|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
With the rapidly expanding world population and limited size of the world's food resources, it is becoming increasingly important that all fish resources should be fully used. It is estimated that, for instance, 5 000 000 tonnes of shrimp by-catch are wasted each year when they are thrown back into the sea. Similar quantities of presently unmarketable and unpopular fish are wasted in other areas. Considerable quantities also of fish offals and waste are either completely wasted or turned into animal feeds or fertilisers. These products have a less significant effect on human nutrition than they would if used directly for human consumption. The search for new products able to make use of these otherwise wasted fish is a continuing one. The products must be acceptable to a consumer willing to pay the price of the processing involved in their production. Presently, the most active field of research and development is the use of fish minces.
The flesh from species of fish which are unmarketable as whole fish or in conventional fish products can be used as a mince. The removal of flesh and subsequent mincing disguises the original nature of the fish and the consumer may well accept a product made from the fish mince when he would not have accepted the original whole fish.
Production of the mince
It is feasible for the flesh to be removed from the fish using hand tools such as filleting knives and then passed through a conventional meat mincer (either hand-operated or powered by an electric motor). On a small scale, and where relatively large fish are concerned, this may well be the most economical way of producing the mince. However, many of the fish that might be used for production of minces are small and occur in relatively large quantities (e.g., shrimp by-catch). Under these circumstances, it may be more advisable to contemplate the use of a mechanical device for removing the flesh from the fish. There are a number of machines capable of achieving this, generally known as 'meat/bone separators'. These are able to separate the soft parts of the fish (flesh, guts, nervous system etc.) from the harder parts (bones, scales, skin etc.). The yield of flesh from a meat/bone separator is greater than that from manual filleting and filleting machines since it will recover the flesh from the head, belly flaps etc., which may not be included in a normal fillet. Indeed, the carcases of fish after fillets have been removed will yield a significant proportion of minced flesh if passed through a meat/bone separator.
It is feasible for whole ungutted fish to be passed through a separator but the guts will also pass through the machine with the flesh, producing a non-white mince containing gut contents, blood etc. In certain cases, this sort of mince may be acceptable but often the fish need to be gutted and washed before separation.
Uses of the mince
Having produced a mince by one means or another what sort of products can be made? In many fish freezing operations where products such as fish fingers, sticks, cakes, balls etc. are being produced, fish mince stripped from the frames after filleting using a separator is incorporated into the final product. This maximises yields, and the production of these types of fish products is almost entirely confined to the developed countries of the world as convenience-type foods. Apart from these conventional products, other uses of minced fish, particularly from underutilised species, may be as additives to the preparation of processed meat products.
In the context of developing countries, however, a major area of interest is the use of minced fish for the production of low-cost salted and dried products. This process may be particularly suitable for the utilisation of shrimp by-catch and other waste fish. Salted fish is an integral part of the diet in many developing countries. With traditional techniques, the salting operation takes several days to complete and during the early stages there may be insufficient salt to prevent spoilage. The need for a more efficient technique to rapidly salt fish was recognised by Del Valle and co-workers who developed a method in which minced fish was mixed with sufficient salt to denature the proteins; the water thus released was pressed out and the pressed cake was sun dried. The technique was designed for use in developing countries and, as such, required little processing equipment. The process was subsequently adopted by several other workers using a similar technique and more sophisticated equipment, producing a blander product more akin to the taste of the western countries. This was the subject of acceptability trials by the World Food Programme in the tropics. This type of product has considerable potential in the tropics if it can be made from more oily fish, the current products being made from species of low fat content. In our own laboratories and overseas programmes, we are attempting to develop a salted/dried fish product using processing methods which could be applied at the village level in the tropics.
