Packaging of Foods

Packaging of Foods AL Brody, Rubbright Brody Inc., Duluth, GA, USA Ó 2014 Elsevier Ltd. All rights reserved. This article is reproduced from the previous edition, volume 3, pp 1611–1623, Ó 1999, Elsevier Ltd.

Packaging is intended to protect foods against environmental invasion. Among the many external variables that may adversely affect foods are an excess or deficiency of moisture, oxygen, dirt, humans (through tampering), dust, animals, insects, and microorganisms. Packaging and processing are increasingly becoming integrated with each other; an example is canning, which is really a packaging and thermal preservation operation in which the can, its product contents, the filling temperature, air removal, closure, heating, cooling, and distribution must be an uninterrupted continuum, or else preservation is not effected. More traditional preservation processes, such as drying and freezing, do not necessarily require close relationships between the product, process, and packaging; the process and the packaging may be separate and the preservation effect still will be achieved. In contrast, in preservation processes, such as thermal pasteurization, modified-atmosphere packaging, aseptic packaging, retort pouch, and tray packaging, it is necessary to integrate all the elements to ensure the optimum preservation of the contained foods. For example, in aseptic packaging, preservation is achieved by sterilization of the product independently of the package, and the packaging equipment and assembly environment therefore must be sterile to exclude microorganisms from the ultimately hermetically sealed package. It is essential that the operations be connected by sterile linkages and that no microorganisms are permitted to contaminate any element. For these reasons, it has become increasingly important that the packaging be incorporated into the system if the objectives of delivering safe and high-quality food are to be achieved. To understand fully the role of packaging in food preservation, it is perhaps instructive to offer a few definitions. ‘Packaging’ is a term describing the totality of containment for the purpose of protecting the food contents and includes the package material, its structure and the equipment that marries the package structure to the food. Package materials are the components that constitute the structures usually known as packages or containers. Package materials are no longer single elements but rather are composites of several different materials. In addition, new forms of packaging increasingly are replacing the traditional cans, bottles, jars, cartons, and cases.

Preservation Requirements of Common Food Categories Meats Fresh Meat Most meat offered to consumers is freshly cut, with little further processing to suppress the normal microbiological flora present from the contamination received during the killing and breaking operations required to reduce carcass meat to edible cuts. Fresh meat is highly vulnerable to microbiological deterioration from indigenous microorganisms. These

Encyclopedia of Food Microbiology, Volume 2

microorganisms can range from benign forms, such as lactic acid bacteria or slime-formers, to proteolytic producers of undesirable odors and pathogens, such as Escherichia coli O157:H7. The major mechanisms that retard fresh meat spoilage are temperature reduction to (or near) the freezing point and a reduced oxygen atmosphere during distribution to retard microbial growth. Reduced oxygen levels could provide conditions for the expression of pathogenic anaerobic microorganisms, a situation usually obviated by the presence of competitive spoilage organisms. Reduced oxygen levels also lead to the color of fresh meat being the purple of myoglobin; exposure to air converts the natural meat pigment to the bright cherry-red oxymyoglobin characteristic of most fresh meat offered to and accepted by consumers in industrial societies. Reduced oxygen packaging is achieved through the mechanical removal of air from the interiors of gas-impermeable multilayer flexible material pouches closed by heat-sealing the end after filling.

Ground Meat About 40% of fresh beef is offered in ground or minced form to enable the preparation of hamburger sandwiches and related foods. Ground beef was originally a by-product – that is, the trimmings from reducing muscle to edible portion size. The demand for ground beef is now so great that some muscle cuts are ground specifically to meet the demand. Grinding the beef further distributes the surface and belowsurface microflora and thus provides a rich substrate for microbial growth even under refrigerated conditions. Relatively little pork is reduced to ground fresh form; however, increasing quantities of poultry meat are being comminuted and offered fresh to consumers, both on its own and as a cheaper substitute for ground beef. The major portion of ground beef is ground coarsely at abattoir level and packaged under reduced O2 levels for distribution at refrigeration temperatures to help retard microbiological growth. The most common packaging technique is pressure-stuffing into chubs, which are tubes of flexible gas-impermeable materials closed at each end by tight-fitting metal clips. Pressurestuffing the pliable contents forces most of the air out of the ground beef, and because there is no head-space within the package, little air is present to support the growth of aerobic spoilage microorganisms, such as Lactobacillus and Leuconostoc spp. At the retail level, the coarsely ground beef is ground finely to restore the desirable oxymyoglobin red color and to provide the consumer with the desired product. In almost all instances, the retail cuts and portions are placed in expanded polystyrene (EPS) trays, which are overwrapped with plasticized polyvinyl chloride (PVC) film. The tray materials are resistant to fat and moisture to the extent that many trays are lined internally with absorbent pads to absorb the purge from the meat as it ages or deteriorates in the retail packages. Because of the prognosis, the PVC materials are not sealed but rather are tacked so that the somewhatwater-vapor-impermeable structure does not permit loss of

http://dx.doi.org/10.1016/B978-0-12-384730-0.00244-5

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significant moisture during short refrigerated distribution. Being a poor gas barrier, PVC film permits the access of air and hence the oxymyoglobin red color is retained for the short duration of retail distribution.

