SCIENCE OF MODIFIED ATMOSPHERE PACKAGING

This section contains summary information on modified atmosphere packaging. More comprehensive treatment is available in a chapter on modified atmosphere packaging, written by the author and Derek McDowell, in the book 'Food Packaging Technology'. Derek McDowell is Head of Supply and Packaging at Loughry Campus and is a packaging specialist.

This book, edited by Cole, Kirwan and McDowell, was published by Blackwell Publishing in August, 2003. The author's chapter deals with the food science and technology of food packaging including packaging materials, their properties, selection for packaging applications, and packaging equipment.

Following requests from students I have added a list of references on modified atmosphere packaging for downloading. Readers may find the article on labeling and the calculator for determining the energy density of foods useful.

Importance of gaseous environment

Many foods spoil rapidly in air due to moisture loss or uptake, reaction with oxygen and the growth of aerobic micro-organisms i.e., bacteria and moulds.  Microbial growth results in changes in texture, colour, flavour and nutritional value of the food.  These changes can render food unpalatable and potentially unsafe for human consumption.  Storage of foods in a modified gaseous atmosphere can maintain quality and extend product shelf life by slowing chemical and biochemical deteriorative reactions and by slowing or in some instances preventing the growth of spoilage organisms.

Modified atmosphere packaging (MAP) is defined as "the packaging of a perishable product in an atmosphere which has been modified so that its composition is other than that of air" (Hintlian and Hotchkiss, 1986).  Whereas controlled atmosphere storage involves maintaining a fixed concentration of gases surrounding the product by careful monitoring and addition of gases, the gaseous composition of fresh MAP foods is constantly changing due to chemical reactions and microbial activity.  Gas exchange between the pack headspace and the external environment may also occur because of permeation across the package material.

Packing foods in a modified atmosphere can offer extended shelf life and improved product presentation in a convenient container, making the product more attractive to the retail customer.  However, MAP cannot improve the quality of a poor quality food product. It is therefore essential that the food be of the highest quality prior to packing in order to optimise the benefits of modifying the pack atmosphere.

Good hygiene practices and temperature control throughout the chill-chain for perishable products are required to maintain the quality benefits and extended shelf life of MAP foods.

Gases used in modified atmosphere packaging

The three main gases used in modified atmosphere packaging are O₂, CO₂ and N₂.  The choice of gas is very dependent upon the food product being packed.  Used singly or in combination, these gases are commonly used to balance safe shelf life extension with optimal organoleptic properties of the food.  Noble or 'inert' gases such as argon are in commercial use for products such as coffee and snack products; however, the literature on their application and benefits is limited.  Experimental use of carbon monoxide (CO) and sulphur dioxide (SO₂) has also been reported.

• Carbon dioxide

Carbon dioxide is a colourless gas with a slight pungent odour at very high concentrations. It is an asphyxiant and slightly corrosive in the presence of moisture. CO₂ dissolves readily in water (1.57 g/kg @ at 100 kPa, 20° C) to produce carbonic acid (H₂CO₃) that increases the acidity of the solution and reduces the pH. This gas is also soluble in lipids and some other organic compounds. The solubility of CO₂ increases with decreasing temperature. For this reason, the antimicrobial activity of CO₂ is markedly greater at temperatures below 10° C than at 15° C or higher. This has significant implications for MAP of foods. The high solubility of CO₂ can result in pack collapse due the reduction of headspace volume. In some MAP applications, pack collapse is favoured, for example in flow wrapped cheese for retail sale.

• Oxygen

Oxygen is a colourless, odourless gas that is highly reactive and supports combustion. It has a low solubility in water (0.040 g/kg at 100 kPa, 20° C). Oxygen promotes several types of deteriorative reactions in foods including fat oxidation, browning reactions and pigment oxidation. Most of the common spoilage bacteria and fungi require oxygen for growth. Therefore, to increase shelf life of foods the pack atmosphere should contain a low concentration of residual oxygen. It should be noted that in some foods a low concentrations of oxygen can result in quality and safety problems (for example unfavourable colour changes in red meat pigments, senescence in fruit and vegetables, growth of food poisoning bacteria) and this must be taken into account when selecting the gaseous composition for a packaged food.

