Spoilage and shelf-life of cod fillets packed in vacuum or modified atmospheres

Microbial growth, sensory and chemical changes and composition of gas atmosphere were studied in vacuum packed (VP) and modified atmosphere packed (MAP) cod fillets stored at 0 degree C. Contrary to previous studies, coccobacilli and pleomorphic Gram-negative microorganisms (2-4 by 2-5 microns) and not Shewanella putrefaciens were found most likely to be the main spoilage organisms. These microorganisms, which may be Photobacterium phosphoreum, can explain the short shelf-life extension of VP and MAP fish products compared to meat products. It is suggested that they may inhibit the typical H2S-producing fish spoilage bacteria, S. putrefaciens, as the maximum concentration of H2S-producing bacteria found in MAP fish products is very low. Compared to VP, a shelf-life extension of 6-7 days was obtained with 48% CO2 in MAP. However, with pure CO2 the shelf life was only extended by 2-3 days. Poor texture and high drip loss indicated that the shelf life of these fillets was limited by chemical reactions and not only by microbial activity.