The nutritional significance in the diet
Suitability for small-scale production
The small-scale fisheries of developing countries are vital because they provide a nutritious food which is often cheaper than meat and therefore available to a larger number of people.
In many countries the bulk of fish is sold fresh for local consumption. Processing, where this is done, is either to supply distant markets or to produce a range of products with different flavours and textures.
Fish is an extremely perishable food. For example, most fish become inedible within twelve hours at tropical temperatures. Spoilage begins as soon as the fish dies, and processing should therefore be done quickly to prevent the growth of spoilage bacteria. Fish is a low acid food and is therefore very susceptible to the growth of food poisoning bacteria. This is another reason why it should be processed quickly. Some methods of preservation cause changes to the flavour and texture of the fish which result in a range of different products. These include:
· Cooking (for example, boiling or frying)
· Lowering the moisture content (by salting, smoking and drying collectively known as curing)
· Lowering the pH (by fermentation)
Lowering the temperature with the use of ice or refrigeration also preserves the fish, but causes no noticeable changes to the texture and flavour.
Fish provides a good source of high quality protein and contains many vitamins and minerals. It may be classed as either white, oily or shellfish. White fish, such as haddock and seer, contain very little fat (usually less than 1%) whereas oily fish, such as sardines, contain between 10-25%. The latter, as a result of its high fat content, contain a range of fat-soluble vitamins (A, D, E and K) and essential fatty acids, all of which are vital for the healthy functioning of the body. The table opposite illustrates some of the main nutritional differences between oily and white fish.
Average composition of fish
|
Composition |
White fish e.g. haddock |
Oily fish e.g. herring |
|
Energy (KJ) |
321 |
970 |
|
Protein (g) |
17 |
17 |
|
Fat(g) |
07 |
18 |
|
Water (g) |
82 |
64 |
|
Calcium (mg) |
16 |
33 |
|
Iron (mg) |
0.3 |
08 |
|
Vitamin A (m a) |
0 |
45 |
|
Thiamine (mg) |
0.07 |
0 |
Certain processing techniques such as boiling leach the water-soluble vitamins into the surrounding liquid. If this is thrown away, a great deal of nutritional value is lost.
Types of fish products
Cooked fish
Cooking provides short-term preservation of fish and it is usually a few days before any deterioration becomes noticeable.
A range of methods are used for cooking fish but the principle of the process remains the same. The flesh of the fish softens, enzymes become inactivated and the process kills many of the bacteria present on the surface of the fish.
Boiling and poaching both involve cooking the fish in hot water whereas frying uses hot oil. The advantage of these techniques is they are very simple and require no more than basic household equipment and are therefore suitable for small-scale production.
Cooked fish products are most usually for immediate consumption and require no sophisticated packaging. The shelf-life can be extended for a few days by using refrigerated storage and the product should be covered to prevent recontamination.
Cooled/frozen fish
The spoilage of fish is directly related to temperature. The higher the temperature, the faster the spoilage up to around 40°C, above which heat will destroy bacteria and enzymes. Any reduction in the temperature prior to processing will maintain the quality of the fish for longer.
Fish can be kept cool by covering it with clean, damp sacking and placing it in the shade. Although this method is simple and requires no special equipment, the fish still begins to deteriorate within a few hours.
An alternative is to pack the fish with ice. This is an effective method and preserves the fish for a longer period of time. Obtaining ice, however, can be difficult for the following reasons:
· Most ice-making machines are power-operated and therefore require some kind of fuel. Obtaining fuel can often be difficult and the machines may prove expensive to operate.· A great deal of ice is required and often the cost of the ice is greater than the actual cost of the fish.
Freezing is an alternative method for cooling fish. This technique provides long-term preservation, but it is relatively expensive in terms of equipment and operating costs. In view of this it is not recommended for the majority of small-scale fisheries.
Cured fish products
Curing involves the techniques of drying, dry salting/brining (soaking in salt solution) or smoking. These may be used alone or in various combinations to produce a range of products with a long shelf-life. For example:
· Drying - Smoking - Drying
· Brining - Smoking - Drying
· Salting - Drying
· Salting - Drying - Smoking
Techniques such as these reduce the water content in the flesh of the fish, and thereby prevent the growth of spoilage microorganisms.
Dried fish
The heat of the sun and movement of air remove moisture which causes the fish to dry. In order to prevent spoilage, the moisture content needs to be reduced to 25 per cent or less. The percentage will depend on the oiliness of the fish and whether it has been salted.
