Novel microwave-freeze drying of onion slices

The present study aimed to introduce the capability of a novel dehydration technique. To do so, slices with various thicknesses (3.5 and 7 mm) of white onions were dried using commercial freeze drier (abs. pressure 0.005 mbar, temp. 45 degrees C), our own designed microwave-vacuum drier (abs. pressure down to 300 mbar) under various microwave powers (120-1200 W) and microwave-vacuum-freeze drier (onion slices kept at -20 degrees C for 2 h). Then, their dehydration rates and some quality parameters, such as rehydration ratio, colour (L*, a* and b*) and micro-structure were investigated. Our findings showed that microwave-vacuum-freeze drier is practically a rapid, simple, efficient, economic and novel dehydration technique which can be used for dehydration of mainly foodstuffs. The quality properties of slices produced by this novel method were also completely comparable and competitive with commercial freeze drier with over 96% saving on processing time and enormous amount on energy and capital investments.