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Fish processing is a fairly wide field, covering a large number of processing techniques, fish species and fish products. Given the purpose of this technical memorandum, the types of fish species, fish products and processing technologies for which detailed information is provided have been restricted to those of interest to small-scale producers in developing countries.

The first section of this chapter describes some of the fish species covered by the memorandum. The following two sections provide a brief description of processing methods, and suggest a few measures for avoiding the spoilage of fish before, during and after processing. The following chapters will provide detailed technical information on the processing methods described in this chapter.

I. FISH SPECIES

Some 20,500 species of fish are known to exist. The majority of these are found in tropical waters. One of the major differences between temperate and tropical fisheries is that the temperate fisheries tend to be based on single species such as cod and herring, while in many tropical fisheries the catch may include 30 or more species, all of which can be consumed fresh or in processed form. The fish curer working in the tropics may thus have to be prepared to preserve a range of species in differing size and fat content, some of which are delicate while others are more robust and less subject to damage. Since it would be impractical to consider more than a small number of fish types in this memorandum, four main groups are considered. In later chapters, where methods and techniques are discussed, it will be possible to be more specific.

The most important factors which affect the suitability of a fish for a particular process are:

(a) Size - Very small fish may be dried whole where weather conditions permit. Larger fish must always be cut so as to increase the surface area available for salt penetration, and/or moisture loss.

(b) Oil content - Fish oils oxidise readily and become rancid. Rancidity gives a bitter flavour to the product and this may be considered objectionable although some communities prefer dried fish to be slightly rancid. Fish which contain much oil do not generally make good salted and/or dried products since the oil acts as a barrier to salt penetration and moisture loss. Satisfactory products may be made by cooking and then drying, or by smoking. In cold climates, such fish are often pickled in salt. If this is attempted in a hot climate, fermentation is rapid, the flesh starts to break down and pastes or sauces result.

(c) Flesh texture - Fish in which the flesh is firm or moderately firm are relatively easily handled; they can be cut without falling apart and the dried product can be transported without breaking up. Fish which have a very soft flesh tend to tear when attempts are made to cut them? if dried products are made, these are very fragile and break up during transport. Fish which have both delicate flesh and a high fat content need especially careful treatment.

I.1. Small pelagic species

Small pelagic species include fish less than 25 cm long. Characteristically, such fish form schools or shoals so that large numbers may be taken at one time. The group includes the herring-like and sardine-like fish, which are slender and have relatively small scales and soft delicate flesh. Many species in this group have a high oil content. These fish are sometimes dried whole without salting, but the products are then fragile and break easily. In many parts of Africa, such fish are smoked and dried, the products again being fragile. It is difficult to control rancidity and unless the products can be marketed soon after processing, its value may be reduced substantially.

The small mackerels, such as the Indian chub mackerel, are also included in this group. These are sold fresh whenever possible, but they may also be boiled or salted and dried. This group also includes anchovies and anchovy-like fish of fresh waters. In Africa, these are often sundried without preliminary treatment while in Asia, products are often made by drying after boiling. The best fermented fish sauces are made from anchovies.

Good canned products may be made from sardines and mackerels, but as noted elsewhere in this memorandum, conditions in the developing countries rarely permit the profitable establishment of a small cannery.

I.2. Large pelagic species

The most important fish in this group are the tunas, some of them weighing 500 kg or more. Many other species reach a weight of 100 kg while others seldom attain 10 kg. The flesh is generally very firm and contains moderate amounts of oil. In some species, the flesh is very dark and many of these bleed heavily when cut. The skin of most species is thin.

Most of the world tuna catch is canned but substantial amounts are sold fresh. Fresh tuna is often highly priced and is not commonly used to make dried salted products. Two special cured products, Maldive fish (much in demand in Sri Lanka) and katsuobushi (which is popular in Japan) are made by a combination of cooking, smoking and drying. The smaller tunas, such as skipjack are most often used for these products.