Traditionally, whole small fish or split large fish are spread in the sun on the ground, or on mats, nets, roofs, or on raised racks. Sun-drying does not allow very much control over drying times, and it also exposes the fish to attack by insects or vermin and allows contamination by sand and dirt. Such techniques are totally dependent upon the weather conditions. The ideal is dry weather with low humidity and clear skies.
Alternatives to sun-drying involve the use of solar or artificial dryers. There has been a great deal of research on the development of solar dryers as an improved method of drying fish. This has shown that by achieving increased drying temperatures and reduced humidities, solar dryers can increase drying rates and produce a lower moisture content in the final products, with improvements in fish quality compared with the traditional sun-drying techniques.
Figure 1: Improved solar dryer
Figure 1 illustrates a solar tent dryer. This was first developed in Bangladesh, but there are now numerous variations in different parts of the world. It is probably one of the most simple designs.
Figure 2: Solar tent dryer
Figure 2 shows an improved solar dryer, with a separate collector and drying chamber. The chimney is painted black to absorb more heat. This will heat the air inside the chimney, thereby increasing the air flow through the dryer.
Figure 3: Tray dryer
Figure 3 illustrates an artificially-heated tray dryer. When rain threatens, the trays, which were previously placed in the sun to dry, are assembled on top of each other over a simple heating compartment. A roof and chimney are placed on top, and drying continues by direct heating.
Both solar and artificial dryers try to overcome the difficulties posed by sun-drying during the rainy season. With these dryers it is possible to minimize drying times and to increase the product quality. It should however, be pointed out that it is only advantageous to use such dryers if there is a market for a higher-quality product or if the fish would otherwise be lost.
Salted fish
Most food poisoning bacteria cannot live in salty conditions and a concentration of 6-10 per cent salt in the fish tissue will prevent their activity. The product is preserved by salting and will have a longer shelf-life. However, a group of micro-organisms known as 'halophilic bacteria' are salt-loving and will spoil the salted fish even at a concentration of 6-10 per cent. Further removal of the water by drying is needed to inhibit these bacteria.
During salting or brining two processes take place simultaneously:
· water moves from the fish into the solution outside
· salt moves from the solution outside into the flesh of the fish.
Salting requires minimal equipment, but the method used is important. Salt can be applied in many different ways. Traditional methods involve rubbing salt into the flesh of the fish or making alternate layers of fish and salt (recommended levels of salt usage are 30-40 per cent of the prepared weight of the fish). There is often the problem, however, that the concentration of salt in the flesh is not sufficient to preserve the fish, as it has not been uniformly applied. A better technique is brining. This involves immersing the fish into a pre-prepared solution of salt (36 per cent salt). The advantage is that the salt concentration can be more easily controlled, and salt penetration is more uniform. Brining is usually used in conjunction with drying.
Ultimately the effectiveness of salting for preservation depends upon:
· uniform salt concentration in the fish flesh
· concentration of salt, and time taken for salting
· whether or not salting is combined with other preservation methods such as drying.
Smoked fish
The preservative effect of the smoking process is due to drying and the deposition in the fish flesh of the natural chemicals of wood smoke. Smoke from the burning wood contains a number of compounds which inhibit bacteria. Heat from the fire causes drying, and if the temperature is high enough, the flesh becomes cooked. Both of these factors prevent bacterial growth and enzyme activity which may cause spoilage.
Fish can be smoked in a variety of ways, but as a general principle, the longer it is smoked, the longer its shelf-life will be.
Smoking can be categorized as:
· Cold smoking. In this method, the temperature is not high enough to cook the fish. It is not usually higher than 35°C.· Hot smoking. In this method, the temperature is high enough to cook fish.
Hot smoking is often the preferred method. This is because the process requires less control than cold processing and the shelf-life of the hot-smoked product is longer, because the fish is smoked until dry. Hot smoking does, however, have the disadvantage that it consumes more fuel than the cold-smoking method.
Traditionally, the fish would be placed with smouldering grasses or wood. Alternatively, fish may be laid or hung on bamboo racks in the smoke of a fire (see below).
There are various types of kiln available in different parts of the world, which are used for smoking. Although traditional kilos and ovens have low capital costs, they commonly have an ineffective air-flow system, which results in poor economy of fuelwood and lack of control over temperature and smoke density. Improved smokers include the oil drum smoker and the chorker smoker.
