Effect of vacuum impregnation and microwave application on structural changes which occurred during air-drying of apple

Apple slices, both vacuum impregnated (VI) with an isotonic solution and non-vacuum impregnated (NVI), were dried at 30 and 50 °C air temperature, with and without microwave (MW) application (0.5 W/g). MW application resulted in an increased water-soluble pectin fraction, especially in VI samples, ranging from 0.313 to 0.390 (expressed as g galacturonic acid/100 g fresh sample). The MW dried slices had a harder texture when dry (water content lower than 10 g water/100 g sample), but softer when rehydrated (to saturation): MW application also implied a slight increase (about 2 °C) of glass transition temperature (Tg) in the samples, especially in the NVI samples. In contrast to the NVI samples, in which Tg changes from about 4 to 32 °C when water activity increases from 0 to 0.22, the VI ones showed a very slight water plasticization effect (mean Tg 40 °C). The changes in Tg could be explained by the increase in the soluble pectin in the aqueous phase of the fruit during drying. There was no correlation between the mechanical properties and Tg.