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This chapter describes two fish processing methods widely used in both developing and developed countries, namely smoking and boiling. The techniques which are described in the following sections are particularly suitable for small-scale processing.
It must be emphasised that smoking alone will not prevent the post-harvest putrefaction in fish. In order to accomplish an effective cure, the water level of the product must also be reduced either by salting or drying, or a combination of the two. Smoked fish is popular in Europe and Africa. Lean fish such as haddock and tilapia, and fatty fish such as mackerel and sardines, are all used to make smoked products. Any fish can be smoked and variations which exist are a result of availability and consumer preference. As for all processed products, the quality of the raw material is important and smoking should never be used as a method for disguising the flavour of stale fish.
Three types of smoking process can be distinguished: cold smoking, hot smoking and smoke drying. In cold smoking, the fish are not cooked and the end product is similar in keeping qualities to fresh fish. Hot smoked fish are cooked, a process which prevents spoilage for only a day or two if the product is not dried. In many traditional processing techniques, there is little distinction between hot smoking and smoke drying which can lead to cooked dried products which tend to break up on handling. Various antimicrobial agents are found in smoke which may have an additional preserving effect on the products, but these are of relatively minor importance when the longer term storage properties of smoked fish are considered.
The actual process for smoking fish will depend to a large extent on the type of kiln used and product required. If, for example, the primary objective of smoking is to produce a product such as banda in West Africa, which has good storage characteristics at ambient temperatures, the drying of the fish is a fundamental aspect of the smoking operation. Effective drying depends on the temperature, velocity and relative humidity of the surrounding air as described in Chapter II.
A number of pre-smoking processes will generally be required prior to the actual smoking of fish, these processes being function of the type of fish used and the type of product required. These processes may include washing, gutting and splitting of fish, salting and drying.
I.1.1. Washing, gutting and splitting
Depending on local customs, the fish may be prepared simply by washing followed by smoking (e.g. herring, bonga). Alternatively, scaling, gutting, splitting and filleting may be carried out prior to smoking. In general, it is the smaller fish which are used whole. This has become a traditional method because of the difficulty in gutting large numbers of small fish. Medium-sized fish, such as tilapia and catfish, are normally split and gutted whereas the larger species, such as sharks and rays, are cut up into chunks or fillets which are scored to increase the surface area for salt and/or smoke contact.
All preparatory work involving washing, heading, gutting and splitting should be carried out in accordance with the recommendations made in Chapter II.
I.1.2. Salting
In Europe, fish are normally salted (usually lightly) before smoking. In traditional African smoking procedures, the fish are not usually salted at all. It is recommended to dip the fish intended for smoking in 70-80% saturated brine for about 15 minutes. This results in a salt uptake of only 2-3%, and produces a good gloss on the surface. Brines should be changed daily and checked regularly with salinometers to ensure that the required degree of saturation is maintained. Such a light salting and smoking necessitates cold and chilled storage of the product, and merely serves to enhance its flavour as well as producing a pleasant glossy surface.
For longer storage life, the use of more salt is required and levels of 8-10% salt or more in the finished product are not uncommon. These levels can be reached either by dry salting or by soaking the fish in saturated brine (see Chapter II).
I.1.3. Pre-drying
Prior to smoking, the fish should be pre-dried preferably in the shade. Pre-drying tends to enhance the surface gloss on the product, and helps prevent case hardening which tends to develop during the smoking process. Drying is best carried out on the frames or hooks on which the fish will eventually be smoked. If frames are used, the fish should be spread out in one single layer and spaced out to allow passage of heat and smoke. However, it is advantageous to hang the fish on hooks since this avoids leaving frame contact marks on the flesh where the smoke is not able to penetrate. Frames also allow a more efficient flow of smoke and hot air especially to the products furthest away from the heat/smoke source.
