What is fish meal?
In the UK the term fish meal means a product obtained by drying and grinding or otherwise treating fish or fish waste to which no other matter has been added. The term white fish meal is reserved for a product containing not more than 6 per cent oil and not more than 4 per cent salt, obtained from white fish or white fish waste such as filleting offal.
These are semilegal definitions, and for convenience fish meal can be defined as a solid product obtained by removing most of the water and some or all of the oil from fish or fish waste. Fish meal is generally sold as a powder, and is used mostly in compound foods for poultry, pigs and farmed fish; it is far too valuable to be used as a fertilizer.
What raw material is used?
Virtually any fish or shellfish in the sea can be used to make fish meal, although there may be a few rare unexploited species which would produce a poisonous meal. The nutritional value of proteins from vertebrate fish differs little from one species to another; whole shellfish would however give a nutritionally poorer meal because of the low protein content of the shell. Most of the world’s fish meal is made from whole fish; the pelagic species are used most for this purpose. Where a fishery catches solely for the fish meal industry, it is known as an industrial fishery.
Countries with major industrial fisheries are Peru,Norway and South Africa. Some countries like the UK make fish meal from unsold fish and from offal, that is the heads, skeletons and trimmings left over when the edible portions are cut off. Other countries like Denmark and Iceland use both industrial fish and processing waste. Fish meal made mainly from filleting offal usually has a slightly lower protein content and a higher mineral content than meal made from whole fish, but a high proportion of small whole fish in the raw material can have the same effect.
The following points are important when selecting species for an industrial fishery:
1. The species must be in large concentrations to give a high catching rate; this is essential because the value of industrial fish is less than that of fish for direct human consumption.
2. The fishery should preferably be based on more than one species in order to reduce the effect of fluctuations in supply of any one species.
3. The total abundance of long lived species varies less from year to year, and
4. Species with a high fat content are more profitable, because the fat in fish is held at the expense of water and not at the expense of protein.
Preservation of the raw material
All fisheries experience periods of glut and scarcity, leaving the fish meal factory at times with no raw material to process and at other times with too much. Large amounts of unprocessed material cause storage and odour problems; moreover spoiled material becomes difficult to process and gives a lower yield.
No cheap, completely safe method of preservation has yet been found. Refrigeration is not usually economic, and the known chemical methods of preservation have some disadvantages. Sodium nitrate with formaldehyde is very effective, but unless its addition is very carefully controlled poisonous nitrosamines can be formed when the nitrite reacts with small amounts of trimethylamine in the fish; for this reason nitrite is not used in the UK. Formaldehyde alone is quite effective in keeping the fish firm enough for processing; it is most useful for species like sand eels that rapidly become semiliquid soon after catching. Although the addition of about 0-2 per cent by weight of formaldehyde is often enough to provide the required toughening effect, the preservative effect is small at this dilution, and more formaldehyde may make the fish too tough to process.
Manufacturing fish meal
There are several ways of making fish meal from raw fish; the simplest is to let the fish dry in the sun. This method is still used, in some parts of the world where processing plants are not available, but the product is poor in comparison with ones made by modern methods. Almost all fish meal is made by cooking, pressing, drying and grinding the fish in machinery designed for the purpose. Although the process is simple in principle, considerable skill and experience are necessary to obtain a high yield of high quality product, and to make the plant efficient. A typical process is shown diagrammatically in figure 1:
Fig. 1. A typical process diagram.
When fish are cooked and the protein is coagulated, much of the water and oil runs off, or can be removed by pressing, whereas raw fish lose very little liquor even under very high mechanical pressure. A commercial cooker consists essentially of a long steam jacketed cylinder through which the fish are moved by a screw conveyor. Some cookers also have the facility for injecting steam into the cooking material. The cooking operation is critical; if the fish are incompletely cooked, the liquor cannot be pressed out satisfactorily, and if overcooked the material becomes too soft for pressing. No drying occurs during the cooking stage.
This stage of the process removes some of the oil and water. The fish are conveyed through a perforated tube whilst being subjected to increasing pressure, normally by means of a tapered shaft on the screw conveyor. A mixture of water and oil is squeezed out through the perforations and the solid, known as press cake, emerges from the end of the press. During the pressing process the water content may be reduced from about 70 per cent to about 50 per cent, and the oil content reduced to about 4 per cent.
After screening to remove coarse pieces of solid material, the liquor from the presses is continuously centrifuged to remove the oil. The oil is sometimes further refined in a final centrifuge, a process known as polishing, before being pumped to storage tanks. The refined oil is valuable and is used in the manufacture of edible oils and fats, for example margarine.
