This chapter describes fish salting, drying and fermenting techniques. Fish salting and drying are described with sufficient details to allow processing on the basis of the provided information. On the other hand, fermenting techniques are only briefly described as fermented fish products are mostly consumed in Asia where the techniques are fairly well mastered.
This chapter contains six main sections. The first five sections refer exclusively to salted and/or dried fish, and cover fish preparation (Section I, including gutting and splitting techniques), salting techniques (Section II), drying techniques (Section III), packaging and storage of dried/salted fish (Section IV) and methods of preparation of specific fish products (Section V). The last section deals exclusively with fish fermenting techniques.
It is important that fish for salting and drying be prepared in a way which allows rapid salt penetration and water removal. Very small fish, such as anchovies, sardines and other species less than 10 cm long, are sometimes cured without any preparatory cutting, with only the guts removed whenever necessary. Fish longer than 15 cm are split open so that the surface area is increased and the flesh thickness is reduced. With fish more than about 25 cm long, additional cuts (scores) should be made in the flesh.
Fish must always be prepared in a manner acceptable to the buyer and consumer. For example, some consumers prefer that the head be cut off, while others prefer that it is left on. In some fisheries, the front two-thirds of the backbone of big fish is taken out once the fish have been split open. Removal of the bone increases the surface available for salt penetration and water removal. Another example refers to fish scales: some consumers prefer scaled fish while others prefer fish with their scales on. It is, however, preferable to scale fish for easy salt penetration and drying.
Fish should never be prepared on the ground as it will pick up dirt even if it were prepared on a board or mat. A table or bench at comfortable working height should be used. The table may be made of wood, metal or concrete. A good design of such a table is shown in Figure II.1 & plate II.2. The surface of the table should be smooth so that it is easily cleaned. Drainage should also be provided to allow scrubbing of the surface with a brush. Whatever the material used for the surface of the table, it is preferable to use a separate wooden cutting board in order to avoid damaging a wooden surface or blunting knives on a metal or concrete one.
Knives are the most important tools for fish preparation. A selection of these is shown in Plate II.1. Short knives should be used for small fish, long flexible knives for filleting and stout knives for splitting big fish. Knives must be kept sharp. Blunt knives tear the fish and slow down the work. If a grind-stone is available, it should be used to shape or profile the cutting edge and to remove nicks. An oilstone or water lubricated stone may then be used to sharpen the cutting edge. A steel should be used to remove burrs on the edge. Proper grindstones are expensive and steels are not easily obtained in some countries. In any case, a fish curer should always have a good sharpening stone available.
It is usual to split lean fish from the belly side, a method known as cod splitting, although all large round-bodied fish can be processed in a similar way.
Plate II.1. Cutting knives and sharpening tools
1. Oilstone (in protective box)
3. Or skinning
5. Block fillet knife
6. Kippering (herring splitting) knife
7. Gutting knife
8. Cod splitting knife
9. Large broad-
10. bladed knives
Plate II.2. Protective clothing and filleting table
Figure II.1. Fish filleting table
This method can only be practised if the fish has been gutted before splitting. In some fisheries, however, splitting from the back is the usual practice. This latter method of splitting is known as mackerel splitting. Whatever the method, all cuts should be made with a clean sweep of the knife as ragged cuts spoil the appearance of the fish and salt penetration and drying are likely to be uneven. Cod-style splitting of a large fish is illustrated in the sequence of Plates II.3 to II.10.
To gut the fish, prior to splitting, a single cut should be made from the vent to the throat. The guts should be pulled out in one piece and cropped into a barrel or other suitable container.
After splitting and if the flesh is thicker than about 2 cm, scoring cuts should be made along the length of the fish at intervals of 2-4 cm depending on the flesh thickness. These scores should not be so deep as to cut through the skin.
All black membranes should be removed from the inside of the fish. It is important that no pieces of gut remain. The fish should then be carefully washed.
Back or mackerel splitting is commonly used with smaller and fatty fish. The head is invariably left on. The method is illustrated in Plates II.11 to II.15 in the case of herring. After splitting, the guts, gills and hearts should be removed and, using a small brush, the dark coloured blood next to the backbone cleaned out. The fish should then be washed thoroughly.
There are three main salting methods: kench salting, pickle curing and brining. The first two methods yield fish with a relatively high salt concentration while the third method (brining) is commonly used for products with a low salt concentration. A method used in some fisheries, whereby fish are rubbed with salt and then hung to dry, is not recommended as it does not produce an even cure.
In kench salting, the fish are mixed with dry crystalline salt and piled up, the brine which forms as the salt takes water from the fish being allowed to drain away. This method is especially popular for large lean fish species. Kenching can be carried out in shallow concrete tanks fitted with a drain, or on raised platforms or racks of approximately 1 m2 area and 8-10 cm off the ground. Starting at the centre of the rack, 2 or 3 rows of prepared fish are laid flesh side up over a bed of salt. Salt is then sprinkled or rubbed all over the fish, more being put on the thick parts of the fish than on the thin parts. Whenever scores have been made, these should be filled with salt. A pile of fish is built up by moving outwards from the centre, and sprinkling each layer of fish with salt before covering with the next layer. To ensure good drainage, the centre of the pile should be about 10 cm higher than the outside edges and it should not be higher than about 2 m.
Plate II.3 to II.10.