Fish which are to be cold smoked are hung after preparation (which usually includes splitting) at a set distance from the smoke source. They should be maintained at a temperature below 29° C which may be raised to 35° C for the last half hour of smoking for some fish products. Temperature constraints tend to limit cold smoking, in its strictest sense, to cool climates. The storage life of cold smoked fish depends on the length of time the fish are smoked, the loss of moisture and whether salt has been used. Fish smoked for just a few hours will not keep much longer than fresh fish. Cold smoking tends to involve more risks than hot smoking and spoilage can occur during processing. Cold smoking requires rigorous quality control, strict hygienic practices, and the product must be kept in chilled or frozen storage. As a means of preservation, it cannot be generally recommended for use in developing countries which lack the required storage and distribution facilities.
The temperature used for hot smoking can vary from 65° C to temperatures as high as 120° C. The fish are partially or wholly cooked within a short time (2-4 hours). In the initial stages of hot smoking, it is important that the fish are not subjected to excessively high temperatures as this will result in the flesh being cooked and breaking up prior to the formation of a surface skin which will hold it together. If hot smoking is continued over an extended period, drying will take place, thus resulting in smoke dried products with an extended storage life. Smoke drying temperatures should be between those used for cold and hot smoking. A range between 45 and 70° C can be suggested although temperatures well above this are often used.
Most types of wood and sawdust, hard or soft, can be used for smoking fish although resinous types such as pine or teak, which produce unpalatable flavours, should be avoided. Red mangrove wood, which is abundant in many tropical countries, burns well even immediately after cutting. Papyrus, palm nut and coconut husks, which burn with a hot flame, are possible alternatives to wood. Local practice and the availability of wood play a major part in determining the type of fuel to be used.
Most traditional kilns used for smoke drying are very simple in design and construction. They range from the simplest type, which is an open fire above which the fish are laid on a grill, to a mud or rush hut in which the fish are placed on racks above a fire. The main disadvantages from which most traditional types of smoke drying kilns suffer are a lack of control over the fire temperature and smoke production, inefficient use of fuel and a low throughput of material.
The following sections describe a number of traditional kilns which have been improved in various ways.
I.5.1. Oil drum kilns
One type of traditional kiln consists of a round or rectangular mud wall about 1 m in height and 1.5 m in diameter or side. The fish are laid on wooden racks placed over the top of the walls, the fire being lit and tended within the kiln through an access hole in one side. An adaptation of this kiln is the simple oil drum kiln, a number of designs of which are found in the tropics. These kilns are both light and portable and are well suited to cases where fishermen migrate along the coast as, for example, in West Africa. The simplest drum kilns, however, are not much more appropriate than the traditional mud type (i.e. a fire is built at the base of the drum, and the fish are laid on racks placed over the open top). The capacity of the drum is often very low, and modifications to allow more fish to be smoked at one time have been suggested.
In one improved kiln, an oil drum with the top removed and the base perforated is placed over a stone or brick-built hearth in which a fire is built. The fish are placed on a series of galvanised wire trays hung within the drum and a loose fitting lid of corrugated iron is fitted to the top. The advantages of this method over the simple drum kiln are an increased capacity, more control over the fire and hence less chance of charring the fish, and an easier maintenance of high temperatures within the kiln. A further modification is obtained by cutting oil drums into three sections and fitting them with handles and rods on which the fish can be hung. An example of a kiln of this type made from two oil drums is shown in Fig. III.1. In the lowest section (i.e. the fire box), the oil drum lid is retained and perforated to allow a well distributed upward passage of smoke, and to prevent charring of the fish being smoked in the lowest sections which can be stacked up to 6 m high. The advantages of this improved kiln include the increased fish smoking capacity and easy portability and flexibility as it is possible to interchange sections to allow even smoking.
Figure III.1. Oven constructed
from two drums (Additional drums may be used to build a 6 m high smoking
kiln)
Figure III.2. Drum smoker
Another type of modified oil drum kiln is the Watanabe type shown in Fig. III.2. It consists of a smoke box which is connected to a separate fire box via a stove pipe of variable length. This facility allows fish to be smoked at different temperatures unlike the other oil drum kilns described above which are used solely for hot smoking.
