Food Science & Technology · Commodity Processing Technologies · also Fish Processing Technology, Seafood Technology, Post-harvest Fisheries
Handling and processing fish and shellfish: post-mortem spoilage and freshness, chilling and freezing at sea and ashore, canning, curing, smoking and drying, surimi and value-added products, and seafood safety hazards.
In the exams: GATE XE-G Section 3 (drying, canning and freezing of fish); ICAR AIEEA-PG Code 18 Unit III (fish and sea foods)
Fish and shellfish are the most perishable of the major animal foods and among the most traded. A fish begins to lose quality the moment it leaves the water: its own enzymes break down the compounds that give fresh flavour, bacteria from the skin, gills and gut multiply even at chill temperatures, and the highly unsaturated oils of fatty species oxidise. Every step from the deck of the boat to the consumer's plate is therefore a race against time and temperature. Fish technology is the set of methods, ancient and modern, by which that race is won: icing and freezing, canning, salting, drying, smoking and fermentation, the making of surimi and other mince products, the handling of shrimp, crabs and molluscs, and the conversion of what is left into fish meal, oil and other by-products.
Seafood also carries hazards that other foods do not: histamine formed in scombroid fish, natural marine biotoxins concentrated by shellfish and reef fish, parasites such as Anisakis in fish eaten raw, pathogenic vibrios and viruses in bivalves eaten raw, and Clostridium botulinum type E in lightly preserved packed fish. Much international trade is governed by importing-country rules on these hazards and on hygiene, traceability and labelling.
This manual covers the composition of fish and shellfish, post-mortem change and freshness assessment, chilling and freezing, canning, curing, fermentation, surimi, shellfish processing, by-products, hazards and trade standards. Freezing equipment and freezing-time calculation are in chilling-freezing-frozen-foods, process lethality in thermal-processing-canning, smoke chemistry in meat-poultry-egg-products, chemical contaminants such as methylmercury in food-toxicology-contaminants-allergens, and the public-health inspection of foods of animal origin in the Veterinary track's milk-meat-hygiene-food-safety. The subject is examined in GATE XE-G and XL-U Section 3 (drying, canning and freezing of fish) and ICAR AIEEA-PG Code 18 Unit III (fish and sea foods), and it is the daily work of technologists in processing plants, export houses and inspection laboratories in every fishing nation.
The edible flesh of fish is mainly water and protein. Water is commonly 60–80% of the fillet, protein about 15–24%, and ash about 1–2%, but fat varies more than anything else, from well under 1% in lean white fish such as cod and haddock, which store fat in the liver, to 10–20% or more in fatty pelagic species such as herring, mackerel, sardine and salmon, which store it in the flesh. In fatty species, fat and water vary inversely: as fat rises with feeding and season, water falls, so the sum of the two stays roughly constant. Fat content also varies with species, sex, maturity, feeding ground and position in the body. Carbohydrate in fish muscle is very low, mainly a little glycogen, which is why post-mortem pH fall is small.
Fish lipids are rich in long-chain n-3 polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These make fish nutritionally valuable but also very prone to oxidation, so fatty fish turn rancid faster than lean fish in chilled and frozen storage. Shellfish are generally lean; molluscs such as oysters and mussels contain glycogen, which gives them a sweetish taste.
Fish muscle is arranged in short blocks called myotomes, separated by thin sheets of connective tissue, the myocommata. Fish contain much less connective tissue than land animals, and their collagen is more heat-labile, so fish flesh is tender and flakes apart on cooking; it also means that rough handling, gaping and freezing damage show up quickly. Most fish have a band of dark muscle along the lateral line, richer in myoglobin, fat and enzymes than the white muscle; dark muscle is more abundant in active pelagic species such as tuna and spoils and oxidises faster.
The muscle proteins are grouped by solubility. Myofibrillar proteins (myosin and actin) make up the larger part and are responsible for gel formation, water holding and texture; they are less stable than those of mammals and denature readily during frozen storage. Sarcoplasmic proteins are water-soluble and include enzymes and myoglobin. Stroma proteins (collagen) form the connective tissue. The low thermal stability of fish myofibrillar proteins is the basis of surimi technology and of the toughening of frozen fish.
Fish flesh contains a high proportion of non-protein nitrogen (NPN) compared with meat, and NPN compounds explain much of seafood flavour and spoilage. Marine fish contain trimethylamine oxide (TMAO), an osmoregulatory compound with little odour, which bacteria reduce to trimethylamine (TMA), the main cause of the 'fishy' smell of spoiling marine fish. Freshwater fish contain little TMAO. Elasmobranchs (sharks and rays) hold large amounts of urea in their tissues, which is broken down to ammonia after death, so they need rapid bleeding and handling. Free amino acids are also present; scombroid and some other fish such as tuna, mackerel and sardine contain high levels of free histidine, which bacteria can decarboxylate to histamine. Nucleotides from the breakdown of ATP contribute to fresh flavour (inosine monophosphate) and to bitterness on spoilage (hypoxanthine).
Seafood is a good source of iodine, selenium and, in small bony fish eaten whole, calcium and phosphorus. Shellfish such as oysters are rich in zinc. Large predatory fish accumulate methylmercury, and bivalves can concentrate metals from their environment; contaminants are covered in food-toxicology-contaminants-allergens.
