Effects of combined osmotic and microwave dehydration of apple on texture, microstructure and rehydration characteristics

The effect of microwave-assisted air-drying with or without osmotic pre-treatment on apple cubes was evaluated. Osmotic pre-treatment in sucrose solution was followed by microwave-assisted air-dehydration at different temperatures (50, 60 and 70 degreesC). The products were compared to the samples that did not undergo any pre-treatment. A puncture test and confocal laser scanning microscopy (CLSM) were used to analyse the effects of these processes on texture and microstructure. Rehydration in water and in yoghurt were studied and compared. Results showed that osmotic pre-treatment before microwave-assisted air-drying increased the final overall quality of the product. Although the drying time to reach 10% moisture content (wb) was reduced, the presence of infused sucrose in the osmotically dehydrated tissue decreased the drying rate during the microwave finish-drying. The effective moisture diffusivity was slightly lower for the osmosed samples than for the non-treated ones at all the studied temperatures. Osmotic pre-treatment had a beneficial effect on the firmness of the rehydrated samples that had been air-dried at 50 degreesC. Microstructural studies showed a thickening of the cell wall after the osmotic treatment. Structural changes in the cell wall due to the final drying could also be detected. Finally, the rehydration capacity in water was significantly higher for non-treated samples than for the pre-treated ones - although rehydration in yoghurt reduced this difference. (C) 2001 Academic Press.