2.1 Resources
2.2 Composition of theFish
2.3 Sampling of RawMaterials
2.4Economic Evaluation of the Raw Material
2.5 Methods of Analysis
2.6Deterioration of the Raw Fish
2.6.1 Autolytic deterioration
2.6.2 Oxidation
2.6.3 Microbial spoilage
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
Fish used for reduction to meal and oil may be divided intothree categories:
A fishmeal industry requires a regular supply of raw material.When planning fishmeal factories, it is necessary to know thetype of fish species available, the length of fishing season, thelocation of the fish, the catchability of the fish by differentfishing gear and, if possible, the attainable catches per yearfor a continuous period.
Practically all fish species as well as most other marineanimal life may, in principle, be converted into fish meal. Awide variety of fish species is used for the production of fishmeal and oil in different countries. Table 1 gives some examplesof constituents of whole fish from different stocks. Thecomposition and quality of the raw material are predominantfactors in determining the properties and yield of the products.A separation of fatty substances (lipids) from the otherconstituents of fatty marine animals is one of the majoroperations in the manufacture of fish meal and oil.
Gadoids (the cod-like fishes) comprise a number of fishspecies which can be classified as lean. It is characteristic ofthese species that most of their fat is located in the liver.Fish meal made from these lean species of fish is called whitefish meal.
Clupeids (the herrings) provide the largest singlesource of raw material for production of fish meal and oil. Theymay be classified as fatty although the fat content may vary from2% to 30%, depending on species and season. The fat is not, as inlean fish, concentrated in the liver, but is generallydistributed throughout the body.
Scombroids (the mackerels) are also fatty fish species.
Elasmobranchs (the sharks and the rays) are notspecially caught for meal and oil production. Some species,however, provide raw material as trash fish and as offal fromprocessing.
Salmonoids (the salmons and other closely related fish)are generally not harvested for fishmeal production, but offalfrom salmon is used. However, there is one species, the capelin,that has become a considerable source of material for meal andoil.
Crustaceans. The carapaces and shells are used, as wellas small crustaceans that are unmarketable for direct humanconsumption.
Table 1 Composition of whole fish; average values overa 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 |
As the composition of the fish may vary widely during theyear, systematic sampling and analysis of seasonal variationsprovide important information when considering the establishmentof a fishmeal industry.
From reliable analyses of the raw material one may estimatethe amount of fish meal and oil which may be produced, and hencethe value of the raw material. The water content gives basicfigures for the cost of drying. There is an interrelationshipbetween fat and water in fish: fat and water are complementaryconstituents inasmuch as fat replaces water in the flesh due toseasonal variation. For a given quantity of raw material,increasing fat content will lead to improved oil yield, reduceddemand on drying energy and increased processing capacity of theplant.
In some countries where the fishing fleet and the fishmealindustry are separate commercial entities, the composition andextent of deterioration of the raw material are often used forevaluating the price. Where the fishing fleet is operated as anintegral part of the industry, such evaluation may be used topredict product yield and quality and as a control of productionefficiency.
Evaluation requires that representative samples from eachcatch be submitted for analyses at a control laboratory. Samplingis not straightforward because considerable variations in fishsize and quality may exist within the same catch. In purseseines, for example, there appears to be a tendency for thesmaller fish to sink to the bottom while the larger fish mass inthe upper part of the nets. When the catch is transferred, thereis therefore a corresponding segregation of fish; consequentlythe collector must exercise care to obtain samples representativeof the whole catch.
Most conveniently the sampling is done during the unloading ofthe fishing vessel. An automatic sampling device, installedimmediately after the weighing equipment, is recommended, butsampling might be done by hand. Stratified spot sampling of 4-5kg samples of fish should be made no less than 30 times duringunloading of a vessel containing industrial fish. The samplingcontainer will finally hold about 250 kg fish which has to beground and thoroughly mixed before further sub-dividing of thesample can take place. The final sample of fish for laboratoryexamination will be approximately 500 g.
The evaluation of raw material requires the carrying out of anumber of proximate methods of analysis, mainly to determineprotein, fat, water and ash contents, and the determination ofvolatile basic nitrogen (usually expressed as mg-N/l00g minced fish) (see Section 2.6.3). Under practical conditions itis often sufficient to make determinations only of water andvolatile basic nitrogen in the samples.
