Many of the odours characteristic of the earlier stages of spoilage of chilled fish muscle have been reproduced by inoculating sterile fish and fish media with pure cultures of bacteria isolated from fish. These organisms belong to the Pseudomonas and the majority are neither proteolytic nor break down trimethylamine oxide. They are chiefly achromogenic, although a few green pigmented species are included. Odour production by these organisms appears not to be inhibited by sodium nitrite. Similar odours, produced by similar types of organisms, have been observed in the past on dairy products, eggs, meat, poultry and other protein foods held in cold storage.
Whole and gutted cod and haddock were divided into three grades of quality by panels of plant foremen and experienced Department of Fisheries personnel. It was found that there was a close correlation between the degree of freshness, as judged by these men, and the trimethylamine values of the fillets cut from the fish. This correlation was less significant when the fish were graded by Department personnel inexperienced in handling and grading of fresh fish.It would appear from these results that the trimethylamine value of cod and haddock muscle may be used with certain limitations, as an objective measure of spoilage in support of an organoleptic system of grading these fish.
The characteristic fruity and onion-like odours produced by Ps. fragi are the result of these organisms acting on substrates containing partially hydrolyzed proteins. It has been shown that the immediate protein derivatives from which the bacteria produce the fruity odours are mono-amino monocarboxylic acids. The fruity smelling compounds are probably volatile esters formed after bacterial deamination of these amino acids.Under some conditions these organisms are able to produce various types of sour-onion and garlic-like odours. It has been found that these odours are also produced through the action of the bacteria on amino acids, but are not restricted to the mono-amino monocarboxylic group. Nothing is yet known of the mechanism by which these onion-like odours are formed.Fruity and oniony odours are only two of many different odours produced by the action of these organisms on amino acids. Among others, they are also able to produce odours suggesting cabbage, sour cabbage and rotten vegetables from methionine; cooked egg yolk, and hydrogen sulphide-like odours from cysteine, cystine, and glutathione; and various sour vegetable-like odours from other amino acids.
Bacteria capable of producing fruity and onion-like odours have been isolated from Atlantic cod and haddock fillets that had developed off-odours of this type. These organisms have been identified as being non-proteolytic strains of Pseudomonas fragi.Compared to other fish-spoiling bacteria, including cultures of Serratia, Proteus, Achromobacter, and green pigmented Pseudomonas, Ps. fragi is quite sensitive to the bacteriostatic action of antibiotics of the tetracycline group.
Bacteria producing musty and potato-like odours have been isolated from fillets of cod and haddock and identified as Pseudomonas perolens Turner. They are psychrophilic and are able to produce their characteristic odour on a wide range of substrates.
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