Effect of inoculation of Carnobacterium divergens V41, a biopreservative strain against Listeria monocytogenes risk, on the microbiological, chemical and sensory quality of cold-smoked salmon
Introduction
The production of CSS within the European community supplies nearly 80% of the world market with an annual production rising 70 000 tonnes (FAO, 2003). France is the first worldwide producer with approx. 19 000 tones/year but other countries such as Denmark, Germany and United Kingdom have recently increased their production. Salting and smoking treatments which were traditionally used as preservation technology are nowadays lighter leading to highly perishable product, with an average salt content of around 5% (water phase) in Europe and phenol content less than 1 mg 100 g− 1 (Cardinal et al., 2004). The major risk associated with this product is Listeria monocytogenes (Bledsoe et al., 2001). L. monocytogenes is of special concern to the CSS industry because it is able to grow at low temperature and in the presence of high NaCl concentration. Indeed, a high prevalence of L. monocytogenes in CSS has been reported (Ben Embarek, 1994, Rorvik et al., 1995, Heinitz and Johnson, 1998, Jorgensen and Huss, 1998) and the processing steps (i.e. salting and smoking) are insufficient to inactivate this pathogenic bacterium (Guyer and Jemmi, 1991, Ribeiro Neunlist et al., 2005). Moreover, growth in the final product stored at chilled temperature is possible (Jorgensen and Huss, 1998, Duffes et al., 1999a, Nykanen et al., 2000) and L. monocytogenes can reach 100 CFU g− 1 which is the maximum tolerable limit till the sell-by date in many European countries. Recently, a biopreservation strategy for this product has been developed. Biopreservation consists in inoculating food product by selected bacteria to inhibit the growth of undesired micro-organisms, without changing quality of the product. The inhibition capacity of three bacteriocin-producing strains isolated from seafood, Carnobacterium divergens V41, Carnobacterium piscicola V1 and C. piscicola SF668, has been demonstrated against a wide collection of L. monocytogenes both in agar plate and in sterile CSS blocks artificially contaminated (Brillet et al., 2004). The three carnobacteria strains gave promising results, C. divergens V41 being the most efficient strain, maintaining the level of L. monocytogenes lower than 50 CFU g− 1 (initial contamination: 20 CFU g− 1) during the 4 weeks of vacuum storage at 4 and 8 °C.
In order to bring the biopreservative technology to a stage where it can become available for the CSS industry, it is necessary to evaluate the effect of the Carnobacterium strains on the quality of CSS. It has been clearly stated that the early quality deterioration of vacuum-packed CSS during vacuum storage at chilled temperature, resulting in the occurrence of off-odours and pasty textures (Stohr et al., 2001), is due to microbial activity (Joffraud et al., 1998). However, spoilage mechanisms are still incompletely understood. Several authors have identified the CSS microflora (Truelstrup Hansen et al., 1998, Stohr et al., 2001) which is highly variable both qualitatively and quantitatively due to difference in the final product composition and in hygienic conditions in the smokehouses (Truelstrup Hansen and Huss, 1998, Leroi et al., 2001). Just after the process, the microflora is often composed of a mixture of Gram-negative marine bacteria such as Shewanella putrefaciens, Photobacterium phosphoreum, Vibrio spp. and psychrotrophic Enterobacteriaceae (Truelstrup Hansen et al., 1995, Leroi et al., 1998, Jorgensen et al., 2000a). During the vacuum storage at refrigerated temperatures, Gram-positive bacteria especially lactic acid bacteria (LAB), represented by Lactobacillus spp. and Carnobacterium spp. become by far the most common variety (Truelstrup Hansen and Huss, 1998, Leroi et al., 1998). Brochothrix thermosphacta and yeasts can also be found in CSS but generally at lower level (Leroi et al., 2001). When inoculated in pure culture in sterile cold-smoked salmon blocks, those micro-organisms do not all participate in spoilage. Lb. sakei, B. thermosphacta, Serratia liquefaciens and P. phosphoreum have been shown to be responsible of off-odours, whereas the spoilage potential of C. piscicola is promisingly weak depending on the strain tested (Stohr et al., 2001).
The aim of the present study was to evaluate the effect of three biopreservative Carnobacterium strains showing a high inhibition potential of L. monocytogenes (Brillet et al., 2004) on microbiological, chemical and sensory quality of CSS. First, the spoilage potential of C. divergens V41, C. piscicola V1 and C. piscicola SF668, was studied on a sterile CSS system by sensory evaluation of off-odour production and analysis of some physico-chemical indices. Then one of the strains, C. divergens V41, was sprayed on slices of commercial CSS batches freshly processed, provided by four different French producers. The effect of inoculation on the sensory characteristics (odour, flavour, texture and colour), physico-chemical parameters (pH, TVBN, biogenic amines) and natural microflora was evaluated during the 4 weeks of storage at refrigerated temperature.
Section snippets
Bacterial strains and subcultures conditions
C. divergens V41 and C. piscicola V1 were isolated from salmon and trout intestine respectively and characterized by Pilet et al. (1995). C. piscicola SF668 was isolated from commercial Norwegian CSS by Leroi et al. (1998), and studied by Duffes et al. (1999b). All strains were stored at − 80 °C in their growth medium with 20% (v/v) sterile glycerol.
Elliker broth (BK 054, Biokar Diagnostics, Beauvais, France) was used as subculture medium to cultivate the Carnobacterium strains for 24 h at 20 °C
Effect of Carnobacterium spp. in sterile CSS blocks
The salt and total phenol concentrations of CSS blocks used in these experiments were respectively 3.2% (w/w, total phase) corresponding to 5.0% (w/w, water phase) and 1.33 mg 100 g− 1. Dry matter and total fat content were 40.1% (w/w) and 15.1% (w/w) respectively.
Fig. 1 represents growth of the three carnobacteria in inoculated CSS during the 4 weeks of vacuum storage at chilled temperature. All strains grew in CSS blocks. For the two C. piscicola strains, growth curves were similar during the
Discussion
A strategy of biopreservation of CSS against L. monocytogenes risk by the use of LAB has been developed by Brillet et al. (2004). In order to allow implementation of this technology in the industry, the effect of the selected LAB on CSS quality had to be considered. The three Carnobacterium strains tested grew in inoculated CSS blocks, with a slight advantage for C. divergens V41. This result confirmed previous observations done by Brillet et al. (2004). However in the present study, final
Acknowledgements
The authors would like to thank Mrs. Josiane Cornet (IFREMER) for the data analysis with Fizz 2.00 software. This work was part of the “Aliment Qualité Sécurité” project (no. R 01/05) and was supported by grants from the French Ministry of Agriculture and Fishery, and from the EU commission under the Integrated Project (IP) SEAFOODplus contract No. FOOD-CT-2004-506359.
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