Application of Radio-Frequency Heating for Inactivation of Foodborne Pathogen

Abstract

The specific objectives of this study were, ( i ) to evaluate the efficacy of RF heating for inactivating foodborne pathogens, such as Salmonella enterica serovar Enteritidis, Typhimurium, and Senftenberg in raw shelled almonds compared to conventional convective heating as well as its effect on product quality, ( ii ) investigate the effect of salt content of samples, packaging material, and electrode gap on the antimicrobial efficacy of RF heating, ( iii ) evaluate the antimicrobial effects of the combination treatment of RF heating with ultraviolet (UV) radiation and organic acid spray against foodborne pathogens on dried foods, ( iv ) develop a computer simulation model and predict the behavior of RF heating in spice products. RF heating can be applied to control internalized pathogens as well as surface-adhering pathogens in raw almonds without affecting product quality. As the salt content of pistachios increased, treatment time required to achieve 4-log reduction of S. enterica decreased and then was maintained when the salt content exceeded a level corresponding to the peak heating rate. PEI film reduced the treatment time required to reduce S. Typhimurium and E. coli O157:H7 by more than 7 log CFU/g (below the detection limit, 1 log CFU/g) in red and black pepper powders. The dielectric constant of PEI film was similar to that of target sample, and the dielectric loss factor of PEI film was relatively low. The heating rate of the sample increased with decreasing electrode gap. RF heating for the treatment time required to reach 90 °C achieved 2.85-, 2.17-, and 2.09-log reductions of C. sakazakii without generating heat-injured cells at the electrode gaps of 8 cm, 10 cm, and 12 cm, respectively. The RF-UV combined treatment showed synergistic effects: the total microbial log unit reduction of the combined treatment was significantly (P < 0.05) different from the sum of the reductions obtained from individual treatments. Qualitative (transmission electron microscopy) and quantitative (leakage of intracellular substances and propidium iodide uptake) analyses provide evidence that damage to the cell membrane was identified as the main factor contributing to the synergistic lethal effect of the combination treatment of RF heating and UV irradiation. RF-UV combined treatment for 60 s did not significantly (P > 0.05) affect the color, moisture content, and sulfhydryl activities of powdered infant formula. As another available hurdle combination, combined treatment of RF heating and LA sprays for 40 s caused 4.94 and 5.48 reductions of S. Enteritidis PT 30 and S. Typhimurium, respectively. The RF-LA combined treatment did not change color and oxidative rancidity of almonds significantly (P > 0.05). A computer simulation was studied to predict the influence of various factors on the inactivation of foodborne pathogens on food samples by RF heating. A finite element-based commercial software, COMSOL Multiphysics, were used to predict electric potential, electric field distribution, and temperature distribution of red pepper powder during RF heating. The computer simulation model was validated by comparing with the experimental temperature profiles of powdered red pepper spices and applied to predict the effect of frequency, electrode gap, and dielectric properties of packaging materials on the antimicrobial effect of RF heating. The simulated results demonstrated that the efficacy of RF heating in reducing foodborne pathogens could be improved using a higher frequency, a bigger electrode area, a similar dielectric constant of packaging material as target sample, and a lower dielectric loss factor of packaging material. The results of this thesis are helpful to establish treatment conditions for maximizing the antimicrobial efficacy of RF treatment, and by extension, to commercial practical application of RF heating. The combination treatment of RF heating with other technology suggest alternatives to conventional decontamination treatments. In conclusion, application of RF heating in the food industry is expected to represent a novel and innovative thermal process for the production of safe foods.