41
Bilgin (2004) reported that the average total number of microorganisms in shrimp
(Palaemon adspersus) was 3.55 log cfu/g, and this number increased to 6.71 log cfu/
day during cold storage. Varlık et al. (2000) examined the changes in the quality of
shrimps stored in cold storage period and determined that the samples were spoiled
in terms of sensory, physical, and chemical on the second day.
2.1.3.4 Freezing
The storage of aquatic products for a long time without spoiling has been achieved
by the development of freezing technique. In freezing technology which is among
the methods of physical storage and the best method for preserving the quality of
products, the water in the structure of the products converts to ice crystals and as a
result of this the chemical, biochemical and microbiological activity slow down by
reducing of the water activity and temperature (Gokoglu 2002).
As a result of the removal of energy from a liquid substance transition from the
liquid phase to the solid phase is called freezing. By removing energy, the free
movement of molecules in the liquid phase is gradually slowed down and the molecules tend to spontaneously aggregate into a regular structure. This is the first sign
of the beginning of phase change. The phase change of a pure substance can only
take place at a specific temperature. For water, this temperature is 0 °C. However,
when a liquid reaches its freezing temperature, it does not cause the phase change
to start. To start phase change, there must be a structure called “nucleus” in the
medium. If there is no nucleus in the medium, the liquid must first cool down to its
original temperature below the critical temperature called the freezing point. This is
called super cooling. There is a second condition other than the super cooling for the
phase change to occur. Accordingly, it is also necessary to have a structure that can
lead to the transformation of molecules into solid phase. This can be either a small
nucleus resulting from the crystalline structure, which is spontaneously formed by
aggregating its molecules in the liquid, or it can be a foreign material similar to the
properties of the crystal structure of that substance. The nucleation formed by the
liquid itself is called homogenous nucleation. Homogeneous nucleation can only
occur in liquids which do not contain any foreign matter, cooling the liquid to below
its freezing level. The energy released because of freezing phenomena occurring by
transition of water from the liquid phase to the solid phase causes heating of froze
mass. However, if this energy is removed by the cooling medium, the temperature
of the freezing mass remains constant at the freezing point until all material is frozen
(Cemeroglu et al. 2003).
As is known, water must be present in the liquid phase to be utilizable by microorganisms. According to this, microorganisms cannot benefit from frozen water.
One of the results obtained by freezing is to make the environment unfit for microorganisms in terms of water. The second effect of freezing is based on the strict
stopping of microorganism activities below certain temperatures. Activity of both
food poisoning and psycrophilic microorganisms stops under −10 °C. To prevent
microbiological deterioration in frozen storage, the maximum temperature that can
2.1 Shrimps/Prawns
Bilgin (2004) reported that the average total number of microorganisms in shrimp
(Palaemon adspersus) was 3.55 log cfu/g, and this number increased to 6.71 log cfu/
day during cold storage. Varlık et al. (2000) examined the changes in the quality of
shrimps stored in cold storage period and determined that the samples were spoiled
in terms of sensory, physical, and chemical on the second day.
2.1.3.4 Freezing
The storage of aquatic products for a long time without spoiling has been achieved
by the development of freezing technique. In freezing technology which is among
the methods of physical storage and the best method for preserving the quality of
products, the water in the structure of the products converts to ice crystals and as a
result of this the chemical, biochemical and microbiological activity slow down by
reducing of the water activity and temperature (Gokoglu 2002).
As a result of the removal of energy from a liquid substance transition from the
liquid phase to the solid phase is called freezing. By removing energy, the free
movement of molecules in the liquid phase is gradually slowed down and the molecules tend to spontaneously aggregate into a regular structure. This is the first sign
of the beginning of phase change. The phase change of a pure substance can only
take place at a specific temperature. For water, this temperature is 0 °C. However,
when a liquid reaches its freezing temperature, it does not cause the phase change
to start. To start phase change, there must be a structure called “nucleus” in the
medium. If there is no nucleus in the medium, the liquid must first cool down to its
original temperature below the critical temperature called the freezing point. This is
called super cooling. There is a second condition other than the super cooling for the
phase change to occur. Accordingly, it is also necessary to have a structure that can
lead to the transformation of molecules into solid phase. This can be either a small
nucleus resulting from the crystalline structure, which is spontaneously formed by
aggregating its molecules in the liquid, or it can be a foreign material similar to the
properties of the crystal structure of that substance. The nucleation formed by the
liquid itself is called homogenous nucleation. Homogeneous nucleation can only
occur in liquids which do not contain any foreign matter, cooling the liquid to below
its freezing level. The energy released because of freezing phenomena occurring by
transition of water from the liquid phase to the solid phase causes heating of froze
mass. However, if this energy is removed by the cooling medium, the temperature
of the freezing mass remains constant at the freezing point until all material is frozen
(Cemeroglu et al. 2003).
As is known, water must be present in the liquid phase to be utilizable by microorganisms. According to this, microorganisms cannot benefit from frozen water.
One of the results obtained by freezing is to make the environment unfit for microorganisms in terms of water. The second effect of freezing is based on the strict
stopping of microorganism activities below certain temperatures. Activity of both
food poisoning and psycrophilic microorganisms stops under −10 °C. To prevent
microbiological deterioration in frozen storage, the maximum temperature that can
2.1 Shrimps/Prawns