The larger mackerels and horse mackerels, or jacks, have moderately firm flesh and a medium oil content. These are popular food fish and are best sold fresh when possible. Alternatively, good quality salted, dried or smoked products may be made. Canned products of good quality could be made from most of these species but they are not usually caught in sufficient quantity for canning.

I.3. Small demersal fish

Small demersal fish include bottom-living fish less than 25 cm long. They constitute a very diverse group including fish of very different shapes, but generally of deeper form than pelagic fish. Most have quite large hard scales and moderately firm flesh. Some, such as the catfish, are scaleless and have soft flesh. The oil content is variable but is generally less than 5% and there is less annual variation in oil content than is found in pelagic fish. The group includes many different types of sea fish such as small mullets, snappers, breams, croakers, jew fish and silver bellies, and small fresh water fish such as carps and breams.

Salted and dried products of good quality can be made from many small demersal fish, but the products fetch generally low prices. These products are useful in that they provide the lower income groups with a source of animal protein food. However, because their market prices are generally low, expensive refinements of processing are not possible. Being relatively lean and bony, these fish are not generally suitable for canning. They are also seldom used for making fermented products.

I.4. Large demersal fish

Large demersal fish also constitute a somewhat diverse group. It includes the sharks and rays as well as bony fish such as mullets, snappers, groupers, jew fish, breams and threadfins. Many of these bony fish are sold most profitably fresh when this is possible They can also be processed into excellent dried salted products when demand for the fresh product is not sufficient. These fish are not usually smoked or used for making boiled or fermented products. The sharks and rays need particularly careful handling otherwise the flesh may smell strongly of ammonia. Good salted dried products can be made out of these two species if processed carefully.

The fresh water fish in this group include tilapia, carp and catfish. In Africa, the latter are commonly smoked. They are also sometimes split or cut into pieces and dried in the sun.

Most of the bony fish have large hard scales, the flesh is moderately firm and the fat content relatively low. Few, if any, could be used for canning and for making fermented or boiled products.

II. PROCESSING METHODS

This section briefly reviews processing methods which will be described in greater detail in subsequent chapters. These methods do not include technologies which are more appropriate for capital-intensive medium and large-scale processing plants.

II.1. Salting

During slating, the flesh of the fish looses some of its water and is impregnated with salt. Rapid penetration of salt into the flesh is desirable for good protection of the product during the curing process. Salting can be done by a number of methods. The obtained results are influenced by such factors as climate, salt quality, type and quality of the fish used, the type of product desired by consumers and cost. Fish may be slated by rubbing dry salt into the flesh or by immersing the fish in a brine (a solution of salt in water). The juices extracted from the fish during dry salting can be allowed to drain away (Kench curing) or they can be contained in order to keep the fish covered by a salty liquid or “pickle” (pickle curing).

II.2. Drying

Simple drying in the sun is one of the commonest methods of curing used in tropical countries. Natural drying using the action of sun and wind constitutes one of the least expensive drying methods. Furthermore, the type of packaging used for dried fish is also fairly inexpensive. Altogether, dried fish is particularly suited for low-income groups which cannot afford expensive fish products. Simple improvements, such as the use of drying racks raised above ground level can increase drying rates and reduce contamination, thus helping to make products of good quality.

Mechanical dryers are relatively expensive to buy and operate. Although the output may be of better quality than that produced by natural drying, artificially dried fish will, most probably, be too expensive for the majority of low-income consumers.

II.3. Smoking

During smoking, the heat from the fire dries the fish while chemicals from the smoke impregnate the flesh. The obtained flavours depend both on the raw materials used and the length of time the fish are smoked.

There are many traditional smoking methods: these range from simple open fires or smoke pits to smokehouses covering a considerable area. Structures used in traditional smoking methods can be built with local materials and labour. They have however a major disadvantage: most of these structures are wasteful of fuel, usually firewood, which has become both scarce and expensive in some countries. A number of modified and improved designs have been produced in order to partially overcome the above disadvantage. The modified structures are easier to use than the traditional pits, and produce smoked fish of a more even quality, using less fuel.

Smoking is one of the most common curing process wherever salt is in short supply, most notably in the inland fisheries of Africa.

