Fish meal is defined as a solid product obtained by removing most of the water and some or all of the oil from fish or fish waste. In the UK, fish meal refers to a product obtained by drying and grinding or otherwise treating fish or fish waste to which no other matter has been added.
Virtually any fish or shellfish in the sea can be used to make fish meal, although a few rare unexpected species may produce poisonous meal.
Based on the type of raw material, fish meal can be classified into two basic types: fish meal from fishery waste associated with the processing of various edible human fishery products and by-products associated with fish or production, and fish meal from industrial fishery, where fish catches are solely for the fish meal industry.
Specific fish (herring, menhaden, pollack, etc.) are harvested solely for producing fish meal. Most of the world’s fish meal is made from whole fish; pelagic species are used most for this purpose.
Selecting Species for Industrial Fish Meal Production
The following points are important when selecting species for an industrial fishery:
- The species must be in large concentration to give a high catching rate; this is essential because the value of industrial fish is less than that for direct human consumption.
- The fishery should preferably be based on more than one species to reduce the effect of fluctuation in the supply of any one species.
- The total abundance of long-lived species varies less from year to year.
- 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.
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Methods of Manufacturing Fish Meal
There are several ways of manufacturing or producing fish meal from raw fish:
- Sun Drying Method: This is the simplest method and is still used in some parts of the world where processing plants are not available. However, the product is poor in comparison with those made by modern methods.
- Heat Transfer Method (HTM): Developed in the USA, this method uses oil added to a slurry of the raw material as a heat transfer medium.
- Press Method: Water and oil are removed by the use of a screw press. The use of this method is increasingly favored for several reasons.
- Centrifugation Method: Presses are replaced by centrifuges.
- Solvent Extraction Method: Oil is removed by solvent extraction.
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.
1. Cooking Process in Fish Meal Production
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.
2. Pressing Process in Fish Meal Production
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. 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% to about 50%, and the oil content reduced to about 4%. In the UK, where white fish offal are the main raw materials, pressing is not essential since the material contains only a small amount of oil. Fish meal can be produced by a simple process of cooking and drying only.
3. Drying Process in Fish Meal Production
There are two main types of dryers: 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 a quicker method, but heat damage is very 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 100°C. The most usual type of indirect dryer consists either of a steam-jacketed cylinder or a cylinder containing steam-heated discs that also tumble the meal.
Although basically a simple operation, considerable skill is required to get the drying conditions just right; if the meal is under-dried, molds or bacteria may be able to grow; if it is over-dried, scorching may occur, and the nutritional value of the meal will reduce.
Much of the unpleasant odor from fish meal originates from the dryers. Indirect dryers, normally used in the UK, cause less nuisance because they use less.
4. Grinding and Packing in Fish Meal Production
The final operation involves grinding to break down lumps and particles of bone and packing the meal into bags or storing it in silos for bulk delivery. The importance of fish meal can best be judged by the fact that some 30% of the world’s catch of fish ends up as fish meal. Peru, the USA, and Japan are the major producers of fish meal.
Fish Meal By-Products
The liquor from the presses is screened to remove coarse pieces of solid material and centrifuged to remove the oil. The oil is sometimes further refined in a final centrifuge, a process known as polishing, to storage tanks. The refined oil is valuable and is used in the manufacture of edible oils and fats, such as margarine.
The water or aqueous portion of the liquor, known as “stick” water, contains dissolved material and fine solids in suspension, which may amount to about 9% by weight. The solids are mostly protein, and stick water can contain as much as 20% of the total solids in the fish, so it is normally well worth recovering.
The material is recovered by evaporating the stick water to a thick syrup containing 30-50% solids and sometimes marketed separately as condensed fish solubles. However, the concentrated product is often added back to the press cake and dried along with it to make what is known as whole meal.
Fish Meal Production in Nigeria
In Nigeria, fish processing is primarily in the hands of artisanal fishermen. In response to an increased demand for “ready-to-eat” fish products, along with a growing awareness of the limited supply of natural fish stocks, the fish industry has developed procedures for more efficient utilization of available raw materials.
Because as much as 70% of harvested fish has traditionally been discarded or converted into cheap animal feeds, initial efforts to conserve fishery resources have focused on the development of edible products from underutilized species.