Cod splitting method
Plate II.3. To hold fish across barrel and cut the throat. To avoid cutting the lug bone out if they are necessary to hang the fish for drying.
Plate II.4. To turn fish over and cut into back of head down to backbone to break off head into barrel.
Plate II.5. With the fish lying on its side, head away and belly to the right hand side, to cut down ventral surface to tail.
Plate II.6. To cut forward alongside backbone and round the outside of the ribs.
Plate II.7. To turn fish round. To cut forward along backbone following the large blood vessel in the bone
Plate II.8. To cut across backbone and then forward under the backbone
Plate II.9. Removal of part of fish backbone
Plate II.10. Split fish
Plates II.11 to II.15.
Herring - splitting methods
Plate II.11. To insert knife into fish close to dorsal fin. To cut forward through head, keeping the knife hard against backbone
Plate II.12. To cut from close to dorsal fin to tail.
Plate II.13. To open out fish with knife, avoiding cutting through the skin.
Plate II.14. To remove guts, gills and heart. To brush out peritoneal (black) lining.
Plate II.15. Clean, washed, split fish.
Care should be taken in making a pile in order to ensure even salting of the fish and a good product quality. Brine should not be allowed to accumulate in some places as this will produce an uneven cure and may discolour the fish. The edges of the kench pile should also be regularly sprinkled with salt to prevent contamination.
In the tropics, fish are usually left in the kench pile for 24 to 48 hours after which it is dried. However, the salt may not have completely penetrated the fish during this time, and penetration may continue during drying. In rainy weather, the fish may be left in the kench pile for longer periods. In this event, the pile should be broken down and a new pile made up so that the top fish from the first pile are placed at the bottom of the new pile. In making the first kench pile, 30-35 parts by weight of salt should be used for each 100 parts of fish.
The advantage of kench salting is that the fluids are drained off leaving the flesh fairly dry. However, it also has a number of disadvantages: oily types of fish become rancid due to exposure to the air, insects and rodents have ready access to the fish, mould and bacterial attack can take place, and salting may not always be even.
In pickle curing, a barrel or tank is used to hold the brine which forms as the salt mixes with the water contained in the fish. From 20 to 35 parts by weight of salt to 100 parts by weight of fish may be used depending on the cure required. Fatty fish, such as mackerel, are commonly pickle-cured.
In this salting method, a layer of dry salt is spread over the bottom of the tank upon which the first layer of fish is laid. There is, however, no need to stack fish higher in the centre as drainage is not required. The layers of salt and fish are stacked up, care being taken to ensure that no fish are overlapped without a salt layer between them since this could cause the fish to stick together. As the pile is built up, the salt layers should become thicker. The top layer of fish must be placed skin side uppermost. A wooden cover should be placed on this top layer so that weights can be used to keep the fish below the surface of the brine which forms.
Pickle curing is recommended in preference to kench salting as it produces a more even salt penetration and provides a better protection of the fish against insects and animals since they are covered with brine.
In brining, or brine salting, the fish are immersed in a solution of salt and water. By varying the strength of the brine and the curing period, it is possible to control the salt concentration in the final product. The method is commonly used in developed countries when a smoked product is to be made and the salt concentration required in the final product must be lower than 3% (e.g. as for hot-smoked mackerel). Brine salting may be used advantageously in developing countries as the process is more uniform and controlable than the dry salting techniques.
A fully saturated brine contains about 360 g of salt to each litre of water (3 lb 10 oz of salt per Imperial Gallon). A sack of salt should be hung in the brine to ensure that the latter remains at full strength. Full strength or saturated brine is called a 100 brine. A 10 brine - which is made up by mixing 1 part of 100° brine with 9 parts of water - is sometimes used to soak fish before salting.
The salt used for curing fish (fishery salt) is a mixture of a number of chemicals. A good fishery salt contains from 95% to 98% of common salt known chemically as sodium chloride. Since fishery salt generally originates from the sea, it contains impurities such as chlorides and sulphates of calcium and magnesium, and sodium sulphate and carbonates. Other types of fishery salt include rock salt (i.e. mined salt) and sun salt or solar salt (i.e. salt obtained through water evaporation from coastal lagoons or ponds).
The type and quality of salt used affect the appearance, flavour and shelf life of cured fish. If pure sodium chloride is used for curing, the product is pale yellow in colour and soft. A small proportion of calcium and magnesium salts is desirable as the latter yield a whiter, firmer cure which is preferred by most people. However, if the proportion of these chemicals is too high, the rate at which the sodium chloride impregnates the fish is slowed down. Furthermore, the salt becomes damp as the chemicals absorb moisture from the air and make the product taste bitter.
The composition of sun or solar salt is determined by various factors outside the control of the processed fish producer. Therefore, if salt from one source proves unsatisfactory another source should be sought or the curer should consider making his own salt.
Solar salt often contains some sand and mud as it is usually scraped up from the bottom of the ponds in which it is made. The cheapest grades contain a large proportion of dirt and these should not be bought for fish curing. Salt should be kept in clean bags or covered bins so that it does not become dirty.
Salt may also contain both moulds and bacteria. The bacteria cause the pink colour sometimes seen in salted fish. These bacteria also make the fish slimy and produce an unpleasant odour. If the salt is kept in storage under dry conditions, for 6 to 12 months, the number of bacteria present will be much reduced. Alternatively, the salt can be baked to kill the bacteria. Both storage and baking will increase the processing costs. These may be avoided if some consumers of traditional products prefer the strong flavours produced in cured fish by mild attacks of pink bacteria.