• Nitrogen

Nitrogen is a relatively un-reactive gas with no odour, taste, or colour. It has a lower density than air, non-flammable and has a low solubility in water (0.018 g/kg at 100 kPa, 20° C) and other food constituents. Nitrogen does not support the growth of aerobic microbes and therefore inhibits the growth aerobic spoilage but does not prevent the growth of anaerobic bacteria. The low solubility of nitrogen in foods can be used to prevent pack collapse by including sufficient N₂ in the gas mix to balance the volume decrease due to CO₂ going into solution.

• Carbon Monoxide

Carbon monoxide is a colourless, tasteless and odourless gas that is highly reactive and very flammable. It has a low solubility in water but is relatively soluble in some organic solvents. CO has been studied in the MAP of meat and has been licensed for use in the USA to prevent browning in packed lettuce. Commercial application has been limited because of its toxicity and the formation of potentially explosive mixtures with air.

• Noble gases

The noble gases are a family of elements characterised by their lack of reactivity and include helium (He), argon (Ar), xenon (Xe) and neon (Ne). These gases are being used in a number of food applications now e.g. potato-based snack products. While from a scientific perspective, it is difficult to see how the use of noble gases would offer any preservation advantages compared with N₂ they are being used.

EFFECT OF MAP ON MAJOR FOODS

• Dairy products   
• Raw red meat
• Raw poultry
• Cooked, cured and processed meat products
• Fish and fish products
• Modified Atmosphere Packaging of fruits and vegetables
• Calculate energy content for a food label
• How to cite this article

Dairy products

MAP has the potential to increase the shelf life of a number of dairy products.  These include fat-filled milk powders, cheeses and fat spreads.  In general these products spoil due to the development of oxidative rancidity in the case of powders and or the growth of micro-organisms, particularly yeasts and moulds, in the case of cheese.

Whole milk powder is particularly susceptible to the development of off-flavours due to fat oxidation.  Commercially the air is removed under vacuum and replaced with N₂ or N₂/CO₂ mixes and the powder is hermetically sealed in metal cans.  Due to the spray drying process air tends to be absorbed inside the powder particles and will diffuse into the container over a period of 10 days or so.  This typically will raise the residual O₂ content to 1%-5% or higher.  Because some markets require product with low levels of residual O₂(<1%) some manufacturers re-pack the cans after 10 days storage.  Use of O₂ scavenging may also be useful. 

English territorial cheeses e.g. Cheddar have traditionally been vacuum packed.  Increasingly MAP is being used with high CO₂ concentration CO₂/N₂ gas mixes.  This has the advantage of obtaining a low residual O₂ content and a tight pack due to the CO₂ going into solution.  It is important to balance this process using the correct N₂ level in the gas mix so as to avoid excessive pressure on the pack seal.

Use of N₂ / CO₂ atmospheres has significant potential for extending the shelf life of cottage cheese. The latter is a high moisture, low fat product that is susceptible to a number of spoilage organisms including Pseudomonas spp.  Use of gas mixtures containing CO₂ balanced with N₂ can increase the shelf life significantly.

Raw red meat

Microbial growth and oxidation of the red oxymyoglobin pigment are the main spoilage mechanisms that limit the shelf life of raw red meats. The packaging technologist has to maintain the desirable red colour of the oxymyoglobin pigment, by having an appropriate O₂- concentration in the pack atmosphere, and at the same time minimise the growth of aerobic micro-organisms. Aerobic spoilage bacteria, such as Pseudomonas species normally constitute the major flora on red meats. Since these bacteria are inhibited by CO₂ it is possible to achieve both red colour stability and microbial inhibition by using gas mixtures containing CO₂ and O_2.  These mixtures can extend the chilled shelf-life of red meats from 2-4 days to 5-8 days.  For information on the gas compositions and the recommended gas/product ratio please see Food Packaging Technology. The maintenance of recommended chilled temperatures and good hygiene and handling practices throughout the butchery, MAP, distribution and retailing chain is of critical importance in ensuring both the safety and extended shelf-life of red meat products. Because raw red meats are cooked before consumption the risk of food poisoning can be greatly reduced by proper cooking.