Case-Ready Meat For many years, attempts have been made to shift the retail cutting of beef and pork away from the retailer’s back room and into centralized factories. This movement has been stronger in Europe than in the United States, but some action has been detected in the latter country in the wake of the E. coli O157:H7 incidents. Case-ready retail packaging in the United Kingdom where the practice is relatively common, involves cutting and packaging meat under extremely hygienic conditions to reduce the probability of microbiological contamination beyond that of the indigenous microflora. Packaging is usually in a gasbarrier structure, typically a gas–moisture barrier foam polystyrene trays heat-sealed with polyester gas-barrier film. The internal gas composition is altered to a high content of O2 (up to 80%) and of CO2 (up to 30%), with the remainder (if any) being nitrogen as a filler gas to ensure against package collapse arising from internal vacuum formation. The high O2 concentration fosters the retention of the oxymyoglobin red color preferred by consumers, while the elevated CO2 level suppresses the growth of aerobic spoilage microorganisms. Using this or similar technologies, refrigerated microbiological shelf lives of retail cuts may be extended from a few days to as much as a few weeks, permitting long-distance distribution, for example, from a central factory to a multiplicity of retail establishments. One thesis favoring the centralized packaging of ground beef is that the probability of the presence of E. coli O157:H7 is reduced. On the other hand, if the pathogen is present at the central location, the probability of it being spread among a number of retailers is increased greatly. Nevertheless, the use of central factories, which probably would be under federal government supervision in the United States, and certainly under technical supervision, would increase the probability of the emerging packaged meat being microbiologically safe. Alternative packaging systems for case-ready beef and pork include the ‘master bag’ system used widely for freshly cut poultry (see Poultry section) in which retail cuts are placed in conventional PVC film–overwrapped EPS trays and the trays are multipacked in gas-barrier pouches whose internal atmospheres are enhanced with CO2 to retard the growth of aerobic spoilage microorganisms. Another popular system involves the use of gas-barrier trays with heat-seal closure using flexible gas and moisture barrier materials. Conventional non-gas-barrier trays such as EPS may be overwrapped with gas–moisture barrier flexible films subsequently shrunk tightly around the tray to impart an attractive appearance. Other systems, all of which involve the removal of O2, include vacuum skin packaging in which a film is heated and draped over the meat on a gas–moisture barrier tray. The film clings to the meat so that no head-space remains, with the result that the meat retains the purple color of myoglobin. In one such system, the drape film is a multilayer whose outer gas-barrier layer may be removed by the retailer, exposing a gas-permeable film that permits the entry of air, which reblooms the pigment and restores the

desired color. Variations on this double film system include packaging systems in which the film is not multilayer but is composed of two independent flexible layers, the outer being impermeable to gas and moisture and the inner layer being gas permeable to permit air entry to restore the red color. In all instances, the microbiological shelf life is extended by reduced temperature plus reduced O2 levels, which incidentally or intentionally may be enhanced by elevated CO2 concentration.

Processed Meat Longer term preservation of meats may be achieved by curing, using agents such as salt, sodium nitrite, sugar, seasonings, spices, and smoke, and by processing methods such as cooking and drying. These treatments alter the water activity, add antimicrobial agents, provide a more stable red color, and generally enhance the flavor and mouth feel of the cured meats. Cured meats often are offered in tubular or sausage form, which means that the shape is dictated by the traditional process and consumer demand. Because of the added preservatives, the refrigerated shelf life of processed meat is generally several times longer than that of the fresh meat. Because cured meats are not nearly so sensitive to oxygen variations as fresh meat, the use of reduced O2 atmospheres to enhance the refrigerated shelf life is quite common. The O2 reduction may be achieved by mechanical vacuum, inert gas flushing, or a combination of methods. Because the conditions have been changed to obviate the growth of anaerobic pathogenic microorganisms, reduced oxygen conditions generally are effective in retarding the growth of aerobic spoilage microorganisms. The containers for reduced O2 packaging of cured meats are selected from a multiplicity of materials and structures depending on the protection required and the marketing needs: Frankfurters generally are sold in twin web vacuum packages in which the base tray is an in-line thermoformed nylon–polyvinylidene chloride (PVDC) web and the closure is a heat-sealed polyester (PET)/PVDC flexible material. Sliced luncheon meats and similar products are packed in thermoformed unplasticized PVC or polyacrylonitrile trays, heat-seal closed with PET/PVDC. Sliced bacon packaging employs one of several variations of PVDC skin packaging (in contact with the surface of the product) to achieve the oxygen barrier. Ham may be fresh, cured, or cooked, with the cooking often performed in the package. The oxygen barrier materials employed are usually a variation of nylon/PVDC in pouch form.

Poultry Poultry meat is most commonly chicken, but turkey is becoming an increasingly significant category of protein. Furthermore, chicken is increasingly penetrating the cured meat market as a less expensive but nutritionally and functionally similar substitute for beef or pork. Since the 1970s, poultry processing in industrial societies has shifted into large-scale, almost entirely automated killing and dressing operations. In such facilities, the dressed birds are chilled in water to near the freezing point, after which they usually are cut into retail parts and packaged in case-ready form: EPS trays overwrapped with printed PVC or polyethylene film.

Packaging of Foods

The package is intended to appear as if it has been prepared at the retailer’s location, but in reality it is only a moisture and microorganism barrier. Individual retail packages, however, may be multipacked in gas-impermeable flexible materials to permit gas flush packaging, thus extending the refrigerated shelf life of the fresh poultry products. Poultry is especially susceptible to infection with Salmonella spp., which are pathogenic in large quantities. Such organisms are not removed or destroyed by the extensive washing and chemical sanitation of current poultry-processing plants, merely reduced in numbers. Modified-atmosphere packaging has relatively little effect on Salmonella and so refrigeration during distribution is critical in the drive to avoid increasing populations of this bacterium. All meat products may be preserved by thermal sterilization in metal cans or, less frequently, glass jars. The product is filled into the container, which is hermetically sealed, usually by double-seam metal end closure (see Figure 2). After sealing, the cans are retorted to destroy all microorganisms present and cooled to arrest further cooking. The metal (or glass) serves as a barrier to gas, moisture, and microbes to ensure indefinite microbiological preservation. Cans or jars do not, however, ensure against further biochemical deterioration of the contents.

