Fish Products and Byproducts Technology

Unit 3- Salting and Drying

3.2. Drying of fish
Drying and Dehydration:
Drying is under the sun by utilizing the atmospheric conditions like temperature, humidity and air flow. This is also called sun drying. The other is dehydration or artificial drying, by using artificial means like mechanical driers for removal of moisture from the fish under controlled conditions.
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
How drying of fish takes place?
Drying of fish is characterized by two different phases.
• Constant rate drying period
• Falling rate drying period
Constant rate drying period
Initially, drying is governed by evaporation from the near surface areas of the fish. Here drying continues at a constant rate equal to the rate of evaporation of free water surface. This phase is referred to as the constant rate drying period