All processing equipment and surfaces must be thoroughly washed with fresh water to help prevent pinking. Light growths can be brushed off from the fish surface and the product redried but severe attack leads to the destruction of the fish.
Solar salt often contains very large pieces which should be broken up by grinding. An ideal salt for dry salting operations contains some very fine grains which will dissolve quickly and some larger ones which will dissolve more slowly and prevent the fish from sticking together. For making brines, very fine salt is preferred because it dissolves quickly.
During drying, water is removed from the fish by evaporation in two phases. During the first phase, only water on the surface of the fish or very close to the surface evaporates. The rate at which the fish dry depends on the surface area of the fish, the air temperature, the speed of the current of air passing over the fish and the relative humidity or wetness of the air. The drying rate during the first phase may be increased by:
- Increasing the fish surface area by splitting the fish and scoring them.
- Choosing a drying site where the air is dry and to avoid, if possible, marshy areas and places where the air has blown over water.
- Choosing a drying site where the wind is strong.
Once the surface is dry, water will evaporate at the rate at which it rises from inside the flesh to the surface of the fish. This rate slows down as the fish gets drier.
During the second phase, the drying rate is function of:
- The type of fish. For example, the rate at which water rises to the surface is slower for fatty fish.
- The thickness of the flesh.
- The temperature of the fish.
- The water content of the fish, and
- The wetness of the surrouding air.
If moisture is removed from the fish surface sufficiently quickly, the drying rate is independent of the level of humidity contained in the air. It depends only on the rate at which water reaches the surface of the fish. If drying is very fast during the early period, the surface may dry too quickly, thus producing a hard layer which will slow down the rise of the water to the surface. This is known as case hardening. When case hardening occurs, the centre of the fish could spoil even though the fish may look as if they have been well dried.
The drying rate during the second phase may be increased by:
- Reducing the thickness of the flesh by splitting and scoring the fish before drying starts, and
- Raising the temperature of the fish.
Natural or air drying uses the combined action of the sun and wind without the help of equipment. It is important to dry the fish quickly before they spoil, and that all surfaces of the fish be open to the drying action of the wind. Where only a few large fish are to be dried, this may be done by hanging the fish up. Split fish may be hung on hooks, by tying them up with string, or by tying the fish in pairs by the tail and hanging them across a pole or line.
Large quantities of fish should be dried on racks. Suitable materials for drying racks include chicken wire, old fishing nets, and thin rods or poles such as reeds or sections of bamboo. The surface of the racks should be at a height of about 1 m from the ground and should slop if split large fish are to be dried. A flat surface is preferred for drying small intact fish. Designs for fixed drying racks are shown in Figure II.2. These racks can be easily covered with plastic sheets to protect the drying fish from the rain. Where large quantities of very small fish are to be dried, a netting rack may be impractical. Suitable drying surfaces may be made instead, with raised floors of wood, concrete, bamboo strip or, where none of these materials are available, well compacted clay.
In the tropics, the air is relatively dry during the day (unless it rains) and relatively wet during the night. From sunrise until about midday, the air becomes gradually drier and, becoming wet again from midday to nightfall. The drying rate - especially in the case of salted fish - is therefore the highest from about 8 or 9 oclock in the morning to 4 or 5 oclock in the afternoon. Fish which have been set to dry during the day should be collected, and stored overnight to avoid them becoming wet by dew or rain. The fish in storage should be piled in a similar manner as for dry salting although no further salt should be added. Wooden boards, weighted with clean rocks or other suitable material, should be placed on the pile of fish in order to flatten them and give them a better appearance. This use of pressure will also speed up the process by which water moves from the inside of the fish to the outside so that they will dry more rapidly when set out the following morning.
Figure II.2. Fixed drying racks with flat and slanding tops
Artificial drying offers better control than natural drying, resulting in greater product uniformity and quality. The initial investment on equipment and expenditures on energy inputs are, however, high and may not always be justified. In general, artificial drying is advantageous when drying by natural means is extremely difficult as, for example, in Southern Brazil where a combination of very humid winters and extremely hot summers - which heat-damage salted fish - do not favour natural drying.
A number of factors can be controlled when drying fish artificially to ensure optimum drying conditions. These are:
- Temperature - the higher the temperature, the quicker the drying. This, however, has to be balanced against the damage which is caused by over-heating the fish and the extra cost of increasing the temperature in a mechanical drier. In general, the initial drying temperature should be restricted to 25 to 45° C. Tropical fish can withstand a higher processing temperature (35-45° C) during drying with no signs of heat damage as compared to temperate fish which may not withstand temperatures higher than 25-30° C.
- Relative humidity (RH) - The moisture content of the air is important for two reasons: it controls the drying and influences the appearance of the final product. The drier the air, that is the lower the relative humidity, the faster the drying rate. If, however, the air is too dry, the surface of the fish will dry too quickly resulting in case hardening. The relative humidity is dependent on local conditions but, as a guideline during initial drying, a 50-65% RH is suitable for optimal drying. This can be lowered by raising the air temperature during the later drying stages.
