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As stakeholders in food safety know, there is no “silver bullet” technology that will eliminate pathogens from the food supply. However, in the past several years significant advances have been made, both in improving existing intervention tools and in developing novel microbial inactivation technologies. Particular attention has been focused on combining multiple intervention technologies to inactivate pathogens in foods, commonly known as the hurdle approach, and some of these successful hybrid strategies promise a silver lining in the industry’s food safety arsenal, if not the elusive bullet.
Intervention technologies, whether thermal, nonthermal or chemical in nature, are designed to provide a significant inactivation or inhibition to a microbiological population and may be used as a kill step to enhance or ensure the safety of foods. At the Food Safety Intervention Technologies Research Unit (FSIT)—part of the U.S. Department of Agriculture Agricultural Research Service’s (USDA ARS) Eastern Regional Research Center (ERRC) in Wyndmoor, PA—researchers’ work in the area of intervention technology development is improving food safety by enabling the food industry to make better decisions about how to reduce or eliminate foodborne pathogens in food processing operations. The FSIT unit is part of the Food Safety Research Program within USDA ARS, which employs approximately 260 scientists involved in 77 long-term research projects and 100 short-term projects in the areas of pre-harvest and post-harvest food safety.
FSIT conducts basic and applied research in food chemistry, food microbiology, food irradiation, food technology and engineering in support of regulatory needs and to improve the safety of meat, poultry, fresh and fresh-cut fruits, vegetables, sprouts and juices while retaining the desired quality attributes. The primary objectives are to develop new processes to improve the safety of the food supply and to determine the efficacy and suitability of new biological (i.e., competitive-exclusion), chemical, and non-thermal physical technologies (i.e., ionizing irradiation, vacuum-steam-vacuum, microwave, or electric field) for the decontamination of these foods.
Currently, the staff of 30 scientists at FSIT focus on the following three research goals to meet these overall objectives:
1. Development and verification/validation of processing technologies to serve as food safety interventions in the production of foods and beverages.
2. Determination of the effectiveness and efficacy of microbial intervention technologies and treatment combinations (multiple hurdle approach) in extending shelf life, ensuring safety and maintaining the quality of food products.
3. Transfer of effective decontamination technology to the food industry to reduce the risk of foodborne illness.
Intervention technology research and development at FSIT is informed by communication and collaboration with two groups of stakeholders in food safety. First, FSIT works very closely with other regulatory agencies when interest areas overlap, such as the USDA Food Safety and Inspection Service (FSIS) in the areas of meat and RTE foods and liquid egg products, and the U.S. Food and Drug Administration’s Center for Food Safety and Applied Nutrition (FDA CFSAN) on its regulatory or data needs in the areas of produce and shell eggs. The second group of stakeholders is the food industry itself. Each year, FSIT hosts stakeholder meetings to help identify industry needs in the areas of food safety research. In January 2007, nearly two dozen food companies were represented at the meeting, assisting researchers in refocusing or adjusting research directions for future projects. FSIT also conducts collaborative research with food companies, the majority of which involve technology transfer.
Many of FSIT’s latest scientific investigations into the effectiveness of food safety intervention technologies are showing new opportunities for application and implementation of nonthermal and advanced thermal methods in food processing facilities. Here, we’ll touch on some of this current research and discuss the tools that are particularly promising for the food processor’s use in the quest for enhanced food safety.
Hot Intervention Technology
Traditionally, food manufacturers have utilized thermal processing technology to inactivate or reduce microbial populations in or on food products. These “heat and kill” sterilization systems include high-temperature, short-time pasteurization and ultra-high temperature (UHT) processing designed for processing of liquid foods, and retorting or canning for processed solid foods. While technologies based on thermal treatment of foods have been around for many years, there are a few that are considered emerging or novel. These include radio frequency, microwave, ohmic and infrared heating during processing.