The work is concentrated on two main areas and two types of product. Firstly, the production of salted minced fish cakes similar to those produced by Del Valle but which do not require excessive pressure during production and, secondly, the production of salted fish powders. It is important that salt is evenly distributed through fish minces if they are to be adequately preserved. When a mincer is being used to comminute fish flesh this can be achieved simply by mincing the fish and salt together. This method has been used successfully in the field to produce salted/ dried fish cakes from waste fish resulting from commercial filleting operations. If other methods of comminution are used, mixing of salt into the mince can be achieved by hand but there are obvious advantages in the use of mechanical mincers. Addition of salt to minced fish muscle releases some of the bound water. In order to reduce subsequent drying times, as much of this water as possible should be expelled before subjecting the material to drying either mechanically or in the sun. This can be done by decanting, pressing or draining through cheesecloth. Once produced, the salted and drained fish mince can be dried either in thin layers in trays or in the form of cakes moulded using a hand-operated hamburger press. The drying operation can be undertaken using a mechanical drier, a solar drier or by simple sun drying. At the village level, simple sun drying would probably be the most economical method provided that the climatic conditions are suitable. Many tropical countries suffer periods of high rainfall and, during these periods, sun drying would not be possible. A non-mechanical kiln of the Ivory Coast type has been used during these periods in the tropics to produce a smoked/dried product. It has been shown that the optimum concentration of salt for the production of fish cake from raw fish is approximately 15 per cent. At salt concentrations below 15 per cent, the water-holding capacity of the proteins in the flesh is high and it is difficult to dry these minces. At higher salt concentrations, the water-holding capacity of the mince is destroyed and, therefore, it is easier to dry but, on the other hand, the gelling capacity of the salt and protein in the muscle is destroyed and the mince will not form cakes using a hand press. The main difference between fish of different salt concentrations as far as the consumer is concerned is in the area of the textural changes which occur. At salt concentrations below 15 per cent, a cohesive mass of fish is produced and it is difficult to remove the salt from the finished dried cake. With fish treated with higher concentrations of salt, a spongy open-textured cake is formed from which the salt can be easily removed. In practice, therefore, higher salted cakes have, after leaching of the water prior to final cooking, lower salt concentrations than the salted mince fish cakes that originally had lower salt concentrations (See Table 1).
There are many sorts of sausage that can be produced using fish. Most recipes are based on recipes for meat sausages where the meat portion is replaced by fish.
The ingredients for this sort of sausage include white fish fillet meat, pork fat, rusk, water, salt, coriander, polyphosphates, pepper, and dye if required. The fillets are chopped in a chopper until the flesh is finely mashed. The other ingredients are added and the chopping is continued for 4 - 5 minutes. The mixture is then filled into edible sausage casings and then twisted into sausages of the lengths required. The product resembles a normal frying sausage in appearance and in taste; it is perishable and should be kept chilled or frozen. The sausages must be cooked before being eaten: they can be fried, grilled or cooked in the same way as meat sausages.
This product resembles a polony sausage. Although it is cooked and ready to eat cold, it can if desired be fried. The ingredients are as follows: skinless white fish fillets, pork fat, water, rusk, salt and pepper, powdered cereal filler, and cayenne pepper. The fillets are chopped in a bowl chopper until finely mashed, then the other ingredients are added to the bowl and mixing continues for another 5 minutes. The sausages are then filled into cellulose casings and tied off. The sausages are then heated for 2 hours in water at 80 - 90°C. To prevent bursting, the water temperature must not rise above 90°C. The sausages are then cooled in iced water for half an hour. The product should be kept chilled or frozen but will keep for longer than a year when frozen and stored at - 30°C.
A variation on the above recipe can be made inasmuch as the fish sausage once prepared can be smoked to produce a smoked slicing sausage.
This product is similar to a frankfurter made with meat. It is only partially cooked during the process and must be cooked by the consumer before it is eaten. The ingredients for these sausages are similar to those for slicing sausages but the proportions are different. Following production, the sausages can be smoked for 3½ hours at 60°C in a mechanical kiln to give them a smokey flavour. They are usually then skinned prior to sale. An acceptable fish sausage can be produced using fish which have been smoked, such as kippers. It is more usual, however, to use nonsmoked fish such as herring to produce a normal sausage and then to hot-smoke the sausage to give it a kippery taste.
Kamaboko is a traditional Japanese fishery product which may be likened to a meat loaf or a sausage without casing. It can be made in various sizes and shapes. The first procedure in the manufacture of kamaboko is the manufacture of a product known as surimi which is a paste or a dough. The prime requirements for making surimi is that the minced fish meat must be elastic. In the main, croakers, lizard fish and conga eel have the desired elasticity. White croakers (Nibea argentata) are generally favoured because the kamaboko made from this fish is highly elastic. The other raw materials, apart from fish, include potato or wheat starch, salt, sugar, ajinomoto or monosodium glutamate, chopped vegetables (carrot and burdock) and vegetable oils (if the product is fried).
The preparation of surimi is as follows:
1. The head, scales and viscera are removed.
2. The flesh is cut into single fillets.
3. The bones and skin are removed. This can be done either manually or in a machine.
4. The meat is washed three times in drums of fresh water to remove the fat. Different kinds of fish may be mixed with the main species at this stage.
