Microwave heat treatment of apple before air dehydration - effects on physical properties and microstructure

Golden delicious apple cubes were heated with microwave energy of high inte nsity (20 W/g), as a pre-treatment before air-dehydration at 40 degrees C, 60 degrees C and 80 degrees C. After the microwave treatment extending for 0.75 up to 5 min, the cubes were finish-dried with only forced air at 2 m/s . Dehydrated and rehydrated samples were analysed with a puncture test usin g a texture analyser. The microstructure of the samples was studied with co nfocal laser scanning microscopy (CLSM). Dried apple pieces were harder and more shrunk when pre-treated with microwaves, compared to only air dehydra tion. Despite the shrinkage and increased firmness, the rehydration capacit y of microwave "blanched" apple cubes was higher than when they were only a ir-dehydrated. Image analysis of the rehydrated apples showed that large vo ids (up to 0.5 mm in diameter) had developed, and that these increased with air temperature. Higher magnification of the images revealed cell separati on and disruption of cell walls, caused by the microwave heating. (C) 2000 Elsevier Science Ltd. All rights reserved.