The rapid deterioration of fish is due to the action ofbacteria from the surfaces and the digestive tract and toautolytic breakdown caused by enzymatic action in the tissues andin the tract. Bacterial and autolytic deterioration result inbreakdown of both the lipid and protein fractions.
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 extensiveautolysis leading to softening of the meat, rupture of the bellywall and formation of considerable amounts of blood watercontaining both protein and oil. This process is aggravated bythe production of large quantities of gastric enzymes at the timeof feeding. Such solubilization causes difficulties in handlingand processing and may lead to serious losses of both protein andoil.
Fat deterioration (lipolysis) caused by different fatsplitting enzymes (lipases) is a general feature in fatty fish.Fish oils are largely composed of glycerol combined with fattyacids to form glycerides. Splitting of the glycerides of the oiland formation of free fatty acids (FFA) result in reduced qualityof the oil with economic consequences.
Oxidation of lipids (rancidity) and browning of the oil occursunder aerobic (in the presence of oxygen) storage conditions; butin transport vessels and storage bins the condition in theinterior of the mass of fish is anaerobic (oxygen is absent).
The anaerobic conditions of bulk storage of whole fish createa complex medium in which microbes can grow, with formation of avariety of chemical spoilage products. Some important endproducts are the volatile basic nitrogenous compounds (mainlyammonia and trimethylamine) and the amount of total volatilebasic nitrogen (TVB-N) is often used as a measure ofdeterioration. Some volatile bases are formed by the bacterialbreakdown of amino acids, in turn derived from protein, but thetrimethylamine is formed by bacterial metabolism oftrimethylamine oxide. The extensive production of ammonia in aload of deteriorating fish may result in significant losses ofprotein. In elasmobranchs ammonia is also produced from the ureawhich is a constituent of their blood and muscle.
The chemical compounds resulting from bacterial activity arenumerous and some are not yet well described, but the sulphurcontaining compounds, which seem to be formed mainly underanaerobic conditions, are significant. Hydrogen sulphide (H2S)and mercaptans may be produced by decomposed fish in lethalconcentrations in the holds of fishing vessels and in closed fishbins. In Denmark, for example, thorough ventilation is compulsorybefore and during the unloading of fish.
Another feature of bacterial deterioration is thetransformation of sulphur from sulphur containing amino acidsinto compounds which are catalyst inhibitors; that is theyinactivate the catalyst used in subsequent hydrogenation of theoil into fat for margarine production.
During meal production, the aqueous phase is particularlyvulnerable to bacterial putrefaction. Bacterial breakdown,besides affecting the yield and quality of the end products andthe production capacity, also results in the formation ofmalodorous compounds. Consequently efforts should be made tominimize bacterial spoilage.
The production of fish meal and oil from fresh raw materialgives the highest yield and the best quality final products.
In many cases, however, it is difficult to avoid partialspoilage as the fish has to be collected from remote areas. Thesearch for economic means of preserving the catch during periodsof transport and storage, exceeding about 30 h, has been acontinuing challenge to the industry. As breakdown of fishprotein and oil are due to, both autolytic and microbialactivity, the preserving method should preferably retard bothbacterial growth and autolysis by digestive and tissue enzymes.The storage life of fish can be extended either by physical orchemical means.
Proper draining of the fish, both aboard and ashore, is asimple and effective method of extending the short-term storagelife of fish. Aboard the fishing vessels proper drainage reducesthe amount of rubbing together and breakage of the fish duringrolling and pitching of the fishing vessel. Moreover, thespreading and rapid growth of bacteria is reduced by restrictingthe presence of free water containing body slime, gut contentsand the bacteria contained therein.
During storage of ungutted fish in bulk, the rate of breakdowncaused by bacteria and by digestive and tissue enzymes doubleswith an increase of temperature of approximately 4�C. Thisbreakdown leads to losses of protein and oil and reduces thequality of the fish for processing. At temperature higher than 5�Chydrogen sulphide is produced by bacteria, and its formationrapidly increases with temperature. At 0�C hydrogen sulphide isnot formed until storage exceeds 9-10 days (Petersen, 1971).