Fish Fish is among the most difficult of all foods to preserve in its fresh state because of its inherent microbiological population, many organisms of which are psychrophilic (i.e., capable of growth at refrigerated temperatures). Furthermore, seafood may harbor a nonproteolytic, quasipsychrophilic anaerobic pathogen, Clostridium botulinum type E. The need to prolong the refrigerated shelf life of fresh fish suggests the application of modified-atmosphere packaging in which reduced O2 levels and elevated CO2 levels are present (Table 1). A reduced O2 atmosphere, however, can permit the expression of type E botulinum, and for this reason, reduced O2 packaging for seafood is discouraged in the United States. This is not the situation in Europe, where gas-barrier flexible and semirigid plastic packaging similar to that described for case-ready fresh beef often is applied. Packaging for fresh seafood is generally moisture resistant but not necessarily resistant against microbial contamination. Simple polyethylene film is employed often as liners in

Table 1 Pathogens of concern in modified-atmosphere-packaged and vacuum-packaged foods Psychrotrophs – growth at 3–4  C Listeria monocytogenes Yersinia enterocolitica Bacillus cereus Nonproteolytic Clostridium botulinum Pseudopsychrotrophs – growth at 7–8  C Escherichia coli O157:H7 Salmonella sp. Mesophiles – growth at >10  C Proteolytic Clostridium botulinum

Table 2

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Ranges for bacterial growth

Organism

pH range

Gram-negative bacteria Escherichia coli Pseudomonas fluorescens Salmonella typhimurium

4.4–9.0 6.0–8.5 5.6–8.0

Gram-positive bacteria Bacillus subtilis Clostridium botulinum Lactobacillus sp. Staphylococcus aureus

4.5–8.5 4.7–8.5 3.8–7.2 4.3–9.2

corrugated fiberboard cases. The polyethylene serves not only to retain product moisture but also to protect the structural case against internal moisture. Seafood may be frozen, in which case the packaging is usually a form of moisture-resistant material in addition to a structure such as polyethylene pouches or polyethylenecoated paperboard cartons. Canning seafood is much like that of meats as all seafoods have a pH above 4.6 and thus require high-pressure cooking or retorting to effect sterility in metal cans (Table 2). One variation unique to seafood is thermal pasteurization, in which the product is packed into plastic cans under reasonably clean conditions, achievable in contemporary commercial seafood factories. The filled and hermetically sealed cans are heated to temperatures of up to 80  C to effect pasteurization to permit several weeks of refrigerated shelf life. The system is usually effective because C. botulinum type E spores are thermally sensitive and may be destroyed by temperatures of 80  C. To ensure against growth of other pathogens that may grow at ambient temperatures, however, distribution at refrigerated temperatures is dictated.

Dairy Products Milk Milk and its derivatives are generally excellent microbiological growth substrates and therefore are potential sources of pathogens. For these reasons, almost all milk is pasteurized thermally as an integral element of processing. Refrigerated distribution generally is dictated for all products that are pasteurized to minimize the probability of spoilage. Milk generally is pasteurized and packaged in relatively simple polyethylene-coated paperboard gable-top cartons or extrusion blow-molded polyethylene bottles for refrigerated short-term (several days to 2 weeks) distribution. Such packages offer little beyond containment and avoidance of contamination as protection benefits; they retard the loss of moisture and resist fat intrusion. Newer forms of milkpackaging incorporate reclosure, a feature that was missing from the traditional gable-top cartons. Furthermore, modern packaging environmental conditions have been upgraded microbiologically to enhance refrigerated shelf life by presterilizing the equipment, shrouding, and using clean air. An alternative, popular in Canada, employs polyethylene pouches formed on vertical form, fill, and seal machines and are heat-sealed after filling. This variant has been enhanced by

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reengineering into aseptic format, a system that has not become widely accepted. Pouch systems generally are less expensive than paperboard and semirigid bottles, but they are less convenient for consumers. Little difference exists between the three packaging systems from a microbiological perspective. In some countries, aseptic packaging is employed to deliver fluid dairy products that are shelf stable at ambient temperatures. The most common processing technology is ultrahigh-temperature short-time thermal treatment to sterilize the product followed by aseptic transfer into the packaging equipment. Three general types of aseptic packaging equipment are employed commercially: vertical form, fill, and seal in which the paperboard composite material is sterilized by high-temperature, high-concentration hydrogen peroxide (removed by mechanics plus heat); erected preformed paperboard composite cartons, which are sterilized by hydrogen peroxide spray (removed by heat); and bag-in-box, in which the plastic pouch is presterilized by ionizing radiation. The former two generally are employed for consumer sizes, while the last is applied to hotel, restaurant, or institutional sizes, largely for ice cream mixes. Fluid milk generally is pasteurized, cooled, and filled into bag-in-box pouches for refrigerated distribution.

Cheese Fresh cheeses such as cottage cheese fabricated from pasteurized milk generally are packaged in polystyrene tubs or polyethylene pouches for refrigerated distribution. Such packages afford little microbiological protection beyond acting as a barrier against recontamination – that is, they are little more than rudimentary moisture loss and dust protectors, but they are adequate because the distribution time is so short. Enhancement of refrigerated shelf life may be achieved by clean filling or the use of a low-O2, high-CO2 atmosphere, all of which retard the growth of lactic acid spoilage microorganisms.

Fermented Milks Fermented milks such as yogurts fall into the category of fresh cheeses from a packaging perspective – that is, they are packaged in polystyrene or polypropylene cups or tubs to contain and to protect minimally against moisture loss and microbial recontamination. Their closures are not hermetic and so gas passes through both the closures and the plastic walls, and microorganisms could enter after the package is opened. Because the refrigerated shelf life is short, however, few measures are taken from a packaging standpoint to lengthen the shelf life. Clean packaging often is used to achieve several weeks of refrigerated shelf life. Aseptic packaging occasionally is used to extend the ambient temperature shelf life of these products. Two basic systems are employed: one uses preformed cups, and the other is thermoform, fill, and seal. In the former, the cups are sterilized by spraying with H2O2 and heating to remove the residue before filling and heat-sealing a flexible closure to the flanges of the cups, which are impermeable to gas and water vapor. In the thermoform, fill, and seal method, a sheet of multilayer barrier plastic sheet (usually polystyrene plus PVDC) is immersed in H2O2 to sterilize it, air-knifed to remove the residual sterilant, heated to softening, and formed into cups by pressure. The web containing the connected cups

