Modified atmosphere

Modified atmosphere packaging (MAP) 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.

The first recorded beneficial effects of using modified atmosphere date back to 1821. Jacques Étienne Bérard, a professor at the School of Pharmacy in Montpellier, France, reported delayed ripening of fruit and increased shelf life in low-oxygen storage conditions. Controlled atmosphere storage (CAS) was used from the 1930s when ships transporting fresh apples and pears had high levels of CO₂ in their holding rooms in order to increase the shelf life of the product. In the 1970s MA packages reached the stores when bacon and fish were sold in retail packs in Mexico. Since then development has been continuous and interest in MAP has grown due to consumer demand.

Atmosphere within the package can be modified passively or actively. In passive MAP, the high concentration of CO₂ and low O₂ levels in the package is achieved over time as a result of respiration of the product and gas transmission rates of the packaging film. This method is commonly used for fresh respiring fruits and vegetables. Reducing O₂ and increasing CO₂ slows down respiration rate, conserves stored energy, and therefore extended shelf life. On the other hand, active MA involves the use of active systems such as O₂ and CO₂ scavengers or emitters, moisture absorbers, ethylene scavengers, ethanol emitters and gas flushing in the packaging film or container to modify the atmosphere within the package.

The mixture of gases selected for a MA package depends on the type of product, the packaging materials and the storage temperature. The atmosphere in an MA package consists mainly of adjusted amounts of N₂, O₂, and CO_2. Reduction of O₂ promotes delay in deteriorative reactions in foods such as lipid oxidation, browning reactions and growth of spoilage organisms. Low O₂ levels of 3-5% are used to slow down respiration rate in fruits and vegetables. In the case of red meat, however, high levels of O₂ (~80%) are used to reduce oxidation of myoglobin and maintain an attractive bright red color of the meat. Meat color enhancement is not required for pork, poultry and cooked meats; therefore, a higher concentration of CO₂ is used to extend the shelf life. Levels higher than 10% of CO₂ are phytotoxic for fruit and vegetables, so CO₂ is maintained below this level.

N₂ is mostly used as a filler gas to prevent pack collapse. In addition, it is also used to prevent oxidative rancidity in packaged products such as snack foods by displacing atmospheric air, especially oxygen, therefore extending shelf life. The use of noble gases such as helium (He), argon (Ar) and xenon (Xe) to replace N₂ as the balancing gas in MAP can also be used to preserve and extend the shelf life of fresh and minimally processed fruits and vegetables. Their beneficial effects are due to their higher solubility and diffusivity in water, making them more effective in displacing O₂ from cellular sites and enzymatic O₂ receptors.

There has been a debate regarding the use of carbon monoxide (CO) in the packaging of red meat due to its possible toxic effect on packaging workers. Its use results in a more stable red color of carboxymyoglobin in meat, which leads to another concern that it can mask evidence of spoilage in the product.