Application of Pulsed Ohmic Heating for Inactivation of Foodborne Pathogens

Abstract

Pulsed ohmic heating has attracted attention as one of the novel thermal technologies enabling rapid and uniform heating. In pulsed ohmic heating, heat is generated inside of food by means of electric current passing through the food component different from conventional heating which heat transfer is implemented with conduction and convection. Even though foodborne pathogens can be inactivated effectively by pulsed ohmic heating compared to conventional heating, food sample still can be damaged by a high temperature of pulsed ohmic heating processing. In this regard, pulse ohmic heating should be optimized before applied in the food industry to the consumer's preference for minimum processing. Applicability of pulsed ohmic heating for inactivation of foodborne pathogens was identified in this thesis, and specific objectives of this study were, (i) to evaluate pulsed ohmic heating technology for inactivation of foodborne pathogens, (ii) to combine pulsed ohmic heating with other sanitizing technologies, (iii) to identify combined inhibitory effect of milk fat and lactose for inactivation of foodborne pathogens by pulsed ohmic heating, (iv) to develop multiphysics model of pulsed ohmic heating for inactivation of foodborne pathogens in tomato juice, and (v) to develop and apply continuous-type pulsed ohmic heating system. <br/> First, it was investigated whether foodborne pathogens can be inactivated effectively by pulsed ohmic heating without causing electrode corrosion and quality degradation of food sample. Buffered peptone water and tomato juice inoculated with pathogens (Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, Listeria monocytogenes) were treated with pulsed ohmic heating at different frequencies (0.06 – 1 kHz). Foodborne pathogens were inactivated effectively by pulsed ohmic heating. Moreover, electrode corrosion and quality degradation of tomato juice were not observed regardless of frequency. Therefore, pulse waveform was used in subsequent studies.<br/> Because high temperature by individual treatment of pulsed ohmic heating can damage the food quality, developed pulsed ohmic heating was combined with various essential oil components or UV-C irradiation to reduce the heating temperature and time. Carvone, eugenol, thymol, and citral, which are registered for use as flavorings, were chosen to combine with pulsed ohmic heating. Combination treatment of pulsed ohmic heating with citral showed the most synergistic bactericidal effect against foodborne pathogens in buffered peptone water followed by thymol, eugenol, and carvone. Cell membrane destruction by combination treatment and the loss of cell membrane potential by essential oil components were proposed as the bactericidal mechanism. When applied in salsa, inactivation of bacterial pathogens was the greatest for the combination treatment with thymol followed by with citral, eugenol, or carvone. Because color (b* values) of salsa were improved by combination treatment with thymol compared to pulsed ohmic treated samples, the combination treatment can be used effectively to pasteurize salsa. Combination treatment of pulsed ohmic heating with UV-C irradiation also showed a synergistic effect for pathogen inactivation. Cell membrane damage increased in all three pathogens synergistically with the simultaneous treatment, while an additive effect was observed for lipid peroxidation values. Therefore, the proposed synergistic bactericidal mechanism of the simultaneous treatment consists of an acceleration of lipid peroxidation, which results in a synergistic effect on cell membrane pore formation. Sequential treatment of UV-C irradiation after pulsed ohmic heating showed a less bactericidal effect than the reversed sequential treatment or the simultaneous treatment in buffered peptone water. Heat shock proteins expressed after pulsed ohmic heating and recovery process after UV-C irradiation were supposed to contribute this phenomenon. On the other hand, the reductions in the levels of all three pathogens in tomato juice were not significantly different between the simultaneous and sequential treatments regardless of the treatment sequence (p > 0.05), and the color and lycopene content of tomato juice were not significantly deteriorated by the simultaneous treatment (p > 0.05). Therefore, not only combination treatment of pulsed ohmic heating with thymol but also with UV-C irradiation can be used as an effective hurdle technology ensuring microbiological safety. <br/> An empirical model was developed to predict the inactivation of foodborne pathogens by pulsed ohmic heating depending on the fat and lactose content (%) in milk. The combined effect of fat and lactose content was analyzed by response surface methodology with a central composite design. Both lactose and fat had an inhibitory effect on the inactivation of all three pathogens by pulsed ohmic heating. Inactivation of E. coli O157:H7 has a quadratic relationship with lactose and fat, whereas the cross product of treatment time with fat or lactose has a significant effect on the inactivation of S. Typhimurium and L. monocytogenes (p < 0.05). The developed model predicted the inactivation of all three pathogens well within the range of experimental conditions, and color change and lipid oxidation were not observed following pulsed ohmic heating. Even though pH values decreased slightly after treatment, the changes would not affect the product quality. Therefore, treatment conditions of pulsed ohmic heating should be decided carefully considering the nutrient content and type of pathogens when using this method to pasteurize milk.<br/> Multiphysics modelling was conducted to analyze the pulsed ohmic heating more precisely. When pulsed ohmic heating processing of tomato juice was analyzed using multiphysics software, a cold spot was observed in the lower part of the pulsed ohmic heating chamber, where some pathogens survived even though all pathogens were inactivated elsewhere of the ohmic heating chamber. The developed computational simulation model was verified for heating rate and pathogen inactivation. Furthermore, inactivation of acid-adapted foodborne pathogens, the heat resistance of which increased significantly, was predicted by the developed simulation model and validated with no significant differences between predicted and experimental results (p > 0.05). Therefore, juice processors can utilize a multiphysics model effectively to adjust processing time to achieve 5 log reductions of foodborne pathogens under the environmental conditions in which heat resistance of pathogens could be altered.<br/> Even though the batch-type pulsed ohmic heating apparatus was used effectively to identify the characteristics of pulsed ohmic heating in the previous chapters, it is well known that continuous-type apparatus ohmic heating is more advantageous for bulk handling of juice products in the food industry. Therefore, continuous-type pulsed ohmic heating was developed, and effects of flow rate, voltage, and preheating on the heating rate and pathogen inactivation were identified. Both heating rate of samples and reduction rates of pathogens increased corresponding to decreased flow rate or increased treatment voltage. Preheating was used as an alternative way to prevent accelerated heating rate by increased voltage, and help inactivate pathogens in the early treatment stage without affecting the heating rate. Because preheating with additional equipment is inconvenient and occupies valuable space, sequential three-cylinder type pulsed ohmic heating was developed. By applying the developed sequential pulsed ohmic heating, 5 log reductions were achieved for all three pathogens without preheating under the same treatment conditions. Therefore, sequential continuous-type pulsed ohmic heating can be effectively utilized to control foodborne pathogens in the juice industry.<br/> In conclusion, the performance of pulsed ohmic heating for inactivation of foodborne pathogens varied significantly depending on the type of treatment to be combined, nutritional conditions of a food sample, and device type. Therefore, it is recommended to optimize pulsed ohmic heating to inactivate foodborne pathogens without quality degradation of food. Moreover, multiphysics modeling can be used effectively in optimizing the pulsed ohmic heating processing.<br/>