The water portion of the liquor, known as stickwater, contains dissolved material and fine solids in suspension which may amount to about 9 per cent by weight. The solids are mostly protein and stickwater can contain as much as 20 per cent of the total solids in the fish so that it is normally well worth recovering. The material is recovered by evaporating the stickwater to a thick syrup containing 30-50 per cent solids, and sometimes marketed separately and known as condensed fish solubles. More usually however the concentrated product is added back to the press cake and dried along with it to make what is known as whole meal.
Although basically a simple operation, considerable skill is required to get the drying conditions just right. If the meal is underdried, moulds or bacteria may be able to grow; if it is overdried, scorching may occur and the nutritional value of the meal will be reduced.
There are two main types of dryer, direct and indirect. In the direct dryer very hot air at a temperature of up to 500°C is passed over the material as it is tumbled rapidly in a cylindrical drum; this is the quicker method, but heat damage is much more likely if the process is not carefully controlled. The meal does not reach the temperature of the hot air, because rapid evaporation of water from the surface of each particle of fish causes cooling; normally the product temperature remains at about 100°C.
The most usual type of indirect dryer consists either of a steam jacketed cylinder or a cylinder containing steam heated discs which also tumble the meal.
Much of the unpleasant odour from fish meal plants originates from the dryers; indirect dryers, which are normally used in the UK, cause less nuisance because they use less air.
In the UK, where white fish offal are the main raw materials, the pressing stage is not essential, since the material contains only very small amounts of oil. White fish meal can be produced by a simpler process of cooking and drying only. However the use of a pressing stage is increaseingly favoured and there are two main reasons for this:
The plant has greater flexibility in that it may be used for white and oily fish, or a mixture of both.
GRINDING AND BAGGING
The final operations are grinding to break down any lumps and particles of bone, and packing the meal into bags or storing it in silos for bulk delivery. From the fish meal factory the meal is transported to the animal food compounder, and from there to the farm. The problems of storage and transport are discussed below.
HYGIENE IN THE FACTORY
Contamination of the material during processing may seriously affect quality; microorganisms like Salmonella that may ultimately cause disease in man have to be kept out. Much can be done by good housekeeping in the plant, for example by keeping floors, walls and conveyors clean and by separating ‘wet’ and ‘dry’ areas of the plant, but the processing machinery itself is often less readily accessible for cleaning. Contaminated water, from a dock for example, should not be used for cooling or other purposes if it can come in contact with the fish or the fish meal.
The temperature during processing is normally high enough to kill any Salmonella present, but when a plant is restarted after a stoppage there is likely to be moist meal standing in the plant that will not reach a high enough temperature; for this reason it is now USA practice to recirculate the meal produced in the first 45 minutes after starting again.
OTHER METHODS OF MANUFACTURE
Of the other processes used, the most well known is the heat transfer method, htm, developed in the USA, where oil added to a slurry of the raw material acts as a heat transfer medium. In some other methods the presses are replaced by centrifuges, and in others the oil is removed by solvent extraction. A very high proportion of the worlds fish meal is however manufactured by the process described above.
Storage and transport of fish meal
Fish meal is not readily spoiled by bacterial action because of its low water content, and it has a very small bulk compared to the fish from which it is made; indeed, these are two of the main reasons for making fish meal. There is no need to refrigerate the meal in storage.
Fish meal is usually stored and transported either in sacks made of paper, hessian or plastics, or in bulk. Fish meal in bulk is sometimes pelletized to make mechanical handling easier, since it does not flow readily as a powder.
Fish oil present in the stored meal can react with oxygen in the atmosphere; the heat generated may damage the meal nutritionally and, on occasion, cause the meal to catch fire. Fortunately this is now a rather rare occurrence, due to the widespread use of antioxidants. Not all fish oils are equally reactive; some oily meals seem to require antioxidant treatment; whilst others do not. The most commonly used antioxidant is ethoxyquin; the amount used varies but is normally in the range 200-1000 mg/kg. Sacks of newly made oily meal are frequently stored in ventilated stacks, particularly in hot climates. White fish meal, with a low oil content, does not require antioxidant treatment.
Fish meal is best kept in a cool dry place protected from rodents and birds. Spoilage is normally very slight even after excessively long periods of storage; fish meal will keep for several years without detectable change in its nutritional value.
Fish meal made from fatty fish, however, will show a gradual decrease in fat content, as measured by extraction with ether, unless antioxidants are present; this is because the fats slowly oxidize during storage and become relatively insoluble in common organic solvents. Oxidized fat is less valuable nutritionally because the animal cannot utilize it for its energy needs. The risk of taint to the animal flesh is much reduced, however, once the fats are oxidized.
Protection against contamination during the manufacture of meal has been mentioned; protection during storage is equally important. The floors, walls and handling equipment in the store must be kept clean, and screens over doors and windows help to keep out birds and rodents that may be carriers of Salmonella organisms. Foot dips are sometimes provided to prevent workers carrying harmful bacteria into the store. The risk of contamination is generally much higher when handling meal in bulk, particularly during loading and unloading of transport.