MICROWAVE DRYING VERSUS HOT-AIR DRYING OF MIXED APPLE AND GINGER POMACE

Apple and ginger pomace is a by-product that can be valorised by drying and further grinding it into flour to be used in cakes, for example, adding fibre and phenols, with antioxidant and antimicrobial properties, to their composition. Drying is one of the most energy- and time-consuming processes and must be optimised to be more sustainable. The pomace was subjected to drying through two different methods: hot-air drying (HAD; Excalibur, model 4900, U.S.A.) and microwave drying (MWD; Samsung, MS23K3513AW, Ireland). HAD was performed at 45, 62, and 70 °C, whereas MWD was carried out at 100, 180, and 300 W. Mathematical models were tested to fit the kinetics of the moisture ratio (MR); the drying time was predicted through the models and the specific energy consumption was calculated for each experiment. Crank’s model presented a good fit (R² > 0.977; χ² < 13.5E-03 RMSE < 11.0E-02) of the HAD kinetics, with an apparent diffusivity coefficient between 2.28E-10 and 4.83E-10 m²/s and an energy of activation of 23.9 kJ/mol; meanwhile, Midili’s model presented a good fit (χ² < 9.5E-05; RMSE < 9.1E-03) of the MWD kinetics. The drying time was calculated to achieve 12 % moisture content using these models with the parameters determined for each experiment. It resulted in being between 125 and 236 min for HAD and between 14 and 61 min for MWD, being 4 to 9 times lower in MWD. The specific energy required was 410.78–453.75 kWh/kg for HAD, whereas it was 1.32 – 2.26 kWh/kg for MWD. Due to its low energy consumption and short drying time, the MWD is more promising for drying apple and ginger pomace than HAD, reducing the environmental impact of the drying process.