11 Safety Considerations in Active Packaging J.H

11 Safety considerations in active packaging J.H. HOTCHKISS

11.1 Introduction

Ever since Appert discovered that heating food in sealed glass jars produced a stable product, a major goal of packaging has been to safely preserve foods for extended periods. A scientific understanding of the relationship between shelf-life, safety, processing/storage conditions, and packaging began to evolve in the late 180Os as the theoretical basis for the thermal inactivation of pathogenic spores was developed (Goldblith, 1989). This understanding is still evolving. The use of packaging to safely protect and preserve foods has remained a central focus of packaging development. The primary roles of packaging in food safety have traditionally been to withstand thermal processing conditions and to act as a barrier to contamination. It would be of little benefit to process food if there was no way to prevent recontamination. The success of the metal can over the last 150 years is due to its ability to withstand thermal processing and provide a barrier against chemical and biological contamination. Modern food packaging can also influence the nutritional and quality attributes of foods and ensure the year-round availability of many foods. These factors are important in the health and nutritional aspects of foods. The major advances in food packaging over the last two decades have been the development of new materials, combinations of materials, and containers with specific technical and economic benefits (Downes, 1989). Most of these new materials and containers are inactive technologies in that they act primarily as passive barriers which separate the product from its environment. However, current research is shifting to the development of packaging which actively contributes to the preservation and safety of foods (Labuza and Breene, 1989). Such packaging interacts directly with the food and the environment to extend shelf-life and/or improve quality.

11.2 Packaging and food safety

Packaging has often been thought of as a source of risk for foods and seldom as a technology which could be used to enhance food safety (Wolf, 1992). Certainly, when packaging fails to preform its protective functions the result is an unsafe product (Downes, 1993). For example, safety may be compromised when package components migrate to a food or when there is a loss of integrity resulting in contamination by pathogenic microorganisms. Table 11.1 lists several general ways in which packaging can detract from safety. However, active packaging can directly enhance food safety. Active packaging can not only prevent contamination but it can also improve food safety in several other ways. Examples of 'active' packaging which improves food safety include antimicrobial polymers and films which inhibit the growth of pathogenic and spoilage microorganisms, packages which react with toxins and indicate their presence, packaging materials which prevent the migration of contaminant, and packages which indicate if packages are leaking. These and other types of active packaging which improve safety and quality are areas of current research and commercial interest (Ishitani, 1994).

Table 11.1 Types of food safety problems associated with packaging Examples Consequences Microbial contamination Loss of integrity Seal rupture, leaking cans, incomplete glass fin- ishes allow contamination by pathogenic m.o. Anaerobiosis Low oxygen environment resulting from product or microbial respiration. Can lead to toxin formation by anaerobic pathogenic microorganisms Chemical contamination Migration Transfer of package components to foods Environmental contamination Environmental toxicants can permeate films Examples include preservatives used in wooden pallets, diesel exhaust Recycled packaging Contamination of post-consumer packaging is transferred to foods after recycling Insect contamination Post packaging Some insects can bore through many common packaging materials Foreign objects Glass shards, metal pieces Injury Exploding pressurized containers Soft drinks, beer in glass, etc. Broken containers Cuts, lacerations Environmental impact Disposal, recycling, CFCs Loss of nutritional and sensory Aroma and nutrient sorption by polymers quality Tamper evidency Malicious and innocuous Inadequate processing Conventional Underprocessing can lead to food poisoning Aseptic Loss of integrity or insufficient sterilization of packaging can lead to food poisoning 11.3 Passive safety interactions

