2. RAW MATERIAL


2.1 Resources
2.2 Composition of the Fish
2.3 Sampling of Raw Materials
2.4 Economic Evaluation of the Raw Material
2.5 Methods of Analysis
2.6 Deterioration of the Raw Fish

2.6.1 Autolytic deterioration
2.6.2 Oxidation
2.6.3 Microbial spoilage

2.7 Preservation of the Raw Material

2.7.1 Draining
2.7.2 Preservation by chilling
2.7.3 Refrigerated (chilled) water systems
2.7.4 Preservation by ice
2.7.5 Chemical preservation


2.1 Resources

Fish used for reduction to meal and oil may be divided into three categories:

  1. fish caught for the sole purpose of fishmeal production (for example by Chile, Peru, Norway, Denmark, South Africa and the USA);
  2. by-catches from another fishery (by most fish-producing countries);
  3. fish offcuts and offal from the consumption industry. (The UK and Germany use these materials to produce white fish meal; South Africa makes rock lobster meal from the carapaces and other parts which are not utilized).

A fishmeal industry requires a regular supply of raw material. When planning fishmeal factories, it is necessary to know the type of fish species available, the length of fishing season, the location of the fish, the catchability of the fish by different fishing gear and, if possible, the attainable catches per year for a continuous period.

2.2 Composition of the Fish

Practically all fish species as well as most other marine animal life may, in principle, be converted into fish meal. A wide variety of fish species is used for the production of fish meal and oil in different countries. Table 1 gives some examples of constituents of whole fish from different stocks. The composition and quality of the raw material are predominant factors in determining the properties and yield of the products. A separation of fatty substances (lipids) from the other constituents of fatty marine animals is one of the major operations in the manufacture of fish meal and oil.

Gadoids (the cod-like fishes) comprise a number of fish species which can be classified as lean. It is characteristic of these species that most of their fat is located in the liver. Fish meal made from these lean species of fish is called white fish meal.

Clupeids (the herrings) provide the largest single source of raw material for production of fish meal and oil. They may be classified as fatty although the fat content may vary from 2% to 30%, depending on species and season. The fat is not, as in lean fish, concentrated in the liver, but is generally distributed throughout the body.

Scombroids (the mackerels) are also fatty fish species.

Elasmobranchs (the sharks and the rays) are not specially caught for meal and oil production. Some species, however, provide raw material as trash fish and as offal from processing.

Salmonoids (the salmons and other closely related fish) are generally not harvested for fishmeal production, but offal from salmon is used. However, there is one species, the capelin, that has become a considerable source of material for meal and oil.

Crustaceans. The carapaces and shells are used, as well as small crustaceans that are unmarketable for direct human consumption.

Table 1 Composition of whole fish; average values over a number of years (in percent)

Fish species

Protein

Fat

Ash

Water

Gadoids  
Blue whiting. North Sea

17.0

5.0

4.0

75.0

Sprat. Atlantic

16.0

11.0

2.0

71.0

Hake. South Africa

17.0

2,0

3.0

79.0

Norway pout

16.0

5.5

3.0

73.0

Clupeids  
Anchoveta

18.0

6.0

2.5

78.0

Herring. spring

18.0

8.0

2.0

72.0

Herring. winter

18.2

11.0

2.0

70.0

Pilchard. South Africa

18.0

9.0

3.0

69.0

Anchovy. South Africa

17.0

10.0

3.0

70.0

Scombroids  
Mackerel, spring, North Sea

18.0

5.5

1.6

75.0

Mackerel, autumn, North Sea

15.0

27.0

1.4

56.5

Horse mackerel, North Sea

16.0

17.0

3.8

62.7

Horse mackerel, South Africa

17.0

8.0

4.0

72.0

Elasmobranchs  
Dogfish

19.0

8.9

2.3

70.0

Salmonoids  
Capelin, Norway

14.0

10.0

2.0

75.0

2.3 Sampling of Raw Materials

As the composition of the fish may vary widely during the year, systematic sampling and analysis of seasonal variations provide important information when considering the establishment of a fishmeal industry.

