Application of near-infrared heating as an antimicrobial intervention for food safety

Abstract

The application of infrared (IR) radiation heating to food processing has gained momentum due to its inherent advantages over the conventional heating systems. Certain characteristics of IR heating such as emissivity and transmissivity set it apart from and make it more effective for industrial applications than others. IR radiation transfers thermal energy in the form of an electromagnetic wave and can be classified into 3 regions, near IR (NIR

0.76 to 2 μm), medium IR (MIR

2 to 4 μm), and far IR (FIR

4 to 1,000 μm). Among them, NIR heating has been gaining wider acceptance because of its higher heat transfer capacity and high energy efficiency compared with MIR and FIR heating. This thesis explored the potential and utilization of NIR heating as an alternative antimicrobial intervention for food safety. The specific objectives of this study were, (ⅰ) to investigate the efficacy of NIR heating to reduce major foodborne pathogens, such as Salmonella enterica serovar Typhimurium, Escherichia coli O157:H7, and Listeria monocytogenes in ready-to-eat (RTE) food (focused on deli meat product) compared to conventional convective heating as well as its effect on product quality, (ⅱ) develop and validate the inactivation kinetic models of the three major pathogens on RTE sliced ham by NIR heating, as a function of the processing parameter, radiation intensity, (ⅲ) investigate the effect of the simultaneous application of NIR heating and ultraviolet (UV) irradiation on inactivation of these pathogens in RTE sliced ham, (ⅳ) elucidate the underlying mechanisms of the synergistic bactericidal action of NIR heating and UV irradiation, (ⅴ) extensively apply the combined treatment to other food systems (dry powdered foods) with a mild NIR heat, and (ⅵ) evaluate the efficacy of organic acid spray along with NIR heating for inactivating Salmonella Enteritidis on dry nut kernel products. A cocktail of three pathogens was inoculated on the exposed or protected surfaces of ham slices, followed by NIR or conventional convective heating at identical conditions (1.8 kW). NIR heating for 50 s achieved 4.1-, 4.19-, and 3.38-log reductions in surface-inoculated S. Typhimurium, E. coli O157: H7, and L. monocytogenes, respectively, without affecting product quality whereas convective heating needed 180 s to attain comparable reductions for each pathogen. There were no statistically significant (P > 0.05) differences in reduction between surface- and internally inoculated pathogens at the end of NIR treatment (50 s). However, when treated with conventional heating, significant (P < 0.05) differences were observed at the final stages of the treatment (150 and 180 s). Thus, NIR heating can be applied to control internalized pathogens as well as surface-adhering pathogens in RTE sliced meats as an alternative to conventional heat treatment. For investigation of the inactivation kinetics as a function of radiation intensity, precooked ham slices inoculated with the three pathogens were treated at different NIR intensities (ca. 100, 150, and 200 μW/cm2/nm). The survival curves of the three pathogens exhibited both shoulder and tailing behavior at all light intensities. Among nonlinear models, the relationship between the scale & shape parameters (α & β values) of the Weibull model and applied radiation intensity was almost a straight line, and single linear equations for the three pathogens were obtained. The final predictive models (tertiary models) for the three pathogens were developed by substituting the secondary linear models into the Weibull primary model and allowed us to predict survival curves at NIR intensities different from those used in this study. The tertiary models were validated with data obtained from further experiments within the range of the experimental domain. The R2, RMSE, Bf, and Af values were within the acceptable range indicating the suitability of the model for predictive purposes. These results would be beneficial to the deli meat industry in selecting the optimum processing conditions of NIR heating to meet the desired target level of pathogen inactivation. Simultaneous application of NIR heating and UV irradiation at ca. 0.9 kW (half of NIR treatment alone) for 70 s achieved 4.17, 3.62, and 3.43 log CFU reductions of S. Typhimurium, E. coli O157:H7, and L. monocytogenes, respectively. For all three pathogens, the simultaneous application of both technologies resulted in an additional log unit reduction as a result of their synergism compared to the sum of the reductions obtained after the individual treatments. To investigate the mechanisms of NIR-UV synergistic injury for a particular microorganism in a food base, I evaluated the effect of four types of metabolic inhibitors using the overlay method and confirmed that damage to cellular membranes and the inability of cells to repair these structures due to ribosomal damage were the primary factors related to the synergistic lethal effect. The NIR-UV combined treatment for a maximum of 70 s did not alter the color values or texture parameters of ham slices significantly (P > 0.05). Additionally, simultaneous NIR-UV combined processing was employed for decontaminating red pepper powder (target pathogens: S. Typhimurium and E. coli O157:H7) and powdered infant formula (target pathogen: Cronobacter sakazakii). Many pathogens have been known to be more resistant in low-water-activity environments. Due to its ability to survive in dry food matrices, controlling pathogens in the final dehydrated product is of great concern to the food industry. NIR-UV combined treatment for 5 min achieved 3.34- and 2.78-log CFU reductions in S. Typhimurium and E. coli O157:H7 in powdered red pepper, respectively, and for 7 min achieved a 2.79-log CFU reduction of C. sakazakii in powdered infant formula without causing any deterioration in product quality due to the lower intensity of NIR. The sum of NIR and UV inactivation was lower than that obtained by the simultaneous application of both technologies due to their synergism. Through qualitative (transmission electron microscopy) and quantitative (propidium iodide uptake) analyses, disruption of the bacterial cell membrane was identified again as the main factor contributing to the synergistic lethal effect of NIR-UV combined treatment. The results of these extensive studies suggest that a NIR-UV decontaminating system can be applied as an alternative to other interventions in various kinds of powdered/granulated foods. As an another available hurdle combination, the efficacy of NIR heating combined with 2% lactic acid (LA) sprays for decontaminating dry nut kernels (almonds and pine nuts) was investigated. Although surface temperatures of nuts treated with NIR were higher than those subjected to NIR-distilled water spray (DW) or NIR-LA treatment, more S. Enteritidis survived after NIR treatment alone. The effectiveness of NIR-DW and NIR-LA was similar, but significantly more sublethally injured cells were recovered from NIR-DW treated samples. I confirmed that the enhanced bactericidal effect of the NIR-LA combination may not be attributed to cell membrane damage per se. NIR heat treatment might allow S. Enteritidis cells to become permeable to applied LA solution. Due to the lower levels of NIR and applied LA, combined NIR-LA treatments did not change quality attributes of nut kernels significantly (P > 0.05). Thus, the NIR-LA treatment may be a potential intervention for controlling food-borne pathogens on nut kernel products. The simultaneous NIR-UV or NIR-LA combinations have some advantages not only regarding the germicidal effect but also in terms of simplified handling, environmental preservation, and reduced costs through lower inputs of energy. Furthermore, the NIR or combined processing techniques can easily be expanded to practical industrial scale on a continuous basis. In conclusion, application of NIR heating in the food industry is expected to represent a novel and innovative antimicrobial process for the production of high-quality & safe foods at low cost.