Convective Drying of Apple Peel as a Preservation Procedure in Fruit Waste Valorization

This study investigated the convective drying process of apple peels, focusing on drying kinetics, energy consumption, and product quality. The apple peels were dried under forced convection at temperatures of 40, 50, 60, and 80 °C, and with microwave assistance of 100 W. The key process parameters that were evaluated included the drying rate, the diffusion coefficient, the activation energy, and the specific energy consumption. Quality of the products was assessed based on moisture content, water activity, overall color change, browning and yellowness indexes, and ascorbic acid content. The results showed that all drying processes occurred during the falling drying rate period. The drying rate and effective diffusion coefficient were positively correlated with the temperature of the drying agent and/or microwave assistance (growth from 2.22 × 10−3 to 7.25 × 10−3 g/s and from 2.64 × 10−10 to 7.80 × 10−10 m2/s respectively). The drying time was negatively correlated with those process parameters (a decrease from 5300 to 1800 s). The lowest energy consumption was recorded for convection-microwave drying (~81 MJ per kg of evaporated moisture). The water activity of the peels dried at lower temperatures (40 and 50 °C) or with microwave assistance (CVMW) was about 0.4 and about 0.2 for those dried at higher temperatures (60 and 80 °C), ensuring microbial stability. Products that dried convectively at 50 and 60 °C, as well as those dried with the assistance of microwaves, showed a slightly higher overall color change compared to products dried using other processes. Ascorbic acid retention was similar in convectively dried products but decreased significantly in the convective-microwave process. In consideration of the positive effects on drying kinetics and the absence of adverse effects on the energy intensity of the process and the quality of the products, it was concluded that high-temperature drying can be the preferred method of preservation of apple peels in the course of valorization.