Microwave and Radio Frequency Electric Field (RFEF) Processing. Microwave and radio frequency electric field heating refers to use of electromagnetic waves of certain frequencies to generate heat in a material. Microwave and radio frequency heating for pasteurization and sterilization may be preferable to conventional heating processes because they require less time to rise to the desired process temperature, particularly for solid and semi-solid foods in the case of microwave processing.
Studies show that the critical factors affecting the effectiveness of microwave heating are the size, shape, composition (moisture, salt, and so on), multiple components (as in a frozen dinner), and liquidity/solidity of the food being treated; the presence of metallic elements in the packaging, such as aluminum foil or susceptor; the power level used, cycling, presence of hot water or air around the food and equilibration time used in the process; and even the dimensions, shape and other electromagnetic characteristics of the oven itself, including the method of agitation of the food and the presence of mode stirrers and turntables. Time is also a factor in the sense that, as the food heats up, its microwave absorption properties can change significantly and the location of cold points can shift.
Although industrial microwave pasteurization and sterilization systems have been reported for three decades, commercial radio frequency heating systems for the purpose of food pasteurization or sterilization are not yet fully commercialized. However, ARS ERRC scientists have conducted several studies into RFEF technology, subjecting liquid foods to its high electric fields, and found it to be both efficient for pathogen inactivation and cost-effective. In one study, FSIT scientists applied RFEF for 3 seconds at 60C to an apple juice sample inoculated with E. coli. The electrical cost of RFEF processing was about 1 cent per decaliter (10.5 quarts), and the procedure was more effective than conventional heating under the same conditions.
Ohmic Heating. Ohmic heating is an advanced thermal processing method by which electrical currents are passed through foods to rapidly increase the temperature for either cooking or sterilization purposes. Its principal advantage is its ability to rapidly and uniformly heat liquid products containing large particulates such as soups, stews, canned fruit in syrup, as well as liquid egg and juice products. By heating the entire mass of the food, the ohmic thermal process provides greater post-processing product quality over conventional thermal processing, which because of its slow conduction and heat transfer can damage desirable sensory attributes of foods. Ohmically processed foods also boast shelf life comparable to that of canned and sterile, aseptically processed products, and the equipment.
Ohmic heating is really an established technology with emerging applications in that several major equipment manufacturers offer commercial ohmic heaters, serving a growing market of food manufacturing companies that produce sliced, diced and whole fruit in sauce. A large number of potential future applications exist for ohmic heating, including its use in blanching, evaporation, dehydration, fermentation and extraction of foods. One emerging application of ohmic heating is fruit peeling, which may greatly reduce the use of lye common to such operations, and results in environmental benefits since no emissions result from ordinary electricity.
Infrared Heating. This is a relatively new process that has been commercialized for the surface pasteurization of ham and other meat products. Infrared lamps radiate heat at a relatively low temperature, helping to reduce bacteria on the surface of ready-to-eat meat product while retaining product quality. Infrared heating systems can be applied effectively to pre-packaged RTE meat products, and infrared sensors combined with other intervention technologies provide a promising tool for inactivation of bacteria on post-packaged product.
Experimental results of a 2004 FSIT study suggested that infrared surface pasteurization is an effective technology to decontaminate the surface of cooked meat products prior to final packaging. The technology was developed to destroy L. monocytogenes inoculated onto the surface of turkey frankfurters. A 3.47, 4.25, and 4.52-log reduction in the population of L. monocytogenes on the frankfurters was achieved after the surface temperature was raised to 70C, 75C and 80C, respectively, and the color of the infrared-treated hot dogs was not adversely affected.
