Paludan-Mueller, C; Dalgaard, P; Huss, HH; Gram, L

International Journal of Food Microbiology [Int. J. Food Microbiol.], vol. 39, no. 3, pp. 155-166, 17 Feb 1998

The microflora on spoiled cold-smoked salmon often consists of a mixture of lactic acid bacteria (LAB) and Gram-negative bacteria. To elucidate the role of the different groups, a storage trial was carried out in which nisin and CO sub(2) were used for the selective inhibition of the two bacterial groups. The shelf-life of vacuum-packed cold-smoked salmon, recorded by sensory evaluation, was four weeks at 5 degree C and the microflora was composed of LAB (10 super(6)-10 super(7) cfu/g) with an associate Gram-negative flora in varying levels (10 super(5)-10 super(7) cfu/g). The addition of nisin and/or a CO sub(2)-atmosphere increased the shelf-life to five or six weeks and limited the level of LAB to about 10 super(4)-10 super(6), 10 super(3)-10 super(6) and 10 super(2)-10 super(4) cfu/g, respectively. CO sub(2)-atmosphere plus or minus nisin inhibited the growth of Gram-negative bacteria, whereas nisin had no effect on these in vacuum packages. The Gram-negative flora on vacuum-packed salmon was dominated by a Vibrio sp., resembling V. marinus, Enterobacteriaceae (Enterobacter agglomerans, Serratia liquefaciens and Rahnella aquatilis) and occasionally Aeromonas hydrophila. Irrespective of the addition of nisin and/or CO sub(2)-atmosphere, the LAB microflora was dominated by Carnobacterium piscicola, which was found to account for 87% of the 255 LAB isolates characterized. Whole-cell-protein patterns analysed by SDS-PAGE confirmed the Carnobacterium species identification. The spoilage potential of C. piscicola isolates was further studied by inoculation of approx. 10 super(6) cfu/g in cold-smoked salmon stored at 5 degree C. The salmon did not spoil within 4 weeks of storage in vacuum- or CO sub(2)-atmosphere, and it is concluded that despite high levels (>10 super(7) cfu/g) of C. piscicola, sensory rejection was caused by autolytic changes. This was supported by the development of soft texture and sour, rancid and bitter off-flavours at the point of spoilage, irrespective of the length of shelf-life and low or high total counts of LAB and Gram-negative bacteria.