EVALUATION OF METHODS FOR DETERMINING THE BACTERIAL POPULATION OF FRESH FILLETS

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Abstract
The use of a nutrient broth consisting of 0.5% tryptone, 0.25% yeast extract, and 0.1% dextrose for blending and diluting fish tissue resulted in a 10 fold higher count compared to that obtained when distilled water was used. No specific preference was shown for yeast extract, beef extract or tryptone for yielding maximum total counts between 0.1 – 0.5% concentrations. A fish infusion agar representing a 1:10 dilution of fish tissue in distilled water yielded lower counts than conventional nutrient media. The addition of 0.1 – 0.5% glucose, glycerol, and sodium lactate to a basal medium of 0.1% yeast extract and 0.2% tryptone failed to increase total counts above those obtained with the basal medium alone. The addition of 0.05% – 0.5% sodium citrate to enumerating media markedly suppressed the total count; 0.3% reduced the count by 64% from fresh haddock and by 95% from stale haddock. The addition of 0.1 – 0.5% NaCl to enumerating media did not influence total counts but was essential for pigment formation by Pseudomonas putrefaciens. Plate counts determined at 20 and 25 C exceeded those from plates incubated at 3 C from fresh haddock while the reverse was true for stale haddock.

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