2.4 Economic Evaluation of the Raw Material

From reliable analyses of the raw material one may estimate the amount of fish meal and oil which may be produced, and hence the value of the raw material. The water content gives basic figures for the cost of drying. There is an interrelationship between fat and water in fish: fat and water are complementary constituents inasmuch as fat replaces water in the flesh due to seasonal variation. For a given quantity of raw material, increasing fat content will lead to improved oil yield, reduced demand on drying energy and increased processing capacity of the plant.

In some countries where the fishing fleet and the fishmeal industry are separate commercial entities, the composition and extent of deterioration of the raw material are often used for evaluating the price. Where the fishing fleet is operated as an integral part of the industry, such evaluation may be used to predict product yield and quality and as a control of production efficiency.

Evaluation requires that representative samples from each catch be submitted for analyses at a control laboratory. Sampling is not straightforward because considerable variations in fish size and quality may exist within the same catch. In purse seines, for example, there appears to be a tendency for the smaller fish to sink to the bottom while the larger fish mass in the upper part of the nets. When the catch is transferred, there is therefore a corresponding segregation of fish; consequently the collector must exercise care to obtain samples representative of the whole catch.

Most conveniently the sampling is done during the unloading of the fishing vessel. An automatic sampling device, installed immediately after the weighing equipment, is recommended, but sampling might be done by hand. Stratified spot sampling of 4-5 kg samples of fish should be made no less than 30 times during unloading of a vessel containing industrial fish. The sampling container will finally hold about 250 kg fish which has to be ground and thoroughly mixed before further sub-dividing of the sample can take place. The final sample of fish for laboratory examination will be approximately 500 g.

2.5 Methods of analysis

The evaluation of raw material requires the carrying out of a number of proximate methods of analysis, mainly to determine protein, fat, water and ash contents, and the determination of volatile basic nitrogen (usually expressed as mg-N/l00 g minced fish) (see Section 2.6.3). Under practical conditions it is often sufficient to make determinations only of water and volatile basic nitrogen in the samples.

2.6 Deterioration of the Raw Fish

The rapid deterioration of fish is due to the action of bacteria from the surfaces and the digestive tract and to autolytic breakdown caused by enzymatic action in the tissues and in the tract. Bacterial and autolytic deterioration result in breakdown of both the lipid and protein fractions.

2.6.1 Autolytic deterioration

In a number of fish species used for fishmeal production, particularly small pelagic fish species such as sardines, anchovies and herring, the digestive enzymes may cause extensive autolysis leading to softening of the meat, rupture of the belly wall and formation of considerable amounts of blood water containing both protein and oil. This process is aggravated by the production of large quantities of gastric enzymes at the time of feeding. Such solubilization causes difficulties in handling and processing and may lead to serious losses of both protein and oil.

Fat deterioration (lipolysis) caused by different fat splitting enzymes (lipases) is a general feature in fatty fish. Fish oils are largely composed of glycerol combined with fatty acids to form glycerides. Splitting of the glycerides of the oil and formation of free fatty acids (FFA) result in reduced quality of the oil with economic consequences.

2.6.2 Oxidation

Oxidation of lipids (rancidity) and browning of the oil occurs under aerobic (in the presence of oxygen) storage conditions; but in transport vessels and storage bins the condition in the interior of the mass of fish is anaerobic (oxygen is absent).

2.6.3 Microbial spoilage

The anaerobic conditions of bulk storage of whole fish create a complex medium in which microbes can grow, with formation of a variety of chemical spoilage products. Some important end products are the volatile basic nitrogenous compounds (mainly ammonia and trimethylamine) and the amount of total volatile basic nitrogen (TVB-N) is often used as a measure of deterioration. Some volatile bases are formed by the bacterial breakdown of amino acids, in turn derived from protein, but the trimethylamine is formed by bacterial metabolism of trimethylamine oxide. The extensive production of ammonia in a load of deteriorating fish may result in significant losses of protein. In elasmobranchs ammonia is also produced from the urea which is a constituent of their blood and muscle.