I.5.2. Altona type kiln
The simplest version of this kiln is shown in Fig. III.3. It consists of a wooden smoke unit placed above a fire box. The fire box is built from clay or sun dried clay blocks. Typically, the inside dimensions of the fire box are 100 × 110 × 90 cm high. Three of the walls are 30 cm thick whereas the fourth front wall is 20 cm thick. This front wall has a 45 × 45 cm hole near the bottom for inserting the fuel and controlling the fire.
The smoke box is constructed from a wooden batten framework covered with corrugated iron sheets. Typical inside dimensions of the framework of such a unit are 120 × 120 cm with a height of 102 cm in the front and 90 cm at the rear. It is fitted with 7 pairs of parallel supports on which the trays carrying the fish slide. The front of the smoke unit can be closed by two corrugated iron doors, hinged onto the framework, in such a manner that small openings are left in the front and the back between the frame and the top cover. These openings allow a relatively small quantity of air and smoke to pass, thus keeping a high temperature and pressure inside the oven which results in a more uniform distribution of heat and smoke throughout the smoke box. A detailed specification of material needed to construct this kiln is given by FAO (1971). Similar types of kiln have been described by Beatty (1964) and Rogers (1970).
I.5.3. Ivory Coast type kiln
A further development of traditional smoking techniques is the adoption of the West African banda kiln known as the “Ivory Coast kiln”. This kiln is fairly easy to construct from limited amounts of expensive materials and, because the fire is contained within an oil drum fire box, the rest of the kiln can be constructed of wood without the risk of fire. In comparison to the Altona type kiln, the Ivory Coast kiln has a number of advantages in that its operation is similar to that of the traditional type oven, the fuel costs are low, the construction is very simple and more uniform smoking is achieved. The Altona type oven on the other hand is more suitable for large quantities of fish and has a superior racking system for holding the fish.
Figure III.3(a). Simple version of
Altona-type oven with fire box built from clay: Overall view
Figure III.3(b). Simple version of
Altona-type oven with fire box built from clay: Ground plan of fire box
Notes: 1. If the fire-box is constructed from sun-dried clay, blocks; the inside dimensions should be 120 × 110 cm, and the walls 90 cm high and 20 cm thick.2. The materials needed to construct a simple Altona-type oven include (for fire-box built from dried clay blocks):
|
Item |
Length |
Pieces |
Item |
Length |
Pieces |
|
Wooden battens |
130 cm |
4 |
Clay blocks |
| |
|
5 × 5 cm |
122.5 cm |
4 |
(dried) |
| |
| |
112.5 cm |
2 |
20 × 10 × 10 cm | |
300 |
| |
100 cm |
2 | | | |
|
4 × 4 cm |
122.5 cm |
14 |
Hinges |
| |
|
4 × 2.5 cm |
112.5 cm |
4 |
10 cm | |
4 |
|
Strip iron |
|
2 |
Nails |
|
1,5 kg |
|
750 × 50 × 60 mm | | |
7.5 cm | |
|
|
Corrugated iron sheets | | |
2.5 cm | |
0.5 kg |
|
60 × 150 cm |
|
10 | | |
|
The Ivory Coast kiln, as shown in Fig. III.4, consists of a walled enclosure which can be constructed from a variety of materials such as locally available stone cemented together with mud, or poles and mud, or wooden posts and corrugated metal sheets. The structure must be air tight and the top rim must be smooth and flat. A fire box made from an oil drum is set into one side of the enclosure. A square baffle plate, made from a steel sheet perforated with 1 cm holes, is suspended above a hole cut in the oil drum by wires attached to each of the four corner posts of the enclosure. This ensures a good distribution of smoke and heat. The fish are laid on wooden framed chicken wire racks which rest on top of the walls of the enclosure, one on top of another. Four or five racks of fish can be smoked at once. During smoking, the upper rack is covered with a sheet metal roof for protection against rain.
