|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
Fish body oils
Fish body oils are usually produced during the wet reduction process used for meal manufacture, the liquor from the press being passed either to a series of settling tanks or to a series of centrifuges. The press liquor is an oil/water emulsion containing dissolved proteins and other substances as well as particulate matter; the quantity of organic matter other than oil depends on the condition of the fish when processed, the degree to which the fish is cooked and the manner in which the press is operated. Pressing stale, soft fish, or over-cooking fish, can result in a press liquor containing large quantities of valuable organic matter and, even in a well conducted plant, the quantities may be considerable.
Both the settling tanks and centrifuges are heated to help to break the emulsion and prevent the solidification of the stearin portion of the oils. Five or more heated tanks may be used in series; the press liquor is admitted to the first tank at a point well below the surface, the oil rises to the top and is passed to the bottom of the second tank of water, the process being repeated in succeeding tanks. The oil is finally heated to dry off remaining water. In a centrifuge system the water phase is spun off and the almost clean oil is heated to about 200°F (94°C), mixed with clean water at the same temperature, then passed to the polishing centrifuge which yields a clean bright oil. In modern plants, centrifuging is more usual since the oil produced is finer, cleaner and brighter and has a lower moisture content than the oil from a settling tank system. The oils produced by settling, being poorer in quality, fetch lower prices than centrifuged oils and are less suitable for some commercial purposes.
It is unusual to attempt to produce oil by pressing the meal scrap from a dry reduction process. Oil extracted in this way would, in most cases, be darkened by contact with the metal surfaces of the drier and a hydraulic press would be required for the extraction.
Fish body oils (and liver oils) consist principally of the esters of fatty acids and glycerol (glycerides) together with unsaponifiable matter; they represent the fishes' energy store. As noted previously, the oil content of a fish species may vary appreciably at different seasons and this variation may be related to either a feeding or spawning cycle. Watts, working with the West African shad, known locally as the bonga (Ethmalosa dorsalis), found that the fat content varied during the year from two to seven per cent of the wet weight. The variation in fat content appeared to be related to the abundance of diatoms in the diet; larger fish also had higher fat contents than smaller ones.
The percentage of oil plus water present in any fish species remains almost constant at different seasons, the water content falling as oil is stored and vice versa; this means that the form of the fish body changes much less than that of land animals which store fat as a food reserve. This is of obvious importance in animals which are streamlined for ease of passage through a relatively dense fluid.
While the oils from different species show considerable variation in their fatty acid composition, a common feature is the high percentage of unsaturated fatty acids present. It is this feature which renders fish oils more reactive than those of most land animals and vegetables. It is generally believed that the characteristic odour of fish oils is at least partly due to the presence of highly unsaturated fats, for when such oils are hydrogenated the odour is lost.
The main features which affect the quantity and quality of fish oils obtained during processing are the fish species, the food consumed, the spawning cycle and the water temperature. Fish are generally able to modify oils taken into the body; if large quantities of a particular oil are ingested this mechanism may fail. Carp fed on maize may thus develop a peculiar flavour due to the presence of quantities of maize oil. With the onset of the spawning season, many fish cease to feed and stored oil is used to build up the gonads as well as for the supply of energy. Fish caught in cold water are reported to show a higher degree of unsaturation in the oil than those of the same species caught in warmer waters.
Deterioration in fish oils results from the development of free fatty acids and the development of oxidative rancidity. The former is brought about by lipases present in the oil and in contaminating micro-organisms, the latter by atmospheric oxidation or by lipoxidases present in the fish or contaminating micro-organisms. Flavour reversion in deodorised oil may also take place. Deterioration may be controlled by heating to 176 - 212°F (80 - 100°C) for 15 - 20 minutes which inactivates the enzymes, by the addition of anti-oxidants, by halogenation or by storage under an inert gas, usually nitrogen. Brody reviews the literature and gives an account of deterioration and its control.
Many fish oils are converted into solid compounds when atmospheric oxygen is absorbed, and such drying oils are suitable for use in paints and varnishes. A few oils are classified as semi-drying and these are not suitable for such purposes. Any fish body oil can be converted into a form suitable for human consumption; examples include canning oils, margarine and cooking fat. These oils are also used in animal feeding especially as carriers for the oil soluble vitamins A and D. Other processes in which fish oils are used include the manufacture of linoleum, detergents, rubber, lubricants, printing inks, leather and cosmetics.
Fish liver oils
Fish liver oils were formerly the most important source of vitamins A and D. Vitamin A can now be manufactured synthetically by cheap processes and there has thus been some decline in interest in the production of liver oils in western countries. Vitamin oil production would of course be of interest in the developing countries where any manufacturing process using local materials could reduce the use of foreign exchange but, unfortunately, the two most important fish species used in the past for vitamin oil production (cod and halibut) are cold water fish. Some tropical fish species, however, possess livers rich in vitamins and these represent a little exploited resource of considerable nutritional significance.
