Location: Characterization and Interventions for Foodborne Pathogens
2023 Annual Report
Objectives
Objective 1: Complete the industrial/commercial implementation of the Radio Frequency (RF) technology (with partners) to enhance the safety of shell eggs. Ensure that the technology is optimized, appreciating the complexity of the intervention process, the processing conditions, the equipment necessary, and the sensory and nutritional qualities of the food product.
Objective 2: Utilization of Cold Plasma, a novel non thermal technology to inactivate microbial contamination on various food products, which can include and is not limited to produce, nuts, meats and ready-to-eat foods. Optimize the technology to allow scale-up to commercial treatment levels, appreciating the complexity of the intervention process in terms of the food to be treated, the processing conditions, the equipment necessary, and the sensory and nutritional qualities of the foods types to be treated.
Sub-objective 2.A: Develop and optimize combined treatments of cold plasma with high-intensity pulsed light-based technologies and/or other antimicrobial processes, including chemical sanitizers to enhance microbial food safety and quality.
Approach
Our research will develop and optimize interventions for reducing the microbial load associated with produce, meats, shell eggs, and other food products. The ARS-patented radio frequency pasteurization (RFP) process for shell eggs is 3X faster and produces a higher quality egg than currently available commercial technologies. We will optimize the existing RFP process to improve efficiency and throughput, leading to a commercial-scale prototype. Research will focus on RFP operating parameters such as: power applied; RF pulse frequency and duration; electrode contact profile; cooling water conductivity, temperature, and flow rate. For many other foods, sanitizing technologies currently available fall short of desirable efficacy goals, such as the FDA’s target of a 5 log kill for fresh and fresh-cut produce. Cold plasma is a novel nonthermal sanitizing process which uses ionized air to inactivate pathogens on a variety of foods and food contact surfaces. We will build on our existing cold plasma expertise to develop and optimize applications, focusing on sanitizing protocols suitable for commercially promising commodities such as meats, fruits and vegetables, and low-moisture foods. We will optimize several different types of cold plasma in parallel, as dictated by the most suitable commodity/pathogen application. The treatments will establish commercialization potential for cold plasma as a standalone process and in combination with other antimicrobial treatments, including a primary focus of combining cold plasma with high-intensity light-based treatments. After we evaluate the combined intervention strategies for their effects on product quality and shelf-life, the most effective, practical treatment combinations will be transferred to industry to reduce the risk of foodborne illness. The outcomes of this project will be new validated means/technologies for producers, processors, and distributors to produce safer shell eggs, meats, fruits and vegetables, and other food related commodities.
Progress Report
For Objective 1, the radio frequency (RF) system for in-shell egg pasteurization has been optimized in several aspects, and the 4th generation is currently under validation. Electrode paddles are 3D printed in nylon to provide strength, electrical insulation, and optimized contact between the egg and the electrode. Upgraded connections are via industrial standard coaxial cable in a waterproof unit, replacing exposed silicone wiring, thereby improving durability, reliability, and cleanability. The internal temperature of the eggs is now monitored during processing in 2 target positions (yolk and albumen) using fiber optic probes inserted into the eggshell. The rotation of the eggs during processing on industry-standard silicone rollers allows for recording the temperature, more uniform treatment, and more uniform cooling via upgraded water spray nozzles. Previous iterations of the RF matching network achieved 25-35% efficiency, losing significant quantities of input power to waste heat. The new RF matching network was redesigned and optimized. Power transfer now is between 97.5-99.8%, meaning a more uniform heating of each treated egg and lower operating costs. During this optimization, some issues were faced regarding the corrosion of electrodes due to the interaction between water, salts, electricity, and metal. A series of optimization studies concluded that nickel-plated copper electrodes would provide higher electrical conductivity and greater corrosion resistance than stainless steel. A cleaning-in-place (CIP) component for the RF system is being evaluated using representative target surfaces for assessing biofilm formation: stainless steel, silicone, and nylon. The biofilm formation was evaluated with Salmonella cells in liquid whole