This article describes a strategy to improve poultry meat shelf life. Traditional packaging methods are described alongside new research, which examines how Modified Atmosphere Packaging (MAP) can be employed to reduce spoilage by microorganisms and minimise product oxidation. In particular, the impact of using a MAP argon/carbon dioxide gas mix to extend the shelf life of turkey meat is assessed.
By Fraqueza M J and Barreto A S

In the last few decades, the retail presentation of meat and processed meat has changed dramatically as a result of the use of MAP and cold storage. This technology has contributed to the reduction of microbial spoilage and microbial and lipid oxidation of meat products. MAP has encouraged new product development and opened up new markets by facilitating alternative meat storage management and distribution practices. Shelf life and consumer handling have also been improved. However, the success of this technology for meat packaging depends on the specificity of gas mixtures being tailored to meet product requirements, the nature and initial quality of the meat, temperature control, the barrier properties of the packaging film, and the efficiency of the equipment used.

MAP gas mixtures approach
The usual gas mixture used for retail sliced poultry under MAP is 20% carbon dioxide (CO2)), 70% oxygen (O2 ) and 10% nitrogen (N2,) yielding a shelf life of approximately eight days. The elevated oxygen levels employed in high oxygen MAP saturate meat pigments with oxygen and slow surface metmyoglobin formation. However, they also accelerate lipid oxidation, off-flavour development and premature browning during cooking [1]. The use of CO2-enriched atmospheres extends the shelf life of raw meat by inhibiting psychrotrophic Gram-negative bacteria and Pseudomonas spp. CO2 is the only gas with a direct antimicrobial effect, resulting in an increased lag phase and generation time during the logarithmic phase of bacterial growth. Eventually, however, meat spoilage and changes in organoleptic characteristics are observed as slower-growing microorganisms begin to proliferate.

Other anaerobic gas mixtures can also extend shelf life. The degree by which shelf life can be extended varies by one or two weeks according to the poultry meat’s quality, colour and temperature [2].However, the aspect (colour) of the meat is not always attractive to consumers at this point. Processors continue to search for alternative technologies in order to match the acceptability of retail fresh meat, whilst extending shelf life and product safety. The only other gas, apart from the CO2, O2 and N2 gas mixtures used in raw meat packaging that has been adopted and studied in red meat is carbon monoxide (CO) . However, its use is forbidden by law in Europe. On the other hand, other gases such as argon (Ar), helium (He), and nitrous oxide ( N2O) are permitted in meat packaging under the European Union (Directive 95/2/CE; EU, 1995).

Recently, there has been great interest shown in the potential benefits of Ar and other gases in MAP applications for meat. According to Morgan [3], Ar is increasingly used in MAP. In fact, Ar is an inert, odourless and tasteless gas, more dense and more soluble than N2. The physical properties of this gas offer certain advantages over the N2, O2, or CO2 atmospheres, and over other MAP gas mixtures typically used to prolong shelf life quality or freshness of packaged foods. Since it is chemically inert, Ar does not react with food constituents, unlike O2 or CO2. Ar also inhibits the action of some oxidase enzymes that cause food spoilage and, since it is denser and exhibits greater solubility in both water and oil, it is more effective than N2 for displacing O2 from the oils and fats in foods. A US patent has already been issued for the use of Ar in the preservation of cut and segmented fresh fruits [4]. Argon gas, introduced as a major constituent of MAP has also been reported to reduce microbial growth and sustain the quality of fresh produce such as broccoli and lettuce [5].

In other studies, Ar has been reported to be biochemically active, probably due to its enhanced solubility in water when compared with N2, and it appears to interfere with enzymatic O2 receptor sites [6]. However, findings presented on the effect of Ar inhibition and control of the growth of certain microorganisms, on the activity of quality-related enzymes, and on degradative chemical reactions in selected perishable food products (such as minimally processed fruit) have proved inconsistant.

A study of sliced cooked ham packaged in Ar found that there was an improvement in oxidation control and microbial inhibition, thereby extending the product’s shelf life. However, there is a wide spread lack of knowledge about the action of this gas on microbial development and prevention of oxidation when applied to raw meat.

An argon mixture used to extend poultry meat shelf life under MAP
The objective of a recent study conducted in our laboratory was to evaluate the effect of an anaerobic gas mixture containing Ar on poultry meat shelf life. Spoilage, flora growth, meat colour and lipid oxidation of turkey meat was evaluated under MAP at 0°C [7]. Sliced breast samples were individually packaged in normal atmosphere and in four modified atmospheres containing different gas mixtures such as 100% N2, 50% Ar/50% N2, 50% Ar/50% CO2 and 50% N2/50% CO2. “HBX-070” bags (a multilayer EVOH-based film) sealed with a packaging machine were used for the MAP samples. The aerobic and MAP samples were immediately stored (0±1ºC in the dark) for 12 and 25 days respectively. Microbiological analysis and lipid oxidation evaluation using a thiobarbituric acid test (TBA) were performed on both samples on days 0, 5 and 12 of storage and then also on days 19 and 25 for the MAP samples.

Results show that the microbial shelf life of sliced turkey meat under MAP is extended by an extra week compared with aerobic packaging (5-d shelf life) when 100% N2 and 50% Ar/50% N2 mixtures are used. The shelf life is extended by two weeks for 50% N2/50% CO2, and three weeks for 50% Ar/50% CO2. The mixture with argon and CO2 was also more efficient at delaying flora development than the CO2/N2 mixture, with 1 log difference on the 25th day of storage for total psychrotrophic, total anaerobic and Brochothrix thermosphacta counts. However, the presence of Ar in gas mixtures had no additional protective effect on lipid oxidation of turkey meat. There is no advantage, therefore, in selecting Ar over N2  in MAP gas mixtures.

References
1. Cornforth D & Hunt M. Low-oxygen packaging of fresh meat with carbon monoxide. Meat Quality, Microbiology, and Safety. AMSA White Paper 2008; Series Number 2: 10p.
2. Fraqueza M J, Ferreira M C and Barreto AS. Spoilage of light (PSE-like) and dark turkey meat under aerobic or modified atmosphere package: microbial indicators and their relationship with total volatile basic nitrogen. British Poultry Science 2008; 49 (1): 12 – 20.
3. Morgan N. Argon–the noble protector. www.gasworld.com/news.php [Accessed July 2007]. Chipping Campden, UK.
4. Powrie W D, Chiu R and Wu H. Preservation of cut and segmented fresh fruit pieces. US patent number 4,895,729. 1990.
5. Jaime P and  Saltveit M E. Postharvest changes in broccoli and lettuce during storage in argon, helium, and nitrogen atmospheres containing 2% oxygen. Postharvest Biol. Technol 2002; 26: 113–116.
6. Spencer K C. Method of preservation foods using noble gases. US patent number 6,342,261. 2002.
7. Fraqueza M J, Barreto A S. The effect on turkey meat shelf life of modified atmosphere packaging with an argon mixture. Poultry Science 2009; 88: 1991-1998.

The authors
Fraqueza M J and Barreto A S
Faculty of Veterinary Medicine
Technical University of Lisbon
Av. da Universidade Técnica
Polo Universitário
Alto da Ajuda, 1300-477
Lisboa, Portugal.
Email: mjoaofraqueza@fmv.utl.pt