Biology Reference
In-Depth Information
spoilage have been identifi ed, it is important to check their colonisation
kinetics on naturally contaminated products and during “real” storage
conditions (temperature, packaging). Potential spoilage fl ora in a model
medium may not have the capacity to develop enough to reach levels
that could cause perceptible sensory deterioration (Jorgensen et al.,
1988). Furthermore, interactions with endogenous fl ora could modify the
spoiling characteristics of microorganisms (Gram and Huss, 1996a).
BACTERIAL METABOLISM
Fish is a matrix that particularly favours microbial development. Despite a
low percentage of carbohydrates (0.2 to 1.5% depending on the species), fi sh
fl esh is rich in non-protein, low molecular weight nitrogenous molecules
that are rapidly metabolised by bacteria. These compounds include
free amino acids, creatine, nucleotides, urea and trimethylamine oxide
(TMAO). The high post-mortem pH (> 6) and the low acidifi cation during
preservation, combined with the small quantity of carbohydrates present,
enable the rapid growth of pH-sensitive psychrotrophic spoilage bacteria
like Shewanella putrefaciens . Lastly, fatty fi sh are rich in polyunsaturated
fatty acids that can be rapidly oxidised by either chemical chain reactions
or lipolysis resulting from autolytic or bacterial enzyme activity.
In aerobiosis, the carbohydrates (ribose and lactate) can be metabolised
into CO 2 and H 2 O. In anaerobiosis, and in the presence of an electron
acceptor such as TMAO some microorganisms are capable of anaerobic
respiration that leads to the production of acetic acid. However, spoilage of
fresh fi sh is rarely linked to the production of organic acids due to the low
concentration of carbohydrates in the fl esh. TMAO, whose concentration
varies according to the species, plays an extremely important role in
spoilage as certain microorganisms such as Shewanella, Photobacterium
and Aeromonas reduced it to trymethyl amine (TMA), a pungent molecule
responsible for the strong amine odour typical of rotten fi sh. TMAO is
occasionally found in freshwater fi sh (Anthoni et al., 1990) but is generally
associated with marine fi sh (Seibel and Walsh, 2002).
Urea found in large quantities in selachians can be metabolised into
ammonia which has a strong, unpleasant odour. Deamination of amino
acids also leads to the production of ammonia.
The breakdown of sulphurous amino acids found naturally in fi sh
leads to the production of H 2 S (hydrogen sulphide) (from cysteine),
methylmercaptan and dimethyl-disulphide (from methionine). These
molecules all play a part in the spoilage process (Shewan, 1974; Lee and
Simard, 1984). Certain microorganisms such as Shewanella, Photobacterium
and lactic acid bacteria (LAB) are capable of this production, but to an
extent that varies depending on the strain.
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