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close this bookSmall-Scale Processing of Fish (ILO - WEP, 1982, 140 p.)
close this folderIII. DRYING
View the documentIII.1. Basic principles
View the documentIII.2. Natural drying
View the documentIII.3. Artificial drying
View the documentIII.4. Solar drying

III.1. Basic principles

During drying, water is removed from the fish by evaporation in two phases. During the first phase, only water on the surface of the fish or very close to the surface evaporates. The rate at which the fish dry depends on the surface area of the fish, the air temperature, the speed of the current of air passing over the fish and the relative humidity or wetness of the air. The drying rate during the first phase may be increased by:

- Increasing the fish surface area by splitting the fish and scoring them.

- Choosing a drying site where the air is dry and to avoid, if possible, marshy areas and places where the air has blown over water.

- Choosing a drying site where the wind is strong.

Once the surface is dry, water will evaporate at the rate at which it rises from inside the flesh to the surface of the fish. This rate slows down as the fish gets drier.

During the second phase, the drying rate is function of:

- The type of fish. For example, the rate at which water rises to the surface is slower for fatty fish.

- The thickness of the flesh.

- The temperature of the fish.

- The water content of the fish, and

- The wetness of the surrouding air.

If moisture is removed from the fish surface sufficiently quickly, the drying rate is independent of the level of humidity contained in the air. It depends only on the rate at which water reaches the surface of the fish. If drying is very fast during the early period, the surface may dry too quickly, thus producing a hard layer which will slow down the rise of the water to the surface. This is known as case hardening. When case hardening occurs, the centre of the fish could spoil even though the fish may look as if they have been well dried.

The drying rate during the second phase may be increased by:

- Reducing the thickness of the flesh by splitting and scoring the fish before drying starts, and

- Raising the temperature of the fish.