In another recent ERRC study, an in-package microwave heating system with a feedback control mechanism was developed and tested for inactivation of L. monocytogenes in beef frankfurters. This method utilized an infrared temperature sensor and a PID process controller to measure and monitor the surface temperature of frankfurters contained in plastic bags in a small household microwave oven. The aim was to raise the surface temperature of frankfurters to a set-point lethal to L. monocytogenes, and then continue to maintain it for an extended period of time. The computer-controlled microwave heating was able to achieve a 7-log reduction of L. monocytogenes in beef frankfurters. If optimized, this study potentially may provide the food industry with a terminal, post-lethality pasteurization technology to kill L. monocytogenes in RTE meats within the final packages.
Innovative Nonthermal Processing Technology
As the food industry strives to meet consumer demand for both fresh, minimally processed, ready-to-eat foods and prepared foods that are conveniently packaged, the demand for innovative nonthermal intervention technology grows. Simply put, food manufacturers want to maintain the quality attributes and extend the shelf life of their products while ensuring product safety. The application of heat to products such as fresh-cut fruits and vegetables or juices not only reduces or eliminates bacteria but often adversely affects many of the organoleptic characteristics of the food associated with fresh foods. FSIT scientists have investigated several alternative processing technologies, including high-pressure processing, pulsed electric fields, radio-frequency electric fields, ultraviolet light, and irradiation, which preserve the quality attributes of product while lowering foodborne pathogen levels. As mentioned earlier, several of these technologies can be combined to to achieve a better kill rate.
High Pressure Processing. This pasteurization method, commercialized since the mid-1990s, has been successfully applied to inactivate microbes in heat-sensitive liquid and solid foods such as guacamole, jams and jellies, fruit juices, tomato salsas, and applesauce. HPP has also been applied to ham, cooked RTE meat products and seafood products such as oysters. Some of the companies using commercialized HPP include Hormel Corp., Avomex, and Coca Cola’s Minute Maid brand division.
Essentially, high pressure kills microorganisms by interrupting their cellular function without the use of heat, which can damage the taste, texture and nutrition of the food. An automated high pressure processing system involves placing a flexible packaged product into a handling basket that is placed into a vessel in which ultra-high hydrostatic pressure of between 80,000-130,000 pounds per square inch (psi) is uniformly applied to both pre- and post-packaged foods. Food samples are pressurized between 2 to 5 minutes, taken out of the chamber and stored or distributed as usual.
HPP will kill most vegetative microorganisms that grow in foods under normal storage conditions. HPP acts instantaneously and uniformly throughout a mass of food independent of size, shape and food composition, which means that the food retains its shape and texture despite undergoing extreme pressure. Compression of foods may shift the pH of the food as a function of imposed pressure and must be determined for each food treatment process. Studies also show that water activity and pH are critical process factors in the inactivation of microbes by HPP.
Studies show that HPP is particularly effective when applied to high-acid content foods to extend shelf life or improve food safety. Low-acid products that are meant to be shelf stable, such as vegetables or soups, are not good candidates for the process, however, due to HPP’s inability to kill spores without added heat. Low-acid refrigerated products fare better when processed with high pressure, both in terms of extended shelf life and pathogen reduction. It is important to note that while HPP can eliminate vegetative microorganisms, it is not effective at killing microbial spores. And at 5-10 cents per pound, it is too pricey to be practical. For foods where thermal pasteurization is not an option (due to flavor, texture or color changes) HPP can extend the shelf life by two- to three-fold over a non-pasteurized counterpart, and improve food safety. As commercial products are developed, shelf life can be established based on microbiological and sensory testing.
In recent research, FSIT scientists found that HPP treatment at 25C can significantly reduce E. coli populations in tomato juice and liquid whole eggs.
Pulsed Electric Field. A commercialized nonthermal intervention technology since 2005, PEF has proved to be an excellent microbial inactivation method for liquid products. PEF may be used to pasteurize fluid and pumpable foods. Currently, the two most common industrial applications of the method, in which intensive electric pulses break down the cell membranes of microorganisms, are the pasteurization of juices and the processing of wastewater. The benefit of PEF is the retention of product quality and freshness. Improvements in PEF technology have resulted in an intervention technology that effectively kills vegetative bacteria, yeasts and molds, retains product quality and freshness, and is inexpensive when compared to other nonthermal systems.
