Experimental procedure
2.4.1. Microwave vacuum drying
Before each experiment, the weighing system was set to zero under each vacuum condition. Fresh durian chips were placed on the two-level tray inside the microwave oven and the vacuum pump was turned on until the required vacuum was obtained by adjusting the control valve. The experimental conditions were performed at the microwave power of 150,200 and 250 W under the vacuum of 10 and 30 kPa. These microwave power settings were electronically controlled by the on-off time system of the magnetron operation. The temperature of the sample was continuously measured by the fibre optic sensor connected to the fibre optic thermometer with an accuracy of ±0.8◦C. Changes in the weight of the sample were recorded continuously during drying by using a load cell connected to a weighing indicator (AD-4329, Korea)with an accuracy of ±1 g. The energy consumption of the drying processes consisting of the vacuum pump and the microwave oven were measured by a three-phase-Watt-hour metre (Mitsubishi, Model MH-96H, Thailand) and a single-phase-Watt-hour metre (Mitsubishi, Model MF-33E, Thailand),respectively.