In tropical and temperate areas where the fish may be caughtat high temperatures and far from the plant, chilling is the mosteffective method of preserving bulk-stored fish. In many casesthe cost of chilling can be recovered by the reduced losses ofprotein and oil.
In principle, two methods of chilling may be considered,namely refrigerated or chilled water systems and mixing of icewith fish.
Sea water is readily available, but prolonged storage of fishin sea water is limited by absorption of salt; high salt levelsare undesired in fish meal. It is therefore preferable to usediluted sea water or fresh water. The method comprisescirculating the refrigerated water through the mass of fish.Merely pumping the water over the fish does not effectively chillthe whole mass, as there is hardly any penetration of the waterthrough the bulk of the fish, most of it being diverted aroundthe sides. Advanced systems are based on pumping the chilledwater upward through the fish from the bottom of the hold. Themethod is expensive and hardly practical except for long voyagesand storage periods.
Mixing fish and ice in the right proportion for chilling thefish at 0�C is an effective method for preserving raw fish. Fishand ice should be mixed before filling the hold. The melt wateris drained off from the bottom leaving the fish dry and compact.The use of ice for preservation is dependent upon the developmentof rapid, preferably automatic, systems for mixing fish and iceat the high rate necessary in industrial fishing.
In Scandinavia, particularly in Denmark, the following icemixing (and grading) system has been installed in more than 100fishing vessels (see Figure l).
The Technological Laboratory of the Danish Ministry ofFisheries has participated in the development of a receiving boxon deck equipped with a conveyor continuously feeding a rotatingdrum. This drum grades the fish into two categories: industrial,which includes fish less than 35 mm thick, and food fish, whichare comprised of thicker specimens.
The receiving box is equipped to remove the few large fish andother large objects before the catch enters the conveyor and thegrader. The box may receive loads of up to 2 t of fish, which areconverted to a continuous flow of up to 1 200 kg/min. The foodfish pass through the length of the cylinder, while theindustrial fish fall through the grader into a trough equippedwith a continuous supply of small pieces of ice to chill the fishto 0�C and maintain this temperature until the fish are landed.A conveyor running along the trough takes the mixture ofindustrial fish and ice to a vertical conveyor that lifts themixture 2 m above the deck and releases it into a funnel. Wideplastic tubes connect the funnel to the ice scuttle on the deckover the section of the hold that is to be filled with icedindustrial fish.
The continuous supply of ice to the trough under the grader istaken from a. store of bulk ice. At the bottom of the store ahorizontal conveyor feeds the ice onto a vertical conveyor to thetrough. The conveyor speed is adjustable so that the supply ofice can be varied. At landing, there should be little or nosurplus ice. Fish at 15�C when caught requires about 23% of itsweight in ice to be chilled and maintained at 0�C for four daysin an insulated hold.
Chemical preservatives for raw fish immediately act on thebacteria on the surface of the fish, but there is some delay inaction on the interior (stomach and intestines) depending uponthe rate of penetration of the preservative.
Sodium nitrite, sodium sulphite, ascorbic acid, benzoic acidand many more preservatives have been evaluated, but they areused only to a very limited extent. Sodium nitrite hasdemonstrated comparatively favourable properties for preservationof species such as herring, as it retards significantly thedevelopment of spoilage microbes and reduces the formation offree fatty acids, but unless nitrites are added in strictlycontrolled small amounts (as is the case in Norway) they mayreact with other components of the raw material and formnitrosamines which are harmful carcinogenic chemical compounds.
Figure 1 Catch handling equipment forseparating industrial from food fish
Special measures must be taken when meal is manufactured fromfish which has been preserved by nitrite. The use of nitrite musttherefore be permitted only under the most careful supervisionand should not be encouraged.
Formaldehyde is widely used and, under certain circumstances,also exerts a beneficial firming effect on the raw material sothat it is rendered more amenable to pressing, after cooking, forremoval of oil. Formaldehyde combines with protein by a mechanismakin to tanning and the reactive amino acid lysine is involved.The use of small amounts, however, (for example 0.05% formalinbased on the weight of the fish) has no detectable deleteriouseffect on protein quality.
