Packaging Materials

Packaging is the main preservation technology in the commercialization of food products. Through the correct selection of both materials and packaging technology, food freshness can be maintained to obtain the required shelf life. Traditionally, a package is considered a passive barrier that retards the adverse effect of the environment on the packaged product. The most visual example of such a package is a glass jar. However, packaging science and technology is in constant evolution, and current trends include the development of packaging materials that positively interact with the environment and the food, or even play an active role in the preservation of foods. The use of permeable or perm-selective plastic packages is an example of the former. Finally, active packaging is understood as a packaging system that acts to preserve and improve food quality. This technology makes use of materials that smoothly release antioxidants or fungicides (emitters) into foods or packages or retain oxygen, ethylene, water, cholesterol, lactose, etc. (absorbers) (1).

The packaging of fermented and dry-cured meats is a well-implanted preservation technology, as for most foodstuffs. However, the package does often play an important role in the meat processing, especially in traditional fermented meats. For some products, packaging materials are used to produce the food, as is the case for salami, pepperoni and other national and local fermented meat products. Once the product is ready for consumption, it is commonly sliced and packaged for retailing.

The variety of packaging materials and structures used to contain these products is so extensive that an exhaustive list is neither feasible nor worthy. It is more convenient to define the diverse possibilities that are commonly applied. First, the natural or artificial materials used for meat production are addressed, then the materials and packaging technologies applied for commercialization.

A. Casings for the Manufacture of Fermented-Meat Products

Sausage-making procedures consist of the stuffing of the mix (meat, fat, starter cultures, and additives) into an elastic tube (casing), which constitutes the skin of the finished product. The characteristics of the tube are highly variable and dependent on the product as well as on the final presentation to the consumer. Many properties of casings such as size, shape, color, printability, and so on can be considered marketing options, but some others should be carefully controlled to obtain the desired product characteristics. The most relevant characteristic of casings for fermented meat products is their permeability to gases (oxygen) and vapors (water, smoke components, etc.). The drying of the product is mostly controlled by water permeating through the casing wall, a mechanism that is also profitable for smoking. Other crucial properties of the skin are (a) the adhesion of the stuffed product to the casing and (b) the elasticity that is basic for the filling process as well as for the shrinkage during product drying.

The most obvious classification of casings is by their nature. Casings can be natural, obtained from biomaterials, or obtained from synthetic polymers. Natural casings are portions of the gastrointestinal tracts of pork, sheep, and beef. At the slaughterhouse, these pieces are cleaned, selected, and stored. The advantages of using natural casings are, among others, good elasticity and tensile strength to facilitate the filling, edibility and easy chewing, good flavor of final product (they do not release off-flavors), and above all, adequate permeability to water and gases (2). Moreover, the sausage shape is variable, resulting in a valuable traditional home-made aspect, although this variation also is often encountered with industrial automation. In general, products manufactured with natural casing are sold as whole pieces.

Artificial casings can be obtained from different sources. Nonedible cellulose-based casings are commonly used in the manufacture of fermented meat products in high-speed industrial lines. Their main characteristics are exceptional uniformity and tensile strength. They are ideal for products that are intended for slicing because they are easily peeled off. This characteristic can be modified by application of diverse coatings; for instance, food-grade protein is used to adhere the casing to the product. Artificial casings present an adequate permeability to water, gases, and smoke; also, the permeation rate through the tube can be adjusted to meet other permeation requirements by perforation (3).

Intermediate between natural casings and paper-derived casings, collagen-derived casings combine the advantages of uniformity and good mechanical resistance with edibility (4). They can be used in industrial stuffing processes as a substitute for cellulose-based materials for dry products because collagen presents enhanced permeability and shrinks with the product. Compared with natural tubes, these casings are not elastic and cannot be overstuffed. Moreover, collagen casings, as a product manufactured from a natural protein, are edible, although they are tough to chew.

For many sausages, the use of plastic tubes is an excellent alternative because they present outstanding shrinkability, processability, and high-barrier properties. However, the latter is a drawback because the release of water through the casing is a must in the manufacture of the final product. As mentioned for cellulose casings, water barrier can be adjusted by a suitable selection of plastic materials or by perforation.

