This section addresses preparation of materials used for food packaging, formation of food packages using these materials, and the advantages and disad-vantages of each package material. A later section in the chapter, Properties of Plastics, will provide a more detailed description of the properties of plas-tics, sample calculations related to permeability and shelf life, and the uses of individual thermoplastics and laminates.
PAPER ANDPAPERBOARD
Types of Paper and Paperboard
Paper is made of plant fiber that is matted or felted into a sheet. The difference between paper and
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Figure 5.1. The four basic levels of packaging (pri-mary, secondary, tertiary, unit load) and two package types defined by destination (consumer package, industrial package).
perboard is related to thickness (caliper) and/or weight (grammage). Paperboard is thicker (>
300μm) and/or weighs more (> 250g/m2) than paper (Hanlon et al. 1998). To make paper and paperboard, wood is made into a pulp (by mechanical, chemical, or combination methods); the pulp may be bleached and additives and sizings added to control desired functional or visible properties; and the resulting
“furnish” is put into a papermaking machine (four-drinier or cylinder) (Soroka 1999). Recycled paper products are generally not used for direct food con-tact applications. Paper is typically made on a four-drinier machine, where the furnish is put on a mov-ing wire screen that allows the water to drain before the wet paper is moved around a series of heated drying drums; then the dried paper is wound into mill rolls. Paperboard is made on cylinder machines, which have a series of six to eight wire-mesh cylin-ders. Each cylinder rotates in an individual vat of furnish and deposits a layer of pulp furnish onto a moving felt blanket. Since the cylinders are in se-ries, six to eight layers of pulp furnish are deposited onto the felt, and the resulting product is thicker than that from the fourdrinier machine. After the paper or paperboard is dried, it may be calendered (passed through a series of heavy rolls) to produce a more dense paper with a glossy, smooth surface.
Types of paper used in food packaging include natural kraft paper, bleached paper, greaseproof paper, glassine paper, parchment paper, waxed paper, tissue paper, label paper, and linerboards. Of these, natural kraft paper is the strongest and most often used. Greaseproof, glassine, parchment, and waxed papers are resistant to grease and oil and are used for baked, greasy, and sometimes wet foods.
Linerboard is a kraft paper used for the liners of cor-rugated paperboard. Types of paperboard used in food packaging include chipboard, white-lined pa-perboard, clay-coated newsback, solid bleached sul-fate, food board, and solid unbleached sulfate.
Chipboard is the lowest cost, lowest quality paper-board and is made from 100% recycled fiber. White-lined paperboard is White-lined with a white pulp on one or both sides to improve appearance and printability.
Clay coatings also are applied to some paperboards to improve appearance and printability. Solid bleached and unbleached sulfate (kraft) paperboards and food board are stronger than other types and are therefore used in high-speed machines and for car-rying containers (carcar-rying baskets for colas and beers in glass bottles). When coated, solid bleached kraft paperboard is used for food contact
applica-tions such as frozen food boxes and butter cartons.
Hanlon and others (1998) provide a good descrip-tion of corrugated paperboard, which is most often used for secondary and/or distribution packaging.
Formation of Paper and Paperboard Packages
Two basic types of food packages are made with paper and paperboard: paper bags and folding car-tons (Fig. 5.2.). Paper bag types include single- and multiwall bags (flat, satchel bottom, square bottom).
Seams and bases of paper bags are sealed with glue.
Flat bags have a lengthwise seam and a folded, glued base; square-bottomed bags have additional bellows folds along the sides; and satchel-bottomed bags have a base folded to provide a flat bottom when opened. Folding cartons are made from paper-board that has been printed, creased, scored, cut, folded, and glued. Common designs include vertical end-filled, horizontal end-filled, and top-filled car-tons. Cartons are often delivered in a collapsed form and set up at the location where they are filled. Paper
Figure 5.2. Examples of paper and paperboard pack-ages.
and paperboard used for bags and cartons may be coated prior to forming (with polyethylene, wax, glassine, etc.) to improve the functional properties of the end package.
