PRINCIPLES OF FOOD PACKAGING

Function of Food Packaging

Packaging is an essential part of processing and distributing foods. whereas preservation is the major role of packaging, there are several other functions for packaging, each of which must be understood by the food manufacturer. Indeed, faulty packaging will undo all that a food processor has attempted to accomplish by the most meticulous manufacturing practices. Packaging must protect against a variety of assaults including physical damage, chemical attack, and contamination from biological vectors including microorganisms, insects, and rodents. Environmental factors such as oxygen and water vapor will spoil foods if they are allowed to enter packages freely. Contamination of foods by microorganisms can spoil foods or cause life-threatening diseases. Many foods would not survive distribution without physical damage were it not for the protection afforded by packaging.

In order to be successful, packaging must also aid consumers in using products. Food packaging should have features which make the product easier to utilize and add convenience. This may be as simple as reclosure after partial use or as complicated as aiding in the microwave cooking of a product (Fig 21.1). Many new products are in reality standard foods package in a new way that aids in preparation or storage. Aseptically packaged milk is an example.

Packaging also serves to unitize or group product together in useful numbers or amounts. In some cases, this might be an amount to be a single time like most canned foods, or in other cases, multiple servings are grouped together such as a six-pack of sodas. Products such as condiments are seldom totally consumed at one time and so reclosure for storage becomes important.

Food packaging must also be able to communicate and educate. It is the package which identifies the product for the consumer. In addition to convincing consumers to buy a product, the package must also inform consumers about how to prepare or use the product, contents or amount of product contained, ingredients, nutritional content, and other pertinent information. Much of this information is required by specific laws in many countries, including the United States.

The packages is also an important part of the manufacturing process and must be efficiently filled, closed, and processed at high speeds in order to reduce costs (Fig. 21.2). It must be made of materials which are rugged enough to provide protection during distribution but be of low enough cost for use with foods. packaging costs, which include the materials as well as the packaging machinery, are a significant part of the cost of manufacturing foods, and in many cases, these costs can be greater than the cost of the raw ingredient used to make the food. Therefore, packaging materials must be economical, given the value of the food product.

Figure 21.1

Packaging of foods has become so complex that an entire industry has developed to satisfy the need. In fact, the packaging industry as a whole is one of the largest industries in the United States. About half of the packaging used in the United States goes for foods, with about 23% being used for industrial products (Table 21.1). Today, most sizable food companies have a packaging division, and universities, offer special curricula leading to a degree in package engineering. The food scientist does not have to become an expert in packaging, but increasingly he/she will be called on to assist with packaging decisions and problems. This commonly involves defining the kinds of protection essential to a specific food product and specifying in quantitative terms what the package must do. There will be considerable help available from suppliers of packaging materials and equipment, but they, in turn, will depend on the food scientist to make them aware of the peculiarities and subtleties of a particular, food system.

Requirements for Effective Food Packaging

Some of the more important general requirements of food packages are that they,

· Be nontoxic,

· Protect against contamination from microorganisms,

· Act as a barrier to moisture loss or gains and oxygen ingress,

· Protect against ingress of odors or environmental toxicants,

· Filter out harmful UV light,

· Provide resistance to physical damage,

· Be transparent,

· Be tamper-resistant or tamper –evident,

· Be easy to open,

· Have dispensing and resealing features,

· Be disposed of easily,

· Meet size, shape, and weight requirements,

· Have appearance, printability features,

· Be low cost,

· Be compatible with the food, and

· Have special features such as unitizing groups of products together.

Figure 21.2

Product must be protected against introduction of microorganism as well as dirt.

In many cases there should be resistance against boring insects and rodents although only glass and metal cans are insect and rodent proof.

All common polymers used in food packaging allow the transfer of moisture and gases such as O2 or water vapor directly through them by a process known as permeation. However, polymer can inhibit permeation to different degrees depending on their chemical makeup and physical structure. Some polymers are high barriers, whereas others offer little resistance. The same holds true for moisture and oxygen transfer through films. Moisture protection is a two=way affair. Dry foods should not absorb moisture from the atmosphere, and moist foods should not lose moisture and dry out. There are exceptions such as permeable films that allow the escape of moisture from respiring vegetables. Barrier against fat migration is needed to keep oils and fats from passing through wrappings. A material that is a high moisture barrier is not necessarily impervious to fat. Similarly, a greaseproof material is not necessarily impervious to moisture.

Gas and odor protection also works two ways. Off-odors should be sealed out, but desirable odors such as the aroma of coffee or the essence of vanilla should be sealed in. for storage stability of many foods, oxygen must b prevented from entering the package: yet some products generate carbon dioxide, which should escape from the package; this is the case with certain gas-evolving dough.

Physical protection prevents breakage of the package and subsequent product contamination. Resistance to product damage from impact or other physical stress (such as protection of crackers from breaking) is often a function of the secondary package.

Transparency and protection from light are contradictory objects. A transparent package is desirable because it allows consumers to see what they are purchasing. Whereas most foods are light sensitive, at least to some degree, the choice of container must take into account the probable normal shelf life of the product and how much damage light will do in this length of time. Colored bottles for beer, wine, and juices are a common compromise.

Tamper-resistance or tamper-evident features are especially important for food packaging. Consider the practice of s shopper opening foods in screw-capped jars and tasting them with the finger for acceptability, and then closing the jar and replacing it on the shelf. This occurred enough times in the past to cause virtually a universal shift away from simple screw top covers to packaging which cannot be easily opened without breaking a seal or leaving other evidence that the jar has been opened. In recent years, many cases of malicious tampering have occurred where poisonous or harmful objects were placed inside contains. Tamper-evident packaging is now seen as a primary deterrent to such practices.

Tamper-indicating devices include plastic bans that sea the closure to the container and membranous films sealed across the mouth of a container beneath the removable hd (Fig 21.3). These also minimize chances of product leakage, gas transfer, and aroma loss.

