Before the “bread bag,” waxed paper served as the preeminent wrapping material for many decades since its introduction in the 1920s. Initially, paraffin wax served as the exclusive coating material for bleached paper stock of suitable weight. Subsequently, the paraffin wax was blended with microcrystalline wax to improve its gloss, flexibility and sealing strength. At the same time, progress in paper processing resulted in a bright, white opaque paper that provided an excellent base for printing. End labels sealed the wrappings.
In the 1930s, cellulose film, or cellophane, was introduced to the baking industry and quickly gained widespread acceptance as a transparent wrapping material. In its original form, the film consisted of a single base sheet of various thicknesses that determined its strength. The application of coatings of nitrocellulose or polyvinylidene chloride (PVdC) greatly improved its oil resistance, gas permeability, heat sealability and moisture proofness. It is an excellent packaging film, but its cost was comparatively high.
By the mid-1950s, plastic polyolefin films in the form of polyethylene and polypropylene began to replace cellulose films as a transparent bread wrapper. Manufacturing of plastic films starts with resins derived by organic chemistry and forms the thin films by either cast or blown extrusion.
Polyethylene and polypropylene differ in properties such as strength, stiffness, temperature sensitivity, permeability, clarity and gloss. Different methods of manufacture result in a broad range of films such as cast oriented, unbalanced oriented, oriented coated and laminated combinations, all with measurably different properties. Their special characteristics and specific suitability to particular applications were discussed at length by Jurist (1964) and Smith (1967). Thomas (1985) provided a summary of packaging as specifically used by the baking industry, along with a comprehensive glossary of commonly used packaging terms.
Polyethylene film, available in low, medium and high densities, is the principal packaging material used for bread and rolls, with the low-density film forming the major base material for preformed bags and roll stock used by flow wrappers and bulk bun lines built to run with polyethylene film. The thickness, or gauge, of polyethylene films, as well as other films, is generally measured in mils, with 1 mil equal to 0.001 in.
Polyethylene and polypropylene films cannot be interchanged in flow wrappers and bulk bun packaging equipment because of the differences in sealing methods and technology required by the different sealing properties of these families of films. Sealing polyethylene requires the film be fused together (melted at the closure point), but polypropylene is sealed using a combination of temperature and pressure, while cold seal uses pressure only.
In general, low-density polyethylene offers adequate strength, gas permeability, moisture resistance and low temperature characteristics to satisfy normal packaging needs.
Where improved heat sealability is required, medium-density polyethylene will normally suffice. Polypropylene possesses properties similar to those of polyethylene but is stiffer and tougher at equivalent thicknesses and possesses superior optical characteristics; however, polypropylene is more susceptible to tearing than polyethylene (once torn, it “runs” with its grain, in line with its direction of initial extrusion). Polyethylene can be used as a component in coextruded films where it contributes resilience to the coextrusion.
To produce finished packages, a converter prepares the film by laminating it with other films, if required by the customer, and applying any needed adhesive coatings. The films are printed and trimmed to the desired size. Some packaging machines (flow wrappers and stand-up pouch systems) require roll stock, while others use pre-formed bags made to the specific size required by the baker. Bagmaking yields gusseted bags, mounted on wickets. Bun bakers generally use wickets of 250 bags because the shorter length gussets cause double thicknesses of film, while wickets of 400 to 500 bags are used for longer bread bags.
Cellulose
Plain cellulose, or cellophane, is a glossy transparent film that is odorless, tasteless and biodegradable when properly composted. To produce cellophane, cellulose fibers from wood, cotton or hemp are treated with alkali and carbon disulfide. It is tough and puncture resistant, although it tears easily. Cellophane has folding properties that make it suit-able for twist-wrapping (as used for small confectioneries); however, it is not heat sealable, and its dimensions and permeability of the film vary with changes in humidity. It is used for foods that do not require a complete moisture or gas barrier, including fresh bread and some types of sugar confectionery. Cellulose acetate is a clear, glossy transparent, sparkling film that is permeable to water vapor, odors and gases and is mainly used as a window material for paperboard cartons.
