Microbial Flora of Cod Fillets (Gadus morhua) Stored at 2°C in Different Mixtures of Carbon Dioxide and Nitrogen/Oxygen

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Abstract
Cod fillets were stored in different combinations of carbon dioxide and oxygen/nitrogen at 2°C and the development of the microflora was studied during storage. The shelf life, expressed as the time for the microflora to reach 1 × 106 CFU/g, successively increased as the CO2 concentration increased. No significant effects were noted whether oxygen or nitrogen was used together with CO2 to comprise the gas mixtures. In 100% CO2, the total aerobic count did not exceed 1 × 106 CFU/g during the storage period. The microflora of the fresh cod fillets consisted of Flavobacterium spp., Alteromonas putrefaciens, Moraxella-like organisms and Pseudomonas spp. After 6 d of storage in air, the total aerobic count had reached 5 × 107 CFU/g and the microflora was dominated by A. putrefaciens (62%) and non-fluorescent Pseudomonas spp. (28%). After 26 to 34 d of storage, Lactobacillus spp. and A. putrefaciens dominated the spoilage flora in the different gas mixtures and the total count amounted to 3 × 105 to 2 × 107 CFU/g. The ratio of Lactobacillus spp. at the termination of storage in the different gas mixtures increased in the order of air 2/50%N2 2/50%O2 2/10%N2 2 2/10%O2, whereas the ratio of A. putrefaciens decreased in the same order. Considering the technical difficulties in using an atmosphere at 100% CO2 and the microflora developing during storage in the different gas mixtures, it is suggested that a suitable gas mixture for retail packages should be 50% CO2/50% O2.