Characteristics of Paper and Paperboard Packages
Paper is one of the most widely used package types, especially for distribution packaging (corrugated shipping boxes, etc.). Advantages for using paper and paperboard for food packages include relative low cost and lightweight. Often, inexpensive prod-ucts, such as flour and sugar, are packaged in paper bags. Products with short shelf life or those rapidly consumed, such as donuts or fast food items, also are given to the consumer in paper or paperboard packages. Paperboard boxes are commonly used for cereals, cake mixes, and many other foods where the food product is contained in a plastic pouch that is placed in the box. Paper also serves as a printable layer in laminate juice box structures. Disadvant-ages of paper include its hygroscopic and hygroex-pansive nature, its viscoelasticity, its poor moisture and gas barrier properties, its lack of resistance to pests, and its limited formability. Paper will absorb and lose moisture as environmental conditions vary;
thus, it will expand in humid summers and possibly create problems in laminated structures, warp, or distort printed graphics. Paper is viscoelastic, mean-ing that paper will distort over time as compressive forces are applied (as in stacked boxes). Unless coated, paper cannot be used for greasy or moist products, and products packaged in paper or paper-board may absorb aromas from the environment.
Pests such as bugs and rodents are easily able to penetrate through paper packages. The sizes and shapes of packages made from paper are more lim-ited than for versatile plastic package designs. Due to these limitations of paper for food packages, products such as cereals and crackers often are packaged in plastic or laminate bags that are placed into paperboard cartons.
METAL
Types of Metal
Four metals are commonly used in food packaging:
steel, aluminum, tin, and chromium. Of these, tin-plate (a composite of tin and steel), electrolytic chromium-coated steel, and aluminum are most widely used. Bare steel, or black plate, corrodes
when exposed to moisture and is therefore not com-monly used for food products. Tinplate is made by electrolytically coating bare steel sheets with a thin layer of tin, which protects the steel from rust and corrosion. A layer of oil is added over the tin to add additional protection against corrosion and to pro-tect the tin during formation and handling. Electro-lytic chromium-coated steel is a bare steel sheet coated with chromium, chromium oxide, and oil to protect the steel, and it is more heat resistant and cheaper than tinplate.
Formation of Metal Packages
Common types of metal packages used for foods in-clude three-piece cans, two-piece cans, and foil pouches. Metallized films are also used in many flexible laminate packages.
Three-Piece Cans. Three-piece cans are made from tinplate or electrolytic chromium-coated steel sheets and two end pieces, as shown in Figure 5.3. A slitter cuts the steel sheets to can-width strips, which then are curled, side seamed, and welded. The cans are transferred to a flanger, which flares the end edges to receive can ends. The body of the can is ribbed or beaded to increase strength and provide re-sistance to collapse due to the external processing temperatures and internal vacuum pressures encoun-tered during thermal processing. The ends of the cans also have concentric beads for the same pur-pose. The ends of three-piece cans are double-seamed onto the can body. A double seam forms a hermetic seal by interlocking the cover and body of a can as shown in Figure 5.4.
Two-Piece Cans. Two-piece cans are often made from electrolytic chromium-coated steel or alu-minum sheets and one end piece, as shown in Figure 5.5. Draw-and-redraw and draw-and-iron processes are used to make two-piece cans. In the draw-and-redraw process, a metal blank, often of electrolytic chromium-coated steel, is stamped (drawn) through a die to form a shallow can shape. The shape from the first draw is forced through additional dies (sec-ond and third draws) to increase the height of the can without decreasing the thickness of the can. For thermal process applications, the cans are beaded, and the end is double-seamed onto the can body. In the draw-and-iron process, a metal blank, often of aluminum, is drawn into a wide cup, which then is
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redrawn to the finished can diameter and ironed to reduce sidewall thickness (Fig. 5.6.). Drawn-and-ironed cans are widely used for carbonated bever-ages that create internal pressure to keep the side-walls from denting. The ends of drawn-and-ironed cans often are necked or narrowed to reduce the size of the ends, thereby requiring a smaller end-piece.
This size decrease in the closure results in reduced packaging costs.
Metal Foils. Metal foils are most commonly made from aluminum and are defined as rolled sections of metal that are less than 0.006 inches thick (Hanlon et al. 1998). Metal foils are most commonly used in multilayer or laminate flexible packages, such as re-tort, meals-ready-to-eat (MRE), and aseptic pouches and juice boxes. An adhesive, often an ionomer, is used to bond a metal foil to other layers in a laminate material. The layers of a retort pouch, from out to in, Figure 5.3. Diagram and photo of
three-piece cans.
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Figure 5.4. A model cross section of a double seam commonly used to hermetically seal lids onto two- and three-piece cans. A cross section of a double seam will contain five layers.
Figure 5.6. Stages of formation (right to left) of drawn and ironed two-piece cans.
