Ohmic Heating Soaking of Millets: Effects of Equilibrium Moisture Content, Soaking Time, Hydration Kinetics, Moisture Diffusivity, Activation Energy, and Surface Morphology

Abstract

The research focused on analyzing the hydration kinetics of Foxtail millet (Setaria italica), Proso millet (Panicum miliaceum), and Kodo millet (Paspalum scrobiculatum) at different soaking temperatures. Millets were studied at different ohmic heating (OH) temperatures (50°C, 60°C, and 70°C). Moisture data were recorded and fitted into hydration models like Page, logarithmic, modified Page, Henderson and Pébis, Lewis, Midilli, and Verma; and SEM microstructure of millets was observed. It was observed that the Foxtail millet soaked at 50°C and 60°C exhibited different moisture absorption patterns than soaked at 70°C. The equilibrium moisture contents (EMCs) of Foxtail millet soaked at 50°C, 60°C, and 70°C were 29.6%, 30%, and 34%, and the soaking times were 180, 120, and 60 min, respectively. The equilibrium EMC of millet soaked at 50°C, 60°C, and 70°C were 30%, 32%, and 35%, and the soaking times were 300, 210, and 180 min, respectively. Similarly, the EMCs of millet soaked at 50°C, 60°C, and 70°C were 27%, 29%, and 31%, and the soaking times were 360, 210, and 150 min, respectively. The hydration kinetics revealed that all seven models were well fitted for Foxtail and Proso millets; only six models were well fitted for Kodo millet, except for the logarithmic model. Among them, the Midilli model showed the coefficient of determination (R²), root-mean-square error (RMSE), and sum of square error (SSE) values for FM; for PM, the R², RMSE, and SSE values from the Midilli model were 0.98817–0.99944, 0.013278–0.008543, and 0.034743–0.000510, and for KM, the Verma model values R², RMSE, and SSE were 0.99519–0.99584, 0.02099–0.02133, and 0.00485–0.0036405 for 50°C, 60°C, and 70°C, which were more suitable. Moisture diffusion coefficients and activation energy ranged from 5.16 × 10⁻¹⁰ to 3.77 × 10⁻¹⁰ m².s⁻¹ and from 105.75 to 262.8 kJ/mol, respectively. The study implies that the OH assisting soaking enhances the hydration kinetics of millets.