PACKAGING AND CANNING, MODERN

PACKAGING AND CANNING, MODERN. Food packaging is an important part of food processing operations and food preservation. "Packaging" ensures safe product delivery to the ultimate consumer in a sound condition and at a minimum cost (Paine and Paine, 1983). In the last quarter of the twentieth century, many important developments in both materials and packaging systems led to the reduction of packaging costs and the development of novel and minimally processed foods.

Packaging serves a number of different functions including preservation, containment, and convenience. Preservation is one of its major roles: packaging protects the contents against environmental, physical, and mechanical hazards (oxygen, water/moisture, light, contamination from microorganisms, rodents, and insects, physical damage, chemical attack, etc.) during storage and distribution. Containment is another important function: packaging contains the food and keeps it secure until it is used. Packaging is also a means of providing useful information to the consumer; communication is its third important function. It provides a way of identifying the contents; attractive or eye-catching packaging helps to sell the product; and it provides a means of fulfilling any regulatory requirements concerning labeling of foods. In addition, food packages provide convenience: they unitize or group products together in useful amounts, have features like easy opening, dispensing, resealing after use, and so on. Finally, a successful, effective food package should fulfill many other requirements. It should have good machinability (that is, it should be easily filled, closed, and processed at high speeds); it should be aesthetically pleasing, recyclable or reusable, nontoxic, tamper-resistant (or tamper-evident); it should have a functional size and shape, be disposed of easily, have low cost, and be compatible with the food it contains.

There are three levels of packaging: primary, secondary, and tertiary. A primary package is in direct contact with the product. Usually, primary packages provide the major protective barrier. A secondary package usually contains several primary packages, and provides the strength for stacking in the warehouse. Like the secondary, a tertiary package contains a number of secondary packages. Its function is to hold together the secondary packages during distribution.

Due to the large variety of food products, a great deal of packaging materials, container types, packaging systems, and techniques exist. The selection and development of a package depend on the nature of the food, the desired shelf life of the product, the storage conditions, and the cost. It is a difficult task and requires in-depth knowledge of the food product and its deterioration mechanisms, transportation hazards, market and distribution requirements, and, finally, the properties and characteristics of all available packaging materials, machines, and systems.

Paperboard

Paper, and some combinations of paper-based packaging material, represents the most economical form of packaging. About 50 percent of all paperboard packaging is used to form corrugated boxes, and another 25 percent goes into fiberboard cartons. Paperboard packaging provides absolutely no oxygen or moisture protection for the product, but it does provide rigidity, mechanical support, and light barrier properties. Fiberboard cartons are popular forms of packaging materials: they are economical, collapsible, and printable; they provide versatility and excellent mechanical handling; they can have dispensing and/or resealing features, windows for product observation, and they can be used in multipacks. Another type of paperboard packaging, the corrugated box, is the most common type of shipping container. Paperboard is also used for the manufacture of composite cans and aseptic cartons, which consist of combinations of thin layers of aluminum foil, paperboard, plastic, adhesive, and coatings. The layers are either wound around a mandrel (composite can) or layered in a sheet (aseptic carton).

Glass

Glass containers are generally classified into two groups: bottles with narrow necks and jars with wide necks. Glass is chemically inert, it provides nearly absolute protection from oxygen, moisture, microorganisms, rodents, and insects, and, if colored properly, can filter out harmful UV light. However, glass has two negative properties: its heavy weight and fragility. Consumers prefer plastic packages to glass containers because plastic is lightweight, convenient, and not fragile. As a result, plastic packaging has replaced glass containers for many products and continues to do so. For example, glass containers for packaging milk, fruit juices, cooking oils, mayonnaise, soda drinks, and salad dressings have been replaced by plastic ones. Recently, the development of high-temperature resistant plastics led to the partial replacement of glass containers for jelly, ketchup, and spaghetti sauces. As plastic technology advances, other products usually packaged in glass containers will also be packed in plastic.

Metal

Metal containers, specifically those called "tin cans," have been widely used in the past and are still used for the production of commercially sterilized food products. The development of the metal can and the sterilization process are closely related. The series of operations that are part of the sterilization process is commonly called "canning." In canning sterilization processes, the product is sealed in a metal container and then treated thermally in order to destroy all pathogenic and spoilage microorganisms. This sequence of operations does not allow recontamination of the product after thermal treatment, and, as a result, it remains shelf-stable for a long period of time.

The use of metal containers has many advantages: they can be sealed hermetically; they provide excellent protection from gases, moisture, microorganisms, rodents, and insects; they are stackable, tamper-proof, and relatively inexpensive; and, in general, they can be thermally processed. On the other hand, the quality of the final product in cans is generally low; there are some safety issues (cut fingers); the containers are heavy, easily damaged, and not microwavable; and they usually do not open easily.

