Potential Spoilage of Packaged and Irradiated Meat

A number of methods can be used to inhibit the growth of microorganisms in meat. These include packaging such as modified atmosphere packaging, vacuum packaging and irradiation. However these methods may encourage the growth of other types of micro-organisms such as Lactic acid bacteria, yeast, etc.

Lactic acid bacteria are mainly responsible for lactic acid, acetic acid and some other organic acids. Lactic acid itself cannot produce offensive off-odour. Its association with other partially volatile substances (including short-chain fatty acids) can result in producing rich off-odour smell in the packaged meat.

During the vacuum storage, the remaining oxygen is quickly used by the present microbes in addition to the respiration of tissue of meat resulting in higher proportion of carbon dioxide. The atmosphere is not entirely anaerobic as the films of vacuum packages are usually oxygen permeable to certain extent. Therefore Gram negative bacteria will have the opportunity to replace with potential Gram positive bacteria (predominantly Lactic acid bacteria).

It has been reported that the foul off-odour in vacuum packed beef cuts may be in connection with the chemical reaction of Clostridium spp.; facilitating the production of hydrogen and carbon dioxide. Vacuum packed meats with excessive oxygen permeability may encourage cheesy, sweet odour in consequence of acetoin synthesis by Brochothrix thermosphacta and Lactobacillus spp. Gram negative bacteria including Enterobacteriaceae and Pseudomonas are capable to develop in large proportion on beef cuts stored in vacuum package.

Significant amount of Enterobacteria can also develop in vacuum packed pork cuts. This type of bacteria is also able to survive on lamb meats stored in vacuum packs. Modified atmosphere packaging (MAP) is mainly based on increasing the amount of CO2 in order to discourage microbial growth by introducing certain percentage of CO2 in conjunction with nitrogen and oxygen into the package containing meat. Although high content of carbon dioxide is remarkably effective on inhibiting a range of bacteria spp, it is very important to use certain quantity as excessive proportion of CO2 may lead to undesirably chemical reaction in the meat and adverse impact on the quality of final product.

Some bacteria spp. such as Enterobacteriaceae and Aeromonas may survive in the MAP and can cause spoilage (depending on pH, storage, temperature, packaging material and initial population of bacteria). It has been detected that the growth of Enterobacteriaceae and Pseudomonas is more significant on pork stored in MAP rather than that in vacuum packs.

Irradiation of meat is also an effective method to suppress most Gram negative bacteria including Pseudomonas and Enterobacteriaceae while certain bacteria such as Moraxella and Psychrobacter immobilis are usually unaffected by irradiation. Furthermore, in anaerobic condition, irradiation-tolerant microorganisms including Lactobacilli can develop in meats of beef, poultry and pork.

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