After death, the circulation stops and the muscle continues to use its energy stores without oxygen. ATP is depleted, actin and myosin lock together and the fish passes into rigor mortis, then later resolves from rigor as structural proteins are degraded. The timing depends on species, fish condition, stress at capture and temperature; struggling and exhaustion shorten the time to rigor, and rigor at high temperature produces strong contraction that can tear the myocommata, causing gaping of fillets. Fish filleted before rigor can shrink as rigor develops. Because fish muscle holds little glycogen, the ultimate pH is relatively high, commonly around 6.2–6.6, compared with about 5.5 in well-rested beef, and this higher pH favours bacterial growth.
Autolysis, the breakdown of tissue by its own enzymes, follows. ATP breaks down through ADP and AMP to inosine monophosphate (IMP), which gives the sweet, meaty taste of very fresh fish; IMP is then converted to inosine and to hypoxanthine, which is bitter, so the fresh flavour fades even before bacteria become important. Gut enzymes in feeding fish can digest the belly wall within hours, causing belly burst, which is why heavily feeding pelagic fish must be chilled at once and some species are gutted at sea.
The flesh of a live, healthy fish is essentially sterile; bacteria are on the skin, gills and in the gut. After death they invade the flesh, and at chill temperatures the cold-tolerant Gram-negative bacteria dominate. Specific spoilage organisms are the minority of the flora that produce the off-odours. In chilled marine fish stored aerobically, Shewanella putrefaciens is important: it reduces TMAO to TMA and produces hydrogen sulphide and other sulphur compounds. Pseudomonas species produce fruity and sulphury off-odours. In fish packed under modified atmosphere with carbon dioxide, Photobacterium phosphoreum often becomes the main TMA producer. In warm tropical waters and at abuse temperatures, mesophilic Enterobacteriaceae and others grow and spoilage is much faster.
The rate of spoilage depends strongly on temperature: fish kept in melting ice at 0 °C keeps many times longer than fish held at ambient temperature in the tropics. Keeping time also depends on species, fat content, catching method and handling. Lean white fish well iced can keep for one to two weeks or more; fatty fish keep less long because of oxidation as well as bacteria.
Fatty fish deteriorate also by lipid oxidation, the reaction of polyunsaturated fatty acids with oxygen, catalysed by heme proteins and metals, producing hydroperoxides and then aldehydes and ketones with rancid, painty or fishy odours. Oxidation continues in frozen storage, so fatty fish have shorter frozen shelf life than lean fish. Lipolysis by lipases releases free fatty acids, which speed oxidation and promote protein denaturation. Other post-mortem changes include discoloration of red and dark muscle as myoglobin oxidises to brown metmyoglobin, browning of tuna flesh, and blackspot (melanosis) in crustaceans, caused by polyphenol oxidase.
Good practice at sea is the foundation of quality: rapid killing to reduce stress, bleeding and gutting where appropriate, washing, immediate chilling, gentle handling to avoid bruising, and protection from sun and contamination. Everything later in the chain can preserve quality but cannot restore it. Records of catch time, hold temperature and landing time let the processor judge how much shelf life remains in each lot and decide its best use.
Sensory assessment is the quickest and most widely used way to judge fish freshness. Assessors look at the eyes (bright and convex in fresh fish, sunken and cloudy later), gills (bright red to brown or bleached, with mucus becoming slimy), skin and mucus (bright, with clear slime, becoming dull and discoloured), flesh firmness and odour (fresh seaweedy to sour, fishy and finally putrid). The Quality Index Method (QIM) gives demerit points to each attribute; the sum, the quality index, rises roughly linearly with storage time in ice, so it can be used to estimate remaining shelf life for a given species. The Torry scale scores cooked fish odour and flavour on a scale from 10 (very fresh) down to 3 (spoiled). The EU grading scheme classes fish into freshness categories for trade.
Sensory methods need trained panels and species-specific schemes, but they are cheap, fast and directly related to what consumers notice. Instrumental devices such as dielectric freshness meters measure changes in skin and tissue properties and are used for quick screening in some fisheries.
Total volatile basic nitrogen (TVB-N) measures the volatile bases, mainly ammonia, trimethylamine and dimethylamine, released as fish spoils. The bases are liberated from an extract or from the fish by alkali, distilled or diffused into standard acid (in Conway microdiffusion or steam distillation) and back-titrated. When the whole distillate from a sample of mass W grams is titrated, TVB-N in mg N per 100 g = (titre − blank, mL) × acid normality × 14 × 100 ÷ W. For a 10 g sample with a titre of 2.5 mL of 0.1 N acid and a blank of 0.1 mL, TVB-N = 2.4 × 0.1 × 14 × 100 ÷ 10 = 33.6 mg N/100 g. TVB-N rises late in storage, so it is better at detecting spoiled fish than at grading fresh fish, and its acceptable limits are set by each regulation and differ by species; the European Union, for example, sets limits for specified species groups.
TMA alone is a more specific index of bacterial spoilage in marine fish, since it comes from bacterial reduction of TMAO; it is low in fresh fish and rises with spoilage. It is not useful for freshwater fish, which contain little TMAO.