11.3.1 Barriers to contamination The major safety and quality function of packaging is to act as a barrier between food and the environment. The purpose is to prevent contamination (or re-contamination after processing) of the food from both environmental chemicals and pathogenic microorganisms. With glass and metal food packaging, which are, for practical purposes, absolute barriers, preventing contamination is usually a function of closure integrity. Considerable experience with such closures has resulted in a remarkably low risk packaging system. The economic and functional disadvantages of metal and glass have led to the development of polymeric packaging materials. The barrier properties of these polymeric materials has been the central focus of packaging development in recent years. Polymers which are high barriers to both oxygen and water vapor are now available. Very recent efforts have focused on improved aroma/flavor barriers. Post-packaging microbial contamination of foods is now not only a function of closure integrity and material integrity (Downes et a/., 1985). There are two aspects of package integrity: strength and completeness. Strength implies that the closure or seal is sufficiently strong to withstand the rigors of distribution without failure. Completeness means that there are no gaps, holes, tears, etc. in the material or the seal. A seal or material can be strong yet incomplete or can be complete yet have insufficient strength for distribution. In some cases, flexible materials can contain minute pin- holes which allow entry to microorganisms yet still not show signs of leakage (Chen et aL, 1991). Flexible packaging can also develop pin holes during shipping. Strength and integrity have become important issues because foods are transported further and stored for extended periods. The canning industry has had considerable experience with double seam closure for metal cans, making a container sealed in this way one of the safest available. The lack of such long-term experience with the heat seal as a means of closure has raised safety concerns. Flexible materials are also more prone to failure during transport and storage. As the change to polymeric packaging has occurred, concern for the integrity of the container has increased. Research has been undertaken in an attempt to improve the testing of integrity of polymer-based packages. Gnanasekharan and Floros (1994) have reviewed methodology for detecting leaks in flexible food packaging. No currently available method is entirely satisfactory for all situations. Increased potential for chemical contamination has become a concern because polymers are permeable to organic vapors, and foods which are hermetically sealed in polymer-based containers can absorb environmental contaminants. The transfer to foods of potentially toxic compounds used to preserve wooden shipping pallets and wooden container floors has been reported and is exacerbated by the increase in long-distance shipment of foods (Whitfield et al, 1994). In addition to toxicological concerns, many environmental organic compounds which permeate films impart undesirable odors to foods. As the pace in the use of recycled materials has gained momentum, concern over microbiological contamination of fiber based packaging materials has also increased (Klungness et al., 1990). The paper making process inactivates most vegetative cells but does not inactivate microbial spores. Foods packaged in recycled materials have the potential to have acquired high spore loads from the packaging (Vaisanen et al, 1994). It is likely that in some cases potentially pathogenic spores could be transferred to foods from recycled materials. Packaging also protects the nutritional and organoleptic quality of foods. While not directly safety issues, foods which have lost their nutritional attributes or are not consumed because of poor taste or appearance, become a health issue. Nutrients and organoleptic properties can be adversely affected in several ways. For example, nutrients can be destroyed when oxygen or light enters a package or when the product is exposed to excessive heat. The loss 'of vitamin C in orange juice when stored in low barrier packaging is a prominent example. Sorption of nutrients by the packaging material is also a mechanism of loss.

