Using PEF to Assure Safety of Fresh Juices
By Hennie Mastwijk and Irene Pol-Hofstad
Despite a small marketshare of about 2% in the beverage category, unpasteurized fruit juices and juice products are associated with a major fraction of reported foodborne outbreaks in the U.S. Food safety policy is aimed at the reduction of the number of infections among certain risk groups, which has led the U.S. Food and Drug Administration (FDA) to warn consumers about the risks from pathogenic organisms such as Salmonella and E. coli involved when consuming untreated fruit juice products. As a result, unpasteurized fruit and vegetable juices must be labeled.
Although such label warnings do contribute to the awareness of potential food hazards among consumers who are susceptible to certain illness risks posed by untreated juices, they do not improve the microbial safety of products. With this in mind, the FDA’s 1998 labeling action was followed by the development of a specific Hazard Analysis and Critical Control Points (HACCP) program for fruit and vegetable juices and juice products. In January 2001, the agency published a final rule designed to improve the safety of fruit and vegetable juice and juice products. Under the rule, juice processors must use HACCP principles for juice processing. It does not matter whether these companies produce unpasteurized or pasteurized juices. The food safety objective is aimed at the reduction in the number of contaminated products.
The final FDA Juice HACCP rule requires that juice processors assess their manufacturing processes to identify any microbiological, chemical, or physical hazards that could contaminate their products. If a potential hazard is identified, processors are required to implement control measures to prevent, reduce or eliminate those hazards. Juice manufacturers also must use processes that achieve a 5-log reduction in the numbers of the most resistant pathogen in their finished products compared to levels that may be present in untreated juice. To aid in achieving this log reduction, operations are allowed to use approved microbial reduction methods other than pasteurization, such as UV irradiation, high pressure processing, or a combination of techniques.
So far, so good. It is clear that the current FDA policy is indisputable, based on evidence that infants and other specific risk groups are in serious danger when consuming unpasteurized juices. However, these rulings will have a negative impact on the development of minimally processed fruit and vegetable products if new technology is not forthcoming that will both assure microbial safety of products and preserve the “fresh” quality of the products. This is important to recognize because the consumption of sufficient amounts of fresh fruit and vegetable products is promoted as a basis of a healthy lifestyle. It is well established that fresh fruit and vegetables not only are a source of vitamins and minerals but also contain dietary fibers and useful flavonoids. Another advantage of fresh fruit products is their low caloric values, unlike commonly used infused fruit preparations that contain added sugars. Moreover, the consumer’s experience of fresh products, and above all, the availability of real alternatives, will be crucial for a transition toward a healthier lifestyle.
The problem is that thermal processing of fresh juices not only kills unwanted microbes to the required 5-log reduction, but heat treatment at certain levels also kills the product’s taste. This is a dilemma for the juice processor, who must comply with the HACCP rule, achieve the 5-log reduction of target organisms for food safety, maintain a good product shelf-life and meet his customer’s expectations of taste. The good news is that industry is working on novel non-thermal microbial reduction, or mild preservation, technologies that will help the juice processor achieve the twin goals of minimal processing to maintain quality attributes and reduction of harmful pathogenic and spoilage organisms.
To date, FDA has approved UV irradiation (21 CFR 179.39) and pulsed light (21 CFR 179.41) for use on fruit and vegetable juices and juice products to achieve the 5-log reduction of target microorganisms as part of HACCP rule compliance. (Processors also may use chemical antimicrobial agents, such as certain sanitizers, on the surface of citrus fruit as long as FDA has approved the chemical agent or it is considered Generally Recognized As Safe (GRAS) by the agency.) According to FDA, alternative treatment technologies “that do not involve the use of a source of radiation or a chemical agent, e.g., high pressure processing, are not likely to require FDA approval.” Along these lines, pulsed electric field (PEF), a mild preservation treatment, is emerging as another commercially viable option for juice processors to use in reducing microbial hazards in fresh juices and juice products.
The Emerging Feasibility of PEF
Pulsed electric field is a nonthermal technology used for the preservation of fluid foods, such as fruit juices, beverages, liquid eggs, milk and sauces. These foods are pumped through a treatment chamber and subjected to high voltage impulses (Figures 1 and 2). A capacitor is charged to a switch, and the pulsed electric discharge takes place between two electrodes where the food is confined in the middle.