Ease of opening is perhaps best exemplified by the pull-tab beer and soda cans and the twist-off crown caps whose forerunners required a can or bottle opener. These technological developments had to balance minimum force for ease of opening against the potential of bursting from the internal pressure of carbon dioxide.

Figure 21.3

Dispensing features apply to containers for many granular, liquid, and particulate solids, from breakfast cereals to salt, as well as liquids. The flow properties of these materials determine the size and type of dispenser. Reseal ability has been applied to baked goods, cheese, and specialty foods in the form of plastic bags with “zipper” or press adhesive seals and clip or twist ties.

Lastly, but importantly, packaging should not have adverse environmental costs. Disposal of solid waste is an environmental problem, and packaging makes up a significant part of the total solids waste stream.

This partial list of requirements and functions is sufficient to illustrate the variability called for in packaging, especially when the tens of thousands of different food items stocked by a modern supermarket are considered. Packaging requirements are even more complex for products destined for harsher conditions of handling and storage than exist in air-conditioned supermarkets. These range from package survival during military or emergency air drips to resistance of the package to moisture and mold deterioration under tropical jungle conditions. Fortunately, very few packages must have all the properties described here.

TYPES OF CONTAINERS

Food packaging can be divided into primary, secondary, and tertiary types. A primary container is one that comes in direct contact with the food, for example, a can or a jar. Obviously, primary container must be nontoxic and compatible with the food and cause no color, flavor, or other foreign chemical reactions. A secondary container is an outer box, case, or wrapper that holds or utilizes several cans, jars, or pouches together but does not contact the food directly. Secondary containers are a necessary part of food packaging. It would not be possible to distribute products in glass jars. For example, without the corrugated secondary carton to protect against breakage. As pointed out above, primary containers have fewer but not less important function. Secondary container must protect the primary from damage during shipment and storage. They must also prevent dirt and contaminant from soiling the primary containers and must utilize groups of primary containers. Corrugated fiberboard is most commonly used to make secondary shipping cartons. There are strict standards on the construction and use of secondary containers to ship products. The size and strength of carton used must be selected for each type of product to be shipped. Cartons come in several different designs, each of which is intended for different types of forms of products (Fig. 21.4). Damage during shipment which can be shown to result from inadequate secondary containers are the responsibility of the food manufacturer, not the shipping agent. Except in special instances, secondary container is not designed to be highly impervious to water vapor and other gases. Depending for this is placed on the primary container.

Tertiary containers group several secondary cartons together into pallet loads or shipping units. The objective is to aid in the automated handling of larger amounts of products. Typically, a forklift truck or similar equipment is used to move and transport these tertiary loads (Fig. 21.5).

Form-Fill-Seal Packaging

containers may be pre formed, that is, fabricated by the packaging manufacturer; or they may be formed in-line by assembly from roll stock or flat blanks just ahead of the filling operation in the food-handling line.

Figure 21.4 and 21.5

This latter approach is called “form-fill-seal” and is one of the most efficient ways to package food. Today most flexible containers, whether made from paper, foil, or plastic, are in-line formed, resulting in great savings in handling labor, container transportation costs, and warehouse storage space. Figure 21.6 illustrates one common way a roll of flexible packaging material can be formed into a package.

Preforming is illustrated by the can-making machine of Fig. 21.7, which may be located in can manufacturing facility distant from the food plant. Formed cylindrical cans are shipped with separate can lids. The lids are seamed onto the cans by the food manufacturer after filling. The handling problems and expense are obvious in this type of operation. For this reason many large food companies set up can-making facilities in a building or area close by the can-filling line, and cans are continuously conveyed to the line. The same problems exist with pre formed glass bottles.

In contrast, flexible foils, papers, plastic films, and laminates in the form of roll stock lend themselves to an endless number of in-line high-speed package forming, filling, and sealing operations. Common techniques employed with particulate foods are illustrated in Fig 21.8. Relate methods are used to package a wide variety of liquid and solid foods, from individual portions of jam and single slices of American cheese to the multipacks of sausage products made skin-tight by vacuum forming.

Figure 21.6

One of the earliest in-line packaging operations was the milk carton system in which cartons were assembled from coated fiber flats, and sealed. Although this is still one of the most important in-line packaging processes, it is now being joined by various systems for aseptically packaging previously sterilize heat-sensitive roll-stock material is described later in this chapter. Some of the advanced methods for packaging milk and other liquids in plastic bottles by high-temperature blow-molding techniques, which at the same time heat sterilize the containers. The automatically filled containers are then heat sealed, again taking advantage of the thermos fusing properties of the plastic.

Hermetic Closure

The term hermetic refers to a container that is sealed completely against the ingress of gases and vapors. Such a container, as long as it remains intact, will also be impervious to bacteria, yeasts, molds, and dirt from dust and other sources, since all of these agents are considerably larger than gas or water vapor molecules. On the other hand, a container that prevents entry of microorganisms, in many instances, will be non-hermetic; that is, it will allow some gases or vapors to enter. Hermetically sealed containers not only protect the product from moisture gain or loss and oxygen pickup from the atmosphere but are essential for vacuum and pressure packaging.

The most common hermetic containers are rigid metal cans and glass bottles, although faulty closures can make them non-hermetic. With rare exceptions flexible packages are not truly hermetic for one or more of the following reasons:

Thin flexible films, even when they do not contain minute pinholes, generally are not completely impermeable to gases and water vapor, although the rates of transfer may be exceptionally slow;

The seals are sometimes imperfect; and

Even when film materials are very high barriers to gases and water vapor (e.g., laminants containing aluminum foil), flexing of packages and pouches can lead to minute pinholes and creases which allow gas and vapor transmission.

