Frédérique Chevalier

Research of quality indices for cold‐smoked salmon using a stepwise multiple regression of microbiological counts and physico‐chemical parameters

Aims: The aim of the study was to assess the relationships between the remaining shelf‐life (RSL) of cold‐smoked salmon and various microbiological and physico‐chemical parameters, using a multivariate data analysis in the form of stepwise forward multiple regression.

Characterisation of the spoilage microbiota in raw salmon (Salmo salar) steaks stored under vacuum or modified atmosphere packaging combining conventional methods and PCR-TTGE.

In order to characterise the spoilage related to microbiota of raw salmon, a combination of culture-dependent and -independent methods, including PCR-TTGE, was used to analyse 3 raw salmon batches stored for 3 days at chilled temperature in modified atmosphere packaging (MAP) (50% CO₂/50% N₂) or under vacuum. Sensory evaluation, microbiological enumeration and chemical analysis were performed after 3, 7 and 10 days of storage. At the onset of spoilage, 65 bacterial isolates were picked from the plates. Thus, 13 different genera or species were identified by phenotypic and molecular tests: Serratia spp., Photobacterium phosphoreum, Yersinia intermedia, Hafnia alvei, Buttiauxella gaviniae, Pseudomonas sp., Carnobacterium maltaromaticum, Carnobacterium divergens, Lactococcus piscium, Lactobacillus fuchuensis, Vagococcus carniphilus, Leuconostoc gasicomitatum and Brochothrix thermosphacta. The PCR-TTGE profiles and band identification enabled a shift of the dominant populations during the storage to be visualised for all the batches, probably due to the temperature change and the packaging. At the beginning of storage, Pseudomonas sp. dominated the raw salmon microbiota while in the following days (7 and 10), P. phosphoreum and L. piscium were identified as the main bacterial groups. This study enhances the knowledge of MAP and vacuum-packed raw salmon spoilage microbiota.

Inhibition of Brochothrix thermosphacta and sensory improvement of tropical peeled cooked shrimp by Lactococcus piscium CNCM I-4031.

Aims:  To investigate the antimicrobial spectrum of Lactococcus piscium CNCM I‐4031 and its protective effect in cooked and peeled shrimp against Brochothrix thermosphacta.

Development of a sterile cold-smoked fish model

J.J. JOFFRAUD, F. LEROI AND F. CHEVALIER. 1998. It is difficult to create a successful sterile cold‐smoked fish model, particularly when very fresh fish are not readily available. This study tested a low‐dose ionization technique (1·5 and 3·0 kGy) as a means of eliminating residual flora and ensuring sterility without altering sensory qualities. Ionized cold‐smoked salmon displayed no bacterial contamination during an 11 week storage period and was not considered as spoiled by a sensory panel. In comparison, non‐ionized smoked salmon, although aseptically processed, contained a bacterial flora responsible for its spoilage. This model allows assessment of the spoilage potential or activity of isolated bacterial strains as well as independent studies of bacterial and non‐bacterial reactions in cold‐smoked fish.

Protective Effect of a Non-Bacteriocinogenic Lactococcus piscium CNCM I-4031 Strain Against Listeria monocytogenes in Sterilized Tropical Cooked Peeled Shrimp

The protective activity of a non-bacteriocinogenic Lactococcus piscium CNCM I-4031 strain against Listeria monocytogenes was investigated in tropical cooked peeled shrimp stored at 8°C in modified atmosphere packaging (50% N2–50% CO2). When inoculated alone (L. piscium 107 CFU g−1 and L. monocytogenes 104 CFU g−1), protective culture and target strain grew very well on shrimp reaching a maximum cell number of 109 CFU g−1 after 7 and 14 days, respectively. In the presence of L. piscium, growth of L. monocytogenes was totally prevented after 3 days of storage. The count was 3.4 log CFU g−1 lower than in the control after 10 days and until the end of storage (31 days). Using the Seafood Spoilage and Safety Predictor Software (http://sssp.dtuaqua.dk), it was shown that pH decrease from 6.58 to 5.94 and lactic acid concentration of 89.65 mM measured in the co-inoculated batch did not fully explain the inhibition observed.