Unlike high pressure processing (HPP) systems, PEF treatment is primarily used in continuous flow process. The high electrical field intensity developed in the food column is needed to inactivate spoilage and pathogenic bacteria by causing structural damage to the cells. The intensity of the electrical field, which is defined by the voltage applied at the electrodes divided by the electrode distance, and the number of pulses received are the main two factors in the successful inactivation of microorganisms. The voltage and the repetition rate of the pulses are controlled electronically, ensuring that the food has actually received a pre-set treatment. PEF-based systems require compact modules that are easily integrated in existing aseptic processes.
PEF is particularly effective in the treatment of high acid food products such as fruit juices to inactivate microorganisms. In a state-of-the-art PEF preservation step, efficient microbial inactivation is achieved within a relatively low temperature process. In the past 15 years, U.S. and European companies and universities have established the scientific and technological basis of PEF treatment.[5-8] The focus of the work has shifted from treatment devices and electronics towards microbiological and product assessment.[5,7,8-10] Equipment with a capacity up to 5,000 liters per hour has been built. This throughput is sufficient for the supply of juices within local markets. Recently, descriptions of economically feasible operations have been outlined, including HACCP programs that are specifically designed for this kind of process.[8,9]
A Dutch consortium of juice companies, research organizations and universities has developed a PEF-based preservation process that fits well in the Dutch market for fresh juices.[8,13] Within The Netherlands, premium quality fruit and vegetable juices are successfully distributed in the refrigerated chain. There are two companies that produce freshly squeezed fruit and vegetable juices for the consumer and foodservice markets. These convenience products are different from products in the shelf stable juice segment and can be distinguished by their excellent sensorial quality and by the absence of additives. The shelf life of these unpasteurized juices is five days and is limited by spoilage organisms. The juices are produced under a strict HACCP plan to exclude pathogens. Despite that these products can be considered safe, the limited shelf life is a drawback. It has been demonstrated that PEF technology can be used to extend of the shelf life of fresh fruit products to approximately three weeks without compromising the initial quality (Figure 3). For juice producers, this extension in shelf live means new opportunities for the expansion of their businesses. For consumers, this means a safety guarantee and more convenience; the juice may be stored in the home for two weeks before consumption.
While the Dutch researchers have made great strides in developing system configurations that will allow the adoption of PEF technology by the juice industry, two major questions have to be answered prior to commercial application: How well is the process known, and is it safe to consume PEF-treated products? Especially since food products are treated by high voltage pulses, these questions must be answered in clear and unambiguous way.
The European Novel Food Regulation and PEF
For the food market within Europe, the Novel Food Regulation (NFR) is the guideline that helps answer those questions. Basically, the NFR is a legislative tool to ensure that foods on the market are safe and reliable. It accounts in particular for food products that are manufactured by processes that have not been previously used. NFR does not prescribe operations nor does it provide a checklist. Rather, it deals with the question, “To what degree is public health affected if state-of-the-art technology is used in practice?” It recognizes the principle of substantial equivalence. In case no significant changes in treated products can be demonstrated, products are assumed to be safe. This principle replaces the demonstration of food safety by long-term clinical studies on the effects of the intake of products by a small panel. The principle of substantial equivalence has been adapted in conjunction with a market surveillance study to ensure that food safety is guaranteed once products have found their way into the marketplace.
Using the NFR guideline, we can assess the current applicability of PEF in terms of microbial safety, toxicological safety, product changes and unintended additives.
Microbial Safety. Since PEF technology is used as a preservation step in the manufacture of beverages, NFR requires that its effectiveness is demonstrated. This includes assessment of target pathogens that are known to date, supplemented with data on a number of pathogens that have recently caused foodborne illness outbreaks in the U.S. Studies have shown that acidic juices (pH ?4.6) can contain enteric bacterial pathogens such as E. coli O157:H7, various Salmonella species and the protozoan parasite Cryptosporidium parvum. Illness-causing organisms that are ubiquitous in nature, such as Listeria monocytogenes, also have been identified as possible contaminants in juice. The ability of some of these pathogens to survive in acidic foods requires adequate control during processing.