The K value measures freshness through the ATP breakdown products, which change from the earliest hours after death and so are useful for grading fresh fish. It is defined as K (%) = (inosine + hypoxanthine) ÷ (ATP + ADP + AMP + IMP + inosine + hypoxanthine) × 100. For example, with ATP 0.1, ADP 0.2, AMP 0.2, IMP 3.0, inosine 1.5 and hypoxanthine 1.0 µmol/g, the total is 6.0 µmol/g and K = 2.5 ÷ 6.0 × 100 = 41.7%. A simplified K1 value uses only IMP, inosine and hypoxanthine in the denominator, because ATP, ADP and AMP disappear quickly: here K1 = 2.5 ÷ 5.5 × 100 = 45.5%. Low K values indicate very fresh fish of sashimi quality in Japanese practice, and high values indicate loss of freshness; the thresholds differ by species.
Other tests include total viable counts and specific spoilage organism counts, histamine for scombroid fish, measures of lipid oxidation such as peroxide value and thiobarbituric acid reactive substances for fatty fish, pH, and texture. No single test describes freshness completely, so inspection laboratories combine sensory and chemical results; analytical methods are detailed in food-analysis.
Chilling to near 0 °C as soon as possible after catch is the single most important step in fish handling. Crushed or flake ice is the traditional medium: it has a large cooling capacity because of its latent heat, keeps the fish moist, and melting water washes away bacteria and slime. Enough ice must be used to cool the fish and to absorb heat entering from the surroundings, so more is needed in the tropics and in poorly insulated holds; fish and ice are layered in boxes so that each fish is in contact with ice and meltwater can drain. Chilled seawater (CSW, seawater cooled by ice) and refrigerated seawater (RSW, seawater cooled by mechanical refrigeration) are used on vessels catching large quantities of small pelagic fish, because they cool bulk catches quickly with little pressure damage; fish can take up salt and water and become discoloured if held too long. Superchilling to just below the freezing point of the flesh, around −1 to −2 °C, extends shelf life further but needs careful temperature control.
Ashore, fish should be kept iced or in chill rooms at every stage of landing, auction, processing and transport, because each period at higher temperature uses up shelf life that cannot be regained.
Freezing preserves fish for months by stopping bacterial growth and slowing chemical change. Freezing methods include plate freezers, in which blocks of fish or fillets are pressed between refrigerated plates and which suit regular packs and on-board freezing; air-blast freezers for irregular products; brine immersion freezing, traditionally used for whole tuna on fishing vessels; and continuous freezers and cryogenic freezers for individually quick frozen (IQF) products such as shrimp and fillets. Fish should be frozen fast through the zone of maximum ice crystal formation and then held at a constant low temperature. Codex standards for quick frozen fish require the product to reach −18 °C or colder at the thermal centre after stabilisation; many processors use colder storage, especially for fatty fish.
Glazing, applying a thin layer of ice by dipping or spraying frozen fish with cold water, protects the surface from dehydration and oxidation during storage. The glaze must be declared or excluded from the net weight in most markets. If 1000 g of glazed shrimp weighs 880 g after the glaze is removed, the glaze is 120 g, or 12% of the glazed weight. Freezer design and freezing-time calculation are covered in chilling-freezing-frozen-foods.
Frozen fish is not unchanging. Protein denaturation and aggregation in frozen storage make the flesh tough, dry and spongy, and increase drip loss on thawing. In gadoid fish such as cod and hake, an enzyme converts TMAO into dimethylamine and formaldehyde during frozen storage; formaldehyde cross-links proteins and accelerates toughening. Fatty fish develop rancid flavours by lipid oxidation, which continues slowly even at low temperature. Freezer burn, dry, pale, spongy patches on the surface, results from sublimation of ice where the product is poorly protected; glazing and good packaging prevent it. Temperature fluctuation in storage causes recrystallisation and moisture migration and worsens all these changes.
Thawing must also be controlled. Thawing in still air at room temperature is slow and lets the surface warm while the centre is still frozen; thawing in running cold water, in controlled humid air or by electrical methods is faster and more uniform. Drip loss, the fluid lost on thawing, is a common quality and economic index: 500 g of fish that gives 470 g after thawing has a drip loss of 30 ÷ 500 × 100 = 6%.
Canned fish is one of the oldest and most traded seafood products: tuna, sardines, mackerel, salmon, herring, anchovies and shellfish are canned worldwide. Fish is a low-acid food, with pH well above 4.6, so it must be given a thermal process in a retort sufficient to achieve commercial sterility, including destruction of Clostridium botulinum spores; process calculation is in thermal-processing-canning. The general sequence is receiving and thawing of frozen raw material, butchering (heading, gutting, cleaning), often precooking, packing into cans, adding a covering medium (brine, oil, tomato sauce or water), exhausting or vacuum seaming, retorting, cooling and inspection.
In tuna canning, whole frozen tuna are thawed, butchered and precooked by steam until the backbone reaches a set temperature; precooking coagulates the protein, removes oil and water that would otherwise separate in the can, and makes it easy to separate the white loin meat from skin, bones and dark meat. The loins are cooled, cleaned and packed as solid, chunk or flake packs. Sardines and small fish are brined, packed, precooked in the can and drained before the medium is added.
