Modified atmosphere packaging technology is capable of doubling the refrigerated shelf-life of fresh pork and beef compared to using vacuum packages. This technology was perfected in New Zealand where intercontinental transport of fresh meat began in the late 1930's. In the simplest form of modified atmosphere systems, package contents are back-flushed with 100% carbon dioxide before being sealed in a film that prevents the passage of oxygen. Provided the volume of gas is twice that of the meat in the package (so that the atmosphere remains saturated with carbon dioxide), fresh pork and beef can have storage lives of 70 and 110 days, respectively.
To take it a step further - individually packaged loins or roasts wrapped with high oxygen transmissible films can be grouped together, over-wrapped with a film having negligible oxygen transmission and this "master package" back-flushed with 100% carbon dioxide to maximize storage life. These packages can be shipped around the world. When they arrive at retail, the meat is simply removed from the master pack and allowed to stand at 4ºC in air for 30 min. The dark purple color of the meat pigment, deoxymyoglobin, which predominates in the absence of oxygen (under vacuum, nitrogen or carbon dioxide) will "bloom" to the cherry red (oxymyoglobin) colour we expect of fresh meat. The individually wrapped meat cuts can then be placed directly on retail display.
Carbon dioxide is used as a packaging gas for meat because it inhibits bacterial growth and delays lipid oxidation or rancidity. This is why it is so effective for storage-life extension of fresh meat. With cured meats such as pastrami or hot dogs, vacuum packaging can produce shelf-lives of 30-60 days because the main curing agent, sodium nitrite, serves as both an antimicrobial and an antioxidant.
Another type of packaging uses nitrogen gas for cured meat products. The next time you check the supermarket meat case, you may find "pillow packages" containing cured sausages of various types. These are not defective vacuum packages. Very often large packages of a dozen or more sausages will be back-flushed using nitrogen which is cheaper than carbon dioxide. The nitrogen gas reduces the pressure generated by packaging films on the meat during vacuum application and retards expression of juices from the tissue, thereby improving product appearance. However, this process does not extend shelf-life longer than vacuum packaging. Sandwiches in convenience stores and potato chips are routinely packaged with 100% nitrogen.
While the meat industry has embraced vacuum packaging as the most important development to occur this past century, there has been minimal interest in taking advantage of the extra shelf-life possible with modified atmospheres containing 100% carbon dioxide except for goat meat that was exported to the U.S. Large retail chains that make rapid product turn-over a key marketing strategy have opted instead to use high oxygen concentrations for packaging fresh meat which yields shorter shelf-life. This approach is dramatically different from previously described systems where maximum attainable shelf-life was the primary goal.
With high oxygen gas atmosphere packaging systems (80% oxygen plus 20% carbon dioxide) the cherry red colour of fresh meat is stabilized and maintained for 9-12 days at 4ºC. As a result of adoption of this technology, on-site butchers and meat kitchens have virtually disappeared from major retail food stores and the large chains will soon be displaying meat cuts on trays in refrigerated cases. Retail or case ready meats are being centrally packaged at modern processing facilities dedicated to portion packaging, where cuts are weighed, labeled, priced and sent simultaneously to many retail stores.
Using comprehensive sanitary controls, meat cuts are prepared and groups of packages are over-wrapped in single large bags made of gas-impermeable film. The larger bag is flushed with the high oxygen gas mixture. Strict temperature control during distribution enables the shelf-life to be tripled to 9 days, which is sufficient for city-wide distribution of fresh products without preservatives. Some outlying areas within a day's drive can also be serviced. The uncooked fresh roasts, steaks, chops and meat patties that we see in the display cases of convenience stores will be a product of this technology.
One of the more recent successes in packaging technology which caters to our desire for convenience as well as freshness with minimal processing, is modified atmosphere packaging of vegetables. Development of bulk storage systems and plastic film materials that exactly suit the respiratory needs of plant tissue which, unlike meat tissue does not respire, have enabled almost year-round distribution and sale of fresh vegetables without preservatives. Washed, chopped salads of almost endless variety are available at retail outlets.
Modified atmosphere packaging technology has contributed to the delivery of high quality, safe fresh and cured meats, produce and bakery products by an industry sensitive to the demands of regulators and consumers. In the Departments of Food Science and Biosystems Engineering, considerable work has been done with this technology in conjunction with Agriculture and Agri-Food Canada researchers at Lacombe, AB to develop portable refrigerant systems using liquid nitrogen to achieve precise temperature control at -1.5º ±0.5ºC.
The adoption of variations of modified atmosphere technology demonstrates both the complexity and continuous evolution within the food industry. To benefit from new process and product development and ensure that future changes do not compromise quality or safety, the industry is heavily reliant on research in food science and processing technology.