is within a sterile environment under positive pressure of sterile air. The cavities are filled with sterile product and a flexible barrier material web, usually an aluminum foil lamination (also sterilized by H2O2 immersion), is heat-sealed to the cup flanges. Filled and sealed cups then pass through a sterile air lock. These aseptic dairy packaging systems also may be employed for juices and soft cheeses. Recently, aseptic packaging of dairy products has been complemented by ultraclean packaging on both preformed cup deposit, fill, and seal and thermoform, fill, and seal systems. In these systems, which are intended to offer extended refrigerated shelf life for low-acid dairy products, the microbicidal treatment is with hot water to achieve a four-dimensional (4D) kill (i.e., four times the decimal reduction time) on the package material surfaces. The same systems may be employed to achieve ambient temperature shelf stability for high-acid products, such as juices and related beverages. Cured cheeses are subject to surface mold spoilage as well as to further fermentation by the natural microflora. These microbiological growths may be retarded by packaging under reduced O2 atmospheres which may or may not be complemented by the addition of CO2. To retain the internal environmental condition, the use of gas-barrier package materials is commercial. Generally, flexible barrier materials such as nylon plus PVDC are employed on horizontal flow wrapping machines or on twin web thermoform, vacuum, and seal machines. On twin-web machines, the flat sealing web is usually a variant of polyester plus PVDC. One problem is that some cured cheeses continue to produce CO2 as a result of fermentation, and so the excess gas must be able to escape from the package or else the package might bulge or even burst. Somewhat less gas-impermeable materials are suggested for such cheeses. In recent years, shredded cheeses have been popularized. Shredded cheeses have increased surface areas that increase the probability of microbiological growth. Gas packaging under CO2 in gas-impermeable pouches is mandatory. One feature of all shredded cheese packages today is the zipper reclosure, which does not represent an outstanding microbiological barrier after the package has first been opened.

Ice Cream Ice cream and similar frozen desserts are distributed under frozen conditions and so are not subject to microbiological deterioration, but the product must be pasteurized before freezing and packaging. The packaging needs to be moisture resistant because of the presence of liquid water before freezing and sometimes during removal from refrigeration for consumption. Water-resistant paperboard, polyethylenecoated paperboard, and polyethylene structures are usually sufficient for containment of other frozen desserts.

Fruit and Vegetables In the commercial context, fruits are generally high-acid foods and vegetables are generally low acid. Major exceptions are tomatoes, which commercially (not botanically) are regarded as vegetables, and melons and avocados, which are low acid. The most popular produce form is fresh, and increasingly fresh cut or minimally processed. Fresh produce is a living,

Packaging of Foods ‘breathing’ entity with active enzyme systems fostering the physiological consumption of O2 and production of CO2 and water vapor. From a spoilage standpoint, fresh produce is more subject to physiological than to microbiological spoilage, and measures to extend the shelf life are designed to retard enzyme– driven reactions and water loss. The simplest means of retarding fresh produce deterioration is temperature reduction, ideally to near freezing point but more commonly to about 4–5  C. Temperature reduction also reduces the rate of microbiological growth, which is usually secondary to physiological deterioration. Since the 1960s, alteration of the atmospheric environment in the form of modified or controlled atmosphere preservation and packaging has been used commercially to extend the refrigerated shelf life of fresh produce items, such as apples, pears, strawberries, lettuce, and now fresh-cut vegetables. Controlled atmosphere preservation has been confined largely to warehouses and transportation vehicles such as trucks and seaboard containers. In this form of preservation, the O2, CO2, ethylene, and water vapor levels are under constant control to optimize refrigerated shelf life. For each class of produce a separate set of environmental conditions is required for optimum preservation effect. In modified-atmosphere packaging, the produce is placed in a package structure and an initial atmosphere is introduced. The normal produce respiration plus the permeation of gas and water vapor through the package material and structure drive the interior environment toward an equilibrium gas environment that extends the produce quality retention under refrigeration. In some instances, the initial gas may be air (passive atmosphere establishment). Produce respiration rapidly consumes most of the oxygen within the package and produces CO2 and water vapor to replace it, generating the desired modified atmosphere. The target internal atmosphere is to retard respiration rate and microbiological growth. Reduced-O2 and elevated-CO2 levels independently or in concert retard the usual microbiological growth on fruit and vegetable surfaces. One major problem is that produce may enter into respiratory anaerobiosis if the O2 concentration is reduced to near extinction. In respiratory anaerobiosis, the pathways produce undesirable compounds, such as alcohols, aldehydes, and ketones, instead of the aerobic end products, such as CO2. To minimize the production of these undesirable end products, elaborate packaging systems are being developed. Most of these involve mechanisms to permit air into the package to compensate for the oxygen consumed by the respiring produce. High-gas-permeability plastic films, microperforated plastic films, plastic films disrupted with mineral fill, and films fabricated from polymers with temperature-sensitive side chains have all been proposed or used commercially. The need for reduced temperature is emphasized in modified-atmosphere packaging because the dissolution rate of CO2 in water is greater at lower temperatures than at higher temperatures. Carbon dioxide is one of the two major gases involved in reducing the rate of respiration and the growth of microorganisms. Since the late 1980s, fresh cut vegetables, especially lettuce, cabbage, and carrots, have been a major product in both the