Figure 21.7

FOOD PACKAGING AND MATERIALS

There are a relatively few materials used in food packaging: metal, glass, paper and paperboard, plastics, and minor amounts of wood and cotton fiber (Table 21.2). However, within each of these categories many types of packaging materials or combinations of materials are available. In the case of polypropylene film alone, there are dozens of types of films and laminates varying in moisture permeability, gas permeability, flexibility, stretch, burst strength, and so on. Often, a new food product requires its own special package since optimum protection, economic considerations, and merchandising requirements change rapidly with variations in product composition, weight and form, and performance demands.

Packaging materials are found in a wide variety of forms including the following:

Figure 21.8

Rigid metal cans and drums; flexible aluminum foils; glass jars and bottles; rigid-and semi-rigid plastic cans and bottles; flexible plastics made from many different films used for bags, pouches, and wraps; paper, paperboard, and wood products in boxes, pouches, and bags; and laminates or multilayers in which paper, plastic, and foil are combined to achieve properties unattainable with any single component.

In addition to the many forms of packaging food packaging also encompasses the equipment and machinery for producing or modifying certain packaging materials, for forming them into the final containers, for weighing and dispensing of food materials, for vacuuming or gas flushing the containers, and for sealing the final packages. Food packages in many instances must withstand additional processing operations such as heat sterilization in pressure retorts, freezing and thawing in the case of frozen foods, and even final cooking or baking in the package.

Two basic types of alloyed metals are used in food packaging: steel and aluminum. Steel is used primarily to make rigid cans, whereas aluminum is used to make cans as well as thin aluminum foils and coatings. Until a few years ago, nearly all steel used for cans was coated with a thin layer of tin to inhibit corrosion; hence, the name “tin can.” The tin was applied electrolytic at a rate of as little as 0.25 lb per 440 ft”. The reason for using tin was to protect the metal can from corrosion by the food. Tin is not completely resistant to corrosion, but its rate of reaction with many food materials is considerably slower than that of steel.

Because of the expense, tin has been replaced in the United States and elsewhere by corrosion-resistant steel alloys, steel alloys with other thinner metallic coatings, as well as improved interior polymeric coatings which help the steel resist corrosion. This material is termed tin-free steel. For example, baseplate steel is coated with very thin layers of chromium followed by chromium oxide that are much thinner than a layer of tin but equally protective. The chromium oxide layer is further covered with an organic coating that is compatible with the food. This means that the nature of the baseplate steel is of major importance; various steels are used depending on the product to be canned. The specifications for five types of base steels used in food canning are listed in Table 21.3. The classes of foods requiring these different steels are also described in Table 21.3. Thus, type L plate is used with the most highly corrosive foods, which are generally acidic. At the other extreme are mildly corrosive or non-corrosive low-acid foods and dry products that may require cans with Type MR or MC steel.

The strength of the steel plate is another important consideration especially in larger cans that must withstand the pressure stresses of retorting, vacuum canning, and other processes. Can strength is determined by the temper given the steel, the thickness of the plate, the size and geometry of the can, and certain construction features such as horizontal ribbing to increase rigidity. This ribbing is known as beading. The user of cans will find it necessary to consult frequently with the manufacturer on specific application, since metal containers like all other materials of packaging are undergoing constant change.

Table 21.3

Aluminum is lightweight, resistant to atmospheric corrosion, and can be shaped or formed easily. However, aluminum has considerably less structural strength than steel at the same gauge thickness. This means that aluminum has limited use in cans such at those used with retorted foods. aluminum works well in very thin beverage cans that contain internal pressure such as soda or beer. This internal pressure from the CO2 gives rigidity to the can. Common types of beverage cans used a ring riveted to the id which is scored to facilitate easy opening. Scored aluminum pulls apart with less force than comparably scored steel. Scored aluminum lids also are being sealed to steel can bodies in great numbers. When this is done, special care must be taken that there is an unbroken enamel coating between the two metals in contact, otherwise bimetallic reactions can occur that can be harmful to the container food. Aluminum in contact with air forms an aluminum oxide film which is resistant to atmospheric corrosion. However, if the oxygen concentration is low, as it is within most food containing cans, this aluminum oxide film gradually becomes depleted and he underlying aluminum metal is then no longer highly resistant to corrosion. This can be overcome with enamel coatings similar to those used to protect steel and tin.

Aluminum is used in very thin gauge (approximately 35 ten-thousands of an inch or 9 µm) as a foil in many packaging applications. When rolled this thin, aluminum acts as a very good barrier to O2 and water vapor transmission but is very fragile. Strength is added by laminating the foil to a stronger material such as paper or plastic films. In this way, the strength of the film or paper can be combined with the barrier of the aluminum foil to produce a high quality package.

The principle disadvantage to aluminum is its requirement for large amounts of electricity for isolation from aluminum-containing ores. For this reason, recycling of aluminum containers has been successful.

As mentioned above, the inside and outside of metal cans is coated with organic coatings to further inhibit corrosion. Common coating materials approved by the FDA and their uses are indicated in Tale 21.4. The coatings not only protect the metals from corrosion by food constituents but also protect the foods from metal contaminations, which can produce a host of color and flavor reactions depending on the specific food. Particularly common are dark-colored when heat-processed. Bleaching of red plant pigments can occur on contact with unprotected steel, tin, and aluminum.

Metal Cans

The hermetic property of the metal can is a remarkable engineering achievement when one considers that cans are manufactured and later sealed at speeds exceeding 1000 units per minute and defective cans are fewer than one is many tens of thousands. The hermetic property of steel is extended to the seals by five folds of metal at the double-seam can ends. Between these folds is an organic sealing compound to ensure gas-tightness.

Can Construction. Metal cans for food and beverage packaging can be divided into two basic types based on method of construction, three-piece and two-piece. Three-piece cans are comprised of a cylindrical body and two end piece. Two-piece cans are made from one single body and end unit and one can end piece which is applied after the can is filled with product (Fig 21.9).

