature consumed intermediate time and energy, and achieved median product quality, they still have the potential to be improved.
Drying apples with a drying curve which followed a pre-designed linear line could improve product quality, save time and energy in a single process. A gradually increasing temperature (from 25 to 50 °C) in the beginning and middle drying stages could achieve a nearly linear drying curve, while real time curve control was omitted. A 3-step temperature control (30, 40, and 50 °C) could also improve the drying effects and greatly simplify the control system. Hence, a controlled drying curve and drying temperature are always recommended in microwave drying practices. Further study is recommended to investigate the microstructure change during different drying methods and in different stages during microwave drying.