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retail trade and the hotel, restaurant, and institutional markets. Cleaning, trimming, and size reduction lead to a greater surface-area-to-volume ratio and expression of fluids from the interior, increasing the respiration rate and offering a better substrate for microbiological growth than the whole fruit or vegetable. On the other hand, commercial freshcutting operations generally are far superior to mainstream fresh produce handling in cleanliness, speed through the operations, temperature reduction, and application of microbicides, such as chlorine. Although some would argue, on the basis of microbial counts found in fresh cut produce in distribution channels, that uncut produce is safer, the paucity of its cleaning coupled with the rarity of adverse incidents related to fresh-cut produce lead to the opposite conclusion – that fresh cut is significantly safer microbiologically. Another argument is that the low-O2 environment within most freshcut produce packages plus the risk of soil contamination lead to ideal conditions for the proliferation of C. botulinum. Furthermore, distribution temperatures are often in excess of 10  C, well within the range of growth and production of spores. However, extensive testing has demonstrated that after responsible fresh-cut processing, pathogenic spores are present in relatively small numbers, distribution temperatures prior to retail level are significantly lower than for uncut produce, and times are too short for pathogenic expression. These data indicate that while anaerobic pathogenic problems may occur, they are significantly less likely in fresh cut than in uncut fruit and vegetables. Uncut produce packaging includes a multitude of materials, structures and forms, ranging from traditional containers such as wooden crates, to inexpensive ones such as injection-molded polypropylene baskets, to polyethylene liners within waxed, corrugated fiberboard cases. Much of the packaging is designed to help retard moisture loss from the fresh produce or to resist the moisture evaporating or dripping from the produce (or occasionally its associated ice), to ensure the maintenance of the structure throughout distribution. Some packaging designs recognize the issue of anaerobic respiration and incorporate openings to allow passage of air into the package, for example, perforated polyethylene pouches for apples or potatoes. Almost none of the contemporary packaging for fresh uncut produce encompasses any specific microbiological barriers or countermeasures. That result is a direct extension of the observation that uncut produce ‘processing’ is virtually nonexistent. Packing-house operations include collection and the removal of debris and gross dirt, and packaging is usually the least expensive structure that will contain the contents during distribution, often at suboptimum temperatures. For freezing, vegetables are cleaned, trimmed, cut, and blanched, before freezing and then packaging (or packaging and then freezing). Blanching and the other processing operations reduce the numbers of microorganisms. Fruit may be treated with sugar to help retard enzymatic browning and other undesirable oxidations. Produce may be individually quick frozen using cold air or cryogenic liquids before packaging, or frozen after packaging as in folding paperboard cartons. Frozen food packages are generally relatively simple monolayer polyethylene pouches or polyethylene-coated paperboard to retard moisture loss. No special effort is engineered to obviate further

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Packaging of Foods

microbiological contamination after freezing, although the polyethylene pouches are generally heat-sealed. Canning of low-acid vegetables to achieve long-term ambient temperature microbiological stability is the same as for other low-acid foods, with blanching before placement in steel cans (today all welded side-seam tin-free steel, with some two-piece cans replacing the traditional three-piece type), hermetic sealing by double-seaming and retorting and cooling. Canned fruit generally is placed into lined three-piece steel cans using hot-filling coupled with postfill thermal treatment. Increasingly, one end is ‘easy open’ for consumer convenience. Newer techniques involve placing fruit hot into multilayer gas- and moisture-impermeable tubs and cups prior to heat-sealing with flexible barrier materials and subsequent thermal processing to achieve ambient temperature shelf stability or extended refrigerated temperature shelf life. These plastic packages are intended to provide greater convenience for the consumer as well as to communicate that the contained product is not ‘overprocessed’ like canned food.

Tomato Products The highly popular tomato-based sauces and pizza toppings must be treated as low-acid foods if they contain meat, as so many do. For marketing purposes, tomato-based products for retail sale commonly are packed in glass jars with reclosable metal lids. The glass jars often are retorted after filling and hermetic sealing; major differences from the technique using metal cans include counterpressured retorting and longer times for heating and cooling, as the thick-walled glass is a thermal insulator.

Juices and Juice Drinks Juices and fruit beverages may be hot-filled or aseptically packaged. Traditional packaging has been hot-filling into steel cans and glass bottles and jars. Aseptic packaging, described previously for paperboard composite cartons, is being applied for polyester bottles using various chemical sterilants to effect the sterility of the package and closure interiors. Much fruit beverage currently is hot-filled into heat-set polyester bottles capable of resisting temperatures of up to 80  C without distortion. Hermetic sealing of the bottles provides a microbiological barrier, but the polyester is a modest oxygen barrier and so the ambient temperature shelf life from a biochemical perspective is somewhat limited. Since the 1970s, high-acid fluid foods such as tomato pastes and non-meat-containing sauces have been hot-filled into flexible pouches, usually on vertical form, fill, and seal machines. The hot-filling generates an internal vacuum within the pouch after cooling so that the contents are generally shelf stable at ambient temperature. Package materials are usually laminations of polyester and aluminum foil with linear lowdensity polyethylene (LLDPE) internal sealant; this resists the relatively lengthy exposure to the high heat of the contents during and immediately following filling. The heat-seal is hermetic. Some efforts have been made to employ transparent gas- and water-vapor barrier films in the structures: polyester– ethylene vinyl alcohol laminations with the same LLDPE sealant. Transparent flexible pouches offer the opportunity for the consumer to see the contents, and for the hotel, restaurant,

or institutional worker to identify the contents without needing to read the label.