I.5.4. Mechanical smoking kilns
The traditional and improved smoking kilns mentioned above rely for their operation on heat convection for air and smoke circulation. As with mechanical drying methods, forced convection is also possible for smoking operations. In most designs, it is possible to regulate the relative humidity, temperature, smoke density and air velocity so as to guarantee product uniformity, a prerequisite for the sophisticated smoked fish market. Mechanical smoking kilns are used extensively in Europe and North America where the production of products such as kippered herring and buckling requires a high degree of control. The main advantage of mechanical smoking is the uniform quality product. However, the equipment is generally very expensive and the extra expense may not be worth considering for most Third World situations.
The smoking procedures given below apply to specific fish or fish types. As a general rule, these methods can also be applied to fish of similar shape, size, fat content, etc. Cold smoked products are not described in detail here as they are generally unsuitable for developing countries (for detailed methods, see FAO, 1970).
Figure III.4. Modified version of traditional oven - Ivory Coast kiln
(a) Slide view
(b) Top view
(c) View showing smoke
baffle. Fish holding racks rest on top of this structure when operating
Notes on construction details for the Ivory Coast kiln
1. The walled enclosure is approximately 1 m high and 1 m2 or 2 m2 in plan section (Figure III.4 shows a 2 m2 section).2. Into one side of the enclosure is set the fire-box. This is most easily made from one or two old oil drums. For small kilns of about 1 m2 two 20 gallon drums are sufficient. For larger kilns, 44 gallon drums will be needed. The drums are set on their sides so that an open end protrudes a few centimetres outside the square at ground level, the rest of the drum being inside the enclosure. The end of the drum inside the square remains closed, but a hole about 0.3 m2 is cut in the uppermost surface of the drum. The square cut in the upper surface of the drum must be located centrally with the outer enclosure.
3. A square baffle sheet is suspended, above the hole in the oil drum, by wires from each of the four corners of the enclosure. This baffle consists of a sheet of iron, about twice the dimension of the hole in the drum, which has holes of about 1 cm diameter punched approximately 4 cm apart. The height of the baffle above the opening must be adjusted in use until a good distribution of smoke and heat is obtained.
4. The fish themselves are supported on racks above the enclosure. The racks are made from 7.5 cm square timber and chicken wire or expanded metal. The size of the racks is exactly the same as the square dimensions of the enclosure. The racks will rest on top of the walls of the enclosure and on top of one another. Four or five racks of fish can be smoked at once.
5. During smoking, the upper rack should be covered with sheet metal or damp sacking; metal sheet is preferable for protection against rain.
The procedures for short term preservation do not involve a long drying stage. It is important that the processor knows whether a short or long storage period is required so that the initial smoking technique can be varied accordingly. Fish smoked at very high temperatures and dried will be very brittle in texture. In commercial practice, accurate temperature measurement and control is not generally easy. The small scale processor should try to adjust smoking temperatures so that well cooked moist products and non-brittle dried products are made.
I.6.1. Hot smoking - smoked shark products (Short term preservation)
A method for smoke drying shark is given by FAO (1978). The various steps are briefly summarised below:
- The shark should be gutted and bled immediately after capture.- After removal of the fins, the skin should be removed by stripping off from head to tail (flaying). The skin can be salt cured and subsequently sold for making leather.
- The flesh should be cut into 10 cm square pieces which are then brined for 30 minutes in a solution containing 10% salt and 2% vinegar. The purpose of the vinegar is to suppress the development of ammoniacal odours from the high residual urea content of the flesh.
- The shark muscle should be pre-dried for 1-3 hours to form a surface skin.
- Smoking should be initially carried out at a temperature of 70-80° C for one hour.
- The temperature should be lowered to 60-70° C and smoking then continued for another 1.5 hours when the product is immediately cooked. This product is very liable to spoil at ambient conditions and should be kept frozen or chilled.
