Evaluation of Diffusion and Azuara Models for Mass Transfer Kinetics during Microwave-Osmotic Dehydration of Apples under Continuous Flow Medium-Spray Conditions

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Abstract
Azuara and diffusion models were evaluated for describing the mass transfer kinetics of apple (Red Gala) cylinders during microwave-osmotic dehydration under continuous-flow medium-immersion (MWODI) and medium-spray (MWODS) conditions as well as immersion (CODI) and spray (CODS) conditions without the microwave heating. Two different sets of experiments were carried out with all four methods. In the first set, osmotic treatments were given at 50°C/50°Brix and 40°C/40°Brix with a solution flow rate of 2800 mL/min and fruit-to-solution ratio of 1:30. The treatment times ranged from 0 to 120 min. In the second set, the MWODS was extended to other conditions (40°C–50°Brix and 50°C–40°Brix). The equilibrium moisture loss and equilibrium solid gains required for the diffusion model were predicted using the Azuara model. Both models well fitted the experimental data for mass transfer kinetics (R2 > 0.92). Higher equilibrium moisture loss and lower solid gain were observed in samples treated with MWODS compared with other methods. The equilibrium moisture loss and solid gain under MWODS were related to solution concentration and solution temperature. The diffusion coefficients representing moisture loss (Dm) and solid gain (Ds) were computed from the diffusion model. The Dm values for were higher and Ds values were lower with MWODS as compared to the other methods. Dm and Ds were dependent on temperature and concentration of the osmotic solution. Half-drying time for moisture loss and solids gain were also computed to compare the different methods. These were inversely related to diffusivity values. Overall, the highest moisture loss and the lowest solids gain were observed in MWODS.

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