Figure 5.5. Diagram and photo of two-piece cans.
Other common two-piece cans are aluminum soda and beer pop-top designs.
are polyethylene terephthalate (PET)/adhesive/foil/
polyolefin/food product. The layers of an aseptic juice box are low-density polyethylene (LDPE)/printed polyethylene (PE)/paper/ LDPE/foil/ionomer/LDPE/
juice product. The metal foil adds barrier properties (to moisture, gases, oils, and light) to the package.
Metallized Films. Metallized films are made by vapor-depositing a thin layer of aluminum onto a plastic film in a high-vacuum chamber. These met-allized plastic films are less expensive than alu-minum foil and also provide a barrier to moisture, gases, oils, and light. Oriented polypropylene (OPP) is the most widely used metallized film, but metal-lized PET and nylon are also used. Metalmetal-lized films are used in laminate structures for snack foods (chips) and coffee. The layers of a snack food pouch are often reverse-printed biaxially oriented polypro-pylene (BOPP)/adhesive/metallized BOPP/sealing polymer/food product. The layers of a pouch for vacuum-sealed coffee are metallized biaxially ori-ented nylon (BON)/LDPE/coffee (Soroka 1999).
Characteristics of Metal Packages
Advantages of using metals for food packaging in-clude thermal stability, mechanical strength and rigidity, ease of processing on high-speed lines, re-cyclability, excellent barrier properties, and con-sumer acceptance. Specific advantages of alu-minum include the highest heat conductivity of any food packaging material, resistance to corrosion, and capacity for being rolled thinner than other metals for use in multilayer film packages (such as aseptic juice boxes)(Hanlon et al. 1998). The intro-duction of pop-top or easy-open ends to metal cans has increased the convenience of using metal pack-ages because the consumer no longer needs a can opener. Disadvantages of metals include the weight of the cans, cost, corrosion, and reactivity with foods (tin will react with acids in foods). Metals in both three- and two-piece cans are coated with an enamel to improve corrosion resistance, package performance and compatibility with a variety of food products, and appearance. Cans are most com-monly used for thermally processed, shelf-stable food products such as soups, fruits, vegetables, and canned meats. Foils and metallized films are used in pouches and laminates, such as retort pouches and snack chip bags, as a barrier layer to moisture, gases, and light.
GLASS
Types of Glass
The most widely used glass for food packaging is soda-lime glass, a rigid, amorphous, inorganic prod-uct. Soda-lime glass contains mostly silica sand (∼73%), limestone (∼12%), soda ash (∼13%), and aluminum oxide (∼1.5%), with small amounts of magnesia, ferric oxide, and sulfur trioxide, which are melted together in a gas-fired melting furnace until fusion occurs (near 1510°C) and cooled to a rigid state without crystallization (Robertson 1993, Sacharow 1976, Soroka 1999). Often cullet, broken or recycled glass, is added as an ingredient. Color-ing additives such as iron or sulfur (amber glass), chrome oxides (emerald glass), and cobalt oxides (blue glass) may be added to control the penetration of specific light wavelengths and thereby help pre-serve product quality (Robertson 1993).
Formation of Glass Packages
Glass is formed into food packages either by the blow-and-blow process (used to form narrow-neck bottles) or by the press-and-blow process (used to form wide-mouth jars) (Fig. 5.7). For both of these processes, a gob (lump) of molten glass is trans-ferred from the furnace to a blank mold (or parison mold). A plunger in the base of the mold is used to form the finish (the threaded part that will receive the closure) and the neck ring of the package. For the blow-and-blow process, air is then blown through the finish to expand the glass into the mold and form the parison. For the press-and-blow process, a metal plunger rather than air pushes the gob into the mold. A completed parison resembles a test tube with a threaded top, as shown in Figure 5.8.
For both processes, completed parisons are trans-ferred into blow molds, where air forces the glass to conform to the shape of the blow mold. The blow mold is the size and shape of the finished package.
Once a glass package is formed, it is transferred to an annealing oven, or lehr, which gradually cools the glass to minimize internal stresses and possible cracking created by uneven cooling of package sur-faces and inner sections. Coatings may be applied to the glass to strengthen the surface and minimize scratching: hot-end coatings (tin or titanium chlo-ride) are applied prior to the annealing oven, and cold-end coatings (waxes, silicones, and polyethyl-enes) are applied at the end of the annealing process (Soroka 1999).