11.3.2 Prevention of migration The second major safety function of packaging is to limit the transfer (i.e., migration) of packaging components to foods. Considerable research has been conducted into the migration of packaging materials to foods (Crosby, 1981). Migrants include inorganic toxicants, primarily lead from soldered cans, as well as organic toxicants such as vinyl chloride monomer which is a known human carcinogen. Both the theoretical and empirical aspects of migration have been studied in detail and in most cases the process is scientifically understood. Concern over migration has been recently heightened because of the use of recycled materials or refillable containers for food and beverage packaging (Begley and Hollifield, 1993). At least two potential problems exist. One is that non-food grade plastics which may contain additives or monomers that are not intended for human food use will enter the recycle stream. These additives or monomers could then migrate to foods. The second problem is the potential contamination of food grade polymeric packages by consumers (gasoline and pesticides are commonly mentioned as potential contaminants). These contaminants could then migrate to foods packaged in containers made from recycled materials. Several solutions to the post-consumer contamination of recyclable plastics have been proposed. First is the use of equipment to detect contaminated containers prior to refilling. These are commonly referred to as 'sniffers' and are designed to sample the air inside the container and determine if volatile organic compounds such as might be found in gasoline are present. Commercial sniffers are available and in use. The second solution is to chemically break down the polymeric structure and subse- quently reform the basic polymer. Any contaminants would be removed during this processing. The third solution is to construct containers in which the recycled polymer is separated from the product by a functional barrier. Such functional barriers are intended to prevent the migration of contaminants from recycled polymers to products. Combining virgin and recycled PET by co-extrusion into a PET bottle has been commercially undertaken. The virgin PET is expected to be a functional barrier to potential contaminants in the recycled layer. The major question is, How effective is this virgin layer at preventing migration? Other treatments of polymers such as cross-linking can retard migration. Migration is also affected by the chemical and physical nature of the migrant. Active packaging materials which can minimize or eliminate migration would be of substantial interest as the concern over the environmental cost of packaging increases. A second recent packaging migration safety concern has resulted from the use of polymer-based packaging as containers for food during heating such as in processing low acid foods in a retort or heating foods in microwave ovens. Initially, food polymer-based containers were designed to store products at ambient or sub-ambient temperatures. Thus, most laboratory work on migration was undertaken at room temperature or lower over extended periods. The advent of the microwave oven has meant that foods are heated in plastic vessels and on plastic surfaces. Heating plastics has two effects on migration. First, migration in general follows Arrhenius-type kinetics and increasing temperature increases migration rates in an exponen- tial fashion. The second effect is that elevated temperature can cause degradation of the polymer and additives which can result in migration of the breakdown products. Each of these issues has been addressed by several regulatory agencies in the USA and Europe.

11.4 Active safety interactions

Active packaging systems face similar barrier and migration safety issues as conventional packaging, as well as some additional issues. While there is concern that some active packaging systems will detract from safety there also is the possibility that new active systems can enhance safety. Materials and containers are being developed specifically to reduce food safety risks. 11.4.1 Emitters and sorbers One the earliest and most successful active packaging concepts was to incorporate a material which either absorbed or emitted vapors or gases inside a package after closure. This might be as simple as water vapor absorbers which are designed to control relative humidity, or more complex substances which absorb ethylene from produce, absorb undesirable odors from foods, or emit ethanol to control molds in bakery products. Particularly desirable types of sorbers are those that remove both residual and ingress oxygen after the package has been sealed (Rooney, 1994). Oxygen absorbers which remove oxygen from the headspace of bottled beer, for example, have been successfully tested commercially. Initially, absorbers/emitters were contained inside packets which were added to the package along with the product. More recent technologies have incorporated the sorber/emitter into the film or container wall. This reduces the likelihood of accidental ingestion. These absorbers are a form of modified atmosphere packaging and can change the microbiology of foods. The safety implications of such changes are the same as those for conventional MAP, as discussed below. There is also the concern that the components making up the absorber/ emitter will migrate to the food.

11.4.2 Active packaging and migration Many active packaging systems incorporate functional additives in food contact materials. These may be as simple as iron oxides which absorb O2 or as complex as systems which react with singlet oxygen (Rooney, 1994). In each case, the potential for and consequences of migration need to be assessed. For example, there has been some reluctance in some parts of the world to allow the use of ethanol emitters in foods which will be consumed without further cooking or processing. The residual ethanol might be considered a food additive and thus be required to undergo the rigors of complete toxicological testing. For those active packaging systems which indirectly add components to foods, the governmental regulatory and health issues will be similar to those related to migration of residual monomers or other polymer components (Crosby, 1981). Laboratory investigations will be required to determine the potential for migration and to quantify the amount of migration. If the amount of additive migrating is considered of potential significance, toxicological testing may be required. In some cases, active packaging systems may involve migrants for which there is little concern in food systems. For example, approved antimycotic agents such as sorbic, benzoic, or propionic acids, would likely be of little regulatory or safety concern if incorporated into antimicrobial films (Giese, 1994). However, in most cases active components and additives will not be common food additives and potential toxicological concerns will need to be addressed. The addition of antimicrobial metal ions to food contact surfaces is likely to result in the migration of small amounts of the metals to foods (Ishitani, 1994). While these metal ions may be of low toxicity, the metals may be classified as food additives and require rigorous toxicological testing. The regulatory consequences of intentional addition of even low amounts of metals will need addressing. In some cases, the use of functional barriers to prevent migration of the active components will be required. Incorporating absorbers or scavengers into the adhesives used to bind layers of inert film as a means of 'burying' the additive is an example.

