Gas packaging of meat

In Norway, a gas mixture with carbon dioxide (CO₂) and nitrogen (N₂) is frequently used for ground beef and other meat for retail. Similar to all anaerobic packaging, the microbiological shelf life of meat in CO₂/N₂ is long, but discoloration can occur due to pigment oxidation by residual oxygen (0₂) in the atmosphere. Vacuum and skin-packaging is increasingly used for case-ready meat in Norway. In the period 1985 to 2004, the Norwegian meat industry successfully developed a gas mixture with low carbon monoxide (CO) and high CO₂ for fresh meat packaging. Due to adoption to EU food regulations, Norway had to discontinue its use of CO. In contrast, food control authorities in the USA permitted the use of 0.4 % CO for case-ready meat in July 2004. There is a great international interest in low CO packaging of meat.

WHAT PACKAGING METHODS ARE AVAILABLE?

Gas mixtures with CO
This gas mixture usually contains 0.4 % carbon monoxide (CO), 30-60 % carbon dioxide (CO₂) and the rest is nitrogen (N₂). At the time gas mixtures with CO were used in Norway for retail packages of meat, approximately 60 % of all fresh meat was packaged in this gas mixture. CO binds strongly to the muscle pigment myoglobin and creates a stable, bright red colour. High concentrations of CO₂ and abscence of O₂ inhibit the growth of many types of bacteria, so that the microbiological shelf life is increased. Meat stored in gas mixtures with low CO implies no toxicological risk for the consumers. Due to the low concentration of CO, gas mixtures with CO are also safe to use in working environments.

High oxygen
A gas mixture containing approximately 70 % O₂ and 30 % N2 is commonly used for fresh meat packaging in many EU countries, Australia and the USA. The high concentration of oxygen makes an initial bright red colour of the meat, but the meat is gradually discoloured during storage. However, colour stability and microbiological shelf life is better than if the meat is wrapped on a tray. Recent research and practical experiences have shown that meat in high oxygen easily becomes rancid. An alternative for counteracting rancidity and discolouration is the addition of antioxidants to the extent they are allowed in different countries and markets. Storage of meat in high oxygen inhibits the normal tendensation of meat. Packaging in high oxygen atmospheres can also involve relatively expensive safety measures in the handling of explosive gas mixtures.

Vacuum
Vacuum can be a simple and quite inexpensive packaging method, but can also be performed on more advanced and expensive machinery. The method has a limited use for display packages of meat, probably because the dark purple colour and exudate in the packages are not very attractive to the consumer. The microbiological shelf life is long, since bacteria that are depending on oxygen will not grow. A more saleable alternative to vacuum is skin-pack, where a thin upper film is shrunk on the top of the product without extensive vacuum to avoid formation of exudate.

Gas mixtures of CO₂/N₂ without CO
Due to the presence of low levels of residual oxygen, the meat pigment can be oxidised and discoloured. If all or almost all residual oxygen is removed, the meat is purple. CO₂/N₂ packaging has the same benefits as other anaerobic packaging in terms of long microbiological shelf life and to avoid rancidity.

Wrapping on trays
This is the simplest and most common packaging method, that is used in food stores all over the world. Wrapping can also be performed in packaging plants with short distribution distances to the markets. The meat is placed on a plastic tray, and then wrapped in an O₂-permeable film, so that the meat does not dry and blooms to bright red colour that holds for a short time. The microbiological shelf life is also short, because access to air makes most spoilage bacteria grow well.

Other packaging methods with CO

Master-bags is a system where one or several trays of meat are wrapped in a gas-permeable film, and placed in a high-barrier pouch with a protective gas mixture, with or without CO. Oxygen absorbers may be included in the master-bag to remove detrimental residual oxygen. CO will stabilise the colour of the meat as long as it stays in the master-bag, but after removing the trays from the master-bag, the colour will destabilise in a similar way to ordinary wrapped meat (Pactiv ActiveTech R3TM process). Meat or other muscle foods can be pre-treated with CO gas mixtures and thereafter placed in vacuum or anaerobic gas mixtures to maintain the bright red colour. Filtered or tasteless smoke containing 15 - 40 % CO is widely used for pre-treatment of tuna (Anova Foods ClearsmokeTM and other processes).