Table 21.4

Two-piece cans do not have side seams. The side seam of cans made from three pieces is most commonly welded in the United States. Side seams which are soldered with a tin-lead alloy are still in common used in parts of the world. Two-piece cans do away with the need for solder, which can contribute undesirable traces of lead to the food. Rigid aluminum containers can also be readily formed without side seams or bottom end seams by draw and ironing techniques. This is the type of can which is commonly used for carbonated beverages. The absence of one of the end seals and the side seal reduces the risk of can failure.

Can Corrosion. In year past, the steel used to make cans was protected from corrosion such as rust pitting by a thin electrolytically deposited coating of tin. The tin reduced the chance of corrosion of the steel (i.e., iron) by acting as an anode in a galvanic-type cell with the food serving as the electrolyte (Fig. 21.10). As the tin dissolves, electrons are transferred to the iron which prevents dissolution,

Tin has largely been replaced by other nonrusting metals such as chromium or the base steel may be given special rust-inhibiting treatment called “passivation.” These types of metals are termed tin-free and used because of their lower cost. The inside and often outside of the can is further protected against rusting, putting, or reaction with the food by a thin layer of nonrusting metal and a baked-on resin. There are several resins that are selected based on the type of food to be canned (Table 21.4).

Figure 21.9 and 21.10

Can Sizing.

Cans are given standard size designations based on their diameter and height in whole inches plus sixteenths of an inch. Thus, a 303 x 404 can has a diameter of 33/16 in and a height of 44/16 in Table 21.5 lists several standard can sizes, their volume, and their standard name.

Glass

As a food-packaging material, glass is chemically inert and an absolute barrier to the permeation of O2 or water vapor. The principal limitations of glass are its susceptibility to breakage, which may be from internal pressure, impact, or thermal shock, it weights which increases shipping costs, and the large amounts of energy required for forming into containers. Glass is primarily formed from oxides of metals, with the most common being silicon dioxide which is common sand.

Table 21.5

Forming glass containers from a carefully controlled mixture of sand, soda ash, limestone, and other materials made molten by heating to about 1500C is seen in Fig. 21.11. After forming, the containers are sent through curing (annealing) ovens to impart toughness or temper to the glass. Apart from influence of chemical composition, optimum shaping of the container, times and temperatures of forming, annealing, and cooling of jars and bottles, and other production practices, the breakage properties of glass containers can be minimized by proper choice of container thickness and coating treatments.

The heavier a jar or bottle of a given volume is, the less likely it is to break from internal pressure. A heavier jar, however, is more susceptible to both thermal shock and impact breakage. The greater sensitivity to thermal shock of heavier jars is due to wider temperature differences which cause uneven stresses between the outer and inner surfaces of the thicker glass. The greater susceptibility to impact breakage of heavier jars is due to less resiliency in thicker walls.

Coatings of various types can markedly reduce breakage by breakage by protecting the surface from scratches and nicks. Scratches and nicks substantially weaken glass. These coating, commonly of special waxes and silicones., impart lubricity to the outside of glass containers. As a result, impact breakage is lessened because bottle and jars glance off one another rather than sustain direct hits when they are in contact in high speed filling lines. Further, after coming from the annealing ovens, the glass surfaces, virtually free of abrasions, quickly acquire minute scratches in normal handling. These scratches are weak points where many of the subsequent internal pressure and thermal shock breaks originate. Surface coating also improves the high-gloss appearance, of glass containers and is said to decrease the noise from glass-to-glass contact at filling lines, probably die to the increased rate of glancing blows rather than direct impacts.

To help prevent thermal shock, it is good practice to minimize temperature differences between the inside and outside of glass containers wherever possible. Some manufacturers recommend that the temperature difference between the inside and outside should not exceed 44C. This requires slow warming of bottles before they are used for a hot fill, and partial cooling before such containers are placed under refrigeration.

Glass Containers

Glass containers come in a wide variety of shapes and sizes. They are hermetic, provided the lids are tight (Fig. 21.12). Lids have inside layers of a soft plastic material which form a tight seal against the glass rim. Many glass containers are vacuum packed, and the tightness of the cover is augmented by the differential of atmospheric pressure pushing down on the cover. Crimping of the covers, as in the case of soda bottle caps which operate against positive internal pressure, can make a gas-tight hermetic seal also. But bottles more often than cans become non hermetic.

Paper, Paperboard, and Fiberboard

The principle differences between paper, paperboard, and fiberboard are thickness and use. Papers are thin, flexible, and used for bags and wraps; paperboard is thicker, more rigid, and used to construct single-layer cartons; fiberboard is made by combining layers of strong papers and is used to construct secondary shipping cartons. The material used to construct shipping cartons is referred to as “corrugated paperboard” because of the wavy inner layer of paperboard used in its construction. “Cardboard” is not a correct packaging terms. When used in primary containers, most paper products are treated, coated, or laminated to improve its strength, especially in high-humidity environments such as are often found around foods. Other additives increase flexibility, tear resistance, burst strength, wet strength, grease resistance, seal ability, appearance, printability, and barrier properties. A few papers are made highly porous to be absorbent, such as the paper in meat and poultry trays.

Kraft paper is the strongest of papers and in it unbleached form is commonly used for grocery bags. If bleached and coated, it is commonly used as butcher wrap. The world “Kraft” comes from the German world for strong.

Acid treatment of paper pulp modifies the cellulose and gives rise to water and oil-resistant parchments of considerable wet strength. These papers are called greaseproof or glassine papers and are characterized by long wood pulp fibers which impart increased physical strength.

Figure 21.12

Paper that comes in contact with food must meet FDA standards for chemical purity and its coatings must be nontoxic. Additionally, the microbiological condition of paper products is rigidly specified by food manufacturers and in certain food ordinances. Thus, the grade A Pasteurized Milk Ordinance of the U.S. Public Health Service –FDA states that paper for milk cartons and caps be made from sanitary virgin pulp and contain no more than 250 colonies per gram of disintegrated stock by a standard bacteriological test.