11.4.3 Barrier to contamination In addition to migration, active packaging systems must fulfill the safety requirement of acting as a barrier to microbial and chemical contamination. The addition of active ingredients to films could decrease their mechanical properties resulting in a higher failure rate during transport. Such failures become safety concerns if they allow for contamination by pathogenic microorganisms or toxic chemicals. For example, the addition of inorganic compounds such as metal-coated zeolites, desiccants, or oxygen scavengers will likely reduce the mechanical properties of films raising the possibility that the contamination barrier will be reduced. The addition of packets or sachets to packages of food raises concern that they will be inadvertently ingested. While sachets and packets have been in use for several years without apparent problems, caution about adding non- edible items to packages should be taken. Incorporation of active ingredients directly into the packaging rather than as sachets seems prudent.

11.4.4 Indirect effects on safety Active packaging often has indirect as well as direct effects on food safety. For example, packaging which absorbs oxygen from inside a package with the goal of reducing deteriorative affects will affect both the types and growth rate of the microorganisms in products. The inclusion of antimicrobial agents in the contact layer of a packaging material may have similar effects. This will result in a change in the microbial ecology of the food. The type of microorganisms present on a product will thus be different from the same product packaged in a conventional manner. This change in microbiology will indirectly influence safety. In some cases, safety may be enhanced such as when carbon dioxide is added to high pH cheeses such as cottage cheese (Chen and Hotchkiss, 1993). In other cases, safety may be compromised as when the growth of Clostridium botulinum is favored. The effect of such changes in the microbial ecology of foods has not been investigated in detail and only a few reports on changes in microbial ecology have been published (Reddy et al.9 1992). Smith and co-workers have investigated the effects of MAP on the microbiology and toxin production by Clostridium botulinum in meats (Lambert et al9 1991). Somewhat surprisingly, toxin was produced most rapidly in samples packaged in air (i.e., 20% O2). This confirms an earlier observation we had made in cooked beef inoculated with both Pseudomonas and Clostridia spp. It is likely that this occurred because the aerobic Pseudomonas grew rapidly and consumed the oxygen rapidly leaving a highly anaerobic environment for the Clostridia. MAP in high carbon dioxide atmospheres inhibited the Pseudo- monas inoculum and left traces of unconsumed O2 which inhibited the Clostridia. These results point out that large changes in microbial popula- tions can result indirectly from altering the gases inside a package. These changes can both detract from safety but can also improve safety.

