Study of the effect of microwave power coupled with hot air drying on orange peel by dielectric spectroscopy

•Sorption isotherms of orange peel dried by HAD + MW at different power were obtained.•A dielectric isotherm to predict the aw in dried samples by using ε' (20 GHz) was developed.•A strong correlation between GAB and dielectric isotherm parameters was found.•The application of MW power increases the adsorption energy of the monomolecular layer.

Monitoring moisture and water activity during drying is crucial for process optimization, avoiding inadequate uses of energy. The main objective of this work was to study the dielectric properties of orange peel during hot air drying at 55 °C (HAD) and microwave power coupled with hot air drying at different power intensities (2 W/g, 4 W/g and 6 W/g). At 5, 15, 40, 60 and 120 min mass, aw, moisture, and permittivity were measured in fresh and dried samples. Results allowed developing a dielectric isotherm technique by adapting the GAB model to predict aw in dried orange peel by using ε' (20 GHz). The physical meaning of the dielectric isotherm parameters (ε0' and Cd) was studied. The value of ε0' at 20 GHz (γ-dispersion) represents the induction effect of the minimum quantity of adsorbed water or the monomolecular moisture layer. The parameter Cd is related with isosteric heat, as well as the C parameter of the GAB model. The application of MW power produced an increase of isosteric heat or adsorption energy of the monomolecular layer, improving surface tension of samples and thus the hygroscopicity, explaining the reduction of the ε0' independently of the quantity of the water molecules adsorbed.