Plastics

The term plastics refers to a broad group of materials that have the common property of being composed of very large long-chain molecules. These may have molecular weights of 100,000 or more and are made by connecting small repeating molecules called “monomers” together in a head-to-tail fashion. Polymers occur naturally, such as starches, proteins, and natural rubber. This molecular arrangement gives plastics some unusual physical properties. Thermoplastic polymers can be melted repeatedly, for example. This means that they can be melted, formed into a desired shape by one of several processes, and solidified on cooling. This allows plastics to be formed in an almost infinite number of shapes, many of which are useful as packages.

Of the few thousands plastics which have be synthesized, only 20 or so are used to make food packaging. However, these 20 polymers are combined in a variety of ways so that several hundreds of different plastic-containing structures are commercially available for food-packaging applications. Among the more important plastics used for films and semi-rigid containers for food packaging are cellulose acetate, polyamide (Nylon), polyesters (PET, Mylar), polyethylene, polypropylene, polystyrene, polyvinylidene chloride (Saran), and polyvinyl chloride. Some important properties of these materials, when made into flexible films are indicated in Tables 21.6 through 21.8. These tables reveal many of the relative strengths and weaknesses of these materials for specific food applications (special uses and restrictions will be mentioned later). These tables do not begin to convey the variety of products that can be made from these materials depending on many variables of their manufacture [e.g., the identity and mixture of polymers, degree of polymerization and molecular weight, spatial polymer orientation, use of plasticizers (softeners) and other chemicals, method of forming such as casting, extrusion or calendaring].

One way to combine polymer is as copolymers. These are plastics that combine different monomers into the same polymer molecules to form materials with combined properties. If the plastic resin is made of just one type of monomer, such as ethylene, it is said to be a homo polymer. If the resin contains more than one type of monomer such as ethylene and vinyl acetate, chemically joined, it is termed a polyethylene-vinyl acetate copolymer, also referred to as ethylene-vinyl acetate. Other copolymers include propylene-ethylene, ethylene-acrylic acid, ethylene-ethyl acrylate, vinyl chloride-propylene, ethylene-vinyl alcohol, and so on. The many variations possible made copolymers an important class of plastics to extend the range of useful food-packaging applications.

Another new class of plastic materials, the ionomers, further illustrates how the properties of plastics can be modified. Carboxylic acid groups can be added to the polymer chains of polyethylene. These acid groups form strong interactions between polymer chain molecules and affect the physical properties of the resulting plastic. The interactions can impart such improved functional properties as greater oil, grease, and solvent resistance, and higher melt strength. The range of applications for ionomers in food packaging is expanding.

Laminates

As noted already, package made of polymer films are not absolute barriers against the transfer of water and O2 through the package, although they may be excellent barriers against microorganisms and dirt. Fortunately, not all foods need absolute hermetic protection. Various flexible materials (papers, plastic films, thin metal foils) differ with respect to water vapor transmission, oxygen permeability, light transmits differ with respect to water vapor transmission, oxygen permeability, light transmission, burst strength, pinhole and crease hole sensitivity, and so on. Multilayers or laminates of these materials that combine the best features of each can be used to produce packaging materials with combined properties such as the strength of paper, heat seal ability of plastics, and barriers properties of aluminum foils (Fig 21.13; Table)

Table 21.6, 21.7 and 21.8

Commercial laminates containing up to as many as eight different layer are commonly custom-designed for a particular product. In the case of a quality instant tea mix, for example, the laminate (progressing from the exterior of the package inward) may have a high quality paper exterior that is printable, a layer of polyethylene to serve as an adhesive to the next layer, and, in the middle, a layer of aluminum foil that serves as the gas barrier, and an innermost layer of polyethylene to provide the thermoplastic material for heat-sealing the package’s inner surfaces. Laminations of different materials may be formed by various processes including bonding with a wet adhesive, dry bonding of layers with a thermoplastic adhesive, hot melt laminating where one or both layers exhibit thermoplastic properties, and special extrusion techniques.

Another new technique for combining different plastics is co extrusion. Co extrusion simultaneously forces two or more molten plastics through adjacent flat dies in a manner that ensures laminar flow and produces a multilayer film on cooling (Fig. 21.14). Such structured plastic films may be complete in themselves or be further bonded to papers or metal foils to produce more complex laminates.

Retort able Pouches and Trays

Flexible materials can be combined to withstand even the adverse conditions of retorting encountered with low-acid foods. such “flexible cans” have become standard containers or some applications such as providing foods to soldiers in the field. The advantages of pouches and trays over cans and jars of equivalent volume include shorter retort times, which can produce higher quality products and save on energy, lighter weight, increased compactness, easier opening, and easier disposability. Retort able pouches are constructed of a three-ply laminate consisting of;

1. An outer layer of polyester film for high-temperature resistance, strength, and printability,

2. A middle layer of aluminum foil for barrier properties, and

3. An inner layer of polypropylene film that provides heat-seal integrity.

Retort able trays are constructed from multilayers of polymers, one of which is ethylene-vinyl alcohol to provide an oxygen barrier. These trays are often sealed with a polymer-foil laminate film.

Edible Films

Edible films have been used for centuries. Sausage casings are one example. More recently there has been renewed interest in such films. For example, food materials can be protected from loss of volatiles or reaction with other food ingredients by being encapsulated in protective edible materials. This can be done by spray drying various flavoring materials emulsified with gelatin, gum Arabic, or other edible materials to form a thin protective coating around each food particle (microencapsulation). The coating of raisins with starches to prevent them from moistening a packaged breakfast cereal and the coating of nuts with monoglyceride derivatives to protect them from oxidative rancidity are additional examples of edible coatings.

Table 21.9

Food materials such as amylose starch and the proteins zein and casein when solubilized can be cast to give sheets of edible films on drying. These films may then be used of fabricate small packets to hold other food ingredients. One application of such films has been to package baking ingredients which can then be added directly to the mixing bowl as an intact packet; on addition of water, the edible film dissolves and releases the packaged ingredients.

