Impact of Microwave-Starch-Blanching on the Drying Kinetics, Transport and Thermophysical Properties of Green Almond

Abstract

This study aimed to investigate the effect of different pretreatment blanching methods on the drying characteristics of green almonds. Microwave blanching at 300, 450, and 600 W power in the water and/or starch solution (0.5% w/w), water blanching (95℃, 5 min), and starch blanching (0.5% w/w) were preferred. Non-blanched samples were considered as the control samples. All samples were dried at a constant air velocity of 1.5 m/s and temperatures of 70, 90, and 110°C. As drying time increased, moisture ratio of all samples exponentially decreased. Nonlinear regression analysis was used to fit the experimental data to drying models. Effective diffusivities, which ranged from 2.238×10−9 to 6.434×10−9 m2 /s were calculated using Fick's second law of diffusion. Activation energies were determined according to the Arrhenius equation and ranged from 12.32 to 15.39 kJ/kg mol. The highest diffusion coefficient was observed in the microwave starch blanched (600 W-110°C) samples. The highest increases in the diffusion coefficient and decreases in the drying time in comparison with control samples were observed in the microwave-starch (600W- 70°C) samples. Thermal conductivity, density, and specific heat of samples ranged from 0.544-0.586 (W/m K), 3643.85-3900.00 (J/kg K), and 835.80-899.44 (Kg/m3), respectively. The friction drag force, convective heat, and mass transfer coefficients varied from 3.965 to 3.972×10−6 N, 66.29 to 66.44 W/m2 K, and 0.03410 to 0.03428 m/s, respectively. The using microwave-starch blanching pretreatment can significantly decrease the drying time and improve the drying process of green almond at the industrial scale.