In recent research at FSIT, PEF has been demonstrated effective to inactivate vegetative bacteria, especially in fluid juices and beverages. In one study, a model salad dressing formulated by Kraft Foods was processed by PEF treatment alone and then by PEF followed with mild heat treatment. While PEF treatment of the aseptically packed product resulted in more than a 7-log reduction of Lactobacillus plantarum, a highly heat resistant spoilage bacterium, it only resulted in microbial shelf stability under refrigerated condition. PEF plus a mild heat, however, produced shelf stability at room temperature. This method may result in improved product quality and extended shelf life for salad dressing and similar products.
Another study subjected applesauce samples to PEF followed by mild heat treatment. The processed applesauce was aseptically packed into plastic cups and stored at room temperature. Evaluations indicated that the processed applesauce maintained high sensory quality during 470 days of storage. That’s a longer shelf life than is currently used in commercial practice. This research demonstrated that following PEF with mild heat may be used in producing high-quality, shelf-stable applesauce products.
With occurrences of egg-related outbreaks of Salmonella in recent years and awareness of the potential for Listeria contamination of these products, research into possible food safety interventions to increase the safety of egg-related products has been another important focus area. Application of pulsed electric fields PEF in liquid whole egg (LWE) or liquid egg white (LEW) pasteurization has been studied by researchers at FSIT, but these showed that the reduction of pathogens was limited. In one study, the objective was to investigate whether a change of egg pH or an increase of treatment temperature would enhance the effectiveness of PEF treatment in inactivating pathogens. S. typhimurium, S. enteritidis and L. monocytogenes cells were inoculated in LWE or LEW, and the samples were treated at various pH levels and temperatures. Ultimately, the results showed that effects of treatment temperature in inactivation of pathogens were dependent on pH of liquid eggs, and increasing treatment temperature at neutral pH would enhance the effectiveness of PEF treatment and provide efficient microbial reduction in liquid eggs.
Ultraviolet (UV) Light. Ultraviolet processing uses radiation from the ultraviolet region of the electromagnetic spectrum. The antimicrobial properties of UV irradiation are believed to be due to damage to bacteria’s DNA, reducing the ability of the microbial population to reproduce in a food product exposed to this light.
Ultraviolet light has been used for the decontamination of air in food factories, for the surface treatment of bakery products and for the treatment of drinking water, water for food and beverage formulation, wash water and wastewater. UV liquid treatment systems are suitable for clear and translucent liquid foods, including fruit and vegetable juices, particularly as some of these change in flavor and color when thermally treated. For example, USDA has approved UV for use on apple juice products, noting that the treatment is shown to be a relatively low-cost pasteurization method and shelf life extender for fresh juices and other liquid products. In one ERRC study, the application of UV processing to apple cider samples inoculated with E. coli and Listeria reduced these bacteria by more than 99% without adverse effect to the flavor profile of the product. Even so, if UV is used, verification and validation through microbiological monitoring should be conducted to ensure that the processing method is working, and such products need to be aseptically packaged to prevent post-processing contamination.
Research is showing that combining UV with other intervention methods reap higher benefits, particularly for products that cannot be treated thermally. Liquid egg whites, for example, are easily denatured and lose their functional properties when exposed to thermal pasteurization methods. At FSIT, researchers built a lab-scale system that combined UV and a low-temperature thermal treatment to inactivate E. coli O157:H7 in liquid egg whites. By using a low-pressure mercury lamp encircled by a coil of UV tubing and treating samples at room temperature, the researchers found that this combined intervention treatment provided a 5-log reduction of E. coli in the liquid egg white samples.
