Molecular dynamics simulation of the mass transfer process for the drying of fruit and vegetable porous media

A large number of nutrients (solutes) can be dissolved in the water inside the porous medium of fruits and vegetables. The migration of solutes can occur simultaneously with the transfer of wet content during drying. It is still lacking in the mass-transfer mechanism of drying in the internal micropores of fruit and vegetable porous media so far. It is a high demand to determine the influence of microporous structure characteristics of fruits and vegetables on the mass transfer process during drying. In this study, a systematic simulation was carried out for the mass transfer in the microporous media of fruits and vegetables during drying using molecular dynamics. The diffusion process model of smooth wall solution and the diffusion process model of rough wall solution were constructed after simulation. The SPC/E water molecular model was also selected under the OPLS-AA all-atomic force field and the regular ensemble. The solution potential function was established with the electrostatic Coulomb and Lennard-Jones interaction. Among them, the initial velocity of the central water molecule was given by the Gaussian distribution. The Velocity-Verlet algorithm was then used to update the position and velocity of atoms. The water molecules were fixed with the SHAKE algorithm. Periodic boundary conditions were applied in the x and y directions of the simulated aperture, whereas, the fixed wall boundary conditions were applied in the z direction. The diffusion process of the internal solution was simulated in the porous media of fruits and vegetables from the molecular level. The improved model was verified using the hot-air drying experiment of potatoes. The experimental value was the closest to the simulated using the KCl solution rough wall model, where the maximum relative error was 17.39%. There was the largest difference between the experimental and the simulated value of the pure water model. Therefore, the rough wall model was much closer to the real pore wall structure, without considering the influence of the presence of solute on the water diffusion coefficient. The radial distribution function demonstrated that K+ and Cl- posed damaging effects on the hydrogen bond structure of water molecules, while both K+ and Cl- shared two hydration layers. The H2O molecule was close to K+ with an O atom, and Cl- with an H atom. The important influences were obtained on the water diffusion coefficient in the pores, including the solute concentration, pore diameter, wall roughness factor, and phase area fraction. The diffusion process of KCl solutions with different mass fractions showed that the larger the solution mass fraction was, the smaller the water diffusion coefficient was. The optimal combination was achieved in the diffusion coefficient of the water during drying. Specifically, there was an increase in the pore diameter, drying temperature, and the area fraction of the rough wall, whereas, a decrease was observed in the roughness factor of the rough wall. The finding can also provide a theoretical basis to analyze the drying quality and process optimization of fruits and vegetables.