The physical and chemical changes in the fruit slices during the pretreatment influenced the drying rates. According to the Brazilian Legislation, the maximum moisture content allowed for dried fruit is 0.3 kg water/kg DM. Thus, if melons are dried to final moisture content of 0.20 kg water/kg DM, which is less than the maximum moisture value allowed to this product, it will take around 47 min to dry the untreated (fresh) melons, while by subjecting the fruit to the pretreatments, the drying time could be reduced to 41 min (US and USVC, pretreatment liquid medium: distilled water) or to 44 min (VC, pretreatment liquid medium: sucrose solution), because the increase in water diffusivity, reducing the drying time up to 12.8% and 6.4%, respectively.
When the samples were immersed in distilled water, the pretreatment with ultrasound generally resulted in lower drying time. As observed by Fernandes et al. (2008b), the ultrasonic step promoted the formation of micro-channels, which reduce the water diffusion resistance and make easier for water to diffuse during drying. However, these authors verified that although pineapple presented micro-channels that might ease water diffusion during the air drying process, the water diffusivity of the osmotically dehydrated fruit decreased. This decrease could be explained by a higher sugar gain. Sugar may have entered into the micro-channel, saturating the channel and creating an extra resistance for water diffusion during air drying.
Thus, processing time can be optimized to reduce air drying time to a minimum, reducing costs and increasing overall productivity, and the pretreatment conditions US and USVC for distilled water and VC for sucrose solution, followed by air drying up to 0.20 kg water/kg DM, where then submitted to physico-chemical analysis