- Air speed - A faster flow of air over the fish results in even and rapid drying. This is due to a more uniform temperature distribution and a quicker removal of moisture from the fish. A compromise must be made between the higher cost of faster air circulation with a mechanical drier and the improved drying rate gained with a high air speed. Therefore, an air speed between 60 and 120 m per minute is normally used when drying fish with a mechanical drier.
- Surface area and volume of fish - Large whole fish take longer to dry than small fish due to the greater difficulty of removing water from inside the flesh of the fish. Large fish should, therefore, be split to increase the surface area. The flesh should also be scored if it is thicker than 2 cm.
III.3.1. General requirements of mechanical driers
To allow control of temperature, air speed and humidity for optimum drying, an enclosed environment in the form of a tunnel or long box is required. The tunnel can be constructed from locally available raw materials such as wood, corrugated iron sheets, etc. The prepared fish are placed on wire mesh trays which allows air flow on both sides of the fish for easier removal of moisture. Layers of mesh trays can be placed in racks or trolleys in the tunnel. A number of these racks or trolleys can be arranged in series. A fan at one end of the tunnel drives the air over a heating element (e.g. an electric heater/steam heater/flame) and the heated air is then blown over the fish and evacuated at the other end of the tunnel. The temperature may be controlled by a thermostat (set at the required drying temperature) placed near the drying fish, so that it automatically switches the heating element on or off as the temperature drops too low or rises too high.
A simple mechanical drier has been tested in Cambodia (Legendre, 1961) using partially dried fish. It was constructed with local material and incorporated a fan and a steam heater. The experimental Cambodian drier (see Figure II.3) was designed to hold 2 tonnes of fish partially sun-dried for about 54 hours, including 6 hours of sun drying at an inland depot. The drier temperature was set up at 43° C and air speed at 108 meters/min. Thus, an effective relative humidity of about 36% was attained, even though the RH of the outside air was 65-72%. Drying under these conditions gave a readily acceptable product. The drier had 6 sections with 20 trays each holding an estimated 16 kg of fish per tray. The total capacity of the drier is therefore approximately equal to two tons.
The design of the drier was kept simple to avoid using expensive or complicated modifications. It is, however, possible to further improve the efficiency of the system by recirculating the air. This will require the use of fans, automatically controlled by humidistats set at the required relative humidity, to bring in or take out air. Such an improvement is advantageous as it substantially reduces heating costs and allows for a more precise control of the relative humidity within the tunnel.
Figure II.4 shows a recirculating air tunnel drier tested in Southern Brazil by FAO (Anon, 1958). This drier was constructed locally of wood and consisted of 5 sections loaded separately with wire mesh trays. Its total capacity was of 700 to 1,400 kg. of salted fish. Above the main body of the tunnel, was the return air duct in which was installed a recirculating fan driven by a 2 HP motor. The air recirculation and linked dampers were reversed half way through the drying process to ensure even drying. A simple paper humidistat, set to operate at 55-60% RH, was used to activate a 3/4 HP centrifugal fan mounted on top of the tunnel in order to introduce fresh air. The temperature of 36° C was controlled by a thermostat located between the second and third drying sections. This thermostat activated a motor driven damper which forced incoming air through a steam heated, finned heat exchanger. Optimum drying conditions were found to be 90-100 meters/min. at a RH of 50%.
More recently, an improved tunnel drier was tested in India by Chakraborty (1977). The drier (see Figure II.5) consisted of a long tunnel divided into an upper air recirculation chamber and a lower product chamber. The lower chamber, which was tall enough for a man to walk through, contained 5 trolleys on rails, fitted with several layers of aluminium trays. These trolleys, loaded at one end of the tunnel (trolley inlet door) and off-loaded at the other end (trolley outlet door) moved at counter current to the flow of heated air. Thus, the fish moved first through warm air moistened by contact with the previous batch of fish, and then through progressively drier air as the trolleys approached the tunnel outlet door. In the upper chamber, the air was heated by steam heaters, the temperature being controlled by a thermostat and the relative humidity by humidistats which activated exhaust fans. The blower fan was capable of delivering 275-285 cubic meters/minute.
Figure II.3. Cambodian Tunnel Dryer
Figure II. 4. Brazilian Tunnel Dryer
Figure II.5. One tonne capacity tunnel dryer
During trials, the fish were prepared by washing, splitting, kench salting in tanks for 18-24 hours, draining and drying in the tunnel to 25-30% moisture for 14-16 hours. Small fish, such as sprats, were simply salted in saturated brine and dried to 15% moisture. Larger fish, such as sharks, were cut into fillets and heavily salted and dried. The products were claimed to be superior to the equivalent sun-dried product: the dried fish did not have a bad smell, its shelf life was equal to 9-12 months when stored in plastic bags and its physical appearance was better than that of sun-dried fish.
The economic viability of mechanical drying of fish depends, to a large extent, on whether adverse weather conditions (e.g. rains, extremely high temperatures) make natural drying very difficult (e.g. long drying periods, high spoilage rates) and on the difference in production costs between natural and mechanical drying at a given project site, taking into consideration the high initial capital investment costs and high energy inputs associated with mechanical drying. Given their generally superior appearance and quality, it would be reasonable to expect that higher prices could be charged for mechanically dried fish products. The higher prices could improve the profitability of these fish products. However, this does not necessarily happen in practice since consumers may not like mechanically dried fish as much as, or more than, traditional sun dried fish. They may not also be willing, or able, to afford the higher retail prices.
