Microwave drying of apple cubes: effect of the osmotic pre-treatment and comparison with hot air drying

The aim of the present work was to study the effects of the osmotic dehydration (OD) on the mass transfer kinetics of the microwave drying (MWD) and some quality parameters of apple cubes. The fit of several mathematical models was tested to describe the drying. A comparison between MWD and hot air drying (HAD) was performed. Apple cubes were osmotically treated with 60ºBrix sucrose or sorbitol solutions at 60ºC and dried at a microwave power between 160 and 850 W. Overall, the increase of the microwave power from 160 to 500 W resulted in an increase of the water loss rate, shown by the A parameter of modified Page’s model, of 123 and 155% for samples osmotically dehydrated with sucrose and sorbitol solutions, respectively. The OD tends to increase the drying rate, but the osmotic agent used did not have any effect. The modified Page's model described well the drying kinetics and MWD was faster than HAD. The water activity of the osmotically dehydrated product was reduced to below 0.4 after MWD at 500 W. The MWD of control samples resulted in a loss of 39% and 28% of the total phenolic content and the antioxidant activity, respectively. For osmotically dehydrated samples, a loss of around 75% was observed for both these quality parameters after the osmotic treatment.