Several defects are characteristic of canned seafood. Struvite, glass-like crystals of magnesium ammonium phosphate, can form in canned fish and shellfish during storage and are mistaken by consumers for glass; they are harmless and dissolve in dilute acid, and their formation is limited by rapid cooling, control of pH and the use of sequestrants where permitted. Blackening or blueing of canned crab, shrimp and some fish is caused by iron or tin sulphide formed from sulphur compounds released from protein; it is prevented by sulphur-resistant lacquer, parchment liners and control of copper and iron contamination. Curd, coagulated protein on the surface of canned salmon, is mainly a cosmetic defect related to raw material handling. Honeycombing, a pitted structure in tuna loins, is a sign that the raw fish was decomposed before processing.
Raw material quality cannot be improved by canning. Fish that has started to spoil may give an acceptable-looking can but with off-flavours, soft texture and, in scombroid species, histamine, which is not destroyed by retorting. Raw tuna is therefore checked for temperature history, sensory quality and histamine before processing.
Most failures of canned fish that cause illness are not due to inadequate heat processes but to post-process leakage, when contaminated cooling water or handling introduces bacteria through defective seams. Cooling water is chlorinated, cans are not handled wet and roughly, and double seams are examined at regular intervals by tear-down or seam projection, as described in thermal-processing-canning. Finished cans are incubated in sample lots and examined for swelling, vacuum, appearance, odour and texture before release. Retort records, including temperature, time and initial temperature for each batch, are kept and reviewed by trained staff.
Because canned fish is traded internationally, importing countries require registration of low-acid canning establishments and their processes, and periodic inspection. Labels state the species, the form of pack and the covering medium, and the net and drained weights, which are checked against declared values. Short drained weight, mixing of species and use of a cheaper species under a premium name are recurring causes of rejection in the canned fish trade.
Salting is among the oldest ways of preserving fish. Salt draws water out of the flesh and enters it, lowering water activity until most spoilage bacteria cannot grow. In dry salting, fish are split or gutted and packed with solid salt; the brine that forms may drain away (kench salting) or be kept so that the fish lie in their own pickle (pickle curing, used for herring in barrels). In wet salting or brining, fish are immersed in a brine of set strength. Salt penetration depends on fish thickness, fat content, salt purity, temperature and brine strength; fatty fish and thick pieces salt more slowly. The amount of salt is often expressed as a ratio of fish to salt; with a ratio of 3:1, 300 kg of fish needs 100 kg of salt. Brine strength is expressed as grams of salt per 100 g of brine: 20 kg of salt in 80 kg of water gives a 20% brine, not 25%.
Impurities in salt matter: calcium and magnesium salts slow penetration but whiten and firm the flesh, and copper and iron promote rancidity. Classic defects of salted fish include pink, red discoloration and slime caused by extremely halophilic bacteria that grow in strong brine and on solar salt, and dun, brown spots of the halophilic mould Wallemia sebi on heavily salted, partly dried fish.
Drying removes water to a level at which bacteria and moulds cannot grow. Sun drying on racks, mats or beaches remains the main method in many fishing communities; it is cheap but slow, depends on weather, and exposes the fish to dust, sand, insects and birds. Blowfly larvae infest wet fish during the early stage, and hide beetles (Dermestes species) attack dried fish in storage, causing large losses. Improved methods include raised racks, solar tent and cabinet driers, and mechanical driers with heated air, which give a cleaner product and dry through the rainy season. Many dried fish are salted first, because salt both lowers water activity at higher moisture and speeds water removal.
Yield follows the solids balance. Fresh fish at 78% moisture dried to 16% moisture gives 100 × 0.22 ÷ 0.84 = 26.2 kg of dried fish per 100 kg; salted fish at 60% moisture dried to 25% moisture gives 100 × 0.40 ÷ 0.75 = 53.3 kg. Dried fish must be stored dry; it is hygroscopic, particularly when salted, and absorbs water in humid weather, after which moulds and spoilage follow. Drier design is covered in evaporation-dehydration.
Smoking combines drying, salting and the deposition of smoke components that flavour, colour and help preserve the fish. Cold smoking is carried out at a temperature below that at which the flesh cooks, commonly below about 30 °C, so the product stays raw in texture; cold-smoked salmon is the main example. Hot smoking cooks the fish as well as smoking it, raising the flesh to a temperature at which the proteins coagulate; hot-smoked mackerel and trout are examples. Traditional kiln smoking in many tropical countries dries and smokes fish heavily for long storage at ambient temperature. The chemistry of wood smoke, including phenols, carbonyls, acids and polycyclic aromatic hydrocarbons, and the use of liquid smoke are covered in meat-poultry-egg-products.
Lightly salted, smoked fish packed in reduced-oxygen packs and kept chilled carry particular hazards. Listeria monocytogenes can grow at chill temperatures in cold-smoked fish, which is not cooked before eating. Clostridium botulinum type E, common in aquatic environments, can grow and form toxin at refrigeration temperatures in vacuum-packed fish if salt is too low. Control relies on salt in the water phase, chill temperature and shelf life. Water-phase salt (WPS) = salt ÷ (salt + water) × 100: a product with 2.0 g salt and 60 g water per 100 g has 2.0 ÷ 62 × 100 = 3.2% WPS. US FDA guidance, for example, specifies minimum water-phase salt levels combined with temperature and other controls for different smoked products; requirements differ by country.
