Biology Reference
In-Depth Information
for several days should now be identifi ed not as Sh. putrefaciens , but as
Sh. baltica . Furthermore, other minor strains have been identifi ed as new
species: Sh. hafniensis , Sh. morhuae , Sh. glacialipiscicola and Sh. algidipiscicola
(Satomi et al., 2006; Satomi et al., 2007) . Tropical sea or freshwater fi sh
are spoiled by Pseudomonas . Sh. putrefaciens has also been isolated from
these products but does not appear to play an important part in spoilage.
This could be due to the inability of this microorganism to develop in the
presence of a large number of Pseudomonas (Gram et al., 1990; Gram and
Melchiorsen, 1996b) . The spoilage caused by this microorganism can be
differentiated from Sh. putrefaciens by the absence of TMA and sulphurous
compounds and the appearance of fruity and rotten odours caused by
aldehydes, ketones and esters. When fi sh is stored at room temperature,
Aeromonas is more likely to spoil freshwater fi sh stored in aerobiosis, but it
has also been shown that Sh. putrefaciens can be involved.
Effects of Storage Temperature on Spoilage and Shelf life
Storage temperature is the most infl uential factor for shelf life. Ratkowsky
et al. (1982) have described models that express the relation between
relative spoilage speed and storage temperature. The relative spoilage
speed at a temperature T is defi ned as the ratio of the shelf life at 0°C to
the shelf life at T°C. For example, if the shelf life of a cod is 14 d at 0°C and
6 d at 5°, the relative speed of spoilage will be equal to 14/6 = 2.3, i.e., the
spoilage will be 2.3 times faster at 5°C than at 0°C. Shelf life at 0°C differs
depending on the species of fi sh and the method of preservation, but the
effect of temperature on the relative spoilage speed R is constant and the
following formula has been established: R = 1 + 0.1 T°C. This “square-
root” model enables the shelf life of a product to be calculated at different
temperatures if its shelf life is known at a certain temperature.
The bacteria responsible for the spoilage of fresh fi sh are different
depending on the storage temperature. Sh. putrefaciens , Sh. baltica
Pseudomonas sp., Aeromonas sp. and Phosphobacterium phosphoreum are the
principal spoilage bacteria found between 0 and 5°C, and their quantity
varies depending on the storage atmosphere. At 15-30°C, Enterobacteriaceae,
Vibrionaceae and Gram-positive bacteria are responsible for spoilage. The
“square-root” model described previously does not take into account these
changes in fl ora. Nevertheless, the estimations of relative spoilage speed
are satisfactory for whole fresh fi sh, vacuum-packed fi sh or modifi ed
atmosphere packaged (MAP) fi sh (Gibson and Ogden, 1986; Dalgaard
and Huss, 1997). However, for tropical fi sh, the relative spoilage speeds
at 20-30°C are more than double those estimated by the model. A separate
model for tropical fi sh has therefore been developed for temperatures
between 0 and 30°C by Dalgaard and Huss (1997). It establishes a linear
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