Raw poultry

Microbial growth, particularly of Pseudomonas and Achromobacter species, is the major factor limiting the shelf life of raw poultry.  These Gram-negative aerobic spoilage bacteria are effectively inhibited by CO₂.  Consequently the inclusion of CO₂ in MAP at a concentration in excess of 20% can significantly extend the shelf-life of raw poultry products. Since poultry meat provides a good medium for the growth of pathogenic micro-organisms, including some that are not inhibited by CO₂, it is critical that recommended chilled temperatures, good hygiene and handling practices throughout the supply chain are adhered to and that products are properly cooked prior to consumption. Early research into gas mixes for MAP of poultry meat reported discolouration of the meat at carbon dioxide concentrations higher than 25%.  This research is at variance with the lack of problems reported from the commercial use of relatively high levels of CO₂ with meat products, up to 100% with some products.  It would appear that the problems that have been occasionally encountered with high levels of CO₂ may simply be due to high residual levels of O₂ refer to Food Packaging Technology for further information. Research into the optimal gas composition and package type and size should be conducted for individual food products.The ratio of headspace pack volume to food product volume is also important as are the types and thickness of the package material and the package design.  Shelf life evaluations must reflect the conditions from manufacture to consumption of the product. It may also be necessary to consider the effect of pack opening on the subsequent shelf life of the product.

Cooked, cured and processed meat products

The principal spoilage mechanisms that limit the shelf life of cooked, cured and processed meat products are microbial growth, colour change, and oxidative rancidity. For cooked meat products, the heating process should kill vegetative bacterial cells, inactivate degradative enzymes, and fix the colour.  Consequently, spoilage of cooked meat products is primarily due to post-process contamination by micro-organisms as a result of poor hygiene and handling practices.  The colour of cooked meats is susceptible to oxidation and it is important have only low levels of residual O₂ in packs.  MAP using CO₂/N₂ mixes along with a recommended gas/product ratio are given in and if used will maximise shelf life and inhibit the development of oxidative off flavours and rancidity. Processed meat products such as sausages, frankfurters and beef burgers generally contain sodium metabisulphite, which is an effective preservative against a wide range of spoilage micro-organisms and pathogens. Cooked, cured and processed meat products containing high levels of unsaturated fat are liable to be spoiled by oxidative rancidity, but MAP with CO₂/N₂ mixtures is effective at inhibiting this undesirable reaction. Potential food poisoning hazards are primarily due microbial contamination or growth resulting from post-cooking, curing or processing contamination.  These can be minimised by using recommended chilled temperatures, good hygiene and handling practices.  The low a_w and addition of nitrite in cooked, cured and processed meat products inhibits many food poisoning bacteria, particularly Cl. botulinum.  This inhibition may be compromised in products formulated with lower concentrations of chemical preservatives than those used in traditional foods.  The potential effects of any changes in product formulation on growth and survival of pathogens should always be considered. Cooked meats stored without any added preservatives will be at risk from growth of Cl. botulinum under anaerobic MAP conditions particularly when held at elevated storage temperatures. 

Fish and fish products

There has been a very significant increase in the sale of MAP fish products in Europe and particularly in the UK.  Nevertheless packaging technologists should be aware of a major concern limiting the development of MAP for this product group, namely Cl. botulinum.  There is also debate about the cost-benefits of MAP since in some applications only relatively small increases in safe shelf life have been reported.  Spoilage of fish results in the production of low molecular weight volatile compounds therefore packaging technologists need to consider the odour barrier properties of packaging films and select appropriate high barrier materials for packaging strong flavoured fresh, smoked and brined fish and fish products. Spoilage of fish and shellfish results from changes caused by three major mechanisms (i) the breakdown of tissue by the fish's own enzymes (autolysis of cells), (ii) growth of micro-organisms, and (iii) oxidative reactions.  MAP can be used to control mechanisms (ii) and (iii) but has no direct effect on autolysis.  Generally the major spoilage bacteria found on processed fish are aerobes including Pseudomonas, Moraxella, Acinetobacter, Flavobacterium and Cytophaga species. There are several micro-organisms that are of particular importance when dealing with MAP fish products, these include Cl. botulinum.  Use of CO₂ can effectively inhibit the growth of some of these species.  The aerobic spoilage organisms tend to be replaced by slower growing and, less odour producing, bacteria particularly lactic acid bacteria such as lactobacilli during storage. Because fish and shellfish contain much lower concentrations of myoglobin, the oxidation status of this pigment is less important than in other meats.  Because of the high moisture content and the lipid content of some species N₂ is used to prevent pack collapse.One of the concerns about MAP of fish is that removal of O₂ and its replacement by N₂ or N₂/CO₂ results in anaerobic conditions that are conducive to the growth of protease-negative strains of Cl. botulinum. Because these bacteria can grow at temperatures as low as 3^oC and do not significantly alter the sensory properties of the fish, there is the potential for food poisoning that can lead to fatalities.  While there is no evidence that CO₂ promotes growth of psychotropic strains of Cl. botulinum there is, as discussed previously, some concerns about CO₂ promoting the germination of spores of this organism. Considerable research has been undertaken to assess, and to control the risks associated with the growth of Cl. botulinum in MAP of fish and other products.  The Advisory Committee on the Microbiological Safety of Food (ACMSF) (Anon, 1992) have recommended controlling factors that should be used singly or in combination to prevent the growth of, and toxin production in, prepared chilled food by psychotropic Cl. botulinum. Some fish processors include O₂ in their MAP to further reduce the risk of growth of clostridia. Since botulinum toxin is relatively heat sensitive, correct cooking of seafood should eliminate any problem with preformed toxin.