Polyethylene
Low-density polyethylene (LDPE) is heat sealable, inert, odor free and shrinks when heated. Polyethylene resin, extruded to make polyethylene film, is produced by the polymerization of ethylene gas under pressure. LDPE is a good moisture barrier but is relatively permeable to oxygen and is a poor odor barrier. It is less expensive than most films and is therefore widely used for bags, for coating papers or boards and as a component in laminates. In different thicknesses, LDPE is also used for shrink- and stretch-wrapping. Additional strength can be obtained by blending in linear low-density polyethylene (LLDPE).
High-density polyethylene (HDPE) is stronger, thicker, less flexible and more brittle than LDPE and a better barrier to gases and moisture. Bags made from HDPE have high resistance to tears and punctures and have good seal strength. Depending on the application, different densities of polyethylene are used for bread bags and bulk bun wrappers.
Polypropylene
Polypropylene (PP) is a clear glossy film with a high strength and good puncture re-sistance. The resin is made by the polymerization of propylene gas. The film has a moderate barrier to moisture, gases and odors, and it is not affected by changes in humidity. It stretches, although less than polyethylene. Oriented polypropylene (OPP) and biaxially oriented polypropylene (BOPP) are also clear glossy films with good optical properties, high tensile strength and improved puncture resistance. These films are widely used to pack baked foods, biscuits, snack foods and dried foods. Polypropylene is frequently used as an inner wrapping and for windows in paperboard bakery cartons.
Perforated polypropylene has been used in Europe to wrap baguettes and rolls. The minute holes (known as micro perfs), spaced in regularly patterns, allow the package to breathe, releasing moisture that would otherwise soften the crust undesirably.
Given a matte finish, polypropylene gives the look of paper to the film with the added advantage of superior barrier qualities and economics.
Coextruded films
Coextrusion is the simultaneous extrusion of 2 or more layers of different polymers to make a single film. Coextruded films have three main advantages compared with other types of film: (a) They have very high barrier properties, similar to laminates but pro-duced at a lower cost; (b) they are thinner than laminates and are therefore easier to use on filling equipment; and (c) the layers do not separate. For baked foods, confectioneries and cereals, low-density and high-density polyethylene, and polypropylene are the poly-mers of choice. If additional strength or barriers (moisture, oxygen, etc.) are required, more expensive polymers such as nylon are added to the coextrusion.
Typically a coextrusion has 3 layers — an outside layer with high gloss and printabil-ity, a middle bulk layer, which provides stiffness and strength, and an inner layer suitable for heat sealing.
Other films
Polyvinyl chloride, a resin produced by the polymerization of vinyl chloride, is very strong and is therefore used in thin films. It has a high barrier to gas and water vapor and is heat shrinkable and heat sealable. However, it has a brown tint, which limits its use in some applications. Polyamides (nylons) are clear, strong films over a wide temperature range from -60 to +200°C (-76 to +392°F) that have low permeability to gases and are greaseproof; however, the films are expensive to produce, require high temperatures to heat seal, and the permeability changes at different storage humidities. They are used with other polymers to make them heat sealable at lower temperatures, to improve the barrier properties and to reduce film cost.
Films can be coated with other polymers or metals to improve their barrier properties or to impart heat sealability. For example, a nitrocellulose coating on both sides of cellulose film improves the barrier to oxygen, moisture and odors, and enables the film to be heat sealed at lower temperature. A thin coating of aluminum, vacuum deposited in a process termed “metallization,” produces a very good barrier to oils, gases, moisture, odors and light; however, the cost of these films is significantly higher than un-coated films.
References
Jurist, M.A. 1964. Films for baked foods. Bakers Digest 38 (1): 74.
Smith, V.O. 1967. Polyolefin films. Bakers Digest 41 (3): 64.
Thomas, G. 1985. The use of flexible packaging materials in the bakery foods industry. AIB Tech. Bull. 7 (12).
More on this topic can be found in “Baking Science & Technology, 4th ed., Vol. II,” Page 589, by E.J. Pyler and L.A. Gorton. Details are in our store.
