Improving Temperature Uniformity Of Radio Frequency Heating Peanut Butter Based On Adjustment Of Dielectric Energy Distribution

The objective of this study was to improve the temperature uniformity of radio frequency (RF) heating low-moisture pasted agri-food material (peanut butter). According to the existing problems of RF heating:1) uneven power distribution; 2) unclear RF heating mechanism; 3) Uniformity Index does not meet the requirement of practical application; 4) difficult to obtain the volumetric temperature distribution, this research used approaches that developing a coupled electromagnetic field and heat conduction numerical model of RF heating agri-food; studying the power distribution from the material, size and shape of treated sample, the electrode gaps, media material and sample position respected to the electrode based on the validated model; proposing some effective methods of adapting and intervening the power domain on the basis of the features of RF power distribution; exploring the best heating protocol and parameters to RF heating peanut butter with desirable temperature distribution by using global optimization algorithm to surrogate model.The RF heating simulated model built in a commercial software based on finite element method, COMSOL Mutliphysics, was validated by comparison of simulation results and experimental results which were conducted with a 27.12 MHz,6 kW RF machine. Experimental temperature profiles of the top surface and the physical center point of sample obtained respectively by infra thermal camera and fiber optical sensor, were in good agreement with the simulated. Analysis based on the numerical model come to conclusions as below:1) the heating rate of RF treat low-moisture agri-food material has a Log linear relationship with the electrode gap, which is independent of material type, geometry shape and size. Though the electrode gap is an important factor to heating rate, it has small impact on temperature uniformity and heating pattern.2) It can not fundamentally change the heating pattern and effectively improve the temperature uniformity by changing the sample size, ratio of sample height to radius and ratio of vacant to sample.3) It has faster heating rate and better temperature uniformity for top surface or bottom surface of sample which is close to the electrode.4)The influence of Polyetherimide (PEI) on temperature distribution of peanut butter has localized feature; The side of sample close to PEI block increases temperature in the direction of height or radius as the thickness or radius of PEI increases, respectively. PEI is an effective mean for intervening to the power domain, since it has remarkable influence on the power distribution and the heating pattern.In order to change the RF heating pattern to achieve better temperature uniformity, Top and Inner First (TIF) heating pattern that the temperature both on the top surface and in the middle part of the sample were higher than the outer side of sample was proposed. For assessing the heating uniformity, two new indexes (DTI and TPD) were developed, which were referred to the severity of the deterioration and the targeted penetration depth needed to be fixed by conventional heat transfer methods after RF heating, respectively. As for peanut butter loaded in a cylindrical Polypropylene container (height:50mm, radius:50mm) with 25.5mm height to the bottom electrode, applying a cylindrical PEI block with thickness of 12.7mm and radius of 40mm on the bottom of the peanut butter could lead to the best heating uniformity among all researched dimensions, which results in DTI (the lower limit:70℃; the upper limit:75℃) and TPD of 4.8445% and 4.940 mm, respectively.Using global optimization algorithm to surrogate model, a perfection protocol based on the TIF heating pattern and its parameters were researched. The best parameters of PEI block optimized with the solution method of satisfied Pareto-optimal were that thickness 9.5 mm, radius 50 mm and height 12 mm. The perfection protocol improved the temperature uniformity greatly with DTI=0.9199% and TPD =3.975 mm.There were complementary heating patterns in RF heating peanut butter which were found in this study. For discussing a new approach to improve temperature uniformity, the strategy of RF combined heating with variable condition and muti-pattern (VCMP) was explored, which numerical model was developed by using resumption computation method. Result shows that RF heating with PEI block of thickness 25.4mm and radius 30 mm for 140s then VCMP to RF heating without PEI block for 110s could obtain PUI 0.172, which is better than non combined heating (RF heating without PEI assistance; RF heating with PEI block of 30 mm radius,25.4 mm thickness; and RF heating with PEI block of 40 mm radius,25.4 mm thickness) in temperature uniformity. The study indicated that RF combined heating with VCMP is an effective method for improving temperature uniformity.