When raw meat is exposed to air, it deteriorates rapidly, even when properly chilled. In red meats, the bright red colour associated with freshness fades to grey-brown as oxymyoglobin is converted to metmyoglobin. Lipid oxidation may also occur, affecting both aroma and flavour acceptability. Growth of spoilage bacteria, such as Pseudomonas spp., exacerbates these effects, whilst also influencing meat texture. The willingness of consumers to purchase meat is greatly reduced by these changes, and so, to ensure commercial success, manufacturers must try to postpone them. Modified atmosphere packaging (MAP) is one way of doing this.
MAP is the enclosure of foods within packs containing gas mixtures that differ in composition from air. Levels of oxygen (O2), nitrogen (N2), carbon dioxide (CO2) and sometimes other gases are adjusted to control biochemical and enzymic reactions, reduce moisture loss and inhibit the growth of microorganisms. Within the meat industry, the use of modified atmospheres is widespread. The exact gas compositions employed vary between products, but there are 2 main types: high O2 and low O2.
High O2 MAP is particularly useful for packaging raw red meat. It uses a combination of O2 and CO2 to enhance meat colour and inhibit microbial spoilage. The elevated O2 levels (70-80 per cent) prolong the period for which the packaged meat exhibits the bright red coloration which consumers associate with freshness. This coloration only occurs when myoglobin is present in its oxygenated form: oxymyoglobin.
In the absence of oxygen, myoglobin is dark purple and less appealing. The problem with high O2 levels is that they encourage the growth of aerobic spoilage organisms and promote oxidation.
Whilst the inclusion of CO2 (20-30 per cent) in packages helps to control the former, the latter means that rancidity may develop despite the product still being desirable in terms of appearance. In addition, should polyunsaturated fatty acids be oxidised, textural and nutritive properties may be adversely affected.
Reductions in tenderness, juiciness and flavour have also been linked with the oxidation of proteins in meat.
With low O2 MAP, these issues are not a problem. O2 is removed from meat packaging and usually replaced with CO2 (20-30 per cent) to prevent the growth of aerobic spoilage organisms and limit the oxidation of proteins and lipids.
Meat packaged in this way is superior to that packaged in high O2 atmospheres in terms of both tenderness and flavour. However, the exudation of fluids from raw meats may be increased by high CO2 concentrations and, because CO2 is highly soluble in water and fats, it can be absorbed by the meat, reducing headspace volume and resulting in packaging collapse.
To counteract this, N2 is typically used as an inert filler wherever CO2 is used in MAP, particularly when semi-rigid containers are employed. N2 also displaces O2, which is desirable for preventing oxidation, but complete exclusion of the gas can have detrimental effects upon consumer appeal.
Myoglobin will not be found in its unattractive oxidized form, but neither will it be oxygenated to form oxymyoglobin.
Use of carbon monoxide (CO), a colourless gas that has no flavour or aroma and binds to myoglobin to produce carboxymyoglobin, may offer a solution to this problem.
Carboxymyoglobin imparts a bright red colour to the surface of meat in much the same way as oxymyoglobin, except that it has greater stability. Thus, modified atmospheres containing CO can maintain desirable meat redness for longer than those simply containing high O2.
CO may find application in both low O2 MAP, where conditions prohibit the oxygenation of myoglobin, and high O2 MAP, where it can prevent discoloration resulting from the eventual oxidation of oxymyoglobin to metmyoglobin.
Low levels of the gas (0.4 per cent) have been approved by the US Food and Drug Administration (FDA) for use in retail meat packaging since 2004, but utilisation remains limited. This is largely due to consumer concerns regarding the toxicity of CO and potential risks associated with the fact that it may cause meat to look fresh despite harbouring other spoilage traits.
Meat quality is undoubtedly multifaceted, and while MAP is able to protect many aspects of it, it may not be able to preserve them all completely.
The key to success is balance, both in terms of the properties targeted and the gases used.
With an appropriate gas composition, and adherence to correct handling protocol and use-by dates, there seems no reason why modified atmosphere packaging meat should not both look attractive and offer good eating quality, and therefore appeal to consumers on several levels.
The International Food Information Service (IFIS) produces the FSTA - Food Science and Technology Abstracts database, a resource specifically developed for the food industry. All information used in this article has been sourced from the FSTA - Food Science and Technology Abstracts database.
Access the database with FSTA Custom Alerts and keep up to date with the latest scientific and technological developments in food science, food technology and nutrition.
Examples of some FSTA - Food Science and Technology Abstracts database records related to meat packaging:
- Books: Improving the sensory and nutritional quality of fresh meat, Woodhead Publishing Ltd, 2009; Food packaging science and technology, CRC Press, 2008
- Journals: Where is MAP going? A review and future potential of modified atmosphere packaging for meat; (Meat Science); Stabilisation of minced meat colour by carbon monoxide (Czech Journal of Food Sciences); Modified atmosphere packaging affects lipid oxidation, myofibrillar fragmentation index and eating quality of beef (Packaging Technology and Science); Consumer acceptability and physicochemical characteristics of modified atmosphere packed beef steaks (Meat Science).
Myra Armson is a Higher Scientific Officer at International Food Information Service (IFIS Publishing), based in Shinfield, Reading, UK.