|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
Drying is the removal of water from fish. Normally the term 'drying' implies the removal of water by evaporation but water can be removed by other methods: for example, the action of salt and the application of pressure will remove water from fish. Since water is essential for the activity of all living organisms its removal will slow down, or stop, microbiological or autolytic activity and can thus be used as a method of preservation.
Where drying has evolved as a traditional method of preserving fish, the action of the sun and wind is used to effect evaporative drying. In recent times, the controlled artificial dehydration of fish has been developed in some industrialised countries so that fish drying can be carried out regardless of weather conditions.
In any process of drying, the removal of water requires an input of thermal energy. If the outward movement of water occurs in the following sequence: migration within the material to the surface - removal from the surface - mixing with the atmosphere surrounding the material - removal from the vicinity of the surface,
it must be accompanied by the inward transfer of heat in the following sequence: emission from the heat source - transfer to the surface of the material - conduction within the material - provision of latent heat of evaporation and the partial enthalpy of dilution of the system which is regarded as a solution.
The thermal energy required to drive off the water can be obtained from a variety of sources, e.g., the sun or the controlled burning of oil, gas or wood. The thermal energy can also be supplied directly to the fish tissue by microwave electromagnetic radiation or ultrasonic heating.
At normal temperatures, fish muscle can be considered to be a gel; it remains a gel until a considerable quantity of water has been removed. During drying, considerable shrinkage takes place, as well as other irreversible changes, and dried fish will not reconstitute to their original condition.
During air drying, water is removed from the surface of the fish and water moves from the deeper layers to the surface. Drying takes place in two distinct phases. In the first phase, whilst the surface of the fish is wet, the rate of drying depends on the condition (velocity, relative humidity etc.) of the air around the fish. If the surrounding air conditions remain constant, the rate of drying will remain constant; this phase is called the 'constant rate period'. Once all the surface moisture has been carried away, the second phase of drying begins and this depends on the rate at which moisture can be brought to the surface of the fish. As the concentration of moisture in the fish falls, the rate of movement of moisture to the surface is reduced and the drying rate becomes slower; this phase is called the 'falling rate period'.
Constant rate drying
During this period the rate of drying depends on the speed at which moisture can be carried away from the surface of the fish. Several factors influence the rate of drying:
(i) Relative humidity (RH) of the air: if the air is fully saturated with water vapour (relative humidity 100 per cent), it cannot carry any more water and no drying of the fish will occur. If the RH is less than 100 per cent, the air has the ability to absorb water and drying will proceed; the lower the RH, the greater the ability to absorb water and the faster the rate of drying.
(ii) Air velocity: the greater the speed of the air over the fish, the greater the drying rate. The air around fish can be considered as three layers: a stationary layer close to the fish, a slowly moving layer outside this and an outer turbulent layer. The stationary layer of air next to the fish is saturated with moisture that passes into the slowly moving layer. The higher the air speed in the outer layer, the thinner the slow moving layer, which allows more rapid movement of water away from the fish.
(iii) Air temperature: the evaporation of water produces a cooling effect. At the beginning of drying, the temperature of the fish is reduced below ambient; after a short while it reaches a steady value. At this steady value, the heat energy required for evaporation is balanced by the heat supplied by the surrounding air. The degree of cooling is related to the wet bulb depression of a hygrometer and reflects the ability of the air to hold water. Warm air can provide more heat energy and, provided that the air speed and relative humidity will allow a high rate of water movement, the rate of drying will be increased.
(iv) Surface area of the fish: the larger the surface area, the faster the rate of drying. If a fish is split, the surface area increases relative to the weight/thickness; the rate of drying will, therefore, be faster.
Falling rate drying
Once the free surface moisture has been removed, the rate of drying depends on the movement of moisture to the surface of the fish. Several factors influence the rate of drying:
(i) The nature of the fish: a high fat content in the fish retards the rate of drying.
(ii) The thickness of the fish: the thicker the fish, the further the water in the middle layers has to travel to reach the surface.
(iii) Temperature of the fish: diffusion of water from the deeper layers to the surface is greater at higher temperatures.
(iv) The water content: as the water content falls, the rate of movement to the surface layers is reduced.
Provided that the air passing over the fish is not fully saturated with water, the rate of drying is independent of the condition of the air. Under certain conditions, where the constant rate drying has been very rapid, the surface of the fish can become 'case hardened' and the movement of moisture from the deeper layers to the surface is prevented. This can result in a fish that is dry on the surface and looks, to all intents and purposes, fully dry but the centre will be wet and spoiled. This can be a particular problem with some designs of mechanical and solar driers.