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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.

III.2. Natural drying

Natural or air drying uses the combined action of the sun and wind without the help of equipment. It is important to dry the fish quickly before they spoil, and that all surfaces of the fish be open to the drying action of the wind. Where only a few large fish are to be dried, this may be done by hanging the fish up. Split fish may be hung on hooks, by tying them up with string, or by tying the fish in pairs by the tail and hanging them across a pole or line.

Large quantities of fish should be dried on racks. Suitable materials for drying racks include chicken wire, old fishing nets, and thin rods or poles such as reeds or sections of bamboo. The surface of the racks should be at a height of about 1 m from the ground and should slop if split large fish are to be dried. A flat surface is preferred for drying small intact fish. Designs for fixed drying racks are shown in Figure II.2. These racks can be easily covered with plastic sheets to protect the drying fish from the rain. Where large quantities of very small fish are to be dried, a netting rack may be impractical. Suitable drying surfaces may be made instead, with raised floors of wood, concrete, bamboo strip or, where none of these materials are available, well compacted clay.

In the tropics, the air is relatively dry during the day (unless it rains) and relatively wet during the night. From sunrise until about midday, the air becomes gradually drier and, becoming wet again from midday to nightfall. The drying rate - especially in the case of salted fish - is therefore the highest from about 8 or 9 o’clock in the morning to 4 or 5 o’clock in the afternoon. Fish which have been set to dry during the day should be collected, and stored overnight to avoid them becoming wet by dew or rain. The fish in storage should be piled in a similar manner as for dry salting although no further salt should be added. Wooden boards, weighted with clean rocks or other suitable material, should be placed on the pile of fish in order to flatten them and give them a better appearance. This use of pressure will also speed up the process by which water moves from the inside of the fish to the outside so that they will dry more rapidly when set out the following morning.

Figure II.2. Fixed drying racks with flat and slanding tops

III.3. Artificial drying

Artificial drying offers better control than natural drying, resulting in greater product uniformity and quality. The initial investment on equipment and expenditures on energy inputs are, however, high and may not always be justified. In general, artificial drying is advantageous when drying by natural means is extremely difficult as, for example, in Southern Brazil where a combination of very humid winters and extremely hot summers - which heat-damage salted fish - do not favour natural drying.

A number of factors can be controlled when drying fish artificially to ensure optimum drying conditions. These are:

- Temperature - the higher the temperature, the quicker the drying. This, however, has to be balanced against the damage which is caused by over-heating the fish and the extra cost of increasing the temperature in a mechanical drier. In general, the initial drying temperature should be restricted to 25 to 45° C. Tropical fish can withstand a higher processing temperature (35-45° C) during drying with no signs of heat damage as compared to temperate fish which may not withstand temperatures higher than 25-30° C.

- Relative humidity (RH) - The moisture content of the air is important for two reasons: it controls the drying and influences the appearance of the final product. The drier the air, that is the lower the relative humidity, the faster the drying rate. If, however, the air is too dry, the surface of the fish will dry too quickly resulting in case hardening. The relative humidity is dependent on local conditions but, as a guideline during initial drying, a 50-65% RH is suitable for optimal drying. This can be lowered by raising the air temperature during the later drying stages.

- Air speed - A faster flow of air over the fish results in even and rapid drying. This is due to a more uniform temperature distribution and a quicker removal of moisture from the fish. A compromise must be made between the higher cost of faster air circulation with a mechanical drier and the improved drying rate gained with a high air speed. Therefore, an air speed between 60 and 120 m per minute is normally used when drying fish with a mechanical drier.

- Surface area and volume of fish - Large whole fish take longer to dry than small fish due to the greater difficulty of removing water from inside the flesh of the fish. Large fish should, therefore, be split to increase the surface area. The flesh should also be scored if it is thicker than 2 cm.

III.3.1. General requirements of mechanical driers

To allow control of temperature, air speed and humidity for optimum drying, an enclosed environment in the form of a tunnel or long box is required. The tunnel can be constructed from locally available raw materials such as wood, corrugated iron sheets, etc. The prepared fish are placed on wire mesh trays which allows air flow on both sides of the fish for easier removal of moisture. Layers of mesh trays can be placed in racks or trolleys in the tunnel. A number of these racks or trolleys can be arranged in series. A fan at one end of the tunnel drives the air over a heating element (e.g. an electric heater/steam heater/flame) and the heated air is then blown over the fish and evacuated at the other end of the tunnel. The temperature may be controlled by a thermostat (set at the required drying temperature) placed near the drying fish, so that it automatically switches the heating element on or off as the temperature drops too low or rises too high.

A simple mechanical drier has been tested in Cambodia (Legendre, 1961) using partially dried fish. It was constructed with local material and incorporated a fan and a steam heater. The experimental Cambodian drier (see Figure II.3) was designed to hold 2 tonnes of fish partially sun-dried for about 54 hours, including 6 hours of sun drying at an inland depot. The drier temperature was set up at 43° C and air speed at 108 meters/min. Thus, an effective relative humidity of about 36% was attained, even though the RH of the outside air was 65-72%. Drying under these conditions gave a readily acceptable product. The drier had 6 sections with 20 trays each holding an estimated 16 kg of fish per tray. The total capacity of the drier is therefore approximately equal to two tons.