Selection of bioprotective cultures for preventing cold-smoked salmon spoilage

Biopreservation is a natural technology of food preservation, which consists of inoculating food with microorganisms selected for their antibacterial properties. The objective of this study was to select lactic acid bacteria (LAB) to improve the quality of cold-smoked salmon (CSS). In this work, different strains representative of the 4 dominant species, identified in a previous study by pyrosequencing the 16S rRNA gene, were isolated and their spoiling potential in CSS blocks, sterilized by ionization, was assessed by twelve trained panelists along the vacuum storage at 8°C. Photobacterium phosphoreum, Brochothrix thermosphacta and Serratia proteamaculans released strong off-odors whereas the spoiling potential of Carnobacterium divergens was weaker. The spoiling capacity of Lactococcus piscium EU2241, Leuconostoc gelidum EU2247, Lactobacillus sakei EU2885, Staphylococcus equorum S030674 and 4 commercial starters was tested by the same method and 2 strains were eliminated due to off-odor production. The effect of the 6 selected LAB against the 4 specific spoiling organisms (SSOs) selected was tested by challenge tests in sterile CSS blocks. The protective effect of the LAB differed from one SSO to another and no correlation could be established between the sensory improvement, SSO inhibition, and the implantation or acidification of protective cultures (PCs). All the PCs except L. piscium reduced the off-odors released by P. phosphoreum although some of them had no effect on its growth. S. equorum, which did not grow in CSS, favored the implantation of P. phosphoreum but prevented its off-odor formation. L. piscium was the only strain that prevented the spoilage of B. thermosphacta and S. proteamaculans although it did not grow very well and did not acidify the product. L. gelidum EU2247 inhibited the growth of these 2 SSOs and lowered the pH but had no effect on the sensory quality. Finally, L. piscium was tested in 2 naturally contaminated products, with a positive effect on 1 batch. This effect was not correlated with the microbial ecosystem as determined by acultural and cultural techniques. Based on these results, the selection strategy is discussed.

Characterization of the spoilage potential of pure and mixed cultures of bacterial species isolated from tropical yellowfin tuna (Thunnus albacares)

The spoilage potential of 28 bacterial strains isolated from spoiled raw yellowfin tuna was evaluated.

Bioprotective effect of Lactococcus piscium CNCM I-4031 against Listeria monocytogenes growth and virulence

Listeria monocytogenes is a Gram-positive pathogen occurring in many refrigerated ready-to-eat foods. It is responsible for foodborne listeriosis, a rare but severe disease with a high mortality rate (20–30%). Lactococcus piscium CNCM I-4031 has the capacity to prevent the growth of L. monocytogenes in contaminated peeled and cooked shrimp and in a chemically defined medium using a cell-to-cell contact-dependent mechanism. To characterize this inhibition further, the effect of L. piscium was tested on a collection of 42 L. monocytogenes strains. All strains were inhibited but had different sensitivities. The effect of the initial concentration of the protective and the target bacteria revealed that the inhibition always occurred when L. piscium had reached its maximum population density, whatever the initial concentration of the protective bacteria. Viewed by scanning electron microscopy, L. monocytogenes cell shape and surface appeared modified in co-culture with L. piscium CNCM I-4031. Lastly, L. monocytogenes virulence, evaluated by a plaque-forming assay on the HT-29 cell line, was reduced after cell pre-treatment by the protective bacteria. In conclusion, the bioprotective effect of L. piscium toward L. monocytogenes growth and virulence was demonstrated, and a hypothesis for the inhibition mechanism is put forward.

Biological Silages from Tunisian Shrimp and Octopus By-Products

ABSTRACT Biological silages were prepared from shrimp head and octopus viscera by-products recuperated from the Tunisian seafood industry. Physical and biochemical changes and microbiological profiles were determined for raw materials during fermentation and on end-products. Results showed that biological silage significantly affected (p < 0.05) moisture, protein, and ash contents of shrimp head (CSHS) and octopus viscera silages (COVS). CSHS and COVS were stable, and their final pH values were 4.31 ± 0.01 and 3.71 ± 0.00, respectively. Proteolysis activity was confirmed by a significant increase (p < 0.05) of soluble nitrogen and low molecular weight of protein (<260 Da) found on the end-products for both silages. Lipid oxidation was delayed by addition of 150 ppm ethoxyquin to the raw material prior to fermentation. Biogenic amines detected in raw shrimp and octopus samples decreased significantly (p < 0.05) during the silage process. Histamine and tyramine, detected at high levels on octopus viscera, were absent in the end-products. Tyramine was produced in CSHS, indicating the possibility of the bacterial decarboxylation of tyrosine. Microbiological profiles showed that both silage products were free from pathogenic and spoilage bacteria. Therefore, biological silage can be used as a conservation procedure of shrimp and octopus by-products. The storage period could be shorter than 30 days, and further analysis should be carried out to ascertain safety and nutritional value of silage products.