The use of oxygen in MAP

16.3.1 Colour retention in fresh meat products

The colour of fresh meat is determined by the condition of myoglobin in the meat. When an anaerobic atmosphere is applied, myoglobin (purplish-red) will be transformed to metmyoglobin, producing a brown colour, which is an undesirable trait for European consumers. It is therefore essential that O2 is included (e.g. 40%) into the applied gas atmosphere when fresh meat, destined for the consumer, is packaged. This will ensure the myoglobin is oxygenated, resulting in an attractive bright red colour. However, by doing this, the microbial shelf life of the packaged meat is decreased compared with meat that is packaged in an O2 free atmosphere.

16.3.2 Inhibition of the reduction of trimethylamineoxide (TMAO) in marine fish

Marine fish contain TMAO, which is an osmo-regulator. In O2 poor conditions (e.g. when stored in ice), TMAO is used by spoilage organisms (e.g. Shewanella putrifaciens) as a terminal electron-acceptor, and is reduced to trimethylamine

(TMA). TMA is the main active component responsible for the unpleasant 'fishy' odour. However, by introducing high levels of O2 in the gas atmosphere, the TMAO-reduction can be retarded, and consequently the shelf-life of the fish is increased . This was clearly demonstrated by Boskou and Debevere (1997, 1998). Therefore, packaging atmospheres for lean marine fish should contain oxygen levels of at least 30%.

16.3.3 Avoiding anaerobic respiration of fresh produce

When fresh produce is packaged in a closed packaging system, it continues to respire. It is of great importance to avoid anaerobic conditions in the package of fresh produce because anaerobic respiration of the plant tissue will result in the production of off-odour compounds such as ethanol and acetaldehyde. The techniques applied to maintain an aerobic atmosphere in the packaging of fresh produce are discussed in detail in section 16.4.2.

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