B. Properties of Packaging Materials

The commercialization and consumer presentation of fermented meat products is highly variable and dependent on the product itself, national or regional market, consumer demands, and so forth. Many products are commercialized in units; that is, they are sold in whole pieces or they are sliced at the grocery according to consumer demand. Most of these products are not closed in a gas-tight package; instead, a net or string is used for hanging. As a consequence of being exposed to air, the products continue changing until the moment of consumption. At present, however, there is an increasing use of modified atmosphere packaging of whole pieces. With this technology, the product quality is maintained more stable during storage and commercialization. Products sliced at the retail level are wrapped with plastic films or plastic-coated paper, or they are packaged in plastic bags. By wrapping, the product is protected only for handling; therefore, the slices must be kept under refrigeration and consumed in a short period. Polyethylenes and flexible polyvinyl chloride are the most popular alternatives for this purpose. Bags can be manufactured with a wide variety of plastic materials. Polyethylene bags closed by a knot, adhesive tape, and so on are the simplest packaging forms. However, some shops have facilities for vacuum packaging. In this case, more complex plastic structures can be used (i.e., polyamide//polyethylene laminates) and the packaged products enjoy an extended shelf life, even at room temperature.

During the last decades of the 20th century, there was a significant change in the distribution channels from small groceries to medium and large supermarkets. This change results in modifications on the type of product presentation. With some exceptions, products are sliced and packaged at the manufacturer plants. Consequently, the packaging materials play an important role in the maintenance of product quality.

Sliced meat products are sold in a wide variety of packages, although they are mostly made of plastic materials. In contrast to glass or metal containers, which present well-defined characteristics, polymers vary widely in their properties. From the hundreds of thousands of different grades of existing polymeric materials, about 30 families are used in the manufacture of packages, and although they may present a very similar visual appearance, their physical, mechanical, thermal, and barrier properties largely vary—from very flexible and tough to rigid and fragile materials (5). Moreover, these materials can be processed with similar technologies, and therefore, combined, blended, or laminated to adjust the final package properties to specific product requirements.

Because barrier properties to gases (oxygen) and vapors (water, smoke, aroma) are crucial in the preservation of product quality, the permeability characteristics of materials and structures are parameters to take into account during packaging design. It is well known that plastic materials allow the passage of small molecular-size substances through the solid matrix. This mass transfer mechanism is responsible for oxygen or water permeability, flavor scalping, and migration ofpolymer residues or additives. Although all these processes must be known and controlled, the permeability to oxygen and water vapor is especially relevant to the shelf life of packaged foodstuffs.

Unlike glass or metal, plastic materials are not impermeable to the transfer of low-molecular-weight substances and consequently, there will always be an exchange of oxygen or water vapor between the external and internal atmospheres in a plastic packaged product. However, the rate of this exchange may vary up to six orders of magnitude among plastics. The barrier properties of plastics are commonly represented by their permeability coefficients (P). The permeability coefficient of a plastic to a gas is defined as the volume of gas (V, measured at standard temperature and pressure conditions) passing through a film of thickness L, per unit of surface area (A), time (t) and gas pressure difference between the atmospheres in contact with both sides of the film (DP). The permeability to condensable substances (water vapor or aroma components) is expressed in mass of substance (m) instead of volume (6).

Many packaging solutions make use of films coated with other polymers, metals (aluminum), or metal oxides (silicon oxide, SiOx, or aluminum oxide, AlOx). Because these materials are heterogeneous, the use of permeability coefficients as parameters to express their barrier properties is substituted by permeance values (i) for oxygen or water vapor transmission rates (WVTR), which take into account the total film thickness.

Table 1 lists the permeability values to oxygen and water vapor of commodity materials commonly used in packaging manufacture, and oxygen permeance and WVTR

Table 1 Permeability to Oxygen and Water Vapor of Common Plastic Materials and Oxygen Permeance and Water Vapor Transmission Rate Values of Complex Structures

Thickness, 1018 Kg.m/(m2.s.Pa)

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