It is possible to harness the suns energy to produce drying conditions superior to those prevailing under natural drying. A number of simple experimental designs have been tested with varying degrees of success. These designs include structures in the form of tents made with wooden or bamboo frames covered with clear and dark polythene, wooden black boxes, or some other simple designs made from wood or brick and glass.
The principle underlying solar drying is simple. Air inside the drier is heated as it flows over dark surfaces which absorb the sunlight, thus resulting in air temperatures higher than those of ambient air. A convection current or upward flow of air takes place as air flows from the vents located at floor level to those located at the top of the structure. The fish, which are placed on wire racks, are dried by this flow of air which gets progressively warmer as it rises upwards and leaves the structure by the top vents. Depend-in on the design of the solar dryer, temperatures of 70° C and over can be achieved if there is no ventilation (Szabo, 1970). The temperature can be lowered by opening the air vents thus allowing free movement of air.
A tent drier, made from a bamboo frame covered with clear and black polythene (e.g. of the type shown in Fig.II.6 and Plate II.16) was evaluated in Bangladesh (Doe et. al, 1972). This drier attained a maximum temperature of 48° C, which is suitable for drying fish, the ambient air temperature being equal to 27° C. Dried fish were produced within a marginally shorter period than in the case of natural drying and were superior in quality, mainly due to the lack of insect infestation. The temperature within the tent was high enough to kill adult flies which would have otherwise laid eggs on the drying fish. Sun dried fish already infested with fly larvae were disinfected to a considerable extent within three hours when placed inside a solar tent drier at about 45°C, twenty hours at this temperature being sufficient for a complete disinfection. In this case the solar drier was more useful for keeping away insects and for the de-infestation of dried fish, than for reducing the drying time. In general, further development work is required with various designs of solar driers, before the method can be widely recommended for commercial use.
Dried fish are sometimes brittle and easily damaged if not handled correctly. In humid conditions, dried fish also absorb moisture and become susceptible to spoilage by moulds and bacteria. They may also be attacked by insects (especially beetles of the genus Dermestes) rats and mice, as well as domestic animals. Packaging methods such as hessian sacks, wooden boxes and baskets are generally inadequate in protecting dried fish from these causes of damage. Thus, losses of fish, especially if transported over long distances, have been reported to be as high as 50% in some parts of the world (Rollings and Hayward, 1963).
Figure II.6. Small polythene tent dryer
Plate II.16. Small polythene tent dryer
To protect dried fish properly, one may adopt some of the following measures:
- To pack the dried fish in a sturdy container such as a wooden or cardboard box fitted with a lid in order to totally enclose the product. Open boxes, although protecting the fish from physical damage, are not effective against high humidity and insect attack. Properly sealed cartons, made from waxed or plastic coated board, should be sufficiently moisture-proof and rigid enough to withstand rough handling. Although this type of packaging is more expensive than traditional packaging, the additional cost should be more than offset by the decrease in the spoilage rate.
- To pack fish in plastic or polythene bags, thus reducing insect attack and the effects of high humidity. Care should be taken not to leave bags containing dried fish in direct sunlight or in hot places since the increased temperature causes sweating (i.e. the removal of water still present in the dried fish). This water condenses on the inside of the polythene bag and will wet the dried fish and make them susceptible to mould attack. A further disadvantage is that some dried fish have sharp, hard points and edges, which puncture and rip the plastic or polythene bags, thus allowing air moisture, dust, of insects to spoil the fish.
- To treat the fish in order to protect it against insect attack. Such treatments may, for example, include heavy salting which protect the fish against attacks by the larvae of beetles. The application of insecticides to fish during processing has also been shown to protect the fish against insect attack. However, considerable care must be exercised with the use of insecticides as indiscriminate use may be harmful to health. Insecticides can be applied as a solution or in powder form. The disadvantage of the latter are the difficulty in achieving an even application of the powder and the poor appearance of the treated fish. Further experimental work is required before a suitable insecticide treatment of dried fish can be recommended.
De-infestation of stored dried fish can be achieved by fumigation (i.e. by vaporising a toxic liquid in an enclosed environment to kill insects). Since the chemicals used for fumigation are also toxic to humans, extreme care is necessary when fumigating any products. Experienced and trained personnel should carry out the process. Phostoxin and methyl bromide are examples of effective fumigants. Fumigation should be carried out in an enclosed fish store or under gas-proof sheets in order to ensure a complete de-infestation of stored fish. 24 g. of methyl bromide per cubic meter has been found to de-infest dried fish successfully when applied over a 24 hours period. However, phostoxin is considered more suitable for use in fish stores at a dose of 0.2 to 0.5 g. phosphine per 50 kg. for 2 or 3 days (FAO, 1981).
Dried fish can also be de-infested by heat treatment such as re-smoking or the use of a solar drier. Forty minutes at 70° C was reportedly required to de-infest cured fish (Szabo, 1970) although some beetle larvae are killed by exposure to temperatures of 50° C for more than 15 minutes. Smoked fish are kept insect free by storing at raised temperatures over smoking kilns in some areas of Africa, and re-smoking of infested smoked fish is a technique in current commercial practice. Further details of the control of insect infestation in fish during processing and storage are to be found in a review by Proctor (1977).
