Biology Reference
In-Depth Information
correlation between the Neperian logarithm of the relative spoilage speed
and the storage temperature (Ln
R
= 0.12
∗
T°C).
Super-chilling consists in storing products at temperatures between
0 and
- 4°C. The “square-root” model gives satisfactory results for products
stored in this way (Dalgaard and Huss, 1997). This technique can greatly
extend the shelf life of the product but often lower the product quality
(water retention, texture, etc.).
The Effects of Storage Atmosphere on Shelf life: Anaerobiosis and
Carbon dioxide (CO
2
)
Vacuum-packaging or MAP with varying amounts of CO
2
(25-100%) are
both widely used to preserve food. Numerous studies have been carried
out on seafood products with very different results. Most frequently, high
concentrations of CO
2
lead to a 30 to 60% longer shelf life. The effi ciency is
closely linked to the temperature, which must be as low as possible so that
the gas can dissolve in the product (Sivertsvik et al., 2002). However, unlike
in meat products, these techniques do not signifi cantly lengthen fi sh shelf
life. In fact, the number of
Pseudomonas
decreases due to the lack of oxygen
in vacuum-packed temperate-water sea fi sh.
Sh. putrefaciens
can respire in
anaerobiosis due to TMAO, and can develop up to 10
6
-10
8
CFU/g, thus
increasing the production of TMA (Gram et al., 1987; Jorgensen et al., 1988;
Dalgaard et al., 1993). Nevertheless, it is probable that, below 10
8
CFU/g,
Sh. putrefaciens
is not the only microorganism responsible for fi sh spoilage
as these authors have also identifi ed the presence of
Ph. phosphoreum
on vacuum-packed cod. This microorganism, which had until recently
escaped detection by microbiologists produces 10 to 100 times more TMA
per cell than
Sh. putrefaciens
. Consequently, these two microorganisms can
provoke the spoilage of vacuum-packed temperate-water sea fi sh and it
is probably the initial quantity of each type that determines which one
will dominate the fl ora. Furthermore, the growth rate of
Ph. phosphoreum
increases in anaerobiosis, which explains why this microorganism plays
such an important part in the spoilage of packaged products such as cod
(Dalgaard et al., 1993).
In the presence of CO
2
, the growth of
Sh. putrefaciens
and
Pseudomonas
is
greatly inhibited whereas
Ph. phosphoreum
is relatively resistant (Dalgaard
et al., 1993; Dalgaard, 1995). It reduces TMAO to TMA without producing
H
2
S, which explains why MAP products are characterised by high levels of
TMA without the odour of H
2
S typical of spoiled fi sh stored in aerobiosis.
Ph. phosphoreum
can be eliminated if
the raw material is fi rst frozen, which
increases the shelf life of thawed cod stored in a modifi ed atmosphere at
2°C from 12 to 20 d (Guldager et al., 1998).
Numerous other seafood products have a shelf life similar to that of
cod. As
Ph. phosphoreum
is widespread in the marine environment, it seems
