A vacuum microwave dryer (WZD4S-1, Nanjing, China for schematic diagram see Fig. 1), with a maximum nominal power of 4.2 kW and an operation frequency of 2450 MHz, was used in this research. The operation power is adjustable between 700 W and 4200 W at 700 W increments.The dimensions of the microwave cavity were 1050 · 1080 · 800 mm. The maximum vacuum degree is -95 kPa. Six plates in the cavity move round at 1 rpm during operation. A temperature-adjustable hot air dryer (SHT series, Shangyu, China) with steam flow tray was used to dry the samples.