Fish sauce is a clear, amber to brown liquid made by fermenting small fish, often anchovies, with a large amount of salt for many months. It is a staple condiment in Southeast and East Asia, known as nuoc mam in Vietnam, nam pla in Thailand and patis in the Philippines, and similar products were made in the ancient Mediterranean. Fish and salt are mixed, commonly at ratios in the range of about 3:1 to 1:1 by weight, and held in tanks or jars at ambient temperature for six months to more than a year. The high salt prevents putrefaction; the fish proteins are hydrolysed mainly by the fish's own enzymes, especially digestive enzymes from the gut, with a contribution from halophilic and halotolerant bacteria that also develop the characteristic aroma. The liquid is drained, sometimes aged and blended, and filtered.
Fish sauce is graded mainly by its total nitrogen and amino nitrogen content, which reflect the degree of protein hydrolysis; first-run sauce is of higher grade than later extractions made by adding brine to the residue. Histamine can form if the raw fish was poorly handled before salting, so raw material quality is the main control.
Fish and shrimp pastes are made from fish or small shrimp salted and partly fermented, then ground and often dried or sun-matured to a thick paste or block. Examples are shrimp paste in Malaysia, Indonesia and Thailand, bagoong in the Philippines and ngapi in Myanmar. Like fish sauce, they rely on salt to suppress spoilage and on enzymes for flavour, and they are used in small amounts as seasoning.
A different group of products ferments fish with a carbohydrate source such as cooked rice, so that lactic acid bacteria produce acid. The acid together with a lower salt level preserves the fish and gives a sour taste. Examples include burong isda in the Philippines, som fak and pla som in Thailand and the traditional narezushi of Japan, the ancestor of sushi. These products depend on a rapid fall in pH; if acidification is slow, pathogens and spoilage organisms can grow, which is why traditional practice uses fresh fish, clean equipment and active fermentation. Fermentation microbiology is covered in food-microbiology and fermentation-food-biotechnology.
The safety of fermented fish rests on salt, acidity and raw material quality. Salt levels in fish sauce are very high, typically well over 20% in the finished product, which prevents the growth of pathogens; lower-salt modern products need other hurdles. Histamine is the most important chemical hazard, because the fish used, such as anchovies and sardines, are rich in histidine, and histamine formed before or early in fermentation is not removed. Other hazards include biogenic amines generally, contamination with pathogens in low-salt and poorly acidified products, and Clostridium botulinum in some traditional fermented fish products made without adequate acidification or salt, which have caused outbreaks.
Process improvements include faster fermentation by adding enzymes or controlling temperature, starter cultures for lactic fermentations, desalting, and standardisation of salt and nitrogen content. International standards, including a Codex standard for fish sauce, set compositional and hygiene requirements; national rules differ. Producers who export fish sauce must also meet the importing country's rules on histamine, salt declaration and additives.
Fish mince is flesh separated from skin and bone by a meat-bone separator, in which fish pressed against a perforated drum leaves the bones and skin outside. Mince makes use of small, bony or low-value fish and filleting trimmings, but it is unstable in frozen storage because it contains fat, blood, enzymes and sarcoplasmic proteins. Surimi is mince that has been washed, dewatered and stabilised so that it keeps its gel-forming ability in frozen storage. It was developed in Japan and is now made from Alaska pollock, Pacific whiting, threadfin bream and many other species.
The mince is washed several times with cold water, which removes sarcoplasmic proteins, fat, blood, pigments and odorous compounds, concentrating the myofibrillar proteins that form gels. It is then dewatered by screw press, refined through a fine screen to remove remaining skin, scales and connective tissue, and mixed with cryoprotectants, commonly about 4% sucrose, 4% sorbitol and a small amount of polyphosphate, which protect the myofibrillar proteins from denaturation during freezing and frozen storage. The paste is frozen in blocks. Yield of surimi from round fish is low, commonly about 25–35%; 300 kg of surimi from 1000 kg of fish is a 30% yield.
Surimi products owe their elastic texture to the gel formed by myofibrillar proteins. The thawed surimi is chopped with salt, about 2–3%, which solubilises myosin and actomyosin into a sticky paste. When this paste is held at a moderate temperature, roughly 30–40 °C or for longer at refrigeration temperature, it forms a weak translucent gel, a stage called setting or suwari, which involves cross-linking catalysed by the endogenous enzyme transglutaminase. Heating to a higher temperature, around 90 °C, then gives a firm, elastic, opaque gel. If the paste is held in the range of roughly 50–70 °C, endogenous proteases degrade myosin and the gel weakens or breaks down; this gel softening is called modori. Some species, notably Pacific whiting infected with myxosporean parasites, contain heat-stable proteases that cause severe modori, and protease inhibitors such as egg white, beef plasma protein or potato extract are added and heating through the modori range is made rapid.
Gel strength is measured by puncture tests (breaking force and deformation), folding tests on slices, and colour and water-holding tests, and surimi is graded and priced accordingly.
Kamaboko is the traditional Japanese steamed fish cake, formed on a wooden board; chikuwa is grilled on a rod; satsuma-age is fried. Crab-flavoured sticks, imitation crab or kanikama, are made by forming thin sheets of surimi gel, folding them into fibres to mimic crab muscle, colouring the surface red, flavouring and cooking. Fish balls, fish sausages, fish burgers and fish fingers are other mince products; fish fingers and portions are often sawn from frozen blocks of fillets or mince, battered, breaded and fried. Starch, egg white and other ingredients are used to adjust texture and cost.