Modified Atmosphere Packaging of fruits and vegetables

Consumers now expect 'fresh' fruit and vegetable produce throughout the year.  MAP has the potential to extend the safe shelf life of many fruits and vegetables.  Packaging fresh and unprocessed fruit and vegetables poses many challenges for packaging technologists. Unlike other chilled perishable foods, fresh produce continues to respire after harvesting.  The products of aerobic respiration include CO₂ and water vapour.  In addition, respiring fruits and vegetables produce C₂H₄ that promotes ripening and softening of tissues.  The latter if not controlled will limit shelf life. Respiration is affected by the intrinsic properties of fresh produce as well as various extrinsic factors including ambient temperature.  It is accepted that the potential shelf life of packed produce is inversely proportional to respiration rate. Respiration rate increases by a factor of 3-4 for every 10^oC increase in temperature.  Hence the goal of modified atmosphere packaging for fruits and vegetables is to reduce respiration to extend shelf life while maintaining quality. Respiration can be reduced by lowering temperature, lowering the O₂ concentration, increasing the CO₂ concentration and by the combined use of O₂ depletion and CO₂ enhancement of pack atmospheres.  If the O₂ concentration is reduced beyond a critical concentration, this is dependant on the species and cultivar, then anaerobic respiration will be initiated.    Anaerobic respiration, or anaerobiosis, is usually associated with undesirable odours and flavours and a marked deterioration in product quality.  While increasing the CO₂ concentration will also inhibit respiration, high concentrations may cause damage in some species and cultivars.The use of low concentrations of O₂ and elevated levels of CO₂ can have a synergistic effect on slowing down respiration, and indirectly, ripening. While the mechanisms whereby MAP can extend the shelf life of fresh produce are not fully understood it is known that the low O₂/high CO₂ conditions reduce the conversion of chlorophyll to pheopytin, decrease the sensitivity of plant tissue to C₂H₄, inhibit the synthesis of carotenoids, reduce oxidative browning and discolouration and inhibit the growth of micro-organisms.  These mechanisms are all temperature dependant.  Packaging technologists should be aware of  several major pathogens as far as MAP fresh produce is concerned, in particular L. monocytogenes and Cl. botulinum.  As previously discussed L. monocytogenes can grow under reduced O₂ levels and is not markedly inhibited by CO₂.  This combined with its ability to grow at temperatures close to 0^oC helps to explain the concern.The use of MAP atmospheres containing low concentrations of O₂ and elevated CO₂ concentrations may permit the growth of psychotropic protease-negative strains of Cl. botulinum. However, provided packs are stored at 3^oC or below for not more than 10 days there is unlikely to be a problem with clostridia.  Temperature control is critical since temperature abuse could lead to pack contents becoming toxic. The environment in which fruits and vegetables are grown may harbour pathogens including Salmonella species, enterotoxegenic E. coli and viruses. While these micro-organisms may not grow in MAP packs, particularly if the storage temperature is maintained around 3^o C, they may survive throughout storage and could cause food poisoning through cross contamination in the home or due to the consumption for raw or under processed product.  Hygienic preparation, sanitation in chilled-chlorinated water, rinsing and dewatering prior to MAP are now considered essential treatments to fruits and vegetables prior to packaging to ensure low microbial counts and assure safety.

Since there is a risk of anaerobic pathogens, such as Cl. botulinum growing in MAP packs, a minimum level of O₂ (e.g. 2-3% ) is usually recommended to ensure that potentially hazardous conditions are not created.

Mullan, W.M.A. (2002). [On-line] UK: Available: Accessed:

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