Ideal food packaging not only contains the product, but also protects against deterioration, transmits information to consumers, and is convenient to use. Clear, air-permeable polyvinylchloride or polyethylene film (stretched tightly over the meat on a polystyrene or foam tray) has, for many years, delivered the desired packaging characteristics for retail meat display cases. A recent innovation in packaging technology is modified atmosphere packaging (MAP). MAP removes or replaces the gases surrounding the meat before sealing it in vapour-barrier materials. Although MAP packaging materials are usually more expensive, they are valued for utilising less retail labour and space, increasing shelf life, improving inventory control, maintaining uniformity and reducing waste.
by Professor Ken McMillin
MAP can include vacuum packaging (VP), which removes almost all the air before the product is enclosed in barrier materials, or the air can be replaced with other gases before the package is sealed by barrier materials. With MAP, the headspace environment and product may alter during storage, but there is no additional manipulation of the internal environment as with controlled atmosphere packaging (CAP). CAP continuously monitors the product environment so it can be maintained at a stable gas atmosphere, temperature, and humidity. CAP has most often been used to control ripening and spoilage of fruits and vegetables in bulk containers larger than retail-sized packages.
Expectations of meat colour
The colour most expected by consumers for red meats is cherry red, arising from the binding of oxygen (O2) (blooming) or carbon monoxide (CO) to myoglobin pigments. Any change in the oxidation-reduction mechanism gives rise to other meat colours. A purple colour occurs when oxygen is totally excluded, as with both vacuum packaging and anoxic packaging containing nitrogen (N2) and/or carbon dioxide (CO2) gases. Brown (oxidized) colours are caused by lack of reducing capability as meat ages or is exposed to light, heat, or other oxidizing conditions.
Innovation in packaging
Many advances in MAP for meat products have been in those areas of technology labelled as active or intelligent packaging. Active packaging incorporates specific compounds into packaging systems in order to maintain or extend product quality and shelf life. Intelligent or smart packaging can sense food properties or the package environment in order to alert the processor, retailer and/or consumer to environment or food status. Other packaging innovations are less conspicuous, but allow for increased shelf life and quality.
No single type of plastic possesses all of the properties required to create the ideal packaging solution. As a result, laminated films (films physically bound in layers) or extruded films (films chemically bound in layers) have been developed. Progress in lamination and extrusion technologies has created a wealth of choice in terms of film type, thickness, porosity, and size. Demands for source reduction and materials that can be recycled has also driven the development of new and different plastics. Alternatives to petroleum-derived plastics such as polylactic acid or edible films have not been readily adaptable to meat products. This is because meat has a high moisture content, is relatively dense, and has a relatively short shelf life. Packaging equipment manufacturers have kept pace with automated systems which allow for high speed assembly, packaging, sorting, labelling and cartoning of meat in MAP.
MAP packages with gaseous atmospheres have a headspace between the product and underside of the top film and also sometimes on the sides of the product. Any temperature differentials between the package interior and the external environment may cause condensation of water droplets on the underside of the film due to the high moisture content in meat. Chemicals to lower the surface tension of the water act as antifogging agents and are either sprayed or dipcoated onto the inner film layer or extruded into the polymer for migration to the film surface. Surface antifog compounds tend to lose effectiveness more rapidly than imbedded antifog agents.
Oxygen concentration is key
O2 levels are important in many MAP packaging systems. In aerobic systems, oxygen above 40% is necessary to provide the red colour bloom of the meat. This ensures that colour is not the first characteristic to deteriorate to an unacceptable level. In anoxic or anaerobic MAP systems, it is necessary to reduce residual O2 levels to below 1%, and preferably below 0.5%, during the packaging process because O2 between 1 and 12% will cause the myoglobin pigment present to change irreversibly to a brown colour (metmyoglobin). Oxygen scavengers are sometimes used in sachet-type packets to reduce O2 to acceptable levels.
Another concern for raw chilled meat in storage is exudate or purge; the fluid that is released from the meat due to pressure from contact with other packages or due to the effects of gravity over time. Absorbent pads are often placed in the bottom of trays to absorb any excess fluid. These may be paper, fiber or plastic composites. Some trays have imbedded absorbent materials and holes in the layer next to the meat to allow wicking of the fluid from the bottom of the tray and into the absorbent material.
