HEATING UNIFORMITY EVALUATION AND IMPROVEMENT OF RADIO FREQUENCY TREATED PREPACKAGED FOOD

Radio frequency (RF) heating has the potential to inactivate pathogens without causing severe food quality degradation due to its fast heating rate and volumetric heating mode. However, heating uniformity is an obstacle that needs to be overcome before commercial applications. In a pasteurization process, underheating may lead to pathogen survival, which should be avoided, but severe overheating normally causes food quality deterioration. To minimize quality losses, RF heating uniformity should be improved. In this study, a new heating pattern of RF heating on low-moisture foods was researched to obtain better heating uniformity: the temperatures on the top surface and in the inner part of a sample were higher than the pasteurization temperature. The temperatures of other parts, including the bottom and side surface, might be lower than the pasteurization temperature, but this could be easily remedied by conventional heat transfer methods, such as a water bath, after RF heating. At the same time, the proportion of overshoot temperatures in the sample should be controlled as small as possible. To assess the heating uniformity, two new indexes (OTCI and TPD) were developed, which refer to the severity of overshoot temperatures and the targeted penetration depth that needs to be achieved by conventional heat transfer methods after RF heating, respectively. The new heating uniformity evaluation method considers that the temperatures between the pasteurization temperature and the control temperature in the treated material meet the requirement for pasteurization, and including these temperatures into a uniformity index does not help with developing a thermal pasteurization protocol. The study indicated that a better RF heating pattern could be achieved when the sample was placed close to the top electrode with an appropriately sized polyetherimide (PEI) block under the sample. For peanut butter loaded into a cylindrical polypropylene container (50 mm height, 100 mm i.d.) with a 25.5 mm height to the bottom electrode and an 84 mm electrode gap in a 6 kW, 27.12 MHz RF machine, adding a cylindrical PEI block with a thickness of 12.7 mm and radius of 40 mm below the peanut butter container led to the best heating uniformity among all studied dimensions, resulting in OTCI and TPD values of 0.0076 and 4.94 mm, respectively.