Other Products A variety of food products that do not fall clearly into the meat, dairy, fruit, or vegetable categories may be described as ‘prepared foods,’ a rapidly increasing segment of the industrial society food market during the 1990s. Prepared foods are those that combine several different ingredient components into dishes that are ready to eat or simply require heating. If the food is canned, the thermal process must be suitable for the slowest heating component, meaning that much of the product is overcooked to ensure microbiological stability. If it is frozen, the components are separate, but the freezing process reduces the eating quality. The preferred preservation technology from a quality retention or consumer preference perspective is refrigeration. Incorporation of several ingredients from a variety of sources correctly implies many sources for microorganisms – aerobic, anaerobic, spoilage, benign, and pathogenic. Where refrigeration is the sole barrier, microbial problems are minimized by reducing the time between preparation and consumption to less than 1 day (under refrigeration at temperatures above freezing) plus a nodding acknowledgment of cleanliness during preparation. As commercial operations attempt to prolong the quality retention periods beyond same-day or next-day consumption, enhanced preservation ‘hurdles’ have been introduced. These microbiological growth retardant factors include elevated salt or sugar concentrations, reduced water activity, reduced pH to minimize the probability of pathogenic microbiological growth, selection of ingredients from reduced microbial count sources, and modified-atmosphere packaging. The last often is suggested as a potential stimulus for the growth of pathogenic anaerobic microorganisms, because the multiple ingredient sources can almost ensure the presence of Clostridium spores, and the reduced O2 low-acid conditions are common to the types of products, such as potato salad and pasta dishes. Furthermore, distribution temperatures often may be in the 5  C range or higher. Packaging for air-packaged prepared dish products generally is oriented thermoformed polystyrene trays with oriented polystyrene dome closures snap-locked into position (i.e., no gas, moisture, or microbiological barriers of consequence). Refrigerated shelf life is measured in days. When the product is intended to be heated for consumption, the base tray packaging may be thermoformed polypropylene or crystallized polyester with no particular barrier closure. For modified-atmosphere packaging, the tray material is a thermoformed, coextruded polypropylene–ethylene vinyl alcohol with a flexible gas– moisture barrier lamination closure heat-sealed to the tray flanges. Refrigerated shelf life for such products may be measured in weeks. For several years, the concept of pasteurizing the contents, vacuum packaging, and distribution under refrigeration has been debated and commercially developed in both the United States and Europe. The sous-vide technique is the most publicized process of this type. In sous-vide processing, the product is packaged under vacuum and is heat-sealed in an appropriate gas- and water-vapor barrier flexible package structure, such as

Packaging of Foods

aluminum foil lamination. The packaged product is processed thermally at less than 100  C to destroy spoilage microorganisms and then chilled for distribution under refrigerated or (in the United States) frozen conditions. The US option is to ensure against the growth of pathogenic anaerobic microorganisms. A similar technology is cook-chill in which pumpable products such as chili, chicken à la king, and cheese sauce are hot-filled at 80  C or more into nylon pouches, which immediately are chilled (in cold water) to 2  C and then distributed at temperatures of 1  C. The hot-filling generates a partial vacuum within the package to virtually eliminate the growth of any spoilage microorganisms that might be present. This listing is only a sampling of the many alternative packaging forms offered and employed commercially for foods subject to immediate microbiological deterioration. An entire encyclopedia would be required to enumerate all of the known options available to the food-packaging technologist with the advantages and issues associated with each.

Package Materials and Structures Package Materials In describing package materials, different conventions are employed depending on the materials and their origins. The commercial conventions are used with some common indicator of quantitative meaning to establish relative values.

Paper The most widely used package material in the world is paper and paperboard derived from cellulose sources, such as trees. Paper is used less in packaging because its protective properties are almost nonexistent and its usefulness is almost solely as decoration and dust cover. Paper is cellulose fiber mat in gauges of less than 250 microns. When the gauge is 250 microns to perhaps as much as 1000 microns, the material is known as paperboard, which in various forms can be an effective structural material to protect contents against impact, compression, and vibration. Only when coated with plastic does paper or paperboard provide any sort of protection against other environmental variables such as moisture. For this reason, despite their long history as packaging materials, paper and paperboard are only infrequently used as protective packaging against moisture, gas, odors, or microorganisms. Paper and paperboard may be manufactured from trees or from recycled paper and paperboard. Virgin paper and paperboard, derived from trees, have greater strength than recycled materials whose fibers have been reduced in length by multiple processing. Therefore, increased gauges or calipers of recycled paper or paperboard are required to achieve the same structural properties. On the other hand, because of the short fiber lengths, the printing and coating surfaces are smoother. Paper and paperboard are moisture-sensitive, changing their properties significantly and thus often requiring internal and external treatments to ensure suitability.

Metals Two metals commonly are employed for package materials: steel and aluminum. The former is traditional for cans and glass

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bottle closures, but it is subject to corrosion in the presence of air and moisture and so is almost always protected by other materials. Until the 1980s, the most widely used steel protection was tin, which also acted as a base for lead soldering of the side seams of ‘tin’ cans. When lead was declared toxic and removed from cans during the 1980s in the United States, tin also was found to be superfluous, and its use as a steel can liner declined. The tin in ‘tin-free’ cans was chrome and chrome oxide. The construction and closure techniques of metal cans are shown in Figures 1–3. In almost every instance, the coated steel is further protected by organic coatings such as vinyls and epoxies, which provide the principal protection. Steel is rigid; is a perfect microbial-, gas-, and water-vaporbarrier; and is resistant to every temperature to which a food may be subjected. Because steel–steel or steel–glass interfaces are not necessarily perfect, the metal often is complemented by resilient plastic to compensate for the minute irregularities. Aluminum is lighter in weight than steel and easier to fabricate; it therefore has become the metal of choice for beverage containers in the United States and is favored in other countries. As with steel, the aluminum must be coated with plastic to protect it from corrosion. It is the most commonly used material for can-making in the United States. Aluminum cans, however, must have internal pressure from CO2 or N2 to maintain their structure, and so aluminum is not used widely for food-canning applications in which internal vacuums and pressures change as a result of retorting. Aluminum may be rolled to very thin gauges (8– 25 microns) to produce foil, a flexible material with excellent microbial-, gas-, and water-vapor barrier properties when it is protected by plastic film. Aluminum foil generally is regarded as the only ‘perfect’ barrier flexible package material. Its deficiencies include a tendency to pinholing, especially in thinner gauges, and to cracking when flexed. In recent years, some applications of aluminum foil have been replaced by vacuum metallization of plastic films, such as polyester or polypropylene.

Canner’s end component

Canner’s end component

Canner’s end seam

Canner’s end seam

Body Side-wall beading

One-piece body

Maker’s end seam Side seam (a)

Maker’s end component

(b)

Figure 1 Metal can construction: (a) three-piece steel can, (b) twopiece steel or aluminum can. From Soroka, W., 1995. Fundamentals of Packaging Technology. Institute of Packaging Professionals, Herndon, Virginia with permission.

