Numbers and types of microorganisms in vacuum-packed cold-smoked freshwater fish at the retail level.

Fifty-four packages (each one belonging to a different lot) of vacuum-packed cold-smoked salmon (30) and trout (24) produced by six Spanish smokehouses were obtained at retail level after 3 weeks storage at 2+/-1 degrees C. Sensorial, chemical, physicochemical and microbiological characteristics were examined. Overall, pH, a(w), salt content in water phase, aerobic plate counts at 30 and 25 degrees C. levels of Enterobacteriaceae, lactic acid bacteria (LAB), fungi and presumptive aeromonads and staphylococci are in agreement with available data on lightly preserved fish products. Psychrotrophic clostridia ranged between 1.71 and 2.21 log CFU/g. Levels of ethanol were highly variable and not significantly related (p > 0.05) to sensory scores or to microbial numbers. Salmonella, Escherichia coli and Listeria monocytogenes were not detected in any sample. Listeriae other than L. monocytogenes were isolated from three packages. Levels of Staphylococcus aureus lower than 4 log CFU/g were also found in three packages. Among 377 bacteria randomly isolated from aerobic 25 degrees C plate counts, LAB predominated, with Carnobacterium (C. piscicola) and Lactobacillus (eight species) being the genera most frequently found. The second and third major groups were Enterobacteriaceae and Micrococcaceae, respectively. Proteus vulgaris, P. mirabilis and Serratia liquefaciens were dominant among Enterobacteriaceae and coagulase-negative staphylococci among Micrococcaceae. Minor microbial groups such as aerobic gram-negative bacilli (Acinetobacter; Moraxella and Pseudomonas), Brochothrix, Aeromonas, Bacillus and Vibrio constituted less than 17% of the total flora.

Hemolytic and proteolytic activities of Aeromonas hydrophila and Aeromonas veronii biovar sobria in broth and salmon extract at different temperatures.

Expression of hemolytic and proteolytic activities throughout the growth cycle was investigated with two enterotoxic aeromonad strains assigned to the species Aeromonas hydrophila and Aeromonas veronii biovar sobria. Although growth kinetic data were dependent on strain, temperature, and substrate, maximum populations attained were higher than 9 log CFU/ml in aerated tryptone soya broth plus yeast extract (TSBYE) and salmon extract within the range 4 to 28 degrees C. For both strains in TSBYE, variable amounts of hemolytic activity were first detected at any temperature when aeromonad counts were over 9 log CFU/ml. Afterwards, this activity increased up to similar levels (109 to 112 hemolytic units per ml) without a significant increase in populations. Salmon extract supported hemolysin synthesis at 28 but not 4 degrees C. Proteolytic activity of the A. hydrophila strain was only expressed in salmon extract at 28 degrees C, whereas A. veronii biovar sobria did at 28 degrees C in both substrates and at 10 degrees C in TSBYE.