11.4.5 Indicators of safety/spoilage In addition to decreasing the safety of food, active packaging holds the promise of reducing risks from certain foods compared to conventional packaging. One example is the use of packaging which shows or in some way indicates the condition or history of a product. One currently available technology is time-temperature indicators. These devices integrate the time and temperature history of a product and give a visual indication if the combination has exceeded some standard or desirable amount (Taoukis et al9 1991). Shelf-life is related not only to how long a product is stored, but just as importantly, to the conditions, such as temperature, under which the product is stored. For example, pasteurized milk will last weeks at O0C but only a few hours at 35°C. Time-temperature indicators can be used on individual packages to warn consumers that a product has been exposed to a combination of time and temperature which may compromise safety or they may be used on shipping cartons to alert store personnel of potential quality/safety problems or allow stock rotations based on both time and temperature. Such devices would be especially useful when combined with other shelf-life technologies such as MAP or sub-sterilization radiation. The next generation of safety/quality indicators may be more specific than integrating time and temperature. In the future it may be possible to directly detect the presence of specific toxins in packaged foods using biosensors. Immunologically based sensors coupled to packaging could find applications in food safety, food processing, and detection of adulteration (Deshpande, 1994). Such sensors could, for example, detect the presence of bacterial toxins in packaged foods. They could also be used to determine if a food had been properly pasteurized or contained enzyme activity. Biosensors which combine electronics with biological specificity and sensitivity may find use in packaging as monitors of safety and quality (Deshpande and Rocco, 1994). In time, it may be possible to incorporate these or similar biosensors into food packaging systems for which the risk of toxin formation exists. Reportedly, methods to quantify the presence of microorganisms on fresh meats are near commercialization (Bsat et #/., 1994). Such systems could eventually be incorporated directly into food packaging. It may likewise be possible to detect the presence of toxic chemicals using similar technologies. The presence of specific pesticides or other environmental contaminants could be detected with immunological-based systems (Deshpande, 1994). Lastly, packaging should provide a margin of safety against tampering. Tamper-indicating packaging has been discussed in detail since several malicious incidents of tampering with drugs and foods have occurred (Hotchkiss, 1983). Several simple and complex tamper-evident packaging systems have been developed and a few implemented for foods.

11.4.6 Direct inhibition of microbial growth Microbial contamination and growth are the major factors in food spoilage and responsible for food-borne disease outbreaks. Two general approaches, heat sterilization and direct addition of antimicrobial additives, have been used to eliminate or minimize microbial growth. In conventional thermal processing, foods are sealed in a package and the combined product- package thermally processed. This is the basis of the canning industry. More recently, the process of the package and the product being sterilized separately then filled and sealed aseptically has been used. This is known as aseptic packaging. Foods can also be dried to reduce microbial growth. Another method to reduce microbial growth is to add antimicrobial additives directly to foods. This approach usually does not inhibit all growth but is selective for certain types of microbial growth, molds for example. The use of these additives is regulated and their use, in most cases, must be stated on the label.

11.4.7 Modified atmosphere packaging Recently, the development of alternative methods of inhibiting microbial growth has resulted from a consumer desire for fresher and more natural foods. The most successful alternative to canning or the direct addition of antimicrobial agents has been modified atmosphere packaging (MAP). The number and type of microorganisms present on a food is governed by five general variables: time, temperature, substrate (food) composition, microbial load (type and number), and gas atmosphere. For a given food product which must be held above freezing, alteration of the gas atmosphere surrounding the product is the most accessible method of inhibiting microbial growth. However, inhibition is not uniform for all types of microbes. In general (although there are exceptions) Gram-negative rods are inhibited by a modified atmosphere containing more than 10% CO2 while Gram-positive organisms are not inhibited and their growth can be promoted. The major goal of MAP is to reduce the growth rate of microorganisms which cause the product to become organoleptically unacceptable. However, organisms which cause disease (i.e., pathogens) do not, in many cases, cause organoleptic changes in foods. Increasing the shelf-life by suppressing spoilage organisms might allow for the growth of pathogenic organisms without development of the normal organoleptic cues of spoilage that warn consumers that a product may not be wholesome (Farber et ai, 1990). Thus, the major safety concern with MAP and controlled atmosphere packaging or other technologies which selectively change the microbiology of a food is that suppression of organoleptic spoilage (i.e., extension of shelf-life) will decrease competitive growth pressure and provide sufficient time for slow growing pathogenic organisms to become toxic or reach infectious numbers (Hotchkiss and Banco, 1992). The knowledge gained over the last decade about pathogenic microorganisms which are capable of surviving and growing at common refrigeration temperatures increases concern about the safety of refrigerated extended shelf-life foods (Gormley and Zeuthen, 1990; Farber, 1991). The effect that this change in microbiology might have on the risk of food-borne disease has been debated (Gormley and Zeuthen, 1990). There are several methods of creating a modified atmosphere inside a package. One is the use of selective or engineered barriers which are used for respiring products such as fruits and vegetables. The combination of product respiration rate (i.e., rate OfCO2 formation and O2 consumption) and CO2 egress and O2 ingress results in the formation of an equilibrium concentration of gases which, if properly designed, will reduce senescence and extend shelf-life. Alternatively, a specific gas mixture can be directly introduced into the package after removal of the air and before sealing. A third method is to use an additional material contained in a sachet or incorporated into the film which will alter the gas composition after sealing. In each case, the change in atmosphere will affect both the growth rate and type of microorganisms present. However, temperature will affect the respiration rate to a much greater extent than the permeability. If the product is stored at an elevated temperature, respiration rates will increase and the O2 content of the package may approach zero. At the same time the growth rate of pathogenic microorganisms substantially increases with the increase in temperature. This could allow for the growth of anaerobic pathogens such as Clostridium botulinum. For example, Lambert et al (1991) have shown that toxigenesis occurs more rapidly in aerobically packaged pork samples compared to anaerob- ically packaged samples when Pseudomonas spp. were present along with the Clostridium botulinum inoculum. It was presumed that the Pseudomonas rapidly consumed the oxygen allowing the C. botulinum to become toxic. These results agreed with earlier results of Hintlian and Hotchkiss (1987) who made a similar observation.