PROPERTIES OF PACKAGED MEAT

Meat colour and colour stability
Gas mixtures with CO, high oxygen and wrapping on trays all have in common that they make a bright red colour of the meat initially. After a short time, the wrapped meat discolours, after a somewhat longer time the same will happen to meat packaged in high oxygen, while meat in gas mixtures with CO maintains a bright red colour over a long time.

Discoloration of bones
Bone-in meat is rapidly discoloured in the bone marrow, provided that the piece of meat is stored in air or gas mixtures with oxygen. Such discolouration does not occur with gas mixtures with CO. Meat with sharp bones is not recommended to be packaged in vacuum due to risk of puncturing of the film.

Premature browning during heating
This is a condition where whole muscle meat or ground meat become gray or brown internally at a lower core temperature than expected, even at 60 degree C. If the consumer relies on internal colour and stops the heat treatment of ground meat too early, pathogenic bacteria can survive and thus pose a health hazard. Packaging in vacuum or gas mixtures with CO prevents premature browning, while high oxygen packaging promotes this condition.

Rancidity
Packaging in high oxygen increases the risk of rancidity or lipid oxidation, in particular in meat with high content of unsaturated fatty acids, meat that has been ground, meat with salt added or frozen meat. To the extent that it is permitted, addition of antioxidants can reduce rancidity and increase colour stability. Oxygen-free packaging with gas mixtures with CO or vacuum prevent rancidity.

Microbiological shelf life and inhibition of pathogenic bacteria

Storage of meat under oxygen-free conditions limits bacterial growth. Use of CO2 yields an additional inhibition of bacterial growth. Perhaps even more important than packaging conditions are good hygiene and storage of the meat at low temperature in an unbroken chill chain. Too high temperatures in display cases is often the cause for poor shelf lives.

The enclosed table shows approximate shelf life times for ground beef, beef loin steaks and pork chops using different packaging methods and storage temperatures. Decisive for the shelf life is the microbiological condition of the raw materials and the bacterial counts at the start of the storage. At higher bacterial counts than listed in the table, the shelf lives are expected to be shorter.

* Gas mixture with CO: 0.4 % CO/ 60 % CO2 / 40% N2

High oxygen: 70 % O2 / 30 % CO2

Oxygen absorber: 60 % CO2 / 40 % N2 with O2 absorber

Reference: Sørheim et al. 1999, Meat Science, 52, 157-164.

Tenderness
Uptake of oxygen in meat during high oxygen storage inhibits the normal tenderisation process that takes place under anaerobic conditions, in vacuum or oxygen-free atmospheres.

Drip loss
This loss can increase as a consequence of both vacuum and use of gas mixtures containing CO2. Skin-pack yields low weight loss and little exudate.

Cooking loss
Inclusion of CO₂ in gas mixtures for ground beef increases the cooking loss of the meat. The causes for this incresed loss is a small pH decrease and formation of small pores and fissures in the meat during heat treatment.

Consumer acceptance
Surveys in markets in different countries have demonstrated that consumers prefer to buy meat with a bright red colour. Meat in gas mixtures with CO either has been evaluated as better or equal to other packaging methods as vacuum or high oxygen. Vacuum is regarded as less attractive at display, due to a dark purple colour and exudate from the meat.

Requirements to packaging films
Storage of meat in gas mixtures with CO or vacuum requires packaging films with a medium to low oxygen penetration. By packaging in high oxygen, films with less oxygen barrier probably can be used. By wrapping, very open films with abundant oxygen penetration must be used.

Packaging costs
Gas packaging requires machinery, gases and films, which often will increase the direct packaging costs compared to vacuum and wrapping in particular. However, be aware that other costs not related directly to the packaging, as for example labour, transportation and losses, can have a large impact on the total costs.

Summary of benefits of packaging in low CO (< 0.5 %) versus high O2 atmospheres for fresh meat
stable bright red colour

• no bone blackening
• no premature browning of cooked meat
• no rancidity (no need for antioxidants)
• tenderness improvement under storage
• extended microbiological shelf life (O₂-free, high CO₂)
• reduction of certain pathogenic bacteria
• no toxicological risk for the consumers
• safe to use in the industry