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Packaging of Foods

combined with each other and with other materials to deliver the desired properties.

Lining compound Can end

Polyethylene

Can body

Can end resting on body

First curl

Finished double seam

Figure 2 Operation of affixing or double-seaming a metal closure to a metal can body. From Soroka, W., 1995. Fundamentals of Packaging Technology. Institute of Packaging Professionals, Herndon, Virginia with permission.

Seaming wall radius

Seaming panel radius

Body hook radius Lining compound Seaming wall Body hook End hook

Chuck wall

Polypropylene Chuck wall radius

End hook radius Body wall Figure 3 Double-seam closure on a metal can. From Soroka, W., 1995. Fundamentals of Packaging Technology. Institute of Packaging Professionals, Herndon, Virginia with permission.

Glass The oldest and least expensive package material is glass, derived from sand. Furthermore, glass is a perfect barrier material against gas, water vapor, microorganisms, and odors. The transparency of glass often is regarded by marketers and consumers as a desirable property. Technologists may view the transparency as less than desirable because visible and ultraviolet radiation accelerates biochemical (particularly oxidative) reactions. Glass is energy intensive to produce; it is heavy and vulnerable to impact and vibration even though it has excellent vertical compressive strength. For these reasons, glass is being displaced by plastic materials in industrial societies.

Plastics

Polyethylene is the most used plastic in the world for both packaging and nonpackaging applications. It is manufactured in a variety of densities, ranging from 0.89 g cm 3 (very low density) to 0.96 g cm 3 (high density) and it is lightweight, inexpensive, impact-resistant, relatively easily fabricated, and forgiving. Polyethylene is not a good gas barrier and generally is not transparent but rather translucent. It may be extruded into film with excellent water-vapor and liquid containment properties. Low-density polyethylene film more commonly is used as a flexible package material. Low-density polyethylene is also extrusion-coated onto other substrates such as paper, paperboard, plastic, or even metal to impart water and water-vapor resistance or heat-sealability. Although used for flexible packaging, high-density polyethylene more often is seen in the form of extrusion blow-molded bottles with impact resistance, good water, and water-vapor barrier, but poor gas-barrier properties. Any of the polyethylene in proper structure functions as an effective microbial barrier.

The term ‘plastics’ describes a number of families of polymeric materials (Table 3), each with different properties. Most plastics are not suitable as package materials because they are too expensive or toxic in contact with food, or they do not possess properties desired in packaging applications. The most commonly used plastic package materials are polyethylene, polypropylene, polyester, polystyrene, and nylon. Each has different properties (Table 4). Plastics may be

Like polyethylene, polypropylene is a polyolefin, but it has better water-vapor barrier properties and greater transparency and stiffness. Although more difficult to fabricate, polypropylene may be extruded into films that are used widely for making pouches particularly on vertical form, fill, and seal machines. In cast film form, polypropylene is the heat-sealant of choice on retort pouches because of its fusion-sealing properties, and because in this form, it is a good microbial barrier. Polypropylene’s heat resistance up to about 133  C permits it to be employed for microwave-only heating trays. Unfortunately, microwave heating alone is insufficiently uniform to be a reliable mechanism for reducing microbiological counts or destroying heat-labile microbial toxins in foods.

Polyester A cyclical polymer that is relatively difficult to fabricate, polyethylene terephthalate polyester is increasingly the plastic of choice as a glass replacement in making food and beverage bottles. Polyester plastic is a fairly good gas and moisture barrier; in bottle, tray, or film form it is dimensionally stable and strong. Its heat resistance in amorphous form is sufficient to permit its use in hot-fillable bottles. When polyester is crystallized partially, the heat resistance increases to the level of being able to resist conventional oven heating temperatures. For this reason, crystallized polyester is employed to manufacture ‘dual ovenable’ trays for heat-and-eat foods (‘dual ovenable’ means that the plastic is capable of being heated in either conventional or microwave ovens). The transparency of polyester makes it highly desirable from a marketing standpoint for foods that are not light sensitive.

Nylon Polyamide or nylon is a family of nitrogen-containing polymers noted for their excellent gas-barrier properties. Moisture permeability tends to be less than in the polyolefin polymers

Packaging of Foods

Table 3

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Package plastic structures

Plastic

Structure

Qualities

Polyethylene (PE)

Three basic types: high-density, linear low-density, low-density Moisture barrier

Polypropylene (PP)

Higher temperature than polyethylene Low density, high yield Very good moisture barrier

Ethylene vinyl alcohol (EVOH)

Excellent O2 barrier resin Moisture sensitive, poor water barrier Used in coextrusion, expensive

Polyvinylidene chloride (PVDC)

Excellent O2, moisture, flavor, fat barrier Dense

Polyvinyl chloride (PVC)

Stiff, clear – without plasticizer Soft with plasticizer No barrier

Polyamide (PA) (Nylon)

Temperature resistant Very good O2 barrier Thermoformable

Polyethylene terephthalate (PET) (polyester)

High temperature after orientation

Polyacrylonitrile (PAN)

Very good O2 barrier Not processable in extrusion unless copolymer

Polystyrene (PS)

Stiff, brittle, clear Very little barrier

and nylon is somewhat hygroscopic, meaning that the gas barrier may be reduced in the presence of moisture. Gasand water-vapor barriers are enhanced by multilayering with polyolefins and high-gas-barrier polymers. Nylons are thermoformable and both soft and tough, and so they often are used for thermoformed processed meat package structures

in which the oxygen within the package is reduced to extend the refrigerated shelf life.