11.4.8 Antimicrobial films Packaging may directly affect the microbiology of foods in ways other than changing atmosphere. In solid or semi-solid foods, microbial growth occurs primarily at the surface. Surface treatment by spraying or dusting with antimicrobial agents for products such as cheeses, fruits, and vegetables is widely practised. Antimycotic agents are commonly incorporated into waxes and other edible coatings used for produce items (Peleg, 1985). More recently, the idea of incorporating antimicrobial agents directly into packaging films which would come into contact with the surface of the food has been developed. Antimicrobial films can be divided into two types: those containing an antimicrobial agent which migrates to the surface of the food and those that are effective against surface growth without migration of the active agent(s) to the food. Several commercial antimicrobial films have been introduced, primarily in Japan. One widely discussed product is a synthetic zeolite which has had a portion of its sodium ions replaced with silver ions. Silver can be antimicrobial under certain situations. This zeolite is incorporated directly into a food-contact film. The purpose of the zeolite apparently is allow for the slow release of silver ions to the food. Only a few scientific descriptions of the effectiveness of this material have appeared and the regulatory status of the deliberate addition of silver to foods has not been clarified in the US or in Europe. Several other synthetic and naturally occurring compounds have been proposed and/or tested for antimicrobial activity in packaging (Table 11.2). For example, the antimycotic (i.e., antifungal) agent, imazalil, is effective when incorporated into LDPE for wrapping fruits and vegetables (Miller et

