A thermodynamic model for hot air microwave drying of orange peel

• A thermodynamic model for microwave-hot air drying was developed.
• Industrial citrus waste treatment was studied to produce by-products.
• Shrinkage-swelling phenomena produced by MW-HAD were described and predicted.

The citrus juice industry produces a great amount of waste that needs innovation and development to become products. There is a continuous demand to develop innovative approaches for the valorization of citrus by-products by applying environmentally and economically sustainable processes. One of the critical steps for by-products stabilization is the drying operation. The aim of this work was to develop a thermodynamic model for understanding internal heating and water transport mechanisms occurring from the inside to the outside of orange peels during hot air-microwave drying, and to predict the chemical and structural transformations. Different microwave energies (2, 4 and 6 W/g) combined with hot air (HAD) at 55 °C were used for drying citrus peels (5, 15, 40, 60 and 120 min). Mass, volume, surface, water activity, moisture, and permittivity were measured in fresh and dried samples. A thermodynamic model was developed to explain the mechanisms involved in mass and energy transports throughout the combined drying by hot air and microwave. This model allows optimizing the traditional hot air drying, by coupling microwave, of orange peel waste as a novel process for citrus by-products valorization, reducing the process time and therefore process costs.