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The objective of this study was to examine how the drying kinetics and physical properties of apples are affected by pre-treatment with 95% ethanol or freezing at -18°C before microwave-assisted air dehydration at 50, 60 and 70°C. Microwave heating was used to obtain these temperatures in the centre of the apple cubes. After dehydration the shrinkage and rehydration capacity were measured. The texture of dehydrated and rehydrated samples was analysed with a puncture test in a texture analyser. Samples were also analysed with confocal laser scanning microscopy to determine the correlation between physical and microstructural properties. Diffusivity in the different dehydration processes was calculated. Ethanol-treated apples showed both high rehydration and high effective rehydration capacity compared with the other samples. Freezing before dehydration increased the diffusivity and reduced the firmness of rehydrated apples compared with no pre-treatment.

The effects on drying rate and texture of treating two plant tissues with calcium, before drying in air with microwave assistance, were studied in this work. The two tissues, potato and apple cubes, which have different structures and composition, were pretreated by immersion in CaCl 2 solutions at 20 or at 70°C before microwave-assisted air dehydration at 50, 60 and 70°C. The pretreatments with calcium influenced the strength of the plant tissue cell wall, producing products of varying hardness after rehydration. The effect of the two calcium pretreatments was quite different for apples and potatoes. For apples, calcium pretreatment at 20°C increased the hardness of rehydrated apples compared with untreated apples, but calcium pretreatment at 70°C had no effect on texture. For potatoes, both calcium pretreatments at 20 and at 70°C significantly increased the hardness of rehydrated potatoes. The water diffusivity during drying varied mainly because of the type of plant tissue, with secondary effects caused by the drying temperature and the type of calcium pretreatment.