Biology Reference
In-Depth Information
was clearly delayed. Conversely, when
Cb. maltaromaticum
and
Vibrio
sp.
were both present (two non-spoiling bacteria in pure culture) spoilage
was increased signifi cantly (Joffraud et al., 2006). Brillet et al. (2005) noted
that
Cb. divergens
did not produce TVBN in pure culture in sterile smoked
salmon but some TVBN was detected when the bacterium was added
to commercial samples containing a natural endogenous fl ora. Some
phenomena of metabolic interaction between LAB and enterobacteria have
been explained by Jorgensen et al. (2000b). For example, a high production
of putrescine (>200 µg/g) could not be due to the simple degradation
of ornithine, whose concentration does not exceed 10 µg/g in salmon
fl esh. Arginine deaminase-positive LAB, like
Cb. divergens
and
Lb. sakei
,
metabolise arginine, found in greater quantity in salmon, into ornithine,
which is then converted to putrescine by ornithine decarboxylase-positive
enterobacteria (i.e.,
Se. liquefaciens
and
Hafnia alvei)
.
All these results show that it is over-optimistic to predict the quality
of lightly preserved seafood products using only one microbiological or
biochemical parameter. Not only is the microfl ora very complex but also
the organoleptic properties of spoilage are greatly varied, with odours
that can be described as amine, sulphur, acidic, sour or “cabbage”. Some
authors succeeded in improving the organoleptic quality of smoked
salmon by modifying the technological parameters, like salt, smoke or the
addition of bioprotective bacteria, but have not demonstrated the inhibition
of a particular group of bacteria (Leroi et al., 1996; Leroi et al., 2000). The
situation is not the same in fresh fi sh, preserved in air or in a modifi ed
atmosphere or vacuum packed, where one or two well identifi ed species
dominate the fl ora and for which a predictive model of sensory spoilage
has been suggested (Dalgaard et al., 2002). However, in the case of smoked
salmon, a multiple approach enabled some authors to correlate quality
to several microbiological and biochemical measurements. For example,
Leroi et al. (2001) have put forward a predictive model for the remaining
shelf life of smoked salmon, based on TVBN content and enumeration of
the fl ora on Mann-Rogosa-Sharp (MRS) medium (selective for lactobacilli)
at pH 5.5. If the TVBN content is less than 35 mg-N/100 g of fl esh, the
product is not spoilt. If it rises above this level, the number of bacteria
on MRS medium must be considered. A product with 50 mg-N/100 g of
TVBN will only be spoilt if the number of colonies on MRS is higher than
10
5
CFU/g. Another model, based on pH and the quantity of histamine
and tyramine, has been developed by Jorgensen et al. (2000a).
Cooked Shrimps Packed in a Modifi ed Atmosphere
France is the second largest importer of shrimps in Europe, and this
product corresponds to the highest value of all seafood imported. Tropical
shrimps account for 80% of the shrimps imported. Refrigerated cooked