The design of the drier was kept simple to avoid using expensive or complicated modifications. It is, however, possible to further improve the efficiency of the system by recirculating the air. This will require the use of fans, automatically controlled by humidistats set at the required relative humidity, to bring in or take out air. Such an improvement is advantageous as it substantially reduces heating costs and allows for a more precise control of the relative humidity within the tunnel.

Figure II.4 shows a recirculating air tunnel drier tested in Southern Brazil by FAO (Anon, 1958). This drier was constructed locally of wood and consisted of 5 sections loaded separately with wire mesh trays. Its total capacity was of 700 to 1,400 kg. of salted fish. Above the main body of the tunnel, was the return air duct in which was installed a recirculating fan driven by a 2 HP motor. The air recirculation and linked dampers were reversed half way through the drying process to ensure even drying. A simple paper humidistat, set to operate at 55-60% RH, was used to activate a 3/4 HP centrifugal fan mounted on top of the tunnel in order to introduce fresh air. The temperature of 36° C was controlled by a thermostat located between the second and third drying sections. This thermostat activated a motor driven damper which forced incoming air through a steam heated, finned heat exchanger. Optimum drying conditions were found to be 90-100 meters/min. at a RH of 50%.

More recently, an improved tunnel drier was tested in India by Chakraborty (1977). The drier (see Figure II.5) consisted of a long tunnel divided into an upper air recirculation chamber and a lower product chamber. The lower chamber, which was tall enough for a man to walk through, contained 5 trolleys on rails, fitted with several layers of aluminium trays. These trolleys, loaded at one end of the tunnel (trolley inlet door) and off-loaded at the other end (trolley outlet door) moved at counter current to the flow of heated air. Thus, the fish moved first through warm air moistened by contact with the previous batch of fish, and then through progressively drier air as the trolleys approached the tunnel outlet door. In the upper chamber, the air was heated by steam heaters, the temperature being controlled by a thermostat and the relative humidity by humidistats which activated exhaust fans. The blower fan was capable of delivering 275-285 cubic meters/minute.

Figure II.3. Cambodian Tunnel Dryer

Figure II. 4. Brazilian Tunnel Dryer

Figure II.5. One tonne capacity tunnel dryer

During trials, the fish were prepared by washing, splitting, kench salting in tanks for 18-24 hours, draining and drying in the tunnel to 25-30% moisture for 14-16 hours. Small fish, such as sprats, were simply salted in saturated brine and dried to 15% moisture. Larger fish, such as sharks, were cut into fillets and heavily salted and dried. The products were claimed to be superior to the equivalent sun-dried product: the dried fish did not have a bad smell, its shelf life was equal to 9-12 months when stored in plastic bags and its physical appearance was better than that of sun-dried fish.

The economic viability of mechanical drying of fish depends, to a large extent, on whether adverse weather conditions (e.g. rains, extremely high temperatures) make natural drying very difficult (e.g. long drying periods, high spoilage rates) and on the difference in production costs between natural and mechanical drying at a given project site, taking into consideration the high initial capital investment costs and high energy inputs associated with mechanical drying. Given their generally superior appearance and quality, it would be reasonable to expect that higher prices could be charged for mechanically dried fish products. The higher prices could improve the profitability of these fish products. However, this does not necessarily happen in practice since consumers may not like mechanically dried fish as much as, or more than, traditional sun dried fish. They may not also be willing, or able, to afford the higher retail prices.

III.4. Solar drying

It is possible to harness the sun’s energy to produce drying conditions superior to those prevailing under natural drying. A number of simple experimental designs have been tested with varying degrees of success. These designs include structures in the form of tents made with wooden or bamboo frames covered with clear and dark polythene, wooden black boxes, or some other simple designs made from wood or brick and glass.

The principle underlying solar drying is simple. Air inside the drier is heated as it flows over dark surfaces which absorb the sunlight, thus resulting in air temperatures higher than those of ambient air. A convection current or upward flow of air takes place as air flows from the vents located at floor level to those located at the top of the structure. The fish, which are placed on wire racks, are dried by this flow of air which gets progressively warmer as it rises upwards and leaves the structure by the top vents. Depend-in on the design of the solar dryer, temperatures of 70° C and over can be achieved if there is no ventilation (Szabo, 1970). The temperature can be lowered by opening the air vents thus allowing free movement of air.

A tent drier, made from a bamboo frame covered with clear and black polythene (e.g. of the type shown in Fig.II.6 and Plate II.16) was evaluated in Bangladesh (Doe et. al, 1972). This drier attained a maximum temperature of 48° C, which is suitable for drying fish, the ambient air temperature being equal to 27° C. Dried fish were produced within a marginally shorter period than in the case of natural drying and were superior in quality, mainly due to the lack of insect infestation. The temperature within the tent was high enough to kill adult flies which would have otherwise laid eggs on the drying fish. Sun dried fish already infested with fly larvae were disinfected to a considerable extent within three hours when placed inside a solar tent drier at about 45°C, twenty hours at this temperature being sufficient for a complete disinfection. In this case the solar drier was more useful for keeping away insects and for the de-infestation of dried fish, than for reducing the drying time. In general, further development work is required with various designs of solar driers, before the method can be widely recommended for commercial use.