The two most obvious possible sources of vitamin oil would be the various tunas and allied species and some of the sharks. Tuna livers are small in relation to the fish body but the liver oils themselves contain relatively large quantities of vitamins A and D. The sharks vary greatly in their liver oil vitamin content: certain species such as the soupfin and hammerhead have high vitamin contents in the liver oils; other species such as the tiger, dusky and leopard sharks have liver oils poor in vitamins. It would, in most situations, be difficult to obtain tuna livers as the fish are gutted and then frozen at sea; in other cases, the fish are landed in small numbers at isolated points on the coast. The second difficulty also applies to the use of shark livers. The utilisation of shark livers presents a further problem in that it would be necessary to sort the livers of the various species in order to avoid the processing of livers having oils of low vitamin content.
Various procedures for the extraction of oil from livers are used depending on the percentage of oil present in the liver and on the vitamin potency of the oil. Livers with high oil content and low vitamin A potency are usually extracted by steaming; the released oil floats and can then be collected by bailing or permitting it to overflow; the cooked mass may be centrifuged. Temperatures of 185 - 190°F (85 - 88°C) are used in Norway when direct steaming is practised; indirect heating may also be employed, the livers being heated to 158 - 167° F (70 - 75°C) and stirred to make them disintegrate more readily. Bailey (See Figure 13) describes a simple small-scale apparatus for the extraction of oil from cod livers; a similar apparatus would appear to be suitable for the extraction of oil from shark livers.
Excessive heating and oxidation must be avoided or the potency of the vitamins may be destroyed. As vitamin A is inactivated by light, the oils must be stored in the dark.
Livers of low oil content cannot be steam-treated satisfactorily since the oil yield would then be too low; suitable methods include alkali and alkali/enzyme digestion processes and solvent extraction. In a typical alkali digestion process, 1 - 2 per cent by weight of sodium hydroxide or 2 - 5 per cent sodium carbonate is added to ground livers and the mass stirred while being heated to 180 - 190°F (77 - 88°C). The liquified mass is centrifuged to extract the oil.
Fish livers spoil very rapidly and oil extraction must take place before spoilage sets in, unless the livers are suitably preserved. Freezing offers the best method of preserving livers as freezing ruptures some of the cells thus releasing the oil. Salting is a cheaper alternative: the livers should be washed and cleansed of blood and slime and then cut into slices 2 - 3 inches in thickness and butt salted using 10 per cent by weight of salt. Salted livers must be stored in airtight containers to prevent oxidation.
Uses of fish skins and scales
A slow setting liquid glue can be made from fish skins and fish heads, which is suitable for furniture making, small repair work, and in book binding, labelling and similar uses. It would almost certainly be impractical to consider the use of fish heads in the developing countries and the use of skins could only be practised in the few countries where skinned fillets are frozen. In the United States, only thick skins from cod and similar species have been used in the past, the skins coming mainly from the cod salting and drying industries. Most tropical fish species have large scales and relatively thin skins and these would be unsuitable for glue manufacture. Fresh skins may be used or the skins may be salted and dried to provide buffer storage.
The skins are washed in cold fresh water, all salt and rubbish being removed. Fresh skins require up to two hours washing, salted skins may need as much as 18 hours. The washed skins are cooked for about eight hours in steam jacketed cookers fitted with perforated plates near the base, a weight of water equal to the skin weight being added. A second run may be made in a similar manner yielding a weaker glue.
The liquid glue can be concentrated in open-heated pans at atmospheric pressure but it is now more usual to use a vacuum evaporator. Concentration should proceed until the liquid contains from 50 - 55 per cent solids. Small amounts of inexpensive volatile essential oils are added to preserve the glue and mask the fishy odour.
An alternative use for fish skins would be to make leather from them. Only shark skins can be used to make an attractive hard wearing leather but suffer from the disadvantage that the shagreen (the shark tooth-like 'scales') must be removed; this cannot be achieved by scraping without damaging the skin and chemical methods must be used. There is no reason why fresh skins should not be processed, given an adequate and regular supply, but it is normal practice to clean, salt and dry the skins. Reader gives details of the skinning and preservation procedures. Skinning sharks is not difficult provided that plenty of space is available and proper preservation is quite simple. Correct skinning does, however, waste quite large quantities of meat. Unless the skin is removed with adhering meat, it is easily cut and thus spoiled; however, a somewhat mutilated carcase results if the main objective is to obtain the skin, and the meat is then less suitable for drying or sale as human food in some other way. The collection of skins for leather production is thus probably only possible where large sharks are not eaten. The carcases could, of course, be reduced to meal but the reduction of shark carcases also offers some problems as the large quantities of cartilage present may cause balling in a drier.
Dried salted skins are freshened in cold water and placed in lime liquor to open the fibre bundles. The liming process may be repeated several times and lime is afterwards removed with ammonium chloride or sulphate and the elastin removed with pancreatic enzymes (bating). Either vegetable or chrome tannage follows, both being preservative processes. Where vegetable tannages are used, an acid milling process follows in which the shagreen is removed. When a chrome tannage is employed, the removal of the shagreen takes place before tanning. Various drying and fat liquoring processes follow, the purposes of the latter being to make the skins more pliable and water resistant; the skins are finally dried and finished.
The skins of some of the smaller cetaceans (dolphins or porpoises) can be used to make very strong and durable leather. In many tropical species, the skin is too thin for this purpose.