Differences Between Fish Meal and Fish Protein Concentrate
Fish meal is a fish protein concentrate (FPC), but the term FPC usually refers to a material suitable for human consumption. Fish meal prepared under hygienic conditions can be FPC in this sense, and small amounts made from white fish meal have been sold for incorporation in other foods.
The specification for some types of FPC demands a very low fat content, and in some plants, it is possible to treat the press cake or the fish meal with a solvent to extract the fat from the raw material rather than from the finished meal when making FPC of low fat content.
Fish meal, as produced throughout the world, is a very cheap potential FPC but not intended for human consumption. It is used for making pig and poultry feeds. Ordinary fish meal is unsuitable for human consumption for three reasons:
- It is not normally made under sufficiently hygienic conditions to rule out the risk of occasional contamination by disease-causing bacteria.
- It usually contains rancid fat, which destroys certain vitamins and may lower the nutritive value of the protein. A fish meal diet may precipitate vitamin deficiency in poorly nourished people. Moreover, the flavor of the rancid fat is unacceptable in many societies, though not all.
- There is a slight risk that the rancid fat may have a cumulative toxic effect if consumed over a long period.
The first of these reasons is the most important. Thus, fish meal made under hygienic conditions is called FPC type posthumously type C.
Uses of Fish Meal in Agriculture
Fish meal in the UK was used as a fertilizer, but its high nutritional value has been far better utilized in animal feeding. Hence, in some parts of the world, it is often the primary supplemental protein source fed to livestock because plant-derived sources are either unavailable or too expensive.
It has been widely used for many years as a supplemental protein source for monogastric animals (e.g., poultry) but more recently is being used more for ruminant animals.
Very small amounts of specially processed meals have been used in prepared foods for humans, and fish meal is also used in the preparation of certain antibiotics for the pharmaceutical industry.
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Quality Standards for Fish Meal
The freshness of fish being processed into fish meal is highly correlated with the quality of the resulting fish meal. If fish has been allowed to degrade prior to processing, the fish meal will be of lower quality. Fish meal prepared from materials that have been allowed to degrade prior to processing can contain high levels of histamine and can be toxic.
Elevated histamine (1000 ppm) can cause gizzard erosion and black vomit in poultry. The crude protein, ash, ether extract, etc., of fish meal can vary depending on the raw material or substrate and the method used to prepare the meal.
Fish meal made from whole fish, with a high portion of small whole fish in the raw material, can have the same effect. The value of protein from vertebrate fish differs little from one species to another; whole shellfish, however, would give a nutritionally poorer meal because of the lower protein content of the shell.
The amino acid quality of fish is excellent, but excessive heating during the drying process can reduce the digestibility of the protein fraction, whereas some amino acids form complexes so that they are not available. The oils associated with fish meal are highly unsaturated and oxidize easily.
Hence, fish meal made from fatty fish shows a gradual decrease in fat content 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. Moreover, the heat generated as fish oil reacts with atmospheric oxygen (during oxidation) may damage the meal nutritionally and, on occasion, cause the meal to catch fire.
This is now a rare occurrence due to the widespread use of antioxidants. The most commonly used antioxidant is ethoxyquin; the amount used varies but is normally in the range of 200-1000 mg/kg.
Raw Materials for Fish Meal Production
Fish meal is produced from a number of raw materials, including skeletal remains of filleted fish, fish cannery waste, fish offal, whole fish generally regarded as inedible, some varieties of herring, pilchards, and many very small fish. Sharks may also be used, and such large fish are cut into small pieces before processing.
Fish meal finds its main, almost exclusive, use as an animal feed supplement, with particular application to poultry and pigs, or as a soil fertilizer. The levels of usage of fish meal in the feed vary, but 2%-10% is the range normally encountered. In general, higher levels of the meal are fed when starting the animals, with reduced levels when finishing.
This is done because consumption of high levels of fish meal just prior to slaughter can cause “fishy” flavors in the animal’s flesh. Also, the use of high levels of fish meal in poultry diets has caused fish flavors in eggs. Fish meal is of particular value because of its high content of essential amino acids, especially lysine.
This makes fish meal of special value in combination with cereal feeds, which are low in lysine content. In addition, fish meal provides vitamins and minerals not provided by cereal grains or plant protein supplements.
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