Irradiation/Ionizing Radiation. This proven intervention technology, which applies a low level of ionizing radiation to inactivate microorganisms, spores, naturally occurring chemical toxins, yeasts, molds and parasites in foods, has had commercial application in the dry spice industry for many years. More recent developments include regulatory approval of irradiation for RTE meat products and juices. Currently, irradiation as a pasteurization process for fresh produce is under intense discussion among the industry and USDA. ERRC research has found that up to one kiloGray (kGy) of ionizing radiation inactivates E. coli inside or on produce sample surfaces to about a 5-log reduction without significantly altering product quality, making the technology a promising solution for the produce industry.
At ERRC, the nation’s only federally funded and operated food irradiation research facility, one of the single most significant research accomplishments within FSIT during FY 2006 was related to radiation inactivation of furan and acrylamide in foods. Furan and acrylamide are two possible carcinogens found in many thermally processed foods. A study was conducted to determine if ionizing radiation could decrease the amount of furan and acrylamide in solution and in real foods. Low-dose ionizing radiation (2-3.5 kGy) completely destroyed furan in water, and reduced furan levels by 25% to 40% in RTE meats. Irradiation completed destroyed acrylamide in water, but had a limited affect on the inactivation of acrylamide in oil and potato chips. This study shows that ionizing radiation significantly reduced the levels of the human carcinogen furan in RTE meats using radiation doses that would be used to inactivate the human pathogen L. monocytogenes. This data will help the FDA evaluate a petition, currently under review, to allow irradiation of ready-to-eat foods.
Researchers at FSIT are also focusing on the effectiveness of irradiation to kill L. monocytogenes in RTE meats—a pathogen that since 1998 has caused approximately 90 million pounds of RTE meat products to be recalled. Although the meat industry has long applied various thermal and nonthermal pasteurization methods to effectively reduce or kill pathogenic bacteria, these treatments do not effectively address the ability of L. monocytogenes to grow in refrigeration temperatures post-processing. Since irradiation can be applied after product has been packaged, it holds promise as an inactivation tool for this pathogen in RTE meat products. FSIT scientists have determined the doses required to kill L. monocytogenes in such products, as well as the irradiation resistance of the strains associated with foodborne illness.
And again, research involving a combined intervention shows the benefit of using a multiple hurdle approach. One FSIT study investigated the feasibility of using hot water treatment of whole melon in combination with low dose irradiation of cut fruit to reduce native microbial populations while maintaining the quality of fresh-cut cantaloupe. Whole cantaloupes were washed in tap water at 20C or 76C for 3 minutes. Fresh-cut cantaloupe cubes, prepared from the washed fruit were then packaged in clamshell containers, and half the samples were exposed to 0.5 kGy of gamma radiation. Microflora and sensory qualities were determined during subsequent storage at 4C over a period of seven days. Results showed that hot water surface pasteurization reduced the microflora population by 3.3-logs on the surface of whole fruits, resulting in a lower microbial load on fresh-cut cubes. Irradiation of cubes prepared from 20 C-washed fruit to 0.5 kGy achieved similar low microbial load of the cubes as those prepared from hot water treated fruit. The combination of the two treatments resulted in further reductions (0.5-0.6 log unit) in bacterial population. Color, titratable acidity, pH, ascorbic acid, firmness, and drip loss were not consistently affected by treatment with irradiation, hot water or the combination of the two. Overall, the results showed that the combination of hot water pasteurization of whole cantaloupe and low dose irradiation of packaged fresh-cut melon can reduce the population of native microflora while maintaining quality of this product.
Cold Plasma/Nonthermal Plasma (NTP). This novel nonthermal intervention method is a process that uses high voltage electricity to ionize a gas (such as air) in order to generate a plasma field. Plasma is considered the fourth state of matter, and can be described as a gaseous form of energy that makes the oxygen molecules of the air passing near electrodes break down into reactive oxygen species. Bacteria, viruses and spores that are exposed to this nonthermal gaseous energy in the form of reactive chemical species are eliminated on contact, and the plasma dissipates immediately when the electrode is turned off.