The most common type of metal container is the three-piece can, which consists of two ends and one body. One of the ends is applied by the can manufacturer and the other by the food packer. Most steel-based, three-piece can bodies are welded together, but they can also be secured mechanically. Aluminum cans cannot be welded economically. As a result, most aluminum cans are two-piece containers. Most metal containers are thermally processed. A series of ridges, known as "cluster beads," are embossed in the sidewall of the can to improve its strength and prevent collapse or paneling when pressure differential is encountered during the thermal process.

Two-piece metal containers consist of a can body and one end applied by the food packer. Two-piece cans are rapidly replacing three-piece cans due to their aesthetic appeal and their lower cost.

The portion of the can formed by rolling the curled edge of the end and the can body together, forming a hermetic seal, is called a "double seam." The double seam is a critical part of any can because it is the weakest point of the can. Each component of the double seam (Figure 1), particularly the overlap, must have the correct dimension and conform to strict guidelines to ensure a tight seal. A thermoplastic sealing compound attached to the cover melts during the formation of the double seam, fills the spaces in the seam, and results in a hermetic seal.

Most metal containers use the "tinplate" as their basic construction material. The tinplate is composed of a thick layer of steel with tin added on either side. The tin layers protect the steel from being corroded by the product

and by atmospheric moisture. Beyond the tin layers, there are coatings that also help to protect the metal from corrosion.

Decreasing tin resources and the resulting increase in the price of tin led to the development of tin-free steel (TFS). Tin-free steel plates use phosphates, chromium, aluminum, or nickel as protective coatings.

Aluminum cans are also coated for protection against corrosion. They are mostly used for carbonated beverages (beer and soda) because the high internal pressure helps the thin, soft metal container hold its shape and withstand mechanical damage.

Plastics

Plastics are long-chain polymers that can be melted, formed into a desired shape, and solidified during cooling. The general advantages of using plastic materials in food packaging include consumer acceptance and preference, excellent safety characteristics (nonfragility), less weight than other materials, good moisture and gas barrier properties, good puncture resistance, low heat conductivity, good sealant properties, recyclability, and microwavability. On the other hand, potential disadvantages include flavor scalping and migration issues. ("Flavor scalping" refers to absorption of the product's flavor compounds by the packaging material, and, conversely, "migration" is the transfer of compounds from the package to the product.)

There are many different plastics available for food packaging. The most important plastics in terms of volume are: (1) low density polyethylene (LDPE); (2) high density polyethylene (HDPE); (3) polypropylene (PP); (4) polyethylene teraphthalate (PET); (5) polystyrene (PS); (6) polyvinyl chloride (PVC); and (7) composite multilayer structures. The properties of the different plastics are presented in Table 1.

In addition to these plastics, other types are also used as food packaging materials, but they are not as popular due to their high cost. As a result, they are used only when their properties are required for the package. These plastics include: polyvinylidene chloride (PVDC), an excellent gas and water vapor barrier material; ethylene vinyl alcohol (EVOH), with excellent gas barrier properties; and some acrylics and nylons.

Laminates

When two or more plastic films are combined together, either with an adhesive or by co-extrusion, they form a laminate. The purpose of laminating materials is to combine the best properties of each film into a single packaging structure. The combination of different films into a laminate can provide a stronger seal, better mechanical properties, machinability, barrier properties for moisture, gas, odor, and light, graphics quality, and, in general, improved characteristics and appearance at a relatively low cost. Some disadvantages of the laminates include their low line speeds and environmental issues, since they are not recyclable.

When very low gas and moisture transmission through the package is desired, the use of an aluminum foil-laminated film is required. However, sometimes, when the foil in the laminate is very thin, it becomes susceptible to flex-cracking and pin-holing, which reduce significantly its barrier properties. A solution to this problem is the use of the metallization process. Vacuum metallization is the deposition of a thin metal layer on a polymeric material under vacuum. Metallized films are not as susceptible to flex-cracking or pin-holing, which gives them a distinct advantage over foil-laminated films. The most common metallized film is oriented polypropylene, which is used widely by the snacks industry, particularly for pretzels and potato chips.

Plastics and their laminations are used to make a variety of packages: bottles, cups, trays, tubs, pouches, bags, films/flexible packages, and composite structures. The large variety of plastics with a wide range of properties, and technological innovations in plastic manufacture, are the main reasons that plastics and their laminations are used increasingly for food packaging. At the same time, these technological innovations have led to the development of many novel and minimally processed products.

Microwavable Packaging

A major area of packaging development is in the microwavable products category. Consumer demands for convenient foods that need minimum time for preparation are satisfied by the use of plastics and their combination with other packaging materials that allow the product to be rapidly heated in a microwave oven. As a result, many shelf-stable, refrigerated, and frozen microwavable products are available in markets.

