Introduction

Fish are an extremely perishable foodstuff. Spoilage occurs as the result of the action of enzymes (autolysis) and bacteria present in the fish, and also chemical oxidation of the fat which causes rancidity. At the high temperatures prevalent in tropical countries, bacterial and enzymic action is enhanced. Fish invariably become putrid within a few hours of capture unless they are preserved or processed in some way to reduce this microbial and autolytic activity and, hence, retard spoilage.

Salting and drying are traditional methods of preserving fish; they have been used for centuries and dried salted products are still popular in many areas, particularly in Africa, SE Asia and Latin America. If the moisture content of fresh fish is reduced during drying to around 25%, bacteria cannot survive and autolytic activity will be greatly reduced, but to prevent mould growth, the moisture content must be reduced to 15%. The presence of salt retards bacterial action and, in addition, it aids the removal of water by osmosis. When salt is added to fish before drying, a final moisture content of 35 - 45% in the flesh, depending on the salt concentration may be sufficiently low to inhibit bacteria.

Traditionally, in tropical countries, many fishermen spread fish on the ground, on rocks or on beaches to dry in the sun. Some fish processors use mats or reeds laid on the ground to prevent contamination of the fish by dirt, mud and sand. Drying fish in this manner has many disadvantages and, in recent years, the use of raised sloping drying racks has been introduced as a simple but often effective improvement (Clucas and Sutcliffe, 1981). A cleaner product is obtained from rack drying since the fish do not come into contact with the ground; also they are less accessible to domestic animals and pests, such as mice, rats and crawling insects, which contaminate or consume them. Protection from rain is simply accomplished by covering the rack with a sheet of waterproof material (e.g. plastic); if fish on the ground are covered, they are protected from falling rain but not from water on the ground itself. Drying rates are higher because air currents are stronger at a metre or so above the ground and air can pass under the fish as well as over them. The use of a sloping rack allows any exudate to drain away.

However, even when racks are used, sun-drying has many limitations: long periods of sunshine without rain are required; drying rates are low and, in areas of high humidity, it is often difficult to dry the fish sufficiently. The quality of sun-dried fish is likely to be low due to slow drying, insect damage and contamination from air-borne dust, and it is difficult to obtain a uniform product.

Thus, in the search for improved drying techniques, the use of solar dryers has been investigated as an alternative to traditional sun drying. Solar dryers employ some means of collecting or concentrating solar radiation with the result that elevated temperatures and, in turn, lower relative humidities are achieved for drying. When using solar dryers, the drying rate can be increased, lower moisture contents can be attained and product quality is higher. The dryers are less susceptible to variations in weather, although drying is obviously slower during inclement weather, and they do provide shelter from the rain. The high internal temperatures discourage the entry of pests into the dryer and can be lethal to any which do enter.

Many forms of solar dryer for use with agricultural and fisheries products have been developed in many parts of the world; only a few of these have been used specifically with fish, and no direct comparative exercise has been reported on the relative performance of any of the types available. It was with this objective that a two-man team, composed of a chemical engineer and a fish processing technologist, from the Tropical Products Institute (TPI) spent seven weeks in Ecuador constructing and operating three types of solar dryer for drying of fish. Relative effectiveness of the dryers was measured by conducting trials with commercially-important local species and the results were compared with sun-drying methods. The evaluation was based on drying efficiency, product quality and basic construction costs. The eventual aim of the programme was to produce a recommended design for a solar dryer (or dryers) for improved drying of fish in many developing countries.

Solar dryers can be categorized into two classes on the basis of the mode of air flow through the dryer, i.e. natural convection or forced convection. Dryers that employ forced convection require a source of motive power, usually electricity, to drive the fan that provides the air flow. In many areas of tropical developing countries, motive power from any source is either unavailable or, at best, unreliable and expensive, and forced-convection dryers would not be a practical proposition for the majority of artisanal fishermen in these areas. Therefore, only natural-convection dryers were investigated in the exercise.

The timing of the programme was chosen to coincide with the period of production of dried salted fish in Ecuador, which occurs during the six months prior to Easter. The fishermen in the Galapagos Islands are the major producers of dried salted fish, but have problems with quality mainly due to inefficient drying. The location for the exercise was the Charles Darwin Research Station (CD RS) on Santa Cruz Island in the Galapagos.