For improved storage characteristics at ambient temperatures, the product should be further dried either in a smoking kiln or in the sun.
I.6.2. Hot smoking - buckling (Short term preservation)
Buckling is a hot smoked product made from herring although other similar fish such as mackerel could be substituted. Medium-sized fresh fat herring are processed as follows:
- To rinse the fish in fresh water to remove blood, slime, scales, etc.- To place the fish in 70-80% saturated brine for 2-3 hours.
- To rinse the fish in fresh water and string them up on sticks which are placed through the eyes or gills and mouth.
- To pre-dry the fish on racks in the sun for 2-4 hours.
- To build up a fire with little smoke and a bright clear flame. To place the fish into the kiln and allow the temperature to rise up to 70-80° C. This takes approximately 1-1½ hours.
- To cover the fire with wood chips to produce heavy smoke and maintain the temperature at 60-70° C for about 1½ hours. The racks should be changed around from time to time.
A weight loss of 20-25% is normal and the product can be kept for 1-2 days at ambient temperatures.
I.6.3. Smoking in an oil drum kiln (After Watanabe and Cabrita, 1971) (Short term preservation)
- The fish are first washed, scaled, split dorsally and the guts removed. They are washed again, and then placed in a saturated brine for between 30 minutes and 2 hours depending on the weight.- After a further washing, the fish are sun dried on a rack, skin side downwards, for 2 days or until a 25% weight loss has been achieved.
- The fish are hot smoked on racks in the smoke drum by using a short connecting pipe (1 m from the fire box) and a temperature of 90-100° C. The fish are smoked for 3-4 hours or until they are brownish-yellow in colour.
- Following smoking, sun-drying is carried out for up to 2 days in order to reduce the fish to 35-40% of their dressed weight.
Products made with this smoking method can have a storage life of 2 months or more.
I.6.4. Hot smoke drying of bonga (Ethmalosa spp.) in the Altona kiln (after Clucas, 1976) (Long term preservation)
- The fish should either be well iced or brought ashore and processed within 6 hours.- The use of fish boxes is recommended. Keeping the fish out of direct sun light by the use of, for example, wet canvas sheets is of great importance in terms of maintaining freshness. Where ice is available, this should be used.
- The fish should be washed in clean running water, or 10% brine to remove slime, blood or any foreign matter. Bonga are not normally headed or gutted for traditional and aesthetic reasons.
- The fish should be hung on wire rods pushed through the cheek bone, leaving small gaps in between the fish to allow the passage of smoke.
- A suitable fire wood, for example, red mangrove, should be cut into 10 cm pieces and arranged in the fire box close to the opening. The fire can be regulated by covering the hole in the fire box with a sheet of corrugated iron. The smoke rising up into the smoke box eventually fans out through small holes in the top of the box. At the start of the process, a low smouldering fire is required.
- After loading the fish into the kiln, the fire should be lit and the temperature for the first 2-4 hours, depending on fish size, maintained at 80° C. For the next 2 hours, it should be raised to 90-110° C to cook the fish. It is then reduced to 80° C again until a sufficiently dry product has been obtained (8-18 hours). Product yield should be in the range or 23-30% with a corresponding moisture content of 8-12%.
The packaging and storage principles applied for salt/dried fish also apply to smoked fish (see Chapter II). However, in the case of smoked fish, re-drying can be carried out if the product absorbs moisture during storage. This can be carried out either by lighting small fires under the storage unit shelves or by re-use of the kiln.
The boiling of fish in water or brine is a method of short term preservation used in many countries, mostly in South-East Asia. The shelf life of the products varies from one to two days to several months, depending on the processing method.
Boiling fish in water at normal pressure denatures (cooks) the proteins and kills many bacteria present in the fish. The normal spoilage that occurs in the fish is thus stopped or drastically reduced. However, given the traditional packaging methods employed with cooked fish, the latter are often re-contaminated with bacteria and the product gets quickly spoiled. Boiling fish in water does not produce a completely sterile product as in the case of canned fish and, even if boiled fish were packed in a completely sealed container, spoilage would still occur. Boiling in water is a form of pasteurisation similar in many respects to that carried out in hot smoking operations.