II.4. Other curing methods

Brine preserved, pickled and fermented fish products such as fish pastes and sauces are widely made in South-East Asia but not elsewhere in the tropics. Spoilage is prevented in these cures by the addition of large quantities of salt. A fish paste is obtained whenever a moderate amount of breakdown of protein occurs. If the breakdown is permitted to continue further, a liquid sauce is obtained. All these products contain large quantities of salt, and can there fore be eaten only in small quantities at one meal.

Boiled fish products are also of considerable importance in South-East Asia. Some products are simply boiled; they can then be kept for only a few days at tropical temperatures. Sometimes, the boiled fish is dried and the products can then be kept for many months. A few products are made by boiling and salting in sealed containers, yielding cured fish with a relatively long storage life.

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Since fish are the most important animal protein in the diet of many people in the tropics, it is important to reduce wastage and losses to the lowest possible level. Fish spoil very quickly and small-scale fish processing enterprises can easily loose profits through wastage. In general, it has been estimated that approximately 25% of a catch of fish may be lost through one cause or another before consumption.

Immediately after a catch, a complicated series of chemical and baterial changes begin to take place within the fish. If these changes are not controlled the fish quickly become spoiled (e.g. within 12 hours at tropical temperatures). Thus, the need to process fish according to some of the curing methods described under section II soon after the catch.

Spoilage of fish may take place before, during or after processing. The reasons for such spoilage and measures to prevent or minimise it are briefly described below.

III.1. Prevention of spoilage of fish before processing

A great deal of spoilage may occur before the fish is processed. The bacterial and chemical changes which cause spoilage proceed rapidly at the temperature at which tropical fish normally live (in the range of 25-30°C). In general, the lower the temperature of the fish, the slower the change which causes spoilage. Furthermore, spoilage may be reduced if fish are handled properly, and good hygienic measures are adopted. A few measures for avoiding or minimising spoilage are briefly described below.

(i) Improvement of landing facilities and distribution. Very often, whenever unexpectedly large catches are taken, landing facilities and the distribution system cannot handle the surplus of fish. Thus, a long period of time may elapse before the fish can be processed. Consequently, a high percentage of the fish may become unsuitable for processing. It is therefore important to expand cold storage facilities in proximity of the catch areas whenever sufficient and/or adequate transport facilities (e.g. trucks equipped with a refrigeration system) are not available. Alternatively, processing plants may be located near the catch areas in order to avoid the need for extensive transport facilities.

(ii) Maintaining the fish at low temperatures. To minimise spoilage, fish should be kept as cool as possible immediately after catching until processing starts. If tropical fish are chilled with ice, they may be kept in an edible condition for an increased period. The actual length of time depends very much on the type of fish, but may be as long as three weeks. However, in many areas far away from major towns, ice may not be available in sufficient quantities. Fish may then be kept relatively cool by other means, including the following:

- keeping the fish in the shade out of direct sun,

- placing damp sacking over the fish. This helps reduce the temperature as the water evaporates. The sacking must be kept wet and the fish must be well ventilated.

- mixing the fish with wet grass or water weeds in an open-sided box so that the water can evaporate and cool the fish. In this method, the fish should be kept continuously wet.

(iii) Maintaining a hygienic environment. Fish which have been handled cleanly and carefully will be in a better condition than fish which have been handled carelessly; they can, therefore, be worth more money.

Before processing starts, attention to the following points is important.

- To keep the fish as clean as possible. Washing with clean water will remove any of the bacteria present on the fish skin, especially in the presence of mud.

- To keep the fish cool, chilled in ice or chilled water, if possible, at all stages before processing starts. Fish spoilage is a continuing process: once a particular stage of spoilage has been reached no amount of good practice or processing can reverse it.

- To avoid damaging fish by careless handling. If the skin is broken this will allow bacteria to enter the flesh more quickly and spoilage will be more rapid. This sort of damage can be caused by walking on fish and by the use of a shovel. If the guts can be removed and the gut cavity washed carefully, this will reduce the number of spoilage bacteria present; however, in some areas, the purchaser requires whole fish so that this practice may lower the value of the catch.