This section briefly summarises the various steps required to prepare dried and/or salted fish. Three types of fish products are described: dried/salted fish, dried-unsalted fish and dried/salted shark.
Only fresh fish make a good product. Mackerels, tunas, etc. should be bled at sea and, if possible, all fish should be iced at sea. The drying/salting of fish should be carried out according to the following steps:
- To scale and split the fish in the way described in Section I. To remove the head before splitting unless it increases the retail value of the processed fish.
- To remove all but the tail third of the back bone.
- To clean carefully the fish by removing all guts, liver, gills, membranes, etc.
- To score to skin with care in order to avoid cutting through the flesh.
- To wash the fish by soaking in a 10% brine for half an hour.
- To drain the fish.
- To dry salt the fish in a shallow box, using the appropriate amount of salt, with more salt on the thick parts of fish than on the thin parts; to fill all scores and rub the salt in thoroughly. One part of salt to three parts of fish by weight is recommended for appropriate salting.
- Saturated brine with an excess of salt may be used as an alternative to dry salting. In this procedure, even piles are made by placing the fish skin side down in the salting vat. The top layer should be skin side up. If brine does not cover the fish within 3-4 hours, to add saturated brine. To place a weight on the fish so that they are below the surface of the brine and to cover the vat.
- To leave the fish in salt for about 12 hours. Fish size, market preferences, weather and working conditions will affect the length of the salting period.
- To wash the fish in 10% brine or sea water, in order to remove all salt crystals. To drain and set to dry. If drying conditions are good, it is advisable to dry the fish in the shade instead of the open sun.
- To leave the fish on the drying racks during the first night. Thereafter, to remove and pile up the fish under pressure each night until drying is complete. Greater pressure and longer press times may be used towards the end of the drying period.
- To continue alternative drying and pressing until no further weight is lost. To store and bale the fish.
In general, only very small fish should be dried without first salting as larger fish will spoil before the drying process is complete. The method to be used would be similar to that described in V.1 above except that the salting stages are omitted.
Shark meat has to be very carefully handled and processed due to the presence of urea in the flesh. The urea can be converted to ammonia by bacterial action and its unpleasant odour can be detected, even at low concentrations. The recommended processing method is as follows:
- It is important that sharks be bled immediately after capture. Small shark are effectively bled by cutting off the caudal fins whilst large sharks can be bled by cutting of the head and putting a water hose into the main vein, thus forcing and washing out the blood. The shark should then be gutted, the belly cavity washed and scrubbed with clean water and iced immediately (if possible).
- Fillets or steaks of the required size are then cut into 2 cm thick pieces from the shark carcase.
- Fillets can be placed in cooled 10% brine solution for 2-6 hours prior to dry salting. Whether the shark meat requires soaking in brine depends on the freshness of the meat and the species of shark. Very fresh shark generally does not require brining, an exception being made for the hammer-head shark which should always be brined. This soaking stage facilitates the removal of ammonia and helps to achieve white dry shark meat. After brining, the fillets are allowed to drain for 10 minutes.
- Each fillet is individually salted by rubbing fine grain salt into the flesh, and cured by either kenching or pickling. The pickle cure is recommended under tropical conditions for the reasons discussed previously. When adequately salted, the meat is briefly washed in water (not soaked) to remove adhering surface salt.
- The salted meat can be dried by either sun drying on sloping racks (and press piling at night) or dried in a mechanical drier. The meat is dried to about 35% water content and, at this stage, it should not be possible to press a thumb mark into it.
In good natural drying conditions, 2 cm thick fillets or steaks should be dry within 3-4 days. If the relative humidity is greater than 75%, then it would be impossible to dry heavily salted shark meat adequately.
A general plan for a fish curing yard is given in Fig.II.7 and an indication of the yields obtained during the preparation of salted dried mackerel is given in Fig.II.8. This plan applies mostly to the production of salt/dried fish.
The scale of the various parts of the curing yard will depend on the amount of fish to be processed and the length of the processing period. These will in turn depend on the nature of the raw material and its supply as well as the process involved. Field evaluation of these factors must take place so that the correct scale of facilities are provided.
Some relevant factors are indicated below for the production of salted dried fish, but similar considerations also apply to other cured products:
- Is the fish supply seasonal or continuous throughout the year and, hence, what quantities of fish will the curing yard be required to process?
- What salting technique will be used, how long does it take and, hence, what scale of salting facilities are required? As a rough guide, 1 tonne of fish will require up to 2 cubic meter vat for the duration of a pickle curing process.
- How long does it take the fish to dry to the required moisture content under the prevailing climatic conditions and, hence, what area should the drying yard cover. As a rough guide, it takes up to 100 m2 of drying area to spread a tonne of fish (wet weight). This figure will vary depending on the shape and size of fish being processed. 6-8 kg fish/m2 of rack should be needed for artificial drying and smoking operations.
It must be noted that there will be fluctuations in the supply of fish on a day to day basis. Adverse drying conditions will also increase the time required for drying, and thus an increased drying area will be necessary. Initially, it would be advisable to allow-2-3 times the estimated minimum scale of facilities required to cope with an average amount of fish and to select a site where further expansion can easily be achieved. These generalisations should not be used as a substitute for an investigation of the actual requirements at a particular location.
