Fish entrails and processing waste as a raw material

by Thomas Jensen
EM5 17 FISH Byproducts

Higher profits through industrial and culinary usage

With the exception of trout, dorade and a few other fish species that are traditionally prepared on the bone, fillets or loins are today the order of the day where enjoyment of fish is concerned. But that doesn’t mean that processing waste and other remains that are often overlooked are worthless: indeed, they often contain valuable ingredients and – if these are processed and prepared correctly – they can definitely find interested buyers. Many of these fish parts are edible and some of them are even considered delicacies in certain regions of the world.

Preferences when it comes to taste are often contradictory and not easy to understand. Dietary preferences have undergone changes in the course of history. What might in one place be seen as waste can somewhere else be considered a culinary delight. In our part of the world no one would think of eating fish entrails, and even the dark strips of meat from the muscle along the lateral line of the fillet are frequently removed. At the same time a lot of these supposedly sensitive fish eaters enjoy eating slimy oysters without considering that they are swallowing a living animal complete with intestine, gills and other guts. What we know, use and appreciate as food is not only regulated by laws and requirements (for the purpose of food safety, for example) but is also influenced by traditions, culture or religion. That explains why by-products like skin, liver, roe and other internal organs are rarely seen on our plates although they are at least just as nutritious as the fillets. Even tolerant people will perhaps turn up their noses at frogs’ legs, scorpions, locusts or insects that are eaten as delicacies in other parts of the world. Our ancestors were much more robust with regard to their food. One only has to think of snipe that was roasted and eaten whole complete with its innards and bowel contents and was seen as the peak of culinary enjoyment. Today this rather dubious pleasure is forbidden in the EU for reasons of hygiene. An unnecessary taboo since most Europeans would probably be quite happy to do without it… With the exception perhaps of some obstinate Italians who in spite of the ban still can’t do without their “merdocchio”.

By-products and waste from fish processing companies contain a large share of organic material. Recycling these raw materials for further usage contributes towards reducing costs, protecting the environment and furthering the sustainability of the whole process chain. Fish wastes contain proteins and fat, minerals, vitamins and enzymes. The spectrum of their potential usage options ranges from animal feed, biodiesel and biogas to natural pigments and cosmetics, medical and pharmaceutical applications such as dietary products, and nutraceuticals. The value that is ultimately to be found in these products can vary greatly. The more processing and the higher the resulting benefit the larger will as a rule be the generated profit. Fish protein hydrolysates, for example, which are converted to protein rich animal feed via biological fermentation are usually less expensive than elaborately produced isolates such as collagen, keratin or chitosan that serve as functional components in numerous products. Although research and development in this field only really got underway just a few years ago it is already revealing that a lot of useful substances are to be gained from fish and seafood waste and that some of them probably have antimicrobial properties and can also be effective against tumours. Fish waste has to be processed quickly, however, for the organic material has a low biological stability (short shelf life) due to its high water content, its oxidation susceptibility and high enzyme activity.

Chitosan and dried cod heads

One of the most popular by-products from seafood processing is chitosan which is produced from the shells of shrimps and other crustaceans. Chitosan is used in the production of cosmetics and pharmaceutical products but also in other fields. Due to its positive charge and coagulating properties chitosan has chelate-building properties. It binds and flocculates the protein-containing residues found in waste water from the food industry. Apart from that, chitosan has germ-killing effects on bacteria, yeasts and fungi and also film-forming properties. The packaging industry is trying to coat film with chitosan to make packaging materials antimicrobially “active”. An elegant possibility to make use of fish oil and fat from slaughter waste is also to process it to biodiesel which can be used in its pure form or as an additive in crude oil-based diesel fuels. Slaughter waste is also very suited to the production of biogas.

A much more effective usage area for the remnants from fish processing, however, is for human consumption. The best known example of this is unsalted air-dried cod heads and backbones that are left over in large quantities during production of stockfish and klippfish in Norway and Iceland. The most important market for this by-product is Nigeria where “okporoko” serves as an aromatic stock for soups and stews: an inexpensive and nutritious food that contains a lot of protein with a long shelf-life plus other nutrients. In Norway alone about 50 to 60 million US dollars revenue is generated with the export of dried Gadidae heads.

Leather and gelatine from fish skin

A raw material that arises during processing and which is often underestimated is fish skins. Part of the skins, particularly of salmon, tilapia, cod and catfish but recently also skate, shark and sturgeon is already processed to very elaborate, resilient and optically attractive leather but, with that, the full potential is by no means exhausted. One reason for this is presumably the small size of the individual fish skins which forces users to intricate patchwork creations. Usage areas are thus mostly limited to handbags, purses, shoes, jewellery and similar decorative fashion accessories.

Fish gelatine is also of great significance. It is produced using hydrolysis of the collagen in fish skin, mostly the skin of cod, haddock and pollock. One advantage of fish gelatine is its chemical composition. Although it contains the same amino acids as other animal gelatines it has a relatively low share of proline and hydroxyproline. This reduces the number of hydrogen bridges between the molecules and lowers gelling temperature. Fish gelatine is a thermo-reversible hydrocolloid with high gelling stability and a good viscosity. In numerous foods it serves as a gelling and thickening agent, emulsifier and stabiliser but it is also used in cosmetics, pharmaceuticals and film material. Because, in contrast to mammalian gelatines, religious Jews and Moslems hardly have any reservations against fish gelatines these are preferably used in kosher and halal products.

The skins of some fish species can also be prepared like “chicharron”, the crisply fried pork skin that is found in Mexican cuisine. For this product the scales and any remaining pieces of meat are removed from the fish skins which are then boiled in salt water, dried and subsequently fried crisp in vegetable oil. Finely seasoned, for example with herbes de Provence, smoked pepper or lemon pepper the fish chicharrones are eaten like crisps. An original but very promising new usage of tilapia skins can currently be seen on Youtube which shows Brazilian doctors using sterilised fish skins as bandages when treating severe burns. They cover the burn completely with the tilapia skin which contains a particularly large quantity of collagen type 1 which accelerates the