Table 11.2 Some antimicrobial agents of potential use in food packaging Class Examples Organic acids Propionic, benzoic, sorbic Bacteriocins Nisin Spice extracts Thymol, p-cymene Thiosulfinates Allicin Enzymes Peroxidase, lysozyme Proteins Conalbumin Isothiocyanates Allylisothiocyanate Antibiotics Imazalil Fungicides Benomyl Chelating agents EDTA Metals Silver Parabens Heptylparaben al, 1984; Hale et al.> 1986). We have demonstrated that the same compound is effective at preventing mold growth on cheese surfaces when incorporated into LDPE films (Weng and Hotchkiss, 1992). Although imazalil is not approved for cheese, this work established that antimycotic films could be effective for control of surface molds in foods. Halek and Garg (1989) chemically coupled the antifungal agent benomyl, which is commonly used as a fungicide, to ionomer film and demonstrated inhibition of microbial growth in defined media. While not directly addressed by the authors, the method used to determine inhibition of growth indicated that the benomyl migrated from the film to the growth media. It is unlikely that benomyl would be approved for food use for toxicological reasons. Reports have appeared which demonstrate the effectiveness of adding common food-grade antimycotic agents to cellulose-based edible films (Vojdani and Torres, 1990). Films were constructed of cellose derivatives and fatty acids in order to control the release of sorbic acid and potassium sorbate. These films would seem to have the greatest application as fruit and vegetable coatings. Cellulose films are not heat sealable are not good barriers in high humidity situations. We have spectroscopically demonstrated that propionic acid, which is a common approved food antimycotic agent, could be coupled to ionomeric films but that antimycotic activity could not be demonstrated on rigorous testing (Weng, 1992). Direct addition of simple antimycotic acids such as propionic, benzoic, and sorbic acids to polymers such as LDPE was unsuccessful because of lack of compatibility between the acid and the non- polar film. This incompatibility is likely to be due to differences in polarity. We have solved this problem by first forming the anhydride of the acid which removes the ionized acid function and decreases polarity (Weng and Hotchkiss, 1993). Anhydrides are stable when dry and relatively thermally stable yet become hydrolysed in aqueous environments such as foods. Hydrolysis leads to formation of the free acid which in turn leads to migration from the surface of the polymer to the food where the free acids can be effective antimycotics. This is an example of 'switched on' packaging; the active ingredient remains in the film until the film comes into contact with a food. The activity is initiated by the moisture in the food. Future work in antimicrobial films may focus on the use of biologically derived antimicrobial materials that are bound or incorporated into films and do not need to migrate to the food to be effective. For example, a group of substances known as bacteriocins, which are proteins derived from microorganisms in much the same way as penicillin is derived from mold, have been described in the literature (Hoover and Steenson, 1993). Bacteriocins are effective against organisms such as Clostridium botulinum and one such compound, nisin, has been approved for food use. These peptides could, theoretically, be attached to the surface of food-contact films. Whether or not such bound bacteriocins would be effective remains unclear. Antimicrobial enzymes might also be bound to the inner surface of food- contact films. These enzymes would produce microbial toxins. Several such enzymes exist, such as glucose oxidase which forms hydrogen peroxide. A third possibility for antmicrobial films is to incorporate radiation- emitting materials into films. Reportedly, the Japanese have developed a material which emits long-wavelengt IR. This is thought to be effective against microorganisms without the risks associated with higher energy radiation. However, little direct evidence for the efficacy of this technology has been published in the scientific literature. In general, several questions, including those dealing with safety, should be considered in developing antmicrobial films: • What is the spectrum of organisms against which the film will be effective? Films which may inhibit spoilage without affecting the growth of pathogens will raise safety questions similar to those of technologies such as a MAP. • What is the effect of the antimicrobial additives on the mechanical and physical properties of the film? It is likely, for example, that effective levels of antimicrobial agents will reduce seal strength. This may adversely affect safety. • Is the antimicrobial activity a reduction in growth rate (yet still a positive increase in cell numbers) or does it cause cell death (decline in cell numbers)? • To what extent does the antimicrobial agent migrate to the food and what, if any, are the toxicological and regulatory concerns? • What is the effect of food product composition? Some antimicrobial agents, for example, are effective only at acid pH while others might require certain product compositions (e.g. aw9 protein, glucose, etc.) to be effective. Each of these questions need addressing before the safety consequences of antimicrobial packaging can be understood.

