Effect of modified atmosphere packaging on major foods
Written by Michael Mullan
The effect of modified atmosphere packaging (MAP) on dairy products, raw meat, raw poultry , cooked meat and fruit and vegetables is discussed.
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 in the moisture in the cheese. 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.
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 O2. 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 (Mullan and McDowell, 2011). 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.
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 (Research into the optimal gas composition and package type and size should be conducted for individual food products. The headspace and pack volume to food product volume are 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 aw 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. There is potential for growth of this organism under anaerobic MAP conditions particularly when held at products are 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 3oC 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 10oC 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 0oC 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 3oC 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, enterotoxigenic E. coli and viruses. While these micro-organisms may not grow in MAP packs, particularly if the storage temperature is maintained around 3o 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. and McDowell, D. (2011) Modified Atmosphere Packaging. In 'Food and Beverage Packaging Technology’. 2nd Edition. Edited by R. Coles and M. Kirwan. Wiley-Blackwell. Oxford.