Another popular application in this category is the metallization of plastics and the development of susceptor technology for microwavable products (susceptors are metallized portions in packages that reflect microwaves). In the microwave, the metallized area creates localized hot spots to enhance heating, and, in some cases, to assist with browning. Examples of susceptor packaging include microwave popcorn, frozen dinners, and frozen pizza.

Modified Atmosphere Packaging

Another recent packaging development involving plastic films is modified atmosphere packaging (MAP). This method involves the alteration of the composition of the air in the package. This can be done by mechanically removing the air and obtaining vacuum, by flushing the package with another gas or mixture of gases, or naturally by the respiring product in the package (Floros, 1990). The modification of the atmosphere has a desired effect on the quality and shelf life of the product. Thus, modified atmosphere packaging can be considered an integral part of the processing operation. The composition of the modified atmosphere depends on the nature of the product and the desired outcome. The barrier properties of plastics play an important role in obtaining the desired result. The gases used for atmosphere modification include oxygen, carbon dioxide, nitrogen, carbon monoxide, sulfur dioxide, ethanol, and argon. The purpose for using each gas varies. Research regarding the use of ozone, chlorine dioxide, and other gases with antimicrobial properties indicates that the use of such gases in modified-atmosphere packaging increases the safety of the products.

The market for fresh-cut and minimally processed produce has grown tremendously over the past ten to fifteen years, and this is the result of the development and availability of plastic materials, packaging systems, and technologies, such as modified atmosphere packaging, that extend the shelf life of these products. Active packaging is a special type of modified atmosphere packaging. It involves the addition of an active substance inside the package that will cause a certain modification during storage. The substances used may be oxygen absorbents, moisture absorbents/regulators, antimicrobial agents, or other compounds with specific properties. An evolving technology is the incorporation of active substances into the packaging material itself (Floros, Dock, and Han, 1997).

Aseptic Packaging

One of the major applications of laminates is in aseptic packaging. The major difference between aseptic packaging and traditional methods of food packaging is that the product and the packaging material are continuously sterilized separately. Then, under aseptic conditions that prevent recontamination of the product, the sterile package is filled with the sterile and cooled product and hermetically sealed to produce a shelf-stable final product with extended shelf life and no need for refrigerated storage. This technique has allowed for substantial improvements in the quality of the final product, mainly due to the much milder heat treatment that the product undergoes compared to the traditional thermal process (Floros, 1993).

A popular aseptic package is the "brick pak," a type of aseptic carton. Its composite structure usually contains both paper and aluminum foil. Paper is a mechanically stable, stiff, and groovable material with good heat resistance. It also provides good light protection and a printable surface. On the other hand, aluminum foil is an excellent gas, water, and light barrier (particularly when laminated between two plastic layers) and it is thermally stable. The combination of these properties has made paper/foil/plastic laminations popular in aseptic packaging. Aseptic cartons are used extensively for products such as fruit juices, milk, and other drinks.

Many changes in the packaging of food took place in the last quarter of the twentieth century, producing a wide variety of materials and technologies. The steady accumulation of research developments indicates that food packaging will continue to evolve and respond to the increased needs and demands of consumers.

See also Fast Food ; Food Safety ; Frozen Food ; Microorganisms ; Microwave Oven ; Military Rations ; Storage of Food .

BIBLIOGRAPHY

Floros, John D. "Controlled and Modified Atmospheres in Food Packaging and Storage." Chemical Engineering Progress 86, no. 6 (1990): 25–32.

Floros, John D. "Aseptic Packaging Technology." In Principles of Aseptic Processing and Packaging, edited by James V. Chambers and Phillip E. Nelson, 2d ed., pp. 115–148. Washington, D.C.: Food Processors Institute, 1993.

Floros, John D., Lotte L. Dock, and Jung H. Han. "Active Packaging Technologies and Applications." Food, Cosmetics, & Drug Packaging 30 (1997): 10–17.

Food and Agriculture Organization of the United Nations. Guidelines for Can Manufacturers and Food Canners: Prevention of Metal Contamination of Canned Foods. Rome, Italy: Food and Agriculture Organization of the United Nations, 1986.

Gnanasekharan, Vivek, and John D. Floros. "Shelf Life Prediction of Packaged Foods." In Shelf Life of Foods and Beverages: Chemical, Biological, and Physical Aspects. Edited by George Charalambous, pp. 1081–1118. New York: Elsevier, 1993.

Paine, Frank A., and Heather Y. Paine. A Handbook of Food Packaging. Glasgow, Scotland: Leonard Hill, 1983.

Potter, Norman N., and Joseph H. Hotchkiss. Food Science. >Reprint, Gaithersburg, Md.: Aspen, 1998.

Robertson, Gordon L. Food Packaging: Principles and Practice. New York: Marcel Dekker, 1993.

John D. FlorosKonstantinos I. Matsos