Durian chips drying using combined microwave techniques with step-down microwave power input

Abstract

The microwave vacuum drying (MWVC) and combined microwave-hot air drying (MWHA) under step-down microwave power input were proposed for drying durian chips. These techniques were compared with multi-stage hybrid drying (MSH) and hot air drying (HA) on drying kinetics, specific energy consumption (SEC) of the drying process and qualities of the dried durian chips in terms of colour, shrinkage percentage, texture profile, water activity, and microstructure. Some properties were also compared with those of commercially-fried durian chips. The results showed that the drying rates of step-down MWHA and MWVC increased in line with increases in the microwave power used during the first stage of drying and with decreases in the vacuum pressure used in the step-down MWVC. These enhanced drying rates led to short drying times and resulted in less SEC in both heat and electricity. The durian chips dried by the step-down MWVC and MWHA had higher values of lightness, crispness, large pore size and void area fraction, but lower values in terms of hardness and shrinkage percentage than those dried by the MSH and HA. Increasing the microwave power in the step-down MWVC and MWHA, and the MSH gave the dried durian chips higher values of lightness, crispness, large pore size and void area fraction, but lower values of hardness and shrinkage percentage. The step-down MWHA is recommended for drying durian chips due to the highest drying rate and the most overall preferential qualities of dried durian chips, although less than the commercially-fried durian chips.