The presence of pathogens
Microbial presence and growth is always a concern in meat, whether as spoilage bacteria that limit shelf life or pathogens that impact product safety. Some antibacterial agents have been tested as coatings to, or imbedded compounds in, films but effectiveness is reliant upon the film contacting the meat surface and transferring sufficient antibacterial agent to destroy or inhibit the microorganism. A difficulty with film-to-meat contact is that many of the plastic polymers initiate oxidation reactions that cause the red colour of bloomed meat to become irreversibly brown. For that reason, films with antibacterial agents will have more use in packaging of processed ready-to-heat or ready-to-eat meats that have a stable colour or are not expected by consumers to have a red colour. Antioxidant compounds imbedded into plastic materials have similar limitations as antimicrobial agents. Contact with the packaging materials is needed for effective control of oxidative deterioration.
Extending shelf life
Several technologies have been proposed to extend the shelf life of meat in MAP whilst providing the conditions that will cause meat to exhibit the expected red colour for retail display. Meat in air (21% O2) has a shelf life of seven days or less. Use of greater than 40% O2 will promote lipid oxidation and sometimes bone darkening in meat without antioxidants earlier than the 14 to 16 day colour display life usually obtained with high oxygen levels. Gas exchange systems that initially package meat in anoxic atmospheres (usually vacuum or CO2 and N2) allow up to 30 days storage and distribution before exchange of the gases with the desired display gas that contains O2.
One system uses meat conventionally overwrapped in air-permeable film with one or more packages placed in a larger master pack that is flushed with the anoxic gas mixture. Removal of the overwrapped packages from the master pack will allow air to permeate through the film and change the meat colour to red. If the air-permeable film is stretched too tightly against the meat, the rate of air contact and the development of colour will be slowed. Other systems rely upon dual lidding films, one permeable and the other acting as a barrier, with the permeable film innermost to the meat and the barrier film on the package exterior. When it is decided to expose the product to room air, the barrier layer is peeled away so air can diffuse through the permeable film and cause the product to bloom. In practice, unless the permeable film has been perforated with microscopic holes that allow air to penetrate, the pores in unperforated film are not sufficient in number or size to allow uniform blooming of the meat surface.
Another difficulty is that the static nature of the anoxic gas makes diffusion of air through the permeable film very slow. These technical deficiencies can be resolved by using equipment that either make a small hole in the barrier film or slices open the seal on barrier packages. Vacuum pumps remove the anoxic atmosphere and high O2 gas mixtures, usually with CO2 added, are flushed back into the package before either the small hole is patched with a label or the end of the package is resealed. The use of equipment for actively exchanging the gases speeds the process and provides a uniform and assured red colour development. However, the use of such machinery is not familiar to retail personnel and has a higher capital cost than most retail meat equipment. As a result, this form of gas exchange technology is not in commercial practice.
Technology in progress
Other advances in MAP technologies include innovations from the Cryovac Sealed Air company. As previously indicated, meat in contact with many of the barrier film types will discolour. The Mirabella package has two co-extruded shrink films where the permeable film layer is created within a barrier lidding film. The desired atmospheric gas is inserted between the two film layers immediately before heat sealing. The MAP gas mix between the two films and also in the headspace film prevents the barrier film from making contact with the meat, while the gas is able to stabilize the desired meat colour within the permeable environment. In the Darfresh system, the package reduces the amount of headspace by placing the product in a vacuum within a permeable flexible film.The product is also placed within a rigid barrier tray. The desired atmospheric gas is flushed into the headspace before sealing of the barrier film onto the tray. Less headspace is required in this MAP because the product is restrained from contact with the barrier film by the permeable film. These technologies are available for commercial use, but the relative cost efficiencies have not been established.
Other systems use CO to cause carboxymyoglobin red meat colour at the time of packaging. This process can take place in barrier MAP or within a master pack system using air-permeable packaging. The consumer controversy over potential exposure to the low levels (less than 0.5%) of CO have partially subsided, so retail sales of meat in these packages continues on a limited scale in the US. To conclude, the food industry requires improved product consistency and cost-effectiveness, a long shelf life, and an acceptable product appearance. As a result, any current, proposed, or future MAP systems must excel in these areas in order to ensure their adoption in commercial practice.
The author
Ken McMillin
Professor of Meat Science
Louisiana State University Agricultural Center, Baton Rouge
Louisiana,
USA
Email: KMcmillin@agcenter.lsu.edu