Cold plasma is a novel technology for sanitizing surfaces which shows promise for treatment of produce. FSIT researchers are conducting studies in partnership with Drexel University that indicate cold plasma decontaminate the surface of produce without destroying organoleptic attributes. Most of this research has focused on the sanitation of apples to determine cold plasma’s antimicrobial efficacy and the efficiency of bacteria inactivation on the fruit’s surface. In initial studies using a gliding arc NTP system designed for treatment of inert surfaces, NTP was applied to apples that had been inoculated with L. innocua which is a surrogate for pathogen L. monocytogenes. The application of NTP led to a 90% reduction of the bacterium on the apple surface without altering the appearance of the treated apples during subsequent storage.
As a result of the initial success of the application, collaborators constructed a test prototype of an improved gliding arc NTP system to be installed at ERRC. A design change has been incorporated into the unit to increase the unit’s flexibility to accommodate future changes in electrode configuration, as needed for testing application to produce with different surfaces. Cold plasma experiments planned for the future will use L. monocytogenes, E. coli O157:H7 or Salmonella.
It will take a few years, but a commercialized NTP system is not far off since the equipment is fast becoming a reality. However, in the near term, the issue remains whether cold plasma is able to reach and treat 100% of the food’s surface, particularly if that surface is irregularly shaped. For now, it promises to be an effective tool for the surface sanitation of produce with regular surfaces.
Again, foods that are very sensitive to heat processes require intervention technologies that do not expose them to the higher temperatures typically associated with kill step methods. On the produce safety side, the choice of microbial inactivation technologies is limited. Active research projects at FSIT involving chemical sanitation treatments such as ozone and chlorine dioxide, biological controls methods such as competitive exclusion and microbiological phage, and antimicrobial packaging hold promise as effective intervention for fresh-cut and minimally processed fruits and vegetables.
As noted, many of FSIT’s research areas involve the combination of processing technologies to achieve greater log reduction of harmful pathogens and spoilage organisms in food and beverage products. Antimicrobial packaging is a good example of one of these areas. Modified atmosphere packaging (MAP) is an established method to limit the oxygen exposure of the product in the package. The goal of MAP is to improve quality retention during storage. Antimicrobial packaging, on the other hand, involves the addition of an antimicrobial agent to packaging, which is released to the product to inhibit the growth of microorganisms in the food during storage.
Also, combinational processing of food that incorporates the use of antimicrobial agents, a limited amount of heating and antimicrobial packaging is another promising intervention development area. Products that have a high tendency of surface contamination, such as hot dogs and sausages, may be effectively treated with surface pasteurization. The ERRC-developed vacuum-steam-vacuum process (based on a rapid heating and cooling procedure in which product is placed in a vacuum chamber, steam is injected into the chamber and held for a fraction of a second, and followed by a second vacuum application to remove the steam and cool the product), and then placed in antimicrobial packaging. The benefit of this process is the heat does not penetrate the interior of the product and potentially reduce the quality of the item, but does effectively decontaminate its surface. By combining interventions, it may be possible to achieve a 5-log reduction for Listeria in these meat products, which is difficult when using a single nonthermal technique.
There is continued interest in the improvement of chemical sanitizers as intervention aids. Using chlorine dioxide as a liquid phase treatment for produce, primarily tomatoes, has been commercialized and in use in industry for the past four years. The gas is a strong oxidizing agent that has been shown to be more effective in eliminating foodborne pathogens from fresh-cut produce than the more commonly used chlorine sanitizer in the washing process. In upcoming FSIT research, scientists aim to establish the degree of efficacy of chlorine dioxide gas applications in disinfecting fresh-cut lettuce and other leafy vegetables, to establish the point of dose-dependent injury to fresh quality and shelf-life so that such injury can be avoided, and to further improve, extend and transfer treatment application methods to end-users.