The chemical compounds resulting from bacterial activity are numerous and some are not yet well described, but the sulphur containing compounds, which seem to be formed mainly under anaerobic conditions, are significant. Hydrogen sulphide (H2S) and mercaptans may be produced by decomposed fish in lethal concentrations in the holds of fishing vessels and in closed fish bins. In Denmark, for example, thorough ventilation is compulsory before and during the unloading of fish.

Another feature of bacterial deterioration is the transformation of sulphur from sulphur containing amino acids into compounds which are catalyst inhibitors; that is they inactivate the catalyst used in subsequent hydrogenation of the oil into fat for margarine production.

During meal production, the aqueous phase is particularly vulnerable to bacterial putrefaction. Bacterial breakdown, besides affecting the yield and quality of the end products and the production capacity, also results in the formation of malodorous compounds. Consequently efforts should be made to minimize bacterial spoilage.

2.7 Preservation of the Raw Material

The production of fish meal and oil from fresh raw material gives the highest yield and the best quality final products.

In many cases, however, it is difficult to avoid partial spoilage as the fish has to be collected from remote areas. The search for economic means of preserving the catch during periods of transport and storage, exceeding about 30 h, has been a continuing challenge to the industry. As breakdown of fish protein and oil are due to, both autolytic and microbial activity, the preserving method should preferably retard both bacterial growth and autolysis by digestive and tissue enzymes. The storage life of fish can be extended either by physical or chemical means.

2.7.1 Draining

Proper draining of the fish, both aboard and ashore, is a simple and effective method of extending the short-term storage life of fish. Aboard the fishing vessels proper drainage reduces the amount of rubbing together and breakage of the fish during rolling and pitching of the fishing vessel. Moreover, the spreading and rapid growth of bacteria is reduced by restricting the presence of free water containing body slime, gut contents and the bacteria contained therein.

2.7.2 Preservation by chilling

During storage of ungutted fish in bulk, the rate of breakdown caused by bacteria and by digestive and tissue enzymes doubles with an increase of temperature of approximately 4C. This breakdown leads to losses of protein and oil and reduces the quality of the fish for processing. At temperature higher than 5C hydrogen sulphide is produced by bacteria, and its formation rapidly increases with temperature. At 0C hydrogen sulphide is not formed until storage exceeds 9-10 days (Petersen, 1971).

In tropical and temperate areas where the fish may be caught at high temperatures and far from the plant, chilling is the most effective method of preserving bulk-stored fish. In many cases the cost of chilling can be recovered by the reduced losses of protein and oil.

In principle, two methods of chilling may be considered, namely refrigerated or chilled water systems and mixing of ice with fish.

2.7.3 Refrigerated (chilled) water systems

Sea water is readily available, but prolonged storage of fish in sea water is limited by absorption of salt; high salt levels are undesired in fish meal. It is therefore preferable to use diluted sea water or fresh water. The method comprises circulating the refrigerated water through the mass of fish. Merely pumping the water over the fish does not effectively chill the whole mass, as there is hardly any penetration of the water through the bulk of the fish, most of it being diverted around the sides. Advanced systems are based on pumping the chilled water upward through the fish from the bottom of the hold. The method is expensive and hardly practical except for long voyages and storage periods.

2.7.4 Preservation by ice

Mixing fish and ice in the right proportion for chilling the fish at 0C is an effective method for preserving raw fish. Fish and ice should be mixed before filling the hold. The melt water is drained off from the bottom leaving the fish dry and compact. The use of ice for preservation is dependent upon the development of rapid, preferably automatic, systems for mixing fish and ice at the high rate necessary in industrial fishing.

