| Fish handling, preservation and processing in the tropics: Part 2 no. G145 |
The energy of the sun and/or the wind is used in many countries to dry fish. To obtain the best possible rate of drying under natural conditions, several factors should be considered:
(i) Air movement at ground level is usually very slow; if fish are raised above the ground, by even one metre, the air movement is greater.
(ii) Drying fish at ground level does not allow air to pass under the fish; drying fish above the ground on raised, slatted or mesh racks allows drying from the upper and lower surfaces.
(iii) Fish placed on racks above the ground are less likely to be contaminated by dust or sand; fish placed on mats on the ground are likely to be contaminated by dust kicked up by people walking past. Raised racks provide some protection from animals.
(iv) Fish dried on racks can be more easily protected from rain by covering them with plastic sheets. Fish on the ground can be covered for protection against rain but not against water on the ground.
(v) Sloping racks allow any surplus water on the surface of the fish to drain away. Water trapped in the gill or body cavities can cause localised spoilage and/or increase the drying time.
The use of drying racks obviously has many advantages. However, they should be located to take the maximum advantage of climatic conditions:
(i) low lying swampy areas with a high relative humidity should be avoided and
(ii) racks should be sited away from forests or high buildings which will reduce the air movement or cast a shadow over the racks.
Salted fish will take up moisture from the surrounding air if the relative humidity rises above 75 per cent. It may, therefore, be necessary to remove the fish from the racks at night, or during rain, when the humidity tends to rise. If the fish are piled and covered in plastic overnight, the absorption of water is minimised until the fish can be put out for further drying the next day. If the fish are press-piled at night by placing weights on top of a stack of fish, movement of water to the surface of the fish is encouraged and the subsequent drying rate will be increased.
Several types of mechanically powered driers have been developed and used commercially in different parts of the world. Fish are dried in a fan driven air-stream; the air is usually heated and, in some cases, the air can be recirculated to control the relative humidity.
Evaporation of moisture from fish placed in a vacuum quickly cools the fish due to the transfer of heat energy. The fish freeze after about 15 per cent of the water has evaporated. If the fish are allowed to freeze during drying, they do not shrink and will dry with an open porous structure. They will rapidly reconstitute to look very similar to fresh fish although the water will not be as tightly bound as in fresh fish. If heat is applied to the fish in a vacuum drier and they are not allowed to freeze, shrinkage similar to that found in normal air dried fish occurs.
For rapid freeze drying, some heat must be supplied to the fish if evaporation is to proceed at a rapid rate. Moisture must also be removed from the vacuum chamber, otherwise it will become saturated and no further drying will be possible.
Freeze drying requires a high energy input and is only feasible for very high value products. Freeze dried products have the advantage that they can be stored under ambient conditions as long as the packaging is impervious to water.
Considerable interest has been shown in the development of solar powered driers in recent years. In these, the energy of the sun is collected and concentrated to produce elevated temperatures and an increased rate of drying. Raising the air temperature increases the amount of water the air can hold, thus the relative humidity will be reduced and the air will be able to absorb additional water vapour. In the humid tropics, the relative humidity is often too high for rapid natural drying and it is hoped that solar powered units which are simple, inexpensive and efficient can be developed for drying fish.
There are two basic methods of collecting and concentrating the sun's energy:
1. Parabolic reflectors: sunlight falling on a mirror is focused to a point, where the temperature becomes much higher. Reflectors have not been applied to fish drying, since the normal requirement is to slightly increase the temperature of large volumes of air.
2. Absorption units: a black surface absorbs heat energy from the sun far more effectively than a light coloured surface. If an insulated box, painted black on the inside and covered with clear glass or plastic, is placed in the sun, the temperature of the enclosed air is increased considerably. If the box has openings at the top and bottom, the air, as it is warmed, will rise and create a flow. Fish placed in the box will, therefore, be exposed to a flow of air that is warmer and of a lower relative humidity than the ambient air.
The black heat collection units can be connected to a drying chamber to supply a flow of warm air and it is not then necessary to expose the fish to the direct rays of the sun which can cause problems with case hardening and cooking of the fish if the temperature is not adequately controlled.
Several experimental designs of solar fish driers have been developed but, at the present time, none are in widespread commercial use.