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.

Composition and nutritional value

Before examining the composition of the finished meal it is interesting to consider the composition of the intermediate products. Figure 2 shows the composition of the material at each stage of its flow through the process and is based on the assumption that the raw fish contain 70 per cent water, 18 per cent solids and 12 per cent fat. It can be seen that more drying occurs in the evaporators than in the dryer. The composition of the intermediate products in this example is as follows:

	water	solids	fat
material	%	%	%
raw fish
press cake
press liquor
dilute stickwater			<1
conc. stickwater
fish meal

Fig. 2. Composition of material during the process.

Figure 2 shows a yield of 21 per cent whole meal from oily fish of the assumed composition; in practice there may be some processing losses which will slightly reduce the actual yield. For example, raw material awaiting cooking will lose some drip as it spoils; since the lost liquor contains protein in solution, yield will decrease when raw material is delayed for long periods. The extent of this loss varies from species to species.

The composition of the final product depends both on the kind of raw material and on the type of process. A whole meal made from fatty fish like herring might contain about 71 per cent protein, 9 per cent fat, 8 per cent water and 12 per cent minerals, whereas a meal made mainly from white fish and white fish offal and dried to the same extent will contain about 66 per cent protein, 5 per cent fat, 8 per cent water and 21 per cent minerals.

Fish meal is valuable not only for the quantity but also the quality of its protein. By this is meant that the amino acids which make up the protein are present in just the right balance for animal or human nutrition. The amino acid composition of typical samples of herring meal and white fish meal might be:

	herring meal	white fish meal
amino acid	g/100 g protein	g/100 g protein
lysine	7·7	6·9
methionine	2·9	2·6
tryptophan	1·2	0·9
histidine	2·4	2·0
arginine	5·8	6·4
threonine	4·3	3·9
valine	5·4	4·5
isoleucine	4·5	3·7
leucine	7·5	6·5
phenylalanine	3·9	3·3
cystine	1·0	0·9
tyrosine	3·1	2·6
aspartic acid	9·1	8·5
serine	3·8	4·8
glutamic acid	12·8	12·8
proline	4·2	5·3
glycine	6·0	9.9
alanine	6·3	6·3

Not only is the balance of amino acids in fish meal suitable for animal feeding, but the availability of the essential amino acids is also greater in fish meal than for example in meat meal; available means capable of being liberated by the animal’s digestive juices and utilized by it, and essential acids are those the animal must have in its diet. The first ten amino acids in the table are those believed to be essential for growing animals. Fish meal is also a valuable source of minerals calcium, phosphorous, magnesium, potassium, of vitamins B₁, B₂, B₆ and B₁₂, and of trace elements, notably zinc, iodine, iron, copper, manganese, cobalt, selenium and fluorine.

In many feeding trials, animals fed on diets containing a similar amino acid composition to fish meal have grown less well than those fed on fish meal itself; this has led to the hypothesis that fish meal contains an unidentified growth factor sometimes abbreviated to UGF. However this ingredient has never been isolated, and in other experimental feeding, results with carefully supplemented vegetable protein diets have been as good as with fish meal. The answer to this problem may be simply that fish meal contains such a wide range of nutritionally valuable materials that whatever is lacking in the diet fish meal can provide it. Thus the attribute of UGF may be due to nutritional balance rather than to the presence of some unknown, and as yet unisolated growth-promoting substance.

How is fish meal used?

Fish meal in the UK was used mainly as a fertilizer until about 1910, but since then its high nutritional value has been far better utilized in animal feeding. The demand in the UK for fish as fish meal is far greater than the demand for fish for direct human consumption; therefore imports of fish meal to the UK are high. The pig and poultry industries producing large amounts of bacon and eggs, pork and chicken, at relatively low prices could not survive without large scale use of high protein animal foods like fish meal. Usually about 10 per cent of the diet of pigs and poultry consists of fish meal; 10 per cent is the upper limit for meal containing 10 per cent fat, because more than about 1 per cent of fish oil in the diet of the animal may taint the taste of its flesh. Much of the UK production is of white fish meal with a fat content low enough to eliminate any risk of taint. Fish meals with an extremely low fat content are sometimes made for certain specialized purposes.

Other uses of fish meal include the feeding of mink, farmed fish, dogs, cats and cattle. 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.

How are fish oils used?

Fish oils are produced whenever fatty fish are processed into meal. In Europe they are widely used in the manufacture of edible oils and fats, for example margarine. Other uses include the paint and varnish industry. In addition, there are several other specialized uses for small quantities of fish OILS. Fish oils usually have to be low in free fatty acids, less than 2 per cent, to obtain the best price; production of high quality fish oils depends on the use of fresh raw material, proper purification and good storage.