Polystyrene Polystyrene is a poor barrier to moisture or gas. It is, however, very machinable and usually highly transparent. Its structural

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Table 4

Packaging of Foods

Properties of plastic package materials

Material

Specific gravity

Clarity or color

Water-vapor transmissiona

Gas transmissionb

Resistance to grease

Polyethylene High density Medium density Low density Polypropylene Polystyrene Plasticized vinyl chloride Nylon

0.941–0.965 0.926–0.940 0.910–0.926 0.900–0.915 1.04–1.08 1.16–1.35 1.13–1.16

Semi-opaque Hazy to clear Hazy to clear Transparent Clear Clear to hazy Clear to translucent

Low Medium Good Good High High to low Varies

High High High High High High Low

Excellent Good Good Excellent Fair to good Good Excellent

Water-vapor transmission rate is measured in gm 2 for 24 h at 38  C and 90% relative humidity. Gas transmission is measured in cm3 ml 1 m 2 for 24 h at 1 atm, 30  C, and 0% relative humidity.

a

b

strength is not good unless the plastic is oriented or admixed with a rubber modifier that reduces the transparency. Polystyrene often is used as an easy and inexpensive tray material for prepared refrigerated foods.

Polyvinyl Chloride PVC is a polymer capable of being modified by chemical additives into plastics with a wide range of properties. The final materials may be soft films with high gas permeabilities, such as used for overwrapping fresh meat in retail stores; stiff films with only modest gas barrier properties; readily blow-moldable semirigid bottles; or easily thermoformed sheet for trays. Gas and moisture impermeability is fairly good but must be enhanced to achieve ‘barrier’ status. This material falls into a category of halogenated polymers, which are regarded by some environmentalists as less than desirable. For this reason, in Europe and to a lesser extent in the United States, PVC has been resisted as a package material.

Polyvinylidene Chloride PVDC is an excellent barrier to gas, moisture, fat, and flavors, but it is so difficult to fabricate on its own that it is almost always used as a coating on other substrates to gain the advantages of its properties.

Metal cans traditionally have been cylindrical (Figures 1–3), probably because of the need to minimize problems with heat transfer into the contents during retorting. Recently, metal – and particularly aluminum – has been fabricated into tray, tub, and cup shapes for greater consumer appeal, with consequential problems with measuring and computing the thermal inputs to achieve sterilization. During the 1990s, shaped cylinders entered the market again to increase consumer market share. Few have been applied for cans requiring thermal sterilization, but barrel and distorted body cans are not rare in France for retorted lowacid foods. Analogous regular-shaped cans are being used for hot-filling of high-acid beverages. Noted for its formability, glass traditionally has been offered in a very wide range of shapes and sizes, including narrow-neck bottles (Figure 4) and wide-mouth jars. Each represents its own singular problems in terms of fabrication, closure, and – when applicable – thermal sterilization. Thread

Sealing surface (land)

Neck ring (bead)

Finish

Neck ring parting line Neck

Ethylene Vinyl Alcohol Ethylene vinyl alcohol (EVOH) is an outstanding gas- and flavor-barrier polymer, which is highly moisture sensitive and so must be combined with polyolefin to render it an effective package material. Often EVOH is sandwiched between layers of polypropylene that act as water-vapor barriers and thus protect the EVOH from moisture.

Neck base Shoulder

Mould seam (parting line) Body

Package Structures Currently, rigid and semirigid forms are the most common commercial structures used to contain foods. Paperboard is most common, in the form of corrugated fiberboard cases engineered for distribution packaging. In corrugated fiberboard, three webs of paperboard are adhered to each other with the central or fluted section imparting the major impact and compression resistance to the structure. Folding cartons constitute the second most significant structure fabricated from paperboard. Folding cartons are generally rectangular in shape and often are lined with flexible films to impart the desired barrier.

Bottom plate parting line

Bottom

Heel Push-up

Base Toe-in

Figure 4 Glass bottle nomenclature. From Soroka, W., 1995. Fundamentals of Packaging Technology. Institute of Packaging Professionals, Herndon, Virginia with permission.

Packaging of Foods

Plastics are noteworthy for their ability to be formed into the widest variety of shapes. Thin films can be extruded for fabrication into flexible package materials. These flexible materials then may be employed as pouch or bag stock or as overwraps on cartons or other structures, or as inner protective liners in cartons, drums, and cases. Thicker films (sheets) may be thermoformed into cups, tubs, and trays for containment. Plastic resins may be injection- or extrusion-molded into bottles or jars by melting the thermoplastic material and forcing it, under pressure, into molds that constitute the shape of the hollow object (e.g., the bottle or jar).

See also: Cheese in the Market Place; Chilled Storage of Foods: Use of Modified Atmosphere Packaging; Food Packaging with Antimicrobial Properties; Fermented Milks: Range of Products; Fish: Spoilage of Fish; Heat Treatment of Foods – Principles of Pasteurization; Ice Cream: Microbiology; Spoilage of Meat; Curing of Meat; Spoilage of Cooked Meat and Meat Products; Milk and Milk Products: Microbiology of Liquid Milk; Milk and Milk Products: Microbiology of Dried Milk Products;

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Microbiology of Cream and Butter; Heat Treatment of Foods: Thermal Processing Required for canning.

Further Reading Brody, A.L., 1989. Controlled/Modified Atmosphere/Vacuum Packaging of Foods. Food & Nutrition Press, Trumbull, Connecticut. Brody, A.L., 1994. Modified Atmosphere Food Packaging. Institute of Packaging Professionals, Herndon, Virginia. Brody, A.L., Marsh, K.S., 1997. Wiley Encyclopedia of Packaging Technology, second ed. John Wiley, New York. Jairus, D., Graves, R., Carlson, V.R., 1985. Aseptic Packaging of Food. CRC Press, Boca Raton, Florida. Paine, F.A., Paine, H.Y., 1983. A Handbook of Food Packaging. Blackie, London. Robertson, G.L., 1993. Food Packaging. Marcel Dekker, New York. Soroka, W., 1995. Fundamentals of Packaging Technology. Institute of Packaging Professionals, Herndon, Virginia. Wiley, R.C., 1994. Minimally Processed Refrigerated Fruits and Vegetables. Chapman & Hall, New York.