III.2. Prevention of spoilage of fish during processing

A number of measures may be adopted in order to minimise the spoilage of fish during processing. They may include the following:

- To keep all tools, fish boxes, boat holds, cutting tables etc. clean by washing with clean water. Where drinking water is available, to use it to wash the fish before and during processing, for example, after gutting or splitting the fish.

- To prevent fish offal (guts, heads, gills, etc.) from coming into contact with cleaned fish. Also, the fish working area should be cleaned regularly, at least once a day by removing all offal and dirt which might contain bacteria or attract insect pests such as flies. All offal should be removed from the working site. It may be used as fertiliser, or buried. It should not be thrown into the water near the work site as this practice fouls the water and may attract insects.

- To ensure that high standards of personal hygiene are maintained. Fish processors are handling food, and hands should always be washed before starting work and particularly after visiting the toilet. People who have infected wounds, stomach complaints or any contagious disease, should not be allowed to handle the fish.

- To ensure speed during processing. The longer the time required by processing, the greater the amount of spoilage which will occur before processing is completed.

- If possible, to keep fish in boxes and off the ground. Work, such as cutting fish prior to salting or drying, must be carried out on tables, not on the ground where the fish will become dirty and pick up bacteria.

- To protect the fish from rain and to use salt during drying in order to avoid the spoilage of fish through bacterial, mould or insect attack.

- To use well-designed smoking kilns or ovens in order to avoid the over-cooking of fish which may catch fire or become excessively brittle.

- To protect the fish against insect infestation during processing. Blow-flies lay their eggs in the fish while they are still moist and the larvae eat the flesh. Beetles, such as the hide beetle, lay eggs in the fish as they are drying and the larvae eat the flesh even when it is quite dry. Damage can be reduced by ensuring that processing waste is properly disposed of so that there are no places for insect to breed. Using better salting techniques may help since insect larvae are not attracted by heavily salted fish. Techniques which speed the drying process are useful in countering blowflies. Temperatures in excess of 45°C decrease infestation by fly larvae although 20 hours at this temperature is required for complete de-infestation. Fumigation or heat treatment such as resmoking can be used to kill beetle pests. 40 minutes at 70° C is generally sufficient to kill insects in a dried product.

III.3. Prevention of spoilage of fish after processing

Although the spoilage process of fresh fish can be inhibited through various curing methods, cured products may still become inedible due to other causes such as mould or insect attack. The subject of losses in cured fish has been reviewed in detail by FAO (1981). It is hoped that the research and development programme recommended in the FAO study will yield improved techniques and reduced losses.

The storage life of cured fish will depend on the adopted curing methods and packaging. Cooking inhibits spoilage by destroying bacteria and preventing certain chemical changes. Tropical fish cook at temperatures over 50° C, although higher temperatures are usually used to reduce the time required to complete the process. Boiling fish in water for a few minutes is, for example, a popular process in South-East Asia. It must be stressed, however, that the preservation of fish due to cooking alone is short term unless recontamination by bacteria is prevented by canning or a similar process. Canned fish can be kept for a long time but the process is expensive and may not be suitable if retail prices are to be kept low. Cooked fish, such as boiled or hot-smoked products, must also be salted and/or dried if a storage life of more than two days at tropical temperatures is required.

The storage life of cured fish may be increased if the following measures were adopted:

- To ensure sufficient drying of the fish in order to avoid attacks by certain bacteria or moulds during storage.

- To use appropriate packaging, and to store packaged goods in cool storage areas protected from dust, insects, etc.

- To avoid excessive smoking and drying of fish if losses are to be avoided through the breaking of fish into small pieces.

- In general, it would be useful to process the right amounts of fish at a time, so that the whole output can be sold within the estimated storage life of the processed fish. It would not make sense to process much more than can be marketed within a given period of time, and have the surplus thrown away because it is not anymore fit for consumption.

More information on the various measures needed to prevent the early spoilage of fish will be further elaborated in the following chapters.