Products made from surimi are cooked, but they are often packed and stored chilled, so they need good hygiene after cooking and controlled shelf life. Their labelling must state the fish content and avoid misleading consumers into thinking they contain crab or shellfish. Fish mince and surimi products also carry allergens, including fish, egg and sometimes crustacean flavourings, which must be declared.
Shrimp is the most valuable traded seafood. Wild-caught and farmed shrimp are washed, chilled and graded by size, expressed as a count, the number of pieces per kilogram or per pound; a shrimp of 25 g average weight gives a count of 40 per kilogram. Products range from whole raw shrimp and headless shell-on shrimp to peeled and deveined, tail-on or tail-off, raw or cooked, frozen as blocks or IQF and glazed. Peeling is done by hand in many countries and by roller peeling machines elsewhere.
The main quality problem of shrimp after harvest is melanosis or blackspot, the black discoloration of the shell and head caused by the polyphenol oxidase system in the shell. It is prevented by rapid chilling and by dipping in permitted anti-melanosis agents, traditionally sulphites, whose residual sulphur dioxide must be declared because sulphites are allergens of concern in many jurisdictions, or 4-hexylresorcinol where permitted. Shrimp are sometimes soaked in phosphate solutions to retain moisture; excessive soaking adds water that consumers pay for, and phosphate use and labelling are regulated. Farmed shrimp must also be controlled for residues of veterinary drugs, a frequent cause of rejection in export trade.
Crabs are cooked whole, cooled, and the meat picked from the body and legs by hand or machine; picked crab meat is chilled, pasteurised in hermetically sealed containers, frozen or canned. Because picking after cooking is a hands-on process, recontamination with Listeria and Staphylococcus aureus is a concern, and pasteurised crab meat is a refrigerated product, not shelf-stable. Lobsters and many crabs are traded live, kept cool and moist, or cooked and frozen; high-pressure processing is used to separate lobster meat from the shell raw.
Cephalopods (squid, cuttlefish and octopus) are exported frozen whole, cleaned, as tubes and rings or as tenderised products. They are lean, high in protein and have low connective tissue but tough muscle that needs mechanical or enzymatic tenderising for some products. Cephalopod processing produces skin, viscera and pens as by-products. Squid and cuttlefish are also prone to discoloration and odour if not chilled quickly, and their ink and viscera must be removed carefully to avoid staining the mantle.
Oysters, mussels, clams and scallops are filter feeders that concentrate bacteria, viruses, toxins and contaminants from the water they live in. Many are eaten raw or lightly cooked, which makes them one of the highest-risk seafoods. Harvesting areas are classified by regulators on the basis of faecal indicator bacteria (Escherichia coli or faecal coliforms) in shellfish flesh or water, and the class decides whether shellfish may go straight to market or must first be depurated, relaid in clean water, or heat treated; the class definitions differ between regulations such as those of the EU and the US. Areas are also monitored for marine biotoxins and closed when toxins exceed limits.
Depuration holds live bivalves in tanks of clean, often UV-treated, seawater for a set time so that they purge bacteria from their gut. It is effective against many bacteria but much less so against viruses such as norovirus and hepatitis A virus, and it does not remove biotoxins. Live bivalves are kept cool and identified with tags showing harvest area and date, so that any illness can be traced. High-pressure processing is used to shuck oysters and to reduce Vibrio vulnificus and V. parahaemolyticus in oysters intended for raw consumption.
A large part of the world's fish catch, mainly small, bony, oily pelagic fish and processing waste, is reduced to fish meal and fish oil rather than eaten directly. The wet reduction process cooks the raw fish to coagulate protein and release oil and water, presses it to give press cake and press liquor, and separates the press liquor by decanter centrifuge (removing solids) and by separator centrifuge (recovering crude oil). The remaining stickwater, which holds dissolved and fine proteins, is concentrated in evaporators and added back to the press cake, giving whole meal. The mixture is dried, cooled, milled and stored. Antioxidants are added to the meal to prevent oxidation of the residual oil, which can generate enough heat to cause self-heating and fires in storage.
Fish meal is a high-protein feed for aquaculture, poultry and pigs. Its quality depends on raw material freshness, which governs protein digestibility and biogenic amine content, and on drying conditions; low-temperature dried meals are valued for aquaculture feeds. The yield follows from the solids balance: if raw fish contains 20% fat-free dry solids and all are recovered in a meal with 82% fat-free solids, 100 t of fish yields 20 ÷ 0.82 = 24.4 t of meal. Fish oil is refined and used in aquafeeds, and refined, deodorised oils rich in EPA and DHA are used in supplements and foods.
Fish silage is a liquid product made by adding acid, usually formic acid or a mixture of acids, to minced fish or waste; the low pH prevents bacterial spoilage while the fish's own enzymes liquefy the tissue. It is a simple way for small plants and fish farms to convert waste into animal feed without a reduction plant. Fermented silage uses lactic acid bacteria and a carbohydrate source instead of mineral acid.