egg at 37°C. Results showed the biofilm as a thick layer of protein with a high number of embedded cells after 24 h attached to the three materials. Nylon led the highest attachment of cells between the three materials. The removal of the biofilm is currently under evaluation. Using peroxyacetic acid at different concentrations and contact times (1, 5, 10 min) and proteases (ficin) to open the protein network and facilitate cell inactivation is showing encouraging results in biofilm removal compared to the traditional sodium hypochlorite treatment. The microbial validation of the RF-system 4th generation is currently under evaluation of 4 target spots to ensure complete inactivation of Salmonella regardless of the position inside the egg. The current inoculation spots are in the upper and lower albumen, an area close to the vitelline membrane and the center of the yolk. The assessment of egg quality is also evaluated after the RF processing. To validate the thermal resistance of a surrogate, avirulent strain of Salmonella Typhimurium in liquid whole eggs (LWE), the surrogate’s decimal reduction time (D-value) and decimal reduction temperature (z-value) were modeled with log-linear and Weibullian-based models in tests ranging from 52 to 60°C. The response values for the avirulent strain were similar to those of the pathogenic strain, thus validating this surrogate for use with the RF pasteurization of eggs. Heat-resistant strains and post-contamination events can lead to Salmonella in pasteurized LWE. The growth of Salmonella Typhimurium in LWE was assessed during storage at retail temperature (7°C) and abuse retail temperature (10°C) using nisin as a natural antimicrobial and ethylenediaminetetraacetic acid (EDTA) as a chelating agent. At both low (10^3) and high (10^5) inoculum loads, Salmonella was suppressed in LWE by nisin and EDTA during storage at 7°C, without significant changes in pH or color. The combination of hurdles and proper storage temperature can minimize the food safety risk of salmonella in pasteurized LWE. For Objective 2, combination of cold plasma (CP) with vortex jet cooling has previously been shown to effectively control excess heat buildup in enclosed systems used to treat seeds and nuts. Initial research with pecans has determined that CP treatment can inactivate Salmonella at levels comparable to those obtained with almonds, 1.1 logs following a 30 s treatment. Sensory evaluation of the color, texture, and aroma of the treated pecans shows no gross changes as a result of CP treatment. In these systems, vortex jet cooling can achieve cooling of 30-45°C below ambient temperature and is driven by the same compressed air plenum as the CP reactor head, resulting in a compact, efficient treatment system. The system is adjustable to allow for uniform CP treatment of nuts and seeds in a rotational tumbler, with the injected cold air from the vortex jet providing uniform temperatures. Further optimization of the combination treatment will use thermal impingement from the CP to augment and/or supplant external heating during drying, pre-roasting, or roasting phases of nut processing. Fr Subojective 2A, in research combining pulsed light (PL) and cold plasma (CP), Escherichia coli (E. coli) O157:H7 was inoculated onto romaine lettuce and subjected to various combination treatments. Short treatments with 10, 20 or 30 s of PL yielded reductions of 2.3, 2.5, and 2.6 log cfu/g, respectively. Comparably short treatments with CP of 15, 30, or 45 s reduced E. coli O157:H7 by 0.5, 1.2, and 2.1 log cfu/g. Combining the most effective treatments (30 s PL, 45 s CP) yielded synergistic reductions of greater than 5 log cfu/g, regardless of sequence (PL-CP or CP-PL). This greater than 99.999% reduction of E. coli O157:H7 persisted during storage at 4°C with no regrowth or recovery of the pathogen up to 7 days in storage. These results on the romaine lettuce model suggest this combination treatment as a fast, effective antimicrobial process for fruits and vegetables.
Accomplishments
1. Radio frequency pasteurization of shell eggs gets stronger, faster, and cheaper. Radio frequency (RF) treatment can effectively inactivate Salmonella inside shell eggs, making them safer for consumers. ARS researchers in Wyndmoor, Pennsylvania, have created a new, 4th generation version of the ARS-patented RF egg process that is more robust, more cleanable, and more efficient than ever before. The heart of the RF egg process is the electronic matching network that pulses power from contact electrodes into the shell egg, killing any Salmonella which might be present. A redesigned and optimized system, based on advanced power flow modeling and advances in materials science for the RF electrode assembly, has led to a 4th generation system which is now up to 99.8% efficient, commercially robust and durable, and hygienically designed for cleaning and maintenance. This 4th gen RF egg system has the potential to improve the safety of shell eggs for the American consumer.
