|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
Methods and equipment
Smoking is a method of preserving fish which combines three effects:
1. Preservative value of the smoke: the smoke produced from burning wood contains a large number of compounds, some of which will kill bacteria, e.g., phenols.
2. Drying: the fire which produces the smoke also generates heat and this will dry the fish.
3. Cooking: if the fish are smoked at a high temperature, the flesh will be cooked and this will destroy the enzymes and kill bacteria.
The long storage life of some smoked fish products is due more to drying and cooking than to the preservative value of the chemical compounds deposited on the fish from the smoke.
The burning of wood or sawdust to produce smoke is extremely complex since the smoke is the result of incomplete combustion and this will vary with the source of the fuel and the ventilation of the fire. A slow burning fire will produce much more smoke than a small intense fire. Wood smoke is a mixture of gases, vapours and droplets. Droplets form the visible part of the smoke although the invisible vapours contribute to the characteristic smell. It has been shown that it is mainly the vapours that are taken up by fish during smoking. The substances in the vapours dissolve in the liquid on the surface of the fish and the rate of uptake depends on the moisture on the surface of the fish and the rate of flow of the smoke.
Smoked fish can be divided into two general categories:
(i) Cold smoked: during the smoking process, the temperature at no time rises to a level where the flesh is cooked (i.e., the protein is denatured). In practice, this means a maximum temperature of approximately 30 - 40°C and is only really possible in temperate climates.
(ii) Hot smoked. during the smoking process, the flesh is cooked. Traditional smoking in tropical countries falls within this category.
Almost all traditional smoked products are heavily smoked and dried; often the fish are salted before smoking. Since the advent of rapid communications (railways) and the use of chilling or freezing to hold perishable commodities, a change towards lighter cured products has occurred in the industrialised countries. In such products, the amount of salt, smoke and drying will not give a long storage life at ambient temperatures and they must be treated as fresh fish to retard spoilage. In developing countries, the fish are heavily smoked and dried so that they can be distributed and stored without specialised facilities.
Methods and equipment
Fish smokers are often very simple and, in the simplest form, fish are suspended above a slowly burning fire. This may be adequate for the subsistence fisherman who wishes to preserve a few fish for his own consumption but it is not suitable for smoking larger quantities of fish caught by a professional fisherman. A variety of kilns have been developed and these fall into two categories: natural convection smokers and mechanical smokers.
Natural convection smokers
With these smokers, the heat from the fire causes a warm column of smoky air to rise; the fish are hung or laid on openwork trays above the fire.
In one of the simplest types, a fire is burnt in a pit over which a table carrying the fish is built. Since the sides of the table are open, a considerable proportion of the smoke and heat can escape without passing over the fish. A number of designs of smoker have been developed in different parts of the world which utilise locally available materials. Although these may be very cheap to construct, they tend to suffer from some, or all, of the following disadvantages:
(i) They have a high fuel consumption compared to output.
(ii) They have a low capacity.
(iii) They require constant attention.
(iv) They are affected by wind and/or rain.
(v) They are difficult to control and the product is not uniform.
(vi) The materials used in construction are often inflammable.
Several designs of improved natural convection smokers have been developed to overcome some of these problems: they range from the small units based on an old 200-litre oil drum to the multiple units based on the 'Altona' design developed by the Food and Agriculture Organization (FAO). Publications by FAO describe the construction and operation of these kilns. The 'Ivory Coast kiln' is a modification of the 'Altona' design and is simple and inexpensive to construct.
In these smokers, electric fans or blowers are used to circulate the smoke instead of natural convection. In most designs, the flow of smoke is horizontal. Trolleys can be used to hold the fish and these reduce the time and labour necessary to load or empty the kiln. The smoke density, air velocity, temperature and humidity of the air may all be controlled. Although mechanical smokers are expensive, they can produce a uniform product and are particularly suitable for large-scale commercial production. Mechanical kilns are not in widespread use.