11.4.9 Rational functional barriers As pointed out, one major safety function of packaging is to act as a chemical and biological barrier. Films are frequently selected for food use based on the highest degree of oxygen and/or water vapor barrier at the lowest cost. More recently, the concept of rational design or engineering of film permeability has evolved. These so-called 'smart' films have barrier properties which are designed to adapt or change permeabilities according to conditions such as a change in gas composition or temperature. These engineered barriers have at least two important safety-related applications. The first is to act as a barrier to permeation of contaminants. Packaged foods can be exposed to contaminants from environmental sources or from the use of recycled plastics in food packaging. Common environmental sources of toxic permeants include chemicals used to treat shipping containers and pollutants. Chlorinated wood preservatives readily permeate through common films and cause taint in foods (Whitfield et al, 1991). Common environmental pollutants such as diesel exhaust and industrial solvents used in printing also permeate many common food-packaging films. The second use of engineered barriers is in passive MAP systems in which an equilibrium in the gas mixture is achieved through the combination of product respiration and package permeability. This equilibrium results from the consumption of O2 and evolution of CO2 by the food product at the same time that O2 is permeating into the package and CO2 is permeating out at a given temperature. At some point these respiration and permeation rates will reach an equilibrium concentration. Selection of a film with the proper permeability will result in the desired gas mixture. Several mathematical models have been developed which predict the proper permeation rates given by a specific product respiration rate (Mannapperuma et ai9 1991). There are two difficulties with this concept. The first is that respiration rates of most produce items vary widely, even within the same type of item. Thus, permeation rates will have to be tailored for each individual product item. The second problem is that CO2 permeation rates for common packaging films is 2-A fold or more higher than for O2. This means that CO2 may egress at a faster rate than O2 will enter, making the atmosphere anaerobic. Engineered films which can independently select CO2 and O2 permeation rates as well as films that change permeation at the same rate that fruits and vegetables change respiration rate with temperature, would be desirable. As pointed out above, the rate of gas change will determine the type of microorganisms on the products and, probably, the safety of such foods. We have recently devised equations for achieving optimum atmosphere concentrations for extending the shelf-life of fresh corn on the cob and head lettuce, each of which illustrates some of the problems with engineered barriers. Head lettuce respires relatively slowly and films which will allow a passive modified atmosphere to be established are commercially available. However, about 90 hours are required for establishment of a suitable atmosphere (Morales-Castro et al, 1994a). During this time considerable deterioration can occur. Sweet corn, on the other hand, respires rapidly and the establishment of a desirable steady-state atmosphere is not possible with normal films because the permeabilities are too low even for very low barrier films (Morales-Castro et aL, 1994b). MAP products such as lettuce and corn or other vegetables could become safety concerns if the atmosphere were to become anaerobic. This might occur if products were stored at a higher than expected temperature. This would cause an increase in respiration beyond that expected and the oxygen might be substantially depleted. This would leave open the possibility of growth of anaerobic pathogens such as Clostridium botulinum.

11.4.10 Combined systems The most successful active packaging materials are likely to combine different technologies. A few examples of such combinations have appeared in the literature. For example, Fu and Labuza (1992) have suggested that MAP might be combined with time-temperature indicators as a means of extending the shelf-life of perishable foods while at the same time minimizing food-borne disease risks. MAP would reduce the deterioration of food while the time-temperature indicator would insure that the product was stored and handled within the time and temperature window for which the product was designed, to insure safety. Labuza et al (1992) have suggested that predictive microbiology should be used to evaluate the safety of MAP foods. Low dose irradiation and MAP have been combined to extend shelf-life (Thayer, 1993). Irradiation reduces the numbers of spoilage and pathogenic vegetative organisms while a modified atmosphere reduces the likelihood that those not destroyed will grow significantly in number. Lambert et al (1992) have demonstrated a substantial increase in the shelf-life of fresh pork treated with both irradiation and MAP. Other combinations such as antimicrobial films combined with MAP or oxygen absorbers combined with antimicrobial films may find commercial uses. Zeitoun and Debevere (1991) have suggested that combining a simple lactic acid dip combined with MAP would enhance the shelf-life of fresh poultry.

11.5 Conclusions

It can be expected that safety will continue to be an important attribute for foods. New packaging technologies which improve quality, usefulness, or reduce environmental impact will also be required to maintain a high level of safety. Active packaging systems will not be an exception. Those active packaging systems which reduce the risks associated with foods may find niche markets for products at the highest risk of deterioration. MAP of non-sterile foods is one example where additional safety measures such as use of microbial inhibitors or indicators of temperature abuse would be useful. Recycled materials for food packaging is another. Active packaging systems which provide benefits for foods will have to adhere to governmental regulatory standards in most of the world. This will inhibit the introduction of some active systems. Antimicrobial films are a prime example. Developers of such materials should understand the safety and regulatory implications of their work early in the process if they expect to be successful.

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