Many variations of the basic processing method are used depending on raw material costs and availability, the required shelf life and consumer preferences. Often, salt is added before, during or after processing. High levels of salt in the final product help extend the shelf life. In hot humind countries, where the drying of fish may be difficult, boiling - which requires low cost facilities and equipment - may provide a method of getting the catch to market in an unspoiled condition. Products where the fish are boiled for a relatively short time with little salt added should be handled in the same way as fresh fish. Where the fish are cooked for several hours with sufficient salt, the product will be relatively stable and can be handled in much the same way as cured fish products.
The examples outline below are processes used in South-East Asia.
II.1.1. Processes used in Cambodia
The fish processed by boiling are usually the Eleutheronema, Stromateus, Polynemus or Sardinella spp. They are placed in small baskets and immersed in boiling brine (5 kg salt per 20 litres of sea water) for three minutes. The same baskets are used for distribution. Prior to consumption the fish are re-boiled in water containing salt (30 kg salt/100 litre water) until cooked. The storage life of such products is one to three days.
II.1.2. Processes used in Malaysia
Fresh Rastrelliger spp., of 14 to 22 cm in length, are washed in sea water and immersed in saturated brine for three to four hours in wooden tubs. The fish are then arranged in bamboo baskets and immersed in boiling brine in a steel trough containing 25% to 34% w/w salt until the fish are cooked. The baskets are removed and allowed to cool for 24 hours. The fish are then either stored in a cold room or distributed immediately. In a cold room (at 0-5° C), the shelf life would be from three to four months whereas, at ambient temperatures, only a short shelf life is to be expected (i.e. one to three days). The yield of product is approximately 70% of the weight of the fresh fish.
II.1.3. Processes used in Indonesia
In Indonesia, various boiled fish products are produced. They are generally known as pindang. Many species of fish, including sharks and Rastrelliger, Decapterus, Euthynnus and Caranx spp., can be used as the raw material for this product.
The fish are first gutted and cut to fit into earthenware pots or tin plate cans. Small fish need not be gutted. The fish are then washed and arranged in the containers in alternating layers of fish and salt. The ratio of fish and salt varies between 20:1 and 3:1 depending on the shelf life and taste required. The more salt used, the longer the shelf life. Freshwater 0.5 to 3 or more litres depending on the size of the container is added and the container is then heated above a fire until the fish is cooked. Most, but not all, of the liquid is drained through a hole in the bottom of the container. More salt is then added to the surface fish and cooking continues until no free water remains in the bottom of the container. The latter is finally sealed with leaves or paper and the product is then ready for distribution. The yield is 80-90% of the weight of the fresh fish and the shelf life varies between a few days and a few months, depending on the quantity of salt used and the effectiveness of the sealing material.
Some concern has been expressed in some countries over the health hazards associated with the consumption of boiled fish, some cases of sickness having been allegedly caused by eating products like pindang. Traditional clay pots have been used for cooking and distribution, but recently these have been replaced, to some extent, by pots made from galvanised metal. These may present problems in terms of heavy metal poisoning especially where lead soldered seams are used. High salt concentrations will erode zinc plate, possibly causing high zinc levels within the product.
In order to avoid the contamination of boiled fish by various bacteria or harmful elements, one should take into consideration the following:
- Good post harvest handling of fish and adequate hygiene in and around all processing areas constitute essential preventive measures. Particular attention should be paid to containers which should be thoroughly cleaned with boiling water, chlorinated water or strong brine.- The correct amount of salt should be used in relation to the storage life required and consumer taste.
- Adequate boiling is required to ensure complete protein coagulation and destruction of all enzymes and most bacteria.
- Prolonged heating should, however, be avoided since the nutritional value of the product may be impaired.
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