Figure II.7. A model lay-out for the preparation of salted sun-dried fish
Figure II.8. Material balance data in the preparation of sun-dried salted mackerel
Fermentation involves the hydrolysis or breakdown of proteins into their constituents peptides and amino acids. The development of the characteristic odours and flavours of putrefaction is prevented by the addition of salt in varying but usually large amounts, similar to those used for pickled fish. The products are not dried after salting. In some circumstances, carbohydrates may be added which result in the formation of acids which further help to impart a characteristic flavour and odour as well as providing a further degree of preservation.
In general, three types of fermented fish products (Subba Rao, 1961) can be distinguished:
(i) Products in which the fish retain substantially their original form or in which large chunks are preserved (e.g. pedah siam (Thailand), makassar (Indonesia) and buro (Philippines));
(ii) Products in which the fish are reduced to a paste (e.g. ngapi (Burma), pra-hoc (Cambodia), belachan/trassi (Malaysia/Indonesia) and bagoong (Philippines));
(iii) Products in which the fish are reduced to a liquid (e.g. budu (Malaysia), patis (Philippines), nuoc-mam (Viet Nam) and nampla (Thailand)).
Few of these products and processes are found outside South-East Asia and, as with other methods of fish curing, traditional practices developed over many years predominate. These vary considerably from place to place, depending on local taste, raw materials available and care taken during processing. Many traditional products are of excellent quality and often rely on traditional skills which are difficult to emulate with modern processing methods. It is unclear whether the fermented fish products could be introduced successfully into other areas due to problems of consumer acceptability. The fermented products of South-East Asia are many and varied and, for the purposes of this review, it is only possible to cover a few of the more important products and processes.
VII.1.1. Makassar (Indonesia)
The fish species used in makassar are anchovies (Engraulis spp. and Stolephorus spp.). These are headed and placed in earthenware pots with an equal weight of salt. After three to four days, a red coloured rice product called angkhak is mixed with the fish and salt. Angkhak is made up of rice fermented with a mould organism imported from China (Monascus purpureus). Ragi (a Japanese preparation made from yeast and rice flour) is then added with spices. After a few days, the mixture becomes red and it is then packed into glass bottles for distribution. The composition of makassar fish includes, in most cases, 66% moisture, 16% protein, 1% fat and 17% ash.
Buro, which is made in the Philippines, is similar to makassar in that it is a rice-fish product mixed with angkhak. A fish often used in the production of buro is milk fish (Chanos chanos), a commonly cultured brackish water fish of the region. Freshwater species such as dalag (Ophiocephalus spp.) can also be used.
VII.1.2. Colombo cure (India)
This processing method, used in the South of India, utilises mackerel (Rastrelliger spp.), seer (Scomberomorus spp.) and large non-fatty sardines. The fish, which should be very fresh, are first gutted, gilled and washed in sea water. They are then mixed with dry salt in large concrete tanks, using a ratio of 1 part salt to 3 parts fish. Malabar tamarind, which is the dried fruit pulp of tamarind (called garikapuli, (Garcinia cambogia)) is added to the mixture in order to increase the acidity level of the fish preparation. As the fish tend to float on the blood pickle produced, they are weighed down with mats on which stones are placed. The fish can remain in the brine for two to four months prior to packing tightly in wooden barrels topped up with the blood pickle. The mango wood barrels, used for this purpose, are very large and can contain up to 5000 large mackerel weighing up to half a tonne. The storage life is well over a year and the product has a peculiar fruity odour. The flesh is flaky but firm. From a nutritional point of view, this product is very economical since the protein lost in the blood pickle is not wasted. After the fish have been unpacked from the barrels, the remaining pickle is used as a fish sauce.
In this processing method, the fish or shrimp are pounded with salt so that a paste results. The paste is then subject to periods of sun drying prior to packing in sealed containers for maturation. Moisture contents range from 35 to 50% so that almost half the water is lost during processing. Fish pastes represent a considerable portion of the protein intake of many people in South-East Asia (FAO, 1971), especially by the poorest sections of the population. In many fish pastes, carbohydrate-rich materials, such as fermented flour, bran or rice are added.
VII.2.1. Ngapi (Burma)
The raw material used in this product is small anchovy (Anchoviella comersonii) or shrimp (preferably the small planktonic types which give a better natural pink colour to the product). There are a number of methods for making ngapi, depending on the type of product required. In one process, which uses one part of salt to three parts of partially dried fish, the fish or shrimps are first washed in sea water and then dried for two days in the sun. About half of the required salt is then added to the fish and mixed in a bamboo basket. This mixture is pounded for several hours until a paste is formed. The paste is then packed into wooden tubs or boxes, care being taken that all air bubbles are removed. Fermentation takes place over 7 days and the paste is then removed, further pounded for three hours during which time the remaining salt is mixed in. The mixture is then spread out to dry in the sun for 3-5 hours. The product is repacked into tubs and the fermentation continues for about a month. After a third pounding, it can be packed for sale in cellophane or brown paper. Artificial dyes are often added to improve the colour. However, their use is not recommended as some may be toxic. When stored anaerobically in the tubs or earthenware pots, the product is said to keep for about 2 years. The average composition of a shrimp or fish ngapi is 43% moisture, 20% protein, 1% ammonia, 2% fat and 22% salt.