Inactivation by PEF is efficient for a number of microorganisms, including some well-known pathogens like Listeria, Bacillus and E. coli. In addition to inactivation data, it is important that industry and researchers provide shelf life data for specific products and include challenge tests.
Toxicological Safety. The most contestable issues related to PEF and substantial equivalence deals with chemical changes that may occur inside the products as a result of PEF treatment. Key questions still to be answered are: What kind of risks are involved? How do we demonstrate that a product is safe from a chemical point of view? To what level can one exclude additions to the food, whether included unintentionally or not? To what degree will the process change the food?
Recently, the first answers have been formulated for PEF-treated products based on relevant scientific data. Chemical changes induced by PEF treatment has been observed in tomato product when analyzed by the method of chemical fingerprinting. Ten compounds were evaluated in more detail. The concentration of three water-soluble compounds (sugars) were higher than in heat processed product. The same for two specific oil soluble compounds. In addition, release of metal into the product stream has been measured in a state-of-the-art system. Metal compounds of ferro (Fe), chromium (Cr), nickel (Ni) were found in the PEF-treated model product. Concentrations were less than 2 microgram per liter. This result is discussed in more detail by comparison with maximum allowed concentration (MAC) values for metals in drinking water. It turns out that the addition of metals by the PEF process is not significant.
Although microbial inactivation is not a result of food additives, it is known that PEF systems introduce compounds by electrochemical effects. The presence of charged electrodes in contact with the food leads to the formation of oxygen and hydrogen by electrolysis of water and release of electrode material into the product stream. It has become apparent that these compounds are not required for bactericidal action and the concentrations of these compounds in the product are very low.[5,14]
Product Changes. The demonstration that treated juices are no different than non-processed juice or heat-processed juice requires an evaluation that accounts for all chemical compounds in the raw product. This can be achieved by chemical fingerprinting, a method that relies on the analysis the overall chemical composition of foods. Evaluation of the acquired data by this method is a complex task. However, by comparison of PEF treated samples with non-treated and heat pasteurized controls detailed information is obtained. This approach allows the observation of minor changes chemical changes induced by PEF treatment in both water- and fat-soluble compounds.
Unintended Additives. In a toxicological assessment, the major risk to public health is associated with the release of metals in the products. Metals present in electrode system end up in the product stream due to electrochemical oxidation of metals. It has been established that minute amount of metals are released into the product as a result of leakage currents through the electronic switch.[17,18] The concentrations of these metals remain well below the MAC values that apply for drinking water.[14,19] From this example and many others, it is now apparent that the particular design of electronics, electrodes and treatment devices used in a PEF system are very important. Legislative authorities that deal with NFR, therefore, require an accurate description of PEF equipment and process design that is supplemented with microbial, toxicological and other relevant product data.
Changes in consumer demand for convenience, quality and health has triggered demand for minimally processed foods. Microbial risks that are involved with these types of products can be overcome by mild preservation technologies. A continuous flow preservation process based on PEF technology has been developed for fruit juices. For this application, PEF technology has reached the point of maturity with respect to equipment, microbiology and economics.
Recently, important issues on food safety and legislation have been addressed, including toxicological assessment of foods and equipment. These results demonstrate that PEF adds a minute amount of metals into the product but that from a product safety point of view, these changes are not significant. PEF treatment has the potential to become a workhorse for the juice market—a market where the demands of modern consumers can be met. PEF technology will unite food safety with a pleasant way of living.
Hennie Mastwijk is senior scientist at the Wageningen University and Research Centre (www.wur.nl) located in The Netherlands. His academic interest is in the field of mild preservation of foodstuffs by high electrical field pulses and surface decontamination by cold plasma. It includes the industrial development of equipment, processes and food products by contract research.
Irene Pol-Hofstad is senior microbiologist with Agrotechnology and Food Innovations B.V. (www.agrotechnologyandfood.wur.nl) located in Wageningen, NL. Her interest is in food microbiology, mild preservation and food safety.
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