Oil drum smoker
Chorker smoker
As well as improved smokers, there are also improved techniques which involve either pre-salting the fish, so that the moisture content is reduced prior to smoking. Alternatively there are a range of improved kiln and oven designs (for details refer to the equipment catalogue section).
Production of cured fish products
The table below outlines the stages in the production of a range of products:
|
Process/product |
Gut |
Wash |
Treat with salt |
Dry |
Smoke |
Dry |
Pack |
|
Dried fish |
* |
* |
|
* |
|
|
* |
|
Dried and salted fish |
* |
* |
* |
* |
|
|
* |
|
Dry-salted and smoked fish |
* |
* |
* |
* |
* |
|
* |
|
Brined and smoked fish |
* |
* |
* Brine solution |
* |
* |
* |
* |
Equipment required
|
Processing stage |
Equipment |
Section reference |
|
Gut |
Cutting equipment |
17.1, 17.2 |
|
Wash |
|
|
|
Salt |
Weighing and measuring equipment |
64.1 and 64.2 |
|
Brine meter |
64.6 |
|
|
Smoke |
Smoking equipment |
60.0 |
|
Dry |
Solar dryer |
23.1 |
|
Fuel-fired dryer |
23.2 |
|
|
Electric dryer |
23.3 |
|
|
Pack |
Packaging materials |
|
|
Sealing machine |
47.1 |
Packaging of cured fish products
The most important concerns regarding packaging for these products are to prevent moisture pick-up and to prevent recontamination by insects and micro-organisms. Traditional packaging materials include cane baskets, leaves, and jute bags. Alternatives include flexible packaging such as polythene bags, or wooden and cardboard packs. Indeed, the two may be combined, as in a polythene bag enclosed in an outer cardboard pack.
Fermented fish
Fermentation is a process by which beneficial bacteria are encouraged to grow. These bacteria increase the acidity of the fish and therefore prevent the growth of spoilage and food-poisoning bacteria. Additionally, salt is used to prevent the action of spoilage bacteria and allow the fish enzymes and the beneficial acid-producing bacteria to soften (break down) the flesh. Fermentation is therefore the controlled action of the desirable micro-organisms in order to alter the flavour or texture of the fish and extend the shelf-life.
The use of fermentation as a low-cost method of fish preservation is commonly practiced all over the world. There are many different types of fermented products and their nature depends largely on the extent of fermentation which has been allowed to take place. They can be categorized as:
· fish which retains its original texture
· pastes
· liquids/sauces.
As with salting, there is little need for equipment other than pans and containing vessels, and the process may easily be carried out on a small scale.
The table below outlines stages in the production of a typical fermented-fish product.
Fish paste (bagoong)
This is a product from Eastern Asia. It is made from whole or ground fish, fish roe, or shellfish. It is reddish brown in colour, although this will depend on the raw materials used, and is slightly salty with a cheese-like odour.
Equipment required
|
Processing stage |
Equipment |
Section reference |
|
Wash |
Clean water |
|
|
Drain |
|
|
|
Gut |
Cutting equipment |
17.1 and 17.2 |
|
Add salt (approx 5 per cent) |
Weighing and measuring equipment |
64.1 and 64.2 |
|
Leave to ferment (for several months) |
Fermentation bin |
03.1 |
|
Add colouring (optional) |
|
|
|
Pack |
Sealing machines |
47.1 |
Packaging of fermented fish
There are almost as many traditional methods of packaging fermented fish as there are ways of making it - such as earthenware pots, oil cans, drums and glass bottles. In the past, the latter have been used because of their low cost, but nowadays, cheaper plastic containers tend to replace the traditional types. The most important function of packaging for fermented fish products is that the containers should be air-tight, helping to develop and maintain the airless conditions required for good fermentation and storage. As the major advantage of these products is their low cost, the type of packaging is necessarily restricted. Glass bottles are often used for the better-quality products, but earthenware pots and even plastic bags are used.
Although traditional processing represents a low-cost option for many small-scale producers, there may be large losses in terms of wasted fish. Improved technologies are usually techniques that require little in the way of expensive equipment, but at the same time increase the quality and the efficiency of the process. Often all that is needed to improve the process and the quality of the final product is the provision of clean water, education and training facilities, simple equipment, or basic materials.
It is important, as with all food processing ventures, to ensure that there is a market for the processed fish. Unfortunately, in areas where fresh fish is a more desirable commodity, small-scale fish processors, with their less-preferred cured products, may face fierce competition from larger-scale processors who have access to refrigeration and transport facilities.