5. Excess water is removed either in a basket centrifuge or by squeezing through a cloth.
6. The meat is kneaded in a machine for about 15 minutes.
7. Salt is added to the meat and the mixture kneaded for a further 15 minutes; 20 - 40 9 of salt/kg are normally used.
8. Potato or wheat starch is added (100 - 250 g/kg of fish) and the mixture kneaded for a further 15 minutes.
9. Sugar may be added at the rate of 30 - 100 g/kg of meat to improve the flavour; monosodium glutamate is added and the mixture kneaded for a further 20 - 30 minutes until it assumes a doughy consistency. Chopped vegetables may be added if desired during the latter stages.
The resulting surimi is a stiff paste. The yield is around 40 per cent of the whole fish.
To produce kamaboko, the surimi must be shaped into half cylinders or into the shape of bread loaves on wooden blocks. The loaves are cut manually into pieces approximately 200 9 each and subjected to infra-red ray treatment, or they may be roasted on the top to get a brown crust similar to bread. They may then be dipped into dilute hydrogen peroxide to sterilise them. The product is then steamed for 40 minutes at a temperature of about 80°C and then cooled for 2 hours in the air. Packaging is in cellophane or polyethylene which is heat sealed. The products can keep for about one week during warm times of the year and up to two weeks in winter.
There are many variations on the basic kamaboko product, the main ones being in size and shape. Hampen is square-shaped and chikuwa is like a tube that is broiled instead of being steamed. There are also products such as fish noodles which are by-products of the kamaboko plant since the process is very similar except that the surimi is extruded through a vermicelli or spaghetti machine (See the flow sheet).
Fresh or frozen tuna, marlin, swordfish, whale meat, pork, salt, spices, wheat or potato starch, and a natural casing are the main ingredients of tuna ham which is produced in Japan. Tuna ham is really a smoked fish sausage similar to salami but cut into thin slices.
The tuna, whale meat and pork are cooked at 86°C and minced. The minced meats are mixed with the salt, sugar, starch and spices in a mechanical mixer and kneaded well for about half an hour. The mixture is put into large-size natural casings and smoked over smouldering oak chips for about 12 hours. The smoked ham is then cut by machine into thin slices and vacuum packed in polyethylene-cellophane bags. The ham will keep for about 10 - 14 days at room temperature.
In China, low price fish such as sharks and lizard fish are used to produce fish balls. The meat is removed from the skin and bone and ground to a paste with water added. Ingredients such as starch, sugar, flavour essence, salt, water and spices are added and the balls are formed either by hand or by machine. Once formed they are put into cold water for a short time and then cooked for 15 minutes in boiling water.
Fish crisps, shrimp crackers or krupuk in Indonesia are produced in considerable amounts. The product is also available in Thailand where it is made from fish. In Indonesia, the product is known as krupuk udang or shrimp krupuk. The krupuk industry is concentrated in East Java.
Shrimp or fish are used but the product made from white shrimp is more popular and higher priced than the others. The fish material should be absolutely fresh. Tapioca flour, sugar and salt are required. Eggs may also be added. To produce 100 kg of krupuk the following amounts of raw materials are required:
The fish or shrimp are mixed with tapioca flour in various proportions depending on the quality of the product desired. The best quality contains at least as much shrimp as tapioca. Lower grade products may contain much less shrimp and may be as low as one part of shrimp to ten of tapioca. The mixture of shrimp meat and tapioca is either pounded with wooden poles in a stone mortar for 1 - 2 hours or mixed mechanically. Machine mixing is not favoured in Indonesia as the product develops a slightly darker colour. During pounding or mixing of the shrimp meat and tapioca, the other materials such as sugar, salt and eggs are also added and mixing continues after addition of water. When mixing is complete the material forms a paste. This paste or dough is put into metal moulds in the shape of half cylinders; the filled moulds are placed in racks in a boiler containing some water. The moulds are exposed to steam but not immersed in the water. The steam gelatinises the starch and coagulates the proteins of the shrimp meat. Steaming continues for about 3 hours. After steaming the moulds are separated and the blocks of cooked fish/tapioca dough are allowed to cool for 2 - 4 hours. When cooled, they may be sliced in a machine to a thickness of 4 - 7 mm. The slices are then dried in the sun. The product is packed in paper cartons of different sizes depending on consumer requirements.
Krupuk is fried in oil before being eaten. This makes it expand several times and it is used as a side dish in Indonesian, Chinese and Thai cookery.
1. SUBBA RAO, G. N. (1961) Fisheries Products Manual. Food and Agriculture Organization of the United Nations, Rome (FAO). Indo-Pacific Fisheries Council.
2. TORRY RESEARCH STATION (1970) Torry Advisory Note No. 43, Torry Research Station, Aberdeen.