In Scandinavia, particularly in Denmark, the following ice mixing (and grading) system has been installed in more than 100 fishing vessels (see Figure l).

The Technological Laboratory of the Danish Ministry of Fisheries has participated in the development of a receiving box on deck equipped with a conveyor continuously feeding a rotating drum. This drum grades the fish into two categories: industrial, which includes fish less than 35 mm thick, and food fish, which are comprised of thicker specimens.

The receiving box is equipped to remove the few large fish and other large objects before the catch enters the conveyor and the grader. The box may receive loads of up to 2 t of fish, which are converted to a continuous flow of up to 1 200 kg/min. The food fish pass through the length of the cylinder, while the industrial fish fall through the grader into a trough equipped with a continuous supply of small pieces of ice to chill the fish to 0C and maintain this temperature until the fish are landed. A conveyor running along the trough takes the mixture of industrial fish and ice to a vertical conveyor that lifts the mixture 2 m above the deck and releases it into a funnel. Wide plastic tubes connect the funnel to the ice scuttle on the deck over the section of the hold that is to be filled with iced industrial fish.

The continuous supply of ice to the trough under the grader is taken from a. store of bulk ice. At the bottom of the store a horizontal conveyor feeds the ice onto a vertical conveyor to the trough. The conveyor speed is adjustable so that the supply of ice can be varied. At landing, there should be little or no surplus ice. Fish at 15C when caught requires about 23% of its weight in ice to be chilled and maintained at 0C for four days in an insulated hold.

2.7.5 Chemical preservation

Chemical preservatives for raw fish immediately act on the bacteria on the surface of the fish, but there is some delay in action on the interior (stomach and intestines) depending upon the rate of penetration of the preservative.

Sodium nitrite, sodium sulphite, ascorbic acid, benzoic acid and many more preservatives have been evaluated, but they are used only to a very limited extent. Sodium nitrite has demonstrated comparatively favourable properties for preservation of species such as herring, as it retards significantly the development of spoilage microbes and reduces the formation of free fatty acids, but unless nitrites are added in strictly controlled small amounts (as is the case in Norway) they may react with other components of the raw material and form nitrosamines which are harmful carcinogenic chemical compounds.

Figure 1 Catch handling equipment for separating industrial from food fish

  1. Receiving box covered with steel bars for protection against the large objects
  2. Conveyor taking the fish continuously into the grader (3) at a rate of up to 1200 kg/min
  3. Rotating grader separating the small industrial fish from the food fish
  4. Trough receiving the small fish and ice, coming from the hold (5) via the conveyor (6)
  5. Ice store in the hold from where the ice is fed into the horizontal part of the conveyor (6)
  6. Ice conveyor built into the central gangway of the fish hold. The horizontal part is covered with a safety grid of. bars spaced at about 80 mm, which will allow the broken ice to fall into the conveyor. The vertical part brings the ice into the mixing trough (4) under the grader (3). The conveyor is continuously variable with a capacity of 0-250 kg of ice/min
  7. Conveyor lifting the mixture of ice and industrial fish 2 m above deck level and discharging it into the funnel (8)
  8. Funnel and plastic tubes reading the mixture to the ice scuttle (9) in the deck
  9. One of the number of ice scuttles installed over the hold sections for industrial fish
  10. Discharge of food fish from the rotating grader (3)

Special measures must be taken when meal is manufactured from fish which has been preserved by nitrite. The use of nitrite must therefore be permitted only under the most careful supervision and should not be encouraged.

Formaldehyde is widely used and, under certain circumstances, also exerts a beneficial firming effect on the raw material so that it is rendered more amenable to pressing, after cooking, for removal of oil. Formaldehyde combines with protein by a mechanism akin to tanning and the reactive amino acid lysine is involved. The use of small amounts, however, (for example 0.05% formalin based on the weight of the fish) has no detectable deleterious effect on protein quality.