Edible films are also used to coat fresh fruits and vegetables to reduce moisture loss and to provide increased resistance to growth of surface molds. The most common and oldest edible film is wax. A wide range of products such as apples are waxed for appearance and improved keeping quality. Newer edible films are being developed which can keep produce longer. All edible films must be approved by the FDA for human consumption.

Figure 21.14

Wood and Cloth Martials

Woven cloth such as jute bags (burlap) and cotton bags are used to a limited extent, mostly for bulk shipment of grains and flours. Wire-wound wood strips have been used to make crates for fresh fruits and vegetables. Solid wooden crates are also used for transporting iced fish.

PACKAGE TESTING

Many test procedures exist to measure quantitatively the protective properties of packaging materials and entire containers. These can be divided into chemical and mechanical parameters. Examples of chemical tests are those used to identify plastics, determine if portions migrate to foods, and measure resistance to greases. Mechanical tests include such things as barrier properties, strength, heat-seal ability, and clarity. The tables in this chapter contain data from several of these tests. Mechanical properties of packaging films (e.g., tensile strength, elongation, tearing strength, bursting strength) are determined on specially designed instruments that precisely measure the forces required to produce these effects. Unfortunately, reporting of test data has lacked standardization and various form of English and metric units continue to be used. Test data in the tables of this chapter have been kept in their original units, since these remain meaningful to suppliers and users of packaging materials. The packaging industry, however, can be expected to gradually replace many of its present designations with standardization metric units. One of the best sources of methods for testing packaging materials are the publications of the American society for Testing and Materials (ASTM).

Water vapor transmission rates (WVTR) can be measure by sealing sheets and films across the opening of a vessel that contains a weighed quantity of a desiccant materials. The vessel is then placed in an atmosphere of controlled temperature and humidity. Periodic weighing of the container of desiccant to determine water pickup gives a measure of water vapor transfer. This measure is commonly expressed in terms of grams per 100 in. of film, of 1 mil (0.001 in) thickness, per 24 h under the defined conditions of temperature, humidity and atmospheric pressure.

Gas transmission rates can be measured by an instrument which uses the test film to separate an inert gas from the test gases. The instrument then continuously measure increase in concentration of oxygen in the inert gas (Fig 21.15). This increase in concentration with time can be used to calculate gas transmission rates. Gas transfer is often expressed in terms of cubic centimeters per 100 in. of film per 24 h under defined conditions of temperature, humidity, and pressure on both sides of the film. Transfer rates of specific gases such as oxygen, carbon dioxide, or nitrogen can be measured with special electrodes fitted into the sealed vessel or by gas chromatographic analysis of the vessel contents.

Resistance of packaging films to acids, alkalis, and other solvents can be measure quantitatively by incubating the films in the solvent under controlled conditions and then determining either the degree of leaching of the film into the solvent or changes in the physical properties of the recovered films by some of the methods already mentioned. Resistance of coated metal cans to acid can be estimated by a colorimetric test for dissolved underlying iron in the acid test solution. Resistance of metal cans to acid also can be established in terms of the rate at which hydrogen is given off by the corroding metal.

Figure 21.15

These are just a few of the approaches used to test package materials. But in the final analysis, although such data permit intelligent initial screening of suitable packaging materials of a particular food applications, the final package and product are best evaluated in actual or simulated use tests. This is especially so when the food will receive additional processing (retorting, freezing, etc.) in the final package.

Actual use tests consist of sending limited numbers of food-filled packages trough he processing, shipping, warehousing, and merchandising chain where they will be exposed to naturally occurring vibrations, humidities, temperatures, and handling abuses. Such packages are then recovered for analysis. Simulated use tests involve machines and devices for producing physical stresses, and incubation cabinets where packages can be subjected to various temperature and humidity cycles comparable to what the packaged food will subsequently experience in trade channels. Simulated use test conditions can often be intensified to arrive more quickly at a judgment of package performance.

PACKAGES WITH SPECIAL FEATURES

As pointed out, one of the newer requirements of food packaging is that it help with the products use. This usually means that the package will have some type of added convenience feature. The “boil-in-bag” package is one of many examples. In addition to protecting the food against microorganisms and dirt, and to a certain degree against moisture and gas transfer, it also is impermeable to grease, nontoxic, compatible with the food, transparent, capable of being evacuated and heat sealed under vacuum, attractive, tamper-evident, easy to open and dispose of, light in weight, requires little storage space, and is low in cost. But this is not all. Its material and seals withstand freezing temperatures and the expansion of foods frozen within it. It then survives frozen storage and the extreme shock of being taken from the home or restaurant freezer and plunged into boiling water for cooking. During boiling, the bag does not burst from steam or allow boiling water in to dilute the food. All of this is made possible by the exceptional properties of polyester and Nylon films including high tensile strength and stability over a range of temperatures from -73 to 150C (Tables 21.6 through 21.8).

The plastic shrink package shown in Fig 21.16 protects food against contamination and yet lets the customer see the meat. In addition, it keeps the meat from drying out. It is made to fit skin-tight by first drying a vacuum on the bagged item, twisting the bag and tying a knot or sealing with a metal clip, and then passing the package through a mild heat tunnel or immersing it in hot water to shrink the plastic. The package may be made from polypropylene film if the product can tolerate or is favored by a moderate oxygen transfer rate. The polypropylene film is specially treated during manufacture to produce a biaxial orientation of its molecules. This contributes to a uniform shrinkage in all directions on heating to about 82C. oriented polyethylene and several other plastic films also have shrink properties. Cryovac “Type L” film is a shrink property is particularly useful in packaging poultry to be frozen because the skin-tight fit excludes pockets of air around the irregularly shaped bird and minimizes voids where water vapor can migrate to the package surface and result in desiccation (i.e., freezer burn) to the skin below. The shrink property also is exploited in the packaging of fragile vegetables and fruits to keep these items from becoming damaged. In this case, several individual items may be packed in a paper tray overwrapped with the shrinkable plastic; when shrunk, this type of package firmly holds the items in place, preventing bruising from loose movement. This differs somewhat from the skin-tight packages used for meat since it does not usually employ vacuum prior to the shrinking step.