2. Validation of a Salmonella surrogate in liquid whole egg. Because live pathogens can’t be safely used in all processing environments, researchers need non-pathogenic organisms which can stand in for those pathogens. USDA-ARS researchers in Wyndmoor, Pennsylvania, tested an avirulent strain of Salmonella Typhimurium for its thermal resistance in liquid whole eggs. Thermal treatment tests were conducted from 52 to 60°C to calculate the decimal reduction time (D-value) and decimal reduction temperature (z-value). Two mathematical models were used to calculate these values, a log-linear, and a Weibullian-based model. The required time to achieve the 99.999% reduction pasteurization standard (aka 5D) was calculated for the avirulent strain at 60°C. The responses of the avirulent surrogate were similar to those of the pathogenic strain. These results mean that studies of egg treatments, including the radio frequency pasteurization process, can be conducted more safely using this validated surrogate, rather than the pathogenic strain. Research will be easier, simpler, and safer, ultimately serving the research community and the American public.
3. Natural antimicrobials control Salmonella in liquid whole egg. Salmonella is sometimes found in pasteurized liquid whole eggs (LWE), due to heat-resistant strains surviving processing as well as to post-process contamination. ARS researchers in Wyndmoor, Pennsylvania, used a combination of natural antimicrobials and temperature control to minimize the risk of Salmonella Typhimurium. LWE samples were prepared with nisin (a natural antimicrobial) and EDTA (a chelating agent). After inoculating the amended LWE at two levels of Salmonella (low and high), the growth of the pathogen was assessed during storage at retail temperature (7°C) and abuse retail temperature (10°C). Nisin and EDTA, in combination with proper storage temperature, inactivated the Salmonella, regardless of the initial inoculum level and without causing significant changes in LWE pH or color. This hurdle combination can minimize the food safety risk of Salmonella in pasteurized LWE, thereby making this product safer for the American consumer.
4. Processing pecans with plasma prevents pathogen progression. Whole nuts like pecans can become contaminated with Salmonella and other pathogens. ARS researchers in Wyndmoor, Pennsylvania, in collaboration with ARS researchers at Byron, Georgia, are developing cold plasma-based treatments to make pecans safer. Research with pecans has determined that cold plasma, applied in short treatments of 30 seconds or less, can inactivate Salmonella by up to 95%. Sensory evaluation of the color, texture, and aroma of the treated pecans shows no gross changes as a result of cold plasma treatment. In these systems, the cold plasma is combined with advanced vortex jet-based cooling, resulting in a compact, efficient treatment system. The system is adjustable to allow for uniform cold plasma treatment of nuts and seeds in a rotational tumbler, with the injected cold air from the vortex jet providing uniform temperatures. This combination system has potential to improve the safety of pecans and other nuts and seeds.
5. Pulsed light sanitizes packaged fresh produce. Postprocessing contamination during packaging and handling is a major food safety issue, yet once packaged, treatment options for fresh produce remain quite limited. ARS researchers in Wyndmoor, Pennsylvania, have developed a high intensity pulsed light intervention that can penetrate the plastic packaging barrier and kill pathogens like Salmonella and Escherichia coli O157:H7 on fresh and fresh-cut produce. In laboratory tests, pulsed light was applied to inoculated produce through various thicknesses of packaging films. The pathogens were reduced by up to 99.9% following an optimized dose of 10.6 J/com^2. The technology demonstrated enhanced microbial safety and shelf life of fresh produce and provides a treatment suitable for application to packaged foods.
Review Publications
Mukhopadhyay, S., Ukuku, D.O., Jin, Z.T., Olanya, O.M., Fan, X. 2023. Evaluation of pulsed light treatment for inactivation of Salmonella in packaged cherry tomato and impact on background microbiota and quality. Journal of Food Safety. https://doi.org/10.1111/jfs.13035.
Bermudez-Aguirre, L.D., Niemira, B.A. 2023. Microbial inactivation models of Salmonella Typhimurium in radio frequency treated eggs. Food Control. https://doi.org/10.1016/j.foodcont.2023.109634.
Bermudez-Aguirre, L.D., Niemira, B.A. 2023. Modeling quality changes and Salmonella Typhimurium growth in storage for eggs pasteurized by radio frequency treatments. Food Control. https://doi.org/10.1016/j.foodcont.2023.109638.
Bermudez-Aguirre, L.D., Niemira, B.A. 2022. A review on egg pasteurization and disinfection: traditional and novel processing technologies. Comprehensive Reviews in Food Science and Food Safety. https://doi.org/10.1111/1541-4337.13088.
Bermudez-Aguirre, L.D., Niemira, B.A. 2022. Pasteurization of foods with ultrasound: the present and the future. Applied Sciences. https://doi.org/10.3390/app122010416.
