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.

COOKING

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.

PRESSING

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.

PRESS LIQUOR

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.

DRYING

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.

The removal of water by pressing, and evaporation of the stickwater obtained, is less expensive because the triple effect evaporators used are more efficient in terms of use of steam than are dryers.

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.