What is Modified Atmosphere Packaging (MAP)

Modified atmosphere is the practice of modifying the composition of the internal atmosphere of a package (commonly food packages, drugs, etc.) in order to improve the shelf life. The need for this technology for food arises from the short shelf life of food products such as meat, fish, poultry, and dairy in the presence of oxygen. In food, oxygen is readily available for lipid oxidation reactions. Oxygen also helps maintain high respiration rates of fresh produce, which contribute to shortened shelf life. From a microbiological aspect, oxygen encourages the growth of aerobic spoilage microorganisms.Therefore, the reduction of oxygen and its replacement with other gases can reduce or delay oxidation reactions and microbiological spoilage. Oxygen scavengers may also be used to reduce browning due to lipid oxidation by halting the auto-oxidative chemical process. Besides, MAP changes the gaseous atmosphere by incorporating different compositions of gases.

The modification process generally lowers the amount of oxygen (O₂) in the headspace of the package. Oxygen can be replaced with nitrogen (N₂), a comparatively inert gas, or carbon dioxide (CO₂).

A stable atmosphere of gases inside the packaging can be achieved using active techniques, such as gas flushing and compensated vacuum, or passively by designing “breathable” films.

Carbon dioxide: The bacteriostatic and fungistatic properties of carbon dioxide have been widely recognized since the 1920s and was used in shipments of beef, mutton and lamb from Australia and New Zealand to England. Carbon dioxide is highly soluble in water, and its’ solubility increases with decreasing temperature and higher meat pH. Greater than 99% of the gaseous carbon dioxide exists as dissolved gas and less than 1% as carbonic acid (H2 CO3 ), which partially dissociates to give H+, HCO3 – and CO32 -. Although drop in surface pH of meat due to dissolution of CO2 in meat does not fully explain the bacteriostatic effects of CO2 , it does contribute significantly to its antimicrobial activity. At lower product temperatures, the solubility of CO2 is greater, and hence is more effective in retarding microbial growth. Thus, CO2 in the product atmosphere in combination with strict temperature control will improve the microbiological quality of the product.

Although older literature indicates that CO2 concentrations greater than 20% result in undesirable brown color in muscle and bone due to either formation of metmyoglobin or precipitation of sarcoplasmic proteins (Seideman and Durland, 1984), this probably was due to small amounts of residual oxygen.

Nitrogen: Nitrogen is normally used as inert filler, and to prevent package collapse when carbon dioxide dissolves into meat tissue (Lambert et al., 1991). While some studies reported extension of shelf life by nitrogen, others did not indicate any antimicrobial/bacteriostatic activity. The variability in shelf life extensions observed in some of the studies could be due to the residual oxygen concentrations in the MAP products due to incomplete evacuation of air from the packages. Thus, inclusion of nitrogen in MAP atmospheres should provide an advantage in terms of exclusion of oxygen and thus, in preventing aerobic spoilage microflora in the meat products.

Carbon monoxide: CO can be used as part of the MAP gaseous mixtures, ranging from 0.3-0.5 %, for stabilization of fresh meat color. Carbon monoxide (CO) has been approved by FDA (2002) for use as a component of a gas mixture in a MAP system as Generally Recognized as Safe (GRAS) up to 0.4%. Although use CO as part of the gaseous mixture of MAP packaged meats has been limited in the United States due to its toxicological effects, the GRAS status should provide the impetus to its’ increased use. In Norway, MAP of meats using mixtures of carbon monoxide has an estimated 50-60% of the retail market share. Carbon monoxide extends the lag phase and slows growth rate of E. coli, Achromobacter and P. fluorescence (Gee and Brown, 1981) at concentrations of 25-30%, while P. aeruginosa is unaffected even at these high concentrations. However, the use of carbon monoxide at the low concentrations normally recommended (1%) would have relatively little effect on bacterial growth on fresh meats (Hunt et al., 2002).

Combinations of carbon monoxide with other gases such as carbon dioxide to control microbial growth provide an excellent opportunity for meat processors to improve shelf lives of the retail packed fresh meats (Kropf, 1980). With an increasing trend towards case-ready meats, carbon monoxide can provide the means to achieve the retail display life of fresh meats for today’s industry.