VII.2.2. Bagoong (Philippines)
Bagoong is one of the major preserved fish products of the Philippines where, in many communities, it constitutes a staple food. The product is also exported as far as the USA to the large ethnic Filipino community. A by-product of bagoong is patis, which is the exuded liquor from the fermentation process and is similar to the Vietnamese nuoc-mam.
Bagoong has a pasty consistency, and is reddish in colour with a slightly fishy cheese-like odour. It can be prepared from fish of the genera Stolephorus, Sardinella and Decapterus, and small shrimp. In the process described by Subba Rao (1961), the fish are washed in clean water, placed in a concrete or wooden vat and mixed thoroughly with salt. The ratio of salt to fish is about one third. The mixture of fish and salt is then transferred to earthenware jars, oil drums or cement tanks and either sealed immediately or, preferably, covered with cheese cloth for five days and then sealed. The sealed containers are held in the sun for one week and the product is then transferred to five gallon cans. These cans are, in turn, sealed by soldering of the lids, and the product is allowed to ferment further for between three months and one year.
The storage life of the product is many years, and the typical composition is not less than 40% total solids, 12.5% protein and 20-25% sodium chloride.
Fish sauces are basically water-extracted solutions of fully fermented fish and are used in a similar manner to soya bean sauce. Indeed, the manufacture and final composition of many fish sauces is similar to that of soya sauce, it being basically a mixture of protein breakdown products (i.e. peptides, amino acids, amines, etc.) in combination with high salt concentrations. Fish sauces may be of limited nutritional value (van Veen, 1965) as their high salt content precludes bulk consumption. However, in some regions, consumption is surprisingly high and in Viet Nam the sauce nuoc-mam can provide up to 20% of the daily protein intake. Fish sacues are rich in hydrolysed proteins and minerals (e.g. sodium chloride and calcium salts) and can be an important source of calcium in the diet.
VII.3.1. Nuoc-mam (Viet Nam)
Nuoc-mam is by far the most important fish sauce in South East Asia, many thousands of tonnes being produced each year, principally in the coastal regions of Viet Nam, Thailand and Cambodia. Nuoc-mam of good quality is a fairly stable, clear dark brown or amber liquid with a distinctive odour and flavour. The lower quality nuoc-mam may, however, have an unpleasant odour and a reduced storage life. Quite often, additional ingredients are added in order to darken the liquid and improve the flavour of the product. These include such materials as caramel, roasted rice, molasses and roasted or boiled corn. Due to its widespread distribution and consumption, legislation has been introduced in some countries in order to guarantee set quality standards.
The fish species used in the production of nuoc-mam are usually of the genera Stolephorus, Engraulis, Dorosoma and Decapterus and clupeoids. Nuoc-mam can also be prepared from shrimp. The processing method is similar to that of bagoong except that the fermentation is generally protracted and the product is the exudate rather than the solid fraction. The actual process varies according to scale. In the small scale operations, whole fish are kneaded lightly by hand, mixed with salt in earthenware pots and buried in the ground for a few months. The nuoc-mam is the clear liquid which settles on top and is carefully decanted off. In the large scale operations the fish (whole and unwashed) are piled, with salt spread between layers, in timber vats. 4 parts of salt to 6 parts fish should be used for this purpose. After three days, the blood pickle (nuoc-boi) is allowed to flow out slowly over a 3-day period into another recipient. The fish are then trampled by foot until a flat surface is obtained. The latter is covered with coconut leaves over which are set two semi-circular bamboo trays, the whole system being wedged down tightly. The nuoc-boi is then poured back over the fish until a 10 cm liquid layer is formed on the top of the trays. It is then left to mature for four months to a year depending on the species of fish. After maturation, the pickle which is run off is the top quality nuoc-mam. The trays and leaves are removed, and fresh salt is added to the top layer of the fish residue. Fresh brine is also added to obtain a lower quality of nuoc-mam.
The yield varies from 2 to 6 parts of nuoc-mam from 1 part of fish, the residual mass being used as fertiliser. Nuoc-mam is normally packed in bottles but may also be stored in earthenware pots.
VII.3.2. Other fish sauces
A number of other fish sauces are also produced in South-East Asia in large quantities. Patis (Philippines) is produced from the bagoong process and is similar to nuoc-mam. Nam-pla is made in Thailand, the preferred fish species being Stolephorus spp. Production of the latter is similar to that of nuoc-mam although less salt is used (i.e. 1 part salt to 4 parts fish). The process may take from 6 to 36 months to complete depending on the quality required. In Malaysia, a sauce known as budu is made from small anchovies. Production involves mixing 1 part of salt and 5 parts of fish in eartheware pots together with tamarind and palm sugar. A dark, sweet-smelling sauce results after 6 months of fermentation. The product has a storage life of 2 years or more.
There are almost as many traditional methods of packaging fermented fish as there are ways of making it. A number of these have been mentioned already, for example earthenware pots, oil cans, drums, glass bottles, wooden barrels, etc. In the past, the latter have been used because of their low cost but nowadays plastic containers tend to replace the traditional containers. The most important function of improved packaging for fermented fish products is that the containers should be air-tight, helping to develop and maintain the anaerobic conditions required for good fermentation and storage. All containers should also, of course, be thoroughly cleaned prior to use. As the major advantage of these products is their low cost, the type of packaging is necessarily restricted. Glass bottles are used for the better quality products, and vacuum packed sealed foil/plastic laminated products pouches might be used in the future.