Fish protein hydrolysates are produced by controlled enzymatic hydrolysis of fish or fish by-products with added proteases, followed by separation and drying; they are used in feeds, pet foods and, increasingly, foods and supplements for their functional and bioactive properties. Fish protein concentrate, made by solvent extraction of fat and water from fish, was developed as a cheap protein for human nutrition but has had limited success because of cost and functionality. Hydrolysates made from by-products of species with allergenic proteins must still be labelled as fish, and their degree of hydrolysis is controlled to avoid bitter peptides.
Fish skins and scales are sources of collagen and gelatin. Fish gelatin has a lower gelling and melting temperature than mammalian gelatin, especially from cold-water species, which limits some uses but suits products where a lower melting point is wanted and where religious or other dietary rules exclude pig or cattle gelatin. Crustacean shells from shrimp and crab processing contain chitin, together with protein, minerals (mainly calcium carbonate) and pigments such as astaxanthin. Chitin is recovered by demineralisation with dilute acid and deproteinisation with dilute alkali; treating chitin with concentrated alkali at high temperature removes acetyl groups to give chitosan, which is used in water treatment, agriculture, medicine and as a food additive where permitted.
Other by-products include fish liver oils, fish bones for calcium supplements, fish heads and frames for soup stocks and pet food, shark cartilage and fins, and fish scales for collagen. Full use of the catch reduces waste and pollution and adds income to processing plants; it also requires the by-products to be handled hygienically if they are intended for food.
Scombrotoxin fish poisoning, also called histamine poisoning, follows eating fish in which bacteria have converted free histidine into histamine. It is associated with fish high in free histidine: tuna, mackerel, bonito, sardines, anchovies, herring, mahi-mahi and others. Histamine-forming bacteria, including Morganella morganii, some Enterobacteriaceae and Photobacterium species, carry histidine decarboxylase; they grow when fish is held above chill temperatures, particularly in the hours after catch. Symptoms appear within minutes to hours and resemble an allergic reaction: tingling or burning in the mouth, flushing, rash, headache, palpitations and gastrointestinal upset.
Histamine is heat-stable, so cooking, canning and smoking do not destroy it, and affected fish may not look or smell spoiled. Control therefore depends on rapid chilling after death and continuous temperature control, monitored through vessel and receiving records and by testing for histamine. Regulatory limits and sampling plans for histamine differ: the EU, for example, sets a sampling plan with limits for fishery products from species associated with high histidine and separate limits for products that have undergone enzyme maturation in brine, and US FDA guidance uses histamine levels both as an indicator of decomposition and of hazard. A concentration of 25 mg per 100 g equals 250 mg/kg (ppm), and results must be compared with the limit in the same units.
Marine biotoxins are produced by certain microalgae and accumulate in filter-feeding shellfish or move up the food chain into fish. They are not destroyed by cooking, freezing or smoking, and contaminated shellfish look and taste normal. Paralytic shellfish poisoning (PSP) is caused by saxitoxins from dinoflagellates such as Alexandrium; it can cause numbness, paralysis and death by respiratory failure. Diarrhetic shellfish poisoning (DSP) is caused by okadaic acid and related toxins and gives gastrointestinal illness. Amnesic shellfish poisoning (ASP) is caused by domoic acid from diatoms of the genus Pseudo-nitzschia and can cause permanent memory loss. Neurotoxic shellfish poisoning is caused by brevetoxins. Ciguatera fish poisoning is caused by ciguatoxins produced by dinoflagellates on tropical reefs and concentrated in large reef predators such as barracuda and grouper; it causes gastrointestinal and neurological symptoms, including reversal of hot and cold sensation, that can last months.
Tetrodotoxin, found in puffer fish and some other animals, is a potent neurotoxin; in countries where puffer fish is eaten, preparation is restricted to licensed specialists. Control of biotoxins relies on monitoring of harvesting areas and closures, not on processing.
Several parasites in fish can infect people who eat raw, lightly cooked, cold-smoked, marinated or lightly salted fish. Nematode larvae of the family Anisakidae (Anisakis, Pseudoterranova) in marine fish and squid cause gastrointestinal illness and allergic reactions; tapeworms of the genus Diphyllobothrium and trematodes (liver flukes such as Clonorchis and Opisthorchis) are transmitted by freshwater and anadromous fish. Thorough cooking kills them. For fish to be eaten raw, freezing to a specified core temperature for a specified time kills parasites, and the conditions are set in regulations and guidance, for example −20 °C for at least 24 hours in EU rules, with longer times in some other guidance. Visual candling of fillets on light tables removes visible worms but is not complete.
Pathogens in seafood include Vibrio parahaemolyticus, V. vulnificus and V. cholerae, especially in raw bivalves in warm waters; norovirus and hepatitis A virus in bivalves from sewage-polluted waters; Listeria monocytogenes in ready-to-eat products such as smoked fish; Clostridium botulinum type E in vacuum-packed lightly preserved fish; and Salmonella and Staphylococcus aureus from handling. Chemical hazards include methylmercury in large predators, dioxins and PCBs in fatty fish from polluted waters, veterinary drug residues in farmed fish and shrimp, and allergens; these are covered in food-toxicology-contaminants-allergens and in the Veterinary track's milk-meat-hygiene-food-safety.
