Thawing food materials is important in terms of hygiene, energy consumption and thawing duration. There are many methods for food thawing and, in recent years, various methods have been researched, including high pressure, microwave, ohmic, acoustic, pulsed electric field, radio-frequency thawing, and thawing using magnetic nanoparticles plus heating. The use of high voltage electrostatic field is one of the new methods of non-thermal thawing and among its advantages over conventional methods is less energy consumption and speed up the process. In this study a laboratory continuous HVEF thawing system was designed in CATIA software and it was constructed in accordance with standards for food machinery. The high voltage electrodes in this system were designed as wires and plate type and in this system a stainless-steel sheet was utilized as the conveyor belt. An electric field was created between the two poles by applying voltage to the electrodes. It was caused by a secondary flow called the corona discharge, which causes turbulence in the air boundary layer on the surface of the tylose gel, which will increase the heat transfer coefficient on the surface ambient. Thawing was performed by placing the frozen material on the surface of the conveyor. Corona flow was created by applying high voltages of 15, 20, and 25 kV and electrode gaps of 6, 8, and 10 cm. To eva‎luate the system, frozen tylose MH4000 gel with a moisture content of 77% w.b. as a meat analog was thawed from −10 to 0 °C. The experiments were performed under a factorial split plot in time in a completely randomized design. Increasing temperature to 0 °C was the thawing criterion. The thawing duration which was mostly devoted to increasing the temperature of the control sample (no electrostatic field), considerably decreased when thawing the gel in the presence of the high voltage. High voltage and EG indicated a significant effect on both thawing duration and specific energy consumption (p˂0.0001). The thawing duration decreased significantly by decreasing the electrode gap and increasing the applied voltage. In terms of energy consumption, the best condition (minimum specific energy consumption of 10.33 kJ kg-1) was obtained for the voltage of 15 kV, and the electrode gap of 10 cm. The governing equations including electrostatic equations, equations of the volumetric force applied on the fluid domain from the electrostatic field, Navier-Stokes equation and the effect of volumetric force on the heat transfer of Taylos gel were investigated. Considering governing equations and, according to the geometry conditions in the present study, appropriate boundary conditions were selected and a part of the system was simulated in the Comsol Multiphysics 5.6 software. The physics of electrostatics, electric currents, laminar flow, heat transfer in fluids and solids were used to the connection of electrostatic field (corona discharge), air-flow, turbulence of the boundary layer on the top surface of the gel, heat transfer into the gel was used to the thawing the gel. To eva‎luate the simulation results, the correlation between experiment temperatures and simulation temperatures were investigated and results showed that the trend of temperature changes in laboratory measurements and simulation is the same. In addition to examining the correlation of the data, bias, root mean square error (RMSE) and coefficient of explanation were used as the validation criteria for the simulation results. The results showed that the simulations at distances between the two poles of 8 and 10 cm at all applied voltages were moderately to well matched with the laboratory data. While the weakness of the simulation model was found for the distance of 6 cm between the two poles. The observed slight differences between the simulation and laboratory results due to systematic error, simplifications performed in the computational parameters, and assumptions were considered.

مرتضي صادقي، امين اله معصومي

ناصر همدمي، علي اكبر عالم رجبي

احمد ميره اي، محمد مهدي مجيدي، بهرام حسين زاده ساماني