Biology Reference
In-Depth Information
trimming) contribute to spreading the bacteria naturally present in fi sh
throughout the muscular tissue, thus accelerating spoilage.
Regarding the bacterial growth, fi rst, there is a lag phase, generally short
for fi sh found in temperate waters but whose length varies depending
on the composition and state of the fi sh, the storage temperature and the
species of bacteria. Next, the microorganisms begin a period of exponential
growth and can reach level of 10
6-8
CFU (Colony Forming Units)/g. Clearly,
the rate of multiplication depends on the factors mentioned previously.
Generally, tropical fi sh kept in ice have a longer lag phase (1 to 2 wk) than
that of fi sh found in temperate waters and growth during the exponential
phase is slower (Gram et al., 1990; Gram, 1995), probably because the
microorganisms cannot adapt well to the low storage temperatures
(Devaraju and Setty, 1985).
THE CONCEPT OF SPECIFIC SPOILAGE FLORA
Sensory spoilage is not always linked to the total number of microorganisms.
In most seafood products organoleptic rejection occurs well after the total
fl ora has reached its maximum. Although the fl ora of fi sh is varied, only
certain microorganisms, called “specifi c spoilage microorganisms” are
responsible for the production of unpleasant odours and fl avours.
In general, in unprocessed fi sh, the chemical modifi cations that lead
to sensory rejection are due to only one bacterial species (Hozbor et al.,
2006). In the case of processed products, however, the mechanisms are
often more complex as several bacterial groups can interact and contribute
to the spoilage of the product (Stohr et al., 2001; Joffraud et al., 2006). The
specifi c spoilage fl ora and associated chemical molecules vary according
to the species of fi sh, the storage temperature, the type of packaging or
processing and even the fi shing season.
Identifying the microorganisms specifi cally responsible for spoilage is
not easy. The common method consists in analysis of the products during
storage (selective microbiological enumeration, sensory and chemical
analyses) and establishment of correlations between different parameters.
However, the enumerations are not always selective enough and it is often
better to collect colonies from the total fl ora at the time of sensory spoilage
and then check their capacity to produce bad odours in a pure culture.
Tests done directly in the fi sh matrix always give more relevant results
than using a laboratory medium or fi sh fl esh extract (Truelstrup Hansen,
1995; Stohr et al., 2001) but are often more diffi cult to carry out as the
fi sh must be sterilised without modifying its composition. The collections
of fi llets in aseptic conditions (Herbert et al., 1971) or low intensity
ionisation (Joffraud et al., 1998; Jorgensen et al., 2000b) are the solutions
most frequently used. Once the microorganisms potentially involved in