Microwave Oven Packaging

One of the most rapidly developing areas of added convenience is packaging designed for the microwave oven (Fig. 21.1). Such packaging must meet all other standard requirements for food packaging, and also must be transparent to microwaves and able to withstand the temperatures encountered in heating foods in the microwave oven. The most commonly used materials for this application are made of plastics. Several plastics such as polyester and Nylon which are capable of withstanding higher temperatures have been used to package microwave foods. these plastics do not deform or char when exposed to temperatures in excess of 100C.

Figure 21.16

One disadvantage of microwave heating is that the heating surface, in this case the package, does not get hot itself. This means that heat is not transferred to the food by conduction and the food does not brown or otherwise behave like conventionally cooked foods. in order to solve this problem, packaging engineers have constructed high-temperature polymeric packaging materials that contain very small aluminum particles (Fig 21.17). These particles get hot during microwaving, which in turn, heats the polymer which further heats the food by conduction. These materials are called “susceptors.” Susceptors cause foods to brown in a microwave oven. this technology improves the quality of popcorn which is popped in a microwave oven, for example. There are many other examples of innovations resulting from the packaging in microwave ovens.

High Barrier Plastic Bottles.

An important development has been the recent introduction of squeezable plastic bottles that have very high barrier properties and are less than one-fourth the weight of glass, do not break when dropped, and can be incinerated without the production of toxic, corrosive, or noxious compounds beyond those found in burning household or municipal trash. This means that products which required the barrier properties of glass can be packaged in plastic. The reduced breakage of plastic bottles not only benefits consumers but also reduces costs throughout production and shipping channels. Bottle breakage is a frequent cause of complete disruption of filling-line operations. Resistance to breakage also permits use of lighter, less expensive corrugated shipping containers. These containers also for easier dispensing of viscous products such as ketchup.

Figure 21.17

Aseptic Packaging in Composite Cartons.

Another development of worldwide significance has been the composite paper carton which is capable of being sterilized and then aseptically filled with sterile liquid products. This process is called aseptic packaging even though it is both a packaging and processing technology. This technology allows foods such as milk to be packaged in relatively inexpensive flexible containers which do not require refrigeration. This means that milk and juices can be distributed in parts of the world where refrigeration is not common. The packaging material is made from laminated roll stock consisting (from the outside inward) of polyethylene, paper, polyethylene, aluminum foil, polyethylene, and a coating of ionomer resin. With equipment shown in Fig. 21.18, the roll stock enclosed in a cabinet a floor level is drawn upward as a continuous sheet through squeeze-rollers to remove excess peroxide, and the descending sheet is formed into a tub that is exposed to radiant heat to complete the sterilization and remove traces of peroxide. Next, the tube is further formed into a rectangular shape, end-sealed at package-size intervals, filled with pre sterilized liquid food, top-sealed, and separated into individual package units in a continuous operation. Commercially sterile liquids have a shelf life of several months at room temperature in the exceptionally light weight form-fill-seal package. Several form-fill-seal systems have been developed to take advantage of the rapidly growing aseptic package market.

Figure 21.18

Military Food Packaging

Special packaging problems have always confronted the military. In addition, to providing protection, packages that simplify preparation and consumption of the food under adverse circumstances often are required. This, one type of military food container is designed with a chemical system separate from the food, which can be made to undergo a rapid exothermic reaction, thus heating the can on opening. It is also possible to design a self-cooling container that might be based on the rapid expansion and release of a compressed refrigerant gas. Such containers presently would be too expensive for general commercial use.

Newer Methods of Cooking and Foodservice

These create special package needs. An example is oven able paperboard, which is impervious to moisture and fat staining and fat staining and is heat resistant to 218C for short periods. These properties, as well as a china like gloss, have been obtained by coating paperboard with PET polyester. Such paperboard containers, in the form of serving dishes, are now used for frozen foods to be reconstituted in conventional and microwave ovens.

Packaging and Communication

The package designer known that the message conveyed by the packages is often the most important single factor that determines a product’s sale or rejection. Among details to be considerable are the package’s color and symbolism. In one’s own country errors are less likely to be made, but when packages are designed for distribution in a less familiar region, special care must be taken and the services of a consultant often employed. Pitfalls are many. Purple is an unlucky color to the Chinese and white connotes mourning. The cherry blossom is a favored symbol of the Japanese, but the chrysanthemum, which connotes royalty in Japan, is to be avoided on package labels. Further examples applying to Asian markets are listed in Table 21.10. One of the subtler examples of regional psychology was experienced in Japan where a U.S. designed tuna fish can picture a tuna with nose turned down toward the water. When the product did not sell, it was learned that to Japanese a tuna with nose turned down meant a fish that was dead. When the picture of the tuna was modified, sales increased.

Distribution Packaging

Tertiary packaging combines several secondary units or cartons into a single unit for greater efficiency in distribution. The most common unit is usually a full pallet lead. Related commercial handling methods involving ship-to-rail, truck-to-air, and other combinations are becoming more sophisticated. The newer methods are not obvious extensions of previous trucking and railroad practices but represent a systems approach to integrating and optimizing packaging, loading, transporting, and unloading practices for greatest efficiency. This involves improved palletizing and stacking techniques which often do away with intermediate-size cartons, drums, and sacks, and put new demands on unit package sizes and shapes for maximum utilization of cargo container space. In many instances the newer packing methods also shift unit package requirements from abrasion, tearing, or puncture resistance to greater emphasis on compression and stacking strength. Major food companies have become increasingly involved in how these advanced modes of containerization and distribution affect package needs and their contents. With some products, pallet loads are delivered directly to supermarkets and customer purchases are made directly from the pallet, further cutting down on handling costs.

