Dielectric properties of water relevant to microwave assisted thermal pasteurization and sterilization of packaged foods

Microwave assisted thermal sterilization and pasteurization systems, as emerging thermal processing technologies, use circulation water to reduce edge heating of the food packages in the microwave heating sections. Yet, the influence of the circulation water on microwave heating efficiency for food is unknown. This research studied the dielectric properties of purified water from a reverse osmosis unit and tap water over microwave frequencies of 150-2500 MHz at 23-120 degrees C and evaluated microwave loses in the circulation water. The values of the dielectric constant of purified and tap water decreased (P < 0.05) with increasing temperature and slightly decreased with increasing frequency. The value of the loss factor of purified water also decreased with temperature (P < 0.05) but increased with frequency. The trend was more complicated for the loss factor of tap water. The penetration depth of 915 MHz microwave power in purified water was 399 and 457 mm at 90 and 120 degrees C, respectively, it was reduced to 161 and 136 mm in tap water. The microwave power loss in the 18-30 mm deep purified water, depending on the thickness of the food packages, was 4-7%, compared to 11-20% in tap water. Thus, using purified circulation water in microwave assisted thermal sterilization (MATS) and pasteurization systems (MAPS) can significantly improve the efficiency of microwave heating of packaged foods. Industrial relevance: The information from this research are highly relevant to industrial operations of microwave-assisted thermal sterilization and pasteurization systems of pre-packaged foods. The dielectric property data will contribute to the literature related to the physical properties of water in microwave heating.