Application of 915 MHz Microwave Heating for Inactivation of Foodborne Pathogen

Abstract

Microwave heating is a form of dielectric heating, generating thermal energy inside food material. In comparison to the conventional heating, as microwave heating shows rapid heat generation and improved heat distribution, microwave can heat food product more effectively with less quality deterioration. There are several factors influencing microwave heating pattern including food composition, packaging material, microwave frequency, and microwave power. Among various factors, research concerning dielectric properties and packaging material has been limited. Thus, in this study, I studied the effect of dielectric properties of food product and plastic packaging material on 915 MHz microwave heating. Also, in order to improve 915 MHZ microwave heating efficiency, I investigated which essential oil can be utilized to induce synergic effect in inactivation of foodborne pathogen. First, I studied the effect of sugar contents on the dielectric properties of hot-chili sauce, and further heating rate when exposed to 915 MHz microwave heating. Sugar contents did not affect the heating rate of hot-chili sauce when subjected to conventional heating (P>0.05), while it is important factor when treated with 915 MHz microwave heating (P<0.05). Under 915 MHz microwave energy, hot-chili sauce with lower sugar contents showed faster heating rate due to higher dielectric constant and dielectric loss factor. This trend linked to inactivation of Escherichia coli O157:H7, Salmonella. Typhimurium, and Listeria monocytogenes. When hot-chili sauce with higher dielectric properties were applied to 915 MHz microwave heating, it took shortest time to eliminate foodborne pathogen under detection limit. Also, I investigated the effect of plastic packaging material on the heating pattern of 915 MHz microwave heating. I utilized PE, PP, Nylon and PEI. PEI had highest value of dielectric constant and dielectric loss factor in this study. No significant difference in heating rate was observed among PE, PP and Nylon (P>0.05), but PEI showed slowest heating rate. Also, this trend linked to inactivation efficiency. PE, PP and Nylon showed better inactivation of foodborne pathogen compared to PEI. As microwave heating is thermal treatment, it can cause quality deterioration. In addition, in mass production, non-uniform heating can be one of problem in food processing, though 915 MHz had improved uniform heating distribution in comparison to 2450 MHz microwave heating and conventional heating. Thus, I researched the method to improve inactivation efficiency of 915 MHz microwave heating. When 3 mM of essential oil including carvacrol, eugenol, carvone, and citral was added to hot-chili sauce, no significant difference in dielectric properties and heating rate was observed. However, carvacrol showed most effective inactivation efficiency among various essential oil. I could observe that individual carvacrol treatment and combination of microwave and carvacrol treatment showed higher value of PI than other essential oils (P<0.05), indicating that these treatment causes most severe damage to cell membrane. This is because carvacrol induce destruction of cell membrane potential as indicated by the results of DiBAC4(3). Carvacrol treatment showed highest value. Thus, I can conclude that dielectric properties and packaging material is important factor for 915 MHZ microwave heating and 915 MHz microwave heating can be improved with addition of natural antimicrobial compound.