Table 21.10

SAFETY OF FOOD PACKAGING

Migration from Plastics

It is important to know that plastics are not completely inert to foods. aside from the permeation of gases and vapors, it is also possible that components of the plastic can migrate to the food and would then be consumed with the food. This raises concern from the safety of some plastics. For this reason, all plastics used in food contact must have specific approval from regulatory agencies for the intended use. Food manufactures must get written assurance from the plastic manufacturer that their container wrap meets all requirements for use in food contact.

Contamination

It is primarily packaging which acts as a barrier to contamination of foods. preventing recontamination of thermally processed low-acid foods which are stored at room temperature is especially serious. Recontamination with pathogenic bacteria such as Clostridium botulinum can lead to outbreaks of food-borne disease. One example occurred when fish had been processed in defective metal cans which contained small holes. Several people ended up with botulism, which is often fatal.

ENVIRONMENTAL CONSIDERATIONS

Packaging of all forms make up approximately 33% of the disposable solid waste in the United States. Foods use about one-half of all packaging (Fig 21.19). The most common way of disposing of solid waste is landfilling. The need to find better ways to dispose of solid municipal waste has prompted interest in both increased recycling of food packaging as well as using less packaging in the first place. Neither of these objectives are as simple as they may seem. It is likely that recycled material will have more chemical and microbiological contaminants than virgin materials. For example, some consumers might use empty plastic bottles to dilute and mix pesticides before recycling the bottle. Traces of the pesticide could remain in the plastic and later enter the food during storage. Recycled paperboard could contain more microbial spores due to contamination.

Figure 21.19

Another strategy to reduce packaging waste is to use less material when packaging foods. This is termed source reduction. However, to do so takes considerable care. Making packages thinner and/or smaller is one approach that has been taken for many products. This reduces the amount of packaging used, but care must be taken so that the package will not break during shipment and allow the food to become contaminated. Making products more concentrated so that they can be packaged into smaller containers is another approach to reducing the amount of packaging used (Fig. 21.20). In some cases, extra packaging is used not to protect the food but for marketing reasons. In these cases, packaging can be reduced, but food manufactures are concerned that they will be at a competitive disadvantage if they eliminate packaging which helps sell their products.

Metal and glass containers are recyclable without worry about recontamination, but they are heavy and require considerable energy to transport and melt and, therefore, have their own negative environmental costs. For this reason, lighter-weight plastics or combined plastic, paper, foil containers are sometimes preferred. Another alternative is to incinerate the trash. This too has negative environmental costs. Highly efficient and clean incinerators can be built, but they are expensive. Burning must not produce toxic or otherwise noxious fumes to pollute the air. Innovation in plastics technology holds promise of yielding more plastics that incinerate cleanly. Plastics that can be recycled are another possibility, but the economics of gathering and sorting plastic containers from other trash may limit the feasibility of this approach. Admirable progress continues to be made in the recycling of steel, aluminum, and polyester containers. Numerous steel can collection stations have been set up by major steel companies and more are planned. Some municipalities mine steel cans from dumps using magnetic separating equipment before or after garbage incineration and then sell the recovered steel scrap. Major aluminum companies continue to invest heavily in reclamation and recycling. Aluminum has greater scrap value than other components of solids waste, about 10 times that of steel and 15 times that of glass. Collectors bringing aluminum cans to Alcoa reclamation centers receive about $0.50 cents per kilogram for the cans. The Reynolds Metals Co. has been operating a fleet of mobile recycling unites in the form of large truck trailers that carry a magnetic separator, electronic scale, and can shredder. The mobile units buy aluminum cans from the public and prepare them for smelting plants. Melting cans to recycle aluminum requires only about 5% of the energy needed to make aluminum from bauxite ore, and the aluminum industry is approaching 70% recycled aluminum in new can production.

Because of its lower economic value and certain other considerations, recycling of glass is not always feasible. One of the largest markets for salvaged glass is in conventional glass-making where crushed waste glass can supply about 30% of the raw materials needed to make new bottles. But such waste glass must be free of food, metal, and other forms of contaminations and needs to be color sorted. Where costs for these handling operations are prohibitive, crushed glass of lesser purity can be used by the construction industries in making glass bricks and for admixture with asphalt to pave roads. After incineration to form a partially melted mass, waste glass also is suitable as an undersurface for road construction. With or without crushing and incineration, waste glass can provide useful landfill.

Paper and paperboard make up nearly half of all packaging materials used. Paper is easily recycled to produce fiber or is burned as fuel. Recycled fibers are used in the manufacture of cartons, boxes, newspaper, and other paper goods. However, more must be learned about recycling of paper and elimination of toxic contaminants that can find their way into paper goods not made from virgin pulp. Recent findings of polychlorinated biphenyls in paperboard food containers emphasize this point. Presently, about 20% of the U.S production of paper is recycled. More economic means of separating paper from other waste materials are being researched to increase this figure.

Innovations that make packaging more “environmentally friendly” are to be encouraged. Development and use of such materials require input by the food scientist.

Figure 21.20

Metal and glass containers are recyclable without worry about recontamination, but they are heavy and require considerable energy to energy to transport and melt and, therefore, have their own negative environmental costs. For this reason, lighter-weight plastics or combined plastic, paper, foil containers are sometimes preferred. Another alternative is to incinerate the trash. This too has negative environmental costs. Highly efficient and clan incinerators can be built, but they are expensive. Burning must not produce toxic or otherwise noxious fumes to pollute the air. Innovation in plastics technology holds promise of yielding more plastics that incinerate cleanly. Plastics that can be recycled are another possibility, but the economics of gathering and sorting plastic containers from other trash may limit the feasibility of this approach. Admirable progress continues to be made in the recycling of steel, aluminum, and polyester containers. Numerous steel can collection stations have been set up by major steel companies and more are planned. Some municipalities mine steel cans from dumps using magnetic separating equipment before or after garbage incineration and then sell the recovered steel scrap.

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