Biology Reference
In-Depth Information
taste from the end of the second week of preservation (Leroi et al., 2001;
Cardinal et al., 2004).
It has been clearly shown that this organoleptic spoilage is mainly
due to the microbial activity, with autolytic or chemical reactions of lipid
oxidation playing a lesser role (Joffraud et al., 1998). For a very long time,
however, the mechanisms of microbial spoilage of smoked fi sh were
poorly understood. Although lactic fl ora have always been found to be
dominant in this product (Magnusson and Traustadottir, 1982; Hildebrandt
and Erol, 1988; Civera et al., 1995), their role in spoilage was not clear
because no correlation between their number, nor that of the total fl ora,
and the sensory deterioration could be identifi ed (Rakow, 1977; Cann et
al., 1984; Hildebrandt and Erol, 1988; Dodds et al., 1992; Huss et al., 1995;
Truelstrup Hansen, 1995; Gram and Huss, 1996a). More recent studies
have specifi ed the composition of the fl ora of these products and their link
with spoilage.
On leaving the factory, product contamination can range from 10
2
to
10
6
bacteria/g, the initial level varies mainly according to the factory
and its hygiene standards and independently of the origin and state
(fresh or frozen) of the raw material (Truelstrup Hansen et al., 1998; Leroi
et al., 2001). Despite the presence of inhibitory factors like salt, smoke,
preservation at low temperature and vacuum-packaging, bacterial growth
in these products can be quite rapid. It is not unusual to observe total fl ora
concentrations of 10
6
CFU/g from the fi rst week of storage, which can
regularly reach 10
7-9
CFU/g before the use-by date (Cardinal et al., 2004;
Dondero et al., 2004; Espe et al., 2004).
The initial fl ora of smoked salmon is often dominated by Gram-negative
bacteria typical of fresh fi sh fl ora, such as
Shewanella, Photobacterium,
and
Aeromonas
,
later identifi ed as
Serratia
(Leroi et al., 1998)
.
Nevertheless,
along with
Photobacterium,
Brochothrix, Yersinia
and
Carnobacterium
were
also found (Olofsson et al., 2007)
.
During vacuum-packed preservation
at low temperatures, the diversity of genera decreases and Gram-positive
bacteria, especially LAB, very often become predominant. Variations
between factories (Leroi et al., 2001) and even between batches from the
same factory have often been observed (Truelstrup Hansen et al., 1998), but
the lactic fl ora always seems to dominate, regardless of the geographical
provenance of the products analysed (processed in Europe). According to
the authors, 50 to 90% of the colonies taken from the total fl ora of samples
at the end of preservation were
LAB (Leroi et al., 1998; Truelstrup Hansen
et al., 1998). The technological parameters of salting and smoking can
infl uence the fi nal proportion of this group of bacteria (Leroi and Joffraud,
2000) so it should be possible to control the level of lactic fl ora by varying
these parameters (Tomé et al., 2007).
