Exercise





What is basis of preservation by
salting?

When salt introduced into the flesh in sufficient quantity, it delays the
activity of bacteria or even inactivates them and reduces enzymatic action by
reducing the water activity. Loss of water due to salting and reduced water
activity limits bacterial growth and enzyme activity, thus preserving the fish.
The high salt content prevents the growth of normal spoilage microflora in the
fish; but halophiles, which can survive 12-15% of salt, will survive.







How salting takes place (Theory of
salting)?

During salting osmotic transfer of water out of the fish and salt into the fish
takes places, which effect fish preservation. It is based on different factors
like diffusion and biochemical changes in various constituents of the fish.
Movement of salt molecules from the brine into the fish takes place through a
layer of salt solution covering the fish but having a salt concentration below
that of the brine. This layer is formed because the water diffuses out at a
faster rate during the initial stages from the fish than the salt solutes,
which enters the fish. As the period proceeds the thickness of this covering
layer diminishes as its salt content grows. At this point, diffusion of water
from the fish ceases and the salt concentration in the layer equals to that of
the salt concentration in the in the brine solution. Penetration ends when the
salt concentration of the fish equals that of the surrounding medium.





What are the
factors related to salt to be considered before using for salting?





  • Physical properties

  • Chemical impurities

  • Microbiological contamination






What is the principle of preservation by
drying?

The moisture content and water activity is reduced and hence microorganisms
cannot grow and multiply to spoil the fish. It can be seen that microbiological
growth is completely arrested below a water activity level of 0.6 whereas
certain other reactions that cause food spoilage continue. Lipid oxidation in
fact increases at very low water activity.To understand the concept of
preservation by drying we should know about the existence of different forms of
water and the concept of water activity.





In what are all the forms the water
exists in fish muscle?

Three layers of water exist in food.

• An adsorption layer

• A diffusion layer

• A free layer

Adsorption layer

• Water surround the surface of a colloidal particle

• Water at the adsorption layer is tightly bound to the particle and is called
bound water

• It does not take part in any chemical reaction

Diffusion layer

• Less tightly bound to the adsorption layer and the further it is away from it

• More it behaves like free water

Free water

• It has all the properties of ordinary water

• It support the growth of microorganisms

• Take part in chemical reactions

• This form of water is important in the drying process





What is smoking of fish?

Smoking of fish refers to subjecting the fish to the smoke generated using
various materials such as wood, coconut husk etc.





What is
cold smoking?

Carrying out the smoking process at a temperature upto 30oC
is called cold smoking, in which cooking of the flesh does not occur. Smoking effect
is mainly due to the chemicals killing off the surface bacteria as reduction in
water is not significant.

Cold smoked products should be cooked before consumption

Cold smoking takes 2-4 hrs when using a
mechanical smoking kiln




What is hot smoking?

Carrying out the smoking process in which the fish is exposed to
increasing temperatures up to at least 70oC is called
hot smoking. It is having following effects





  • The fish will be cooked

  • Enzymes will be destroyed and chemical
    spoilage will be reduced

  • Surface bacteria will be killed



How
hot smoking is carried out?

In a mechanical kiln, hot smoking is carried out in three stages.





  • A preliminary drying period at about 30°C.
    During this time the skin is toughened to prevent breakage occurring in
    the remainder of the process.

  • A smoking and partial cooking
    period at 50°C

  • A final cooking period at 80°C



The total time could
be up to 3 hours. The actual times of each stage will depend upon:





  • The species

  • The size and thickness

  • The fat content of the product

  • The loading of the kiln

  • The colour and taste requirements






How smoking exerts
preservative action?

Spoilage of fish can be reduced by reducing some of the important substances
like water content, enzyme content and fat content.

The smoking process is effective in reducing the water by
evaporation.

This occurs due to:





  • The movement of the gases in the smoke
    over the surface of the fish

  • Raising the temperature of the fish



The chemical
components such as phenols and formaldehyde exist in the form of vapor in the
smoke exert antioxidant and antibacterial activity. In addition
the smoking process results in the penetration of the chemicals into
the fish and loss of water from the flesh. A layer is formed which reduces the
effect of oxygen that enters the fish from the surrounding air. Therefore the
rancidity of the fat is reduced. This layer is called “barrier layer”.