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3.5. Modified atmosphere packagingModified atmosphere packaging (MAP) has proven to be successfulin extending the shelf-life of fresh meat by the inclusion of oxygen(>13%) to prolong its bloomed, cherry-red appearance and the inclusionof carbon dioxide to decrease the microbial activity (McMillin,2008). Carbon monoxide has also been shown to improve the colourof meat by binding to myoglobin to form a bright pink red colour. Thecarbon monoxide also acts to decrease the redox potential of theenvironment in which the meat is packaged, thus decreasing microbialactivity (Mancini & Hunt, 2005). However, there is a huge controversyabout the use of CO in packaging with a number of countries notallowing the use thereof (Anonymous, 2004). Nitrogen has also beenused as a filler gas, as it is inert and helps inhibit package collapseas the carbon dioxide and the oxygen are absorbed and utilised bythe meat and the microbes (McMillin, 2008). Therefore, MAP holdspotential in mitigating the negative effects on the colour stability ofmeat brought about by freezing and thawing. The inclusion of highconcentrations of oxygen or low concentrations of carbon monoxidecould restore or maintain the bloomed cherry red appearance of themeat (Leygonie et al., 2011).The major drawback of oxygen inclusion (>21%) in MAP isincreased oxidation of the lipid and protein fractions. As freezingand thawing lead to accelerated oxidation under refrigerated storagepost thawing, research is required to establish the levels at whichoxygen should be included in order to improve the colour, but notto cause accelerated lipid and protein oxidation. Nevertheless, meatpurchasing decisions are influenced more by colour than any otherquality factor(s). Therefore, if meat colour can be enhanced, themarket demand for the product is likely to simultaneously improve(Mancini & Hunt, 2005). Carbon monoxide does not increase oxidationbut the EU has limited its use due to health risks to the consumerand meat plant workers (Anonymous, 2004).The use of carbon dioxide during thawing and post thawing mightmitigate the increased rate of spoilage due to the decreased lag phaseof the spoilage organisms and the abundance of nutrients from thepurge loss, because of the antimicrobial action of the gas (McMillin,2008). Carbon dioxide gas has been proven to be very effectiveagainst the most common meat spoilage bacteria, Pseudomonas andAchromobacter (Gill & Tan, 1980). Therefore, the correct compositionof modified atmospheric packaging could result in a significantimprovement in the physicochemical properties of frozen/thawedmeat under refrigerated storage conditions.
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