Seafood was one of the first foods for which HACCP became a legal requirement in international trade. The United States requires seafood processors, domestic and foreign, to operate HACCP plans under its seafood HACCP regulation, and the European Union requires fishery products to come from approved establishments in listed third countries whose competent authorities give equivalent guarantees on hygiene, residues and contaminants. Other importing countries such as Japan, Canada, China and Australia have their own requirements. A processor exporting to several markets must therefore meet several sets of rules, often audited by both the national competent authority and the importer.
In India, the Export Inspection Council (EIC) and its agencies certify the quality and safety of seafood for export and approve processing plants, while the Marine Products Export Development Authority (MPEDA) promotes exports and supports the industry; domestic food safety is under FSSAI. Comparable arrangements exist in other exporting countries, where a designated competent authority inspects plants and issues health certificates for each consignment.
Codex Alimentarius standards cover many seafood products, including quick frozen fish fillets, quick frozen shrimps or prawns, canned tuna and bonito, canned sardines, salted fish, smoked fish, surimi, fish sauce and live and raw bivalve molluscs, together with a code of practice for fish and fishery products. These standards define products, set essential composition and quality factors, list permitted additives, and set hygiene, labelling, sampling and defect criteria. Defects assessed in frozen fish include dehydration, foreign matter, bones, parasites, discoloration, and the net content after removal of glaze. National standards follow or adapt Codex.
Commercial grading of seafood is by species, size or count, form of product (whole, headed and gutted, fillet, peeled), quality grade and pack. Buyers write detailed specifications covering temperature history, chemical residues, histamine where relevant, microbiology, glaze, net weight and labelling. Net weight disputes, especially over glaze and added water, are among the most common causes of trade claims.
Seafood labels in most markets must state the commercial name and often the scientific name of the species, the production method (caught at sea, caught in fresh water or farmed), the catch area (for example the FAO fishing area) or country of production, whether the product has been defrosted, and allergens. Species substitution, selling a cheap species as an expensive one, is a well-documented fraud; DNA-based methods are now routinely used to verify species identity in fillets and processed products.
Traceability from vessel or farm to consumer is required by many regulations and by retailers, both for safety recalls and to prevent illegal, unreported and unregulated (IUU) fishing; the EU, for example, requires catch certificates for imported fishery products. Voluntary sustainability certification schemes for wild fisheries and aquaculture are widely demanded by retailers in some markets. For the processor, these requirements mean keeping lot records that link raw material, processing and dispatch, and labelling that is accurate for every market served.
A student uses this subject to answer the fish questions that appear in food technology papers: why fish spoils faster than meat, what TMAO and TMA are, how the K value and TVB-N are calculated, what happens during freezing and frozen storage, why tuna is precooked, what struvite and blackening are in canned seafood, how surimi gels form, and which hazards are specific to seafood. In the laboratory the same student scores fish by the Quality Index Method, runs a TVB-N distillation, calculates drying yields and glaze, and tests the gel strength of a fish paste.
Five years into a seafood plant or inspection laboratory, a technologist controls raw material intake (temperature, sensory grade, histamine for scombroid species), sets icing, freezing and glazing procedures, writes and verifies the HACCP plan required by each export market, and investigates rejections for histamine, residues, glaze shortfall or species substitution. The same person runs the canning line records or surimi gel tests, trains handlers, and prepares the documentation that inspectors and buyers audit before each shipment is released. When a buyer reports a defect, the technologist traces the lot through the plant's records to find its cause.
After twenty-five years a senior manager decides what a seafood business processes and for which markets: whether to invest in on-board freezing, IQF lines, surimi or value-added products; how to secure traceable, legally caught or certified raw material; how to keep approvals for several export markets at once; and how to turn heads, frames, shells and viscera into meal, oil, collagen or chitosan. The manager also handles crises such as a histamine outbreak, a biotoxin closure or a residue rejection, deciding on recalls, supplier changes and communication with regulators.
A researcher with long industry experience studies the biochemistry and microbiology behind seafood quality and safety: specific spoilage organisms and their metabolism, rapid freshness sensors, mechanisms of protein denaturation and formaldehyde formation in frozen fish, the enzymology of surimi setting and modori, growth and toxin formation by C. botulinum type E and Listeria in lightly preserved fish, histamine formation kinetics, and methods for detecting biotoxins and species fraud. Such work supports regulators in setting limits and helps industry design processes for new species and products.
This manual does not give numerical regulatory limits for TVB-N, TMA, histamine, biotoxins or parasites beyond naming the jurisdictions that set them and one example of an EU freezing condition for parasites, because limits, sampling plans and species groups differ between the EU, the US, Codex, India and other countries and are revised. It does not give thermal process schedules for canned fish (see thermal-processing-canning), freezer design and freezing-time calculations (see chilling-freezing-frozen-foods), smoke chemistry (see meat-poultry-egg-products) or detailed contaminant limits (see food-toxicology-contaminants-allergens). Aquaculture production, fishing gear and vessel design, and seaweed processing are outside its scope, and species-specific QIM schemes and harvest-area classification criteria are described only in principle.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Every numerical answer was computed by script. Regulatory limits for TVB-N, histamine, biotoxins and parasite freezing differ between jurisdictions; questions name the jurisdiction or avoid a number.
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