Artificial pearl manufacture
Artificial pearls are made by coating glass or alabaster beads with guanine crystals in a lacquer base, or by coating the inside of hollow glass beads with the same material and then filling these with wax. The guanine crystals from which the lacquer is made are obtained from a variety of species of silvery fish, principally the clupeoids, such as herrings and sardines. It is these crystals, which are brilliantly lustrous, which provide such fish with their sky camouflage. Most of the guanine is in the skin but some adheres to the scales which are the source of the guanine used in pearl lacquer manufacture. Pearl essence is used in the manufacture of a number of articles other than artificial pearls, such as plastic trays, door furniture, fishing rods and textile finishes.
The types of fish which yield suitable scales are among the commonest of all fish species and it is unlikely that a developing country would find an export market for pearl essence. Lacquers could, however, be produced for local use.
Scales are collected from the holds of fishing boats, usually by providing these with a false bottom. The scales may be preserved by storing in weak brine but they must not be dried. The guanine crystals are removed by mechanical scrubbing and centrifuging; they are then cleaned and suspended in water or an organic solvent or acid, acetone and amyl acetate being commonly used.
Pharmaceutical and biochemical products
Several of the substances commonly used in medicine which are obtained from mammals could be obtained instead from fish in developing countries where meat animals are not available. The most important of these are bile salts and insulin; since the insulin-containing islets of Langerhans are found attached to the gall bladder which contains the bile salts, the manufacture of the two products could be undertaken conveniently in the same small plant. This, of course, pre-supposes that fish are being marketed in such a way that they pass through a plant where the entrails are removed or that a sufficient number of large fish are gutted at sea to yield a worthwhile number of large gall bladders. Such situations are at present somewhat rare. The raw material must be taken from freshly dead fish and suitably preserved. The best way to preserve the insulin-containing caps would be to freeze in dry ice and hold the material thus frozen until it could be processed; an alternative is to hold the material in 95 per cent alcohol acidified with 0.3 per cent hydrochloric acid but even then the material must be chilled with crushed ice, kept in the dark and processed in less than 24 hours. Brody gives details of the preparation of insulin, and he also gives a method for the collection of bile.
Proteolytic enzymes suitable for use in leather bating, meat tenderising or in preparing fermented, liquified products from meal or fish could be obtained from the pyloric caecae of the larger tropical species of carnivorous fish. However, the collection of such material would be subject to the difficulties already discussed.
In those few developing countries where substantial quantities of egg albumin are used in the food industry, fish albumin, which would have similar physical and chemical properties, could be manufactured from fish scrap, fillet waste or unwanted small fish. Unrefined or technical grades for use in the manufacture of foam rubber, paper, cosmetics, textiles and a number of similar industrial products could also be made.
According to Brody, a technical grade could be manufactured by mincing the raw material, heating it to 160 - 176°F (70 - 85°C) for an hour in an aqueous solution with 0.5 per cent acetic acid in order to produce partial hydrolysis. The partially hydrolysed material should be washed in cold water, then pressed to leave less than 40 per cent of water in the cake. Any oil present is then removed from the cake using ethyl alcohol or trichloroethylene chloride, following which it is dried under vacuum at about 120°F (50°C). The food grade is produced by caustic digestion of the technical grade.
These are also known as air bladders, sounds and fish maws. The Chinese use the dried bladders as a base for soups; the principal international market is for isinglass, which is used to clarify wines and beers. In the UK, the better grades are used for beer fining; continental markets accept lower grades for wine fining.
Possible sources are the polynemids (thread fins), sciaenids (jewfish), Lates spp., catfish and carps. Generally speaking, fish of 25 - 100 lb (10 - 40 kg) are used.
Preparation: The bladders should be removed and all blood and adhering fat scraped off; they are washed and air dried. They may be dried whole or split. They should be stored in dry conditions.
Shipment: Shipments of several hundredweights are packed in wooden boxes, crates etc. Samples of 2 - 3 lb (1 - 1½ kg) will establish a value.
Prices: Top quality £1.50 per pound (£3.30 per kg); low grades £0.20 per pound (£0.44 per kg).
There are still a few minor tropical fisheries in which small fish or otherwise unutilisable species are reserved for manure; usually the unsalted carcases are sun dried for ease of transport. Fillet waste from freezing or drying operations could also be used; the fillet waste from Maldive fish processing is dried beside the smokehouse fire for use in this way. The heads and shells of sun dried shrimps can be ground to make a useful fertiliser as can the carapaces of crawfish. Crawfish waste should never be discarded on the fishing grounds as the breakdown products are through" to drive other crawfish off the grounds.
Drying operations of this kind are certain to attract insects and should therefore be carried out at a distance from any processing for human consumption.
There is a small market for green turtle as soup. In view of the conservation issue, it is very doubtful whether the turtle should at present be exploited at all in most areas. Exploitation for this luxury (high value) trade may be a better alternative than local consumption of eggs.
Tortoise shell from the hawksbill finds a small market; local crafts for the tourist trade possibly offer the best outlet. Items produced must be attractive and of first rate workmanship.