Characteristics of Glass Packages
Advantages of using glass for food packaging include chemical inertness, nonpermeability, strength, resist-ance to high internal pressure, optical properties, and surface smoothness (Sacharow 1976). Applesauce is often hot-filled into glass bottles that withstand high temperatures but allow the consumer to see the prod-uct. Disadvantages of glass packages include fragil-ity, brittleness, and heavy weight (Sacharow 1976).
The heavy weight of glass and/or safety concerns re-lated to broken or chipped glass in foods have de-creased the use of glass for many food products, such as carbonated cola beverages, which are now pack-aged in aluminum cans and PET bottles. However, the nonpermeability trait of glass outweighs its disad-vantages for applications such as beer bottling.
PLASTICS
Types of Plastic
Plastics are a group of synthetic and modified natu-ral polymers that can be formed into a wide variety of shapes using heat and pressure. Most polymers used for food packaging originate from the petro-chemical industry. The type and arrangement of monomer units in a polymer and the processing con-ditions are used to identify types of plastics. There are two basic classes of plastic polymers: thermoset and thermoplastic. Thermoset plastics are formed by irreversible polymerization of monomers into highly cross-linked three-dimensional structures. Thermo-plastic Thermo-plastics can be reversibly solidified and melted; therefore, thermoplastic materials are
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Figure 5.7. Narrow-neck glass bottles formed by the blow-and-blow process and wide-mouth jars formed by the press-and-blow process.
Figure 5.8. Photo of an injection-molded parison (preform) used for producing bottles. The bit of plastic on the bottom of the parison indicates this was made using an injection process in which a bit of plastic remains at the gate point.
clable, and scrap can be recovered. These thermo-plastic materials are the most widely used in food packaging.
Types of thermoplastics used in food packaging include polyolefins (polyethylenes, polypropylene);
substituted olefins (polystyrene, polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride, polyte-trafluoroethylene); copolymers of ethylene (ethylene-vinyl acetate, ethylene-(ethylene-vinyl alcohol); polyesters (polyethylene terephthalate); polycarbonates;
polyamides (nylons); and acrylonitriles (styrenes) (Robertson 1993). Refer to Table 5.1 for properties of select polymers. The chemical structures com-prising these polymers have a significant influence on the barrier and functional properties of the plas-tics. Polymers with more nonpolar structures (poly-ethylene and polypropylene) interact with water differently than polymers with polar structures.
Varying the density of the polymers alters the prop-erties the polymer: low-density polyethylene (LDPE) is more flexible but has poorer barrier prop-erties than high-density polyethylene (HDPE).
Orientation processes also influence properties: ori-ented and bioriori-ented polypropylene (OPP and BOPP) have better strength and barrier properties than polypropylene (PP). Refer to the plastics struc-tural and mechanical property sections, below, for further discussion of these topics.
Formation of Plastic Packages
Plastics are formed into packages by various meth-ods: compression molding, extrusion, thermoform-ing, injection moldthermoform-ing, and blow molding (extrusion blow molding, injection blow molding, and injection stretch blow molding).
Compression Molding. Compression molding is used to mold thermoset resins into closures by plac-ing a set weight of resin into a heated mold, closplac-ing the mold, and allowing the pressure and heat of the mold to cure and set the resin into the desired shape.
Compression molding also is commonly used to mold thermoplastics into closures, with the largest application being the screw caps for plastic soda bottles. No visible markings on the final package are produced by the compression molding process.
Extrusion. All of the plastic-forming techniques, except compression molding, require an extruder.
Thermoplastics are formed into sheets, films, or
tubes using screw extrusion. A powdered plastic resin is fed into a screw extruder and passed through a die to form a sheet or tube. The flat film (cast film) process is used to make thin films of plastic, while the tubular or blown film process is used to make thin tubes of plastic that are commonly slit into a film. Due to the faster rate of cooling, the cast film process produces films with better surface thickness and uniformity and a more amorphous structure than films produced by blown film extrusion (Soroka 1999). The blown film process has lower equipment
tubes using screw extrusion. A powdered plastic resin is fed into a screw extruder and passed through a die to form a sheet or tube. The flat film (cast film) process is used to make thin films of plastic, while the tubular or blown film process is used to make thin tubes of plastic that are commonly slit into a film. Due to the faster rate of cooling, the cast film process produces films with better surface thickness and uniformity and a more amorphous structure than films produced by blown film extrusion (Soroka 1999). The blown film process has lower equipment