Effect of heat on microorganisms isolated from freshly caught chilled scad (Trachurus trachurus)

The effects of heat on bacteria making up part of the microflora of the sea-fish Trachurus trachurus were studied. From 80 bacterial isolates which were subject to primary identification four were selected. The bacteria were subjected to a heat stress of 55°C for 20 s both in pure and mixed suspensions, the D-values and post-stress recovery in nutrient broth were measured. Using differential plating on selective agents to identify sites of injury and recovery times, the growth parameters dissolved oxygen, redox potential and pH were measured during recovery. Bacteria presumptively identified as Shewanella putrefaciens and Pseudomonas putida were more sensitive to heat than other bacteria, suggesting that most heat treatments of fish may reduce the number of these bacteria. After dissolved oxygen was used up during recovery in nutrient broth, Eh declined from positive to negative values typical of facultative bacteria showing that, after initial aerobic growth, the bacteria changed their metabolism to grow anaerobically. When dissolved oxygen reached zero, about 120 h after inoculation, the numbers of all bacteria were about 10⁷ mL^-1, and corresponds to a minimum in the pH values, except for the most heat sensitive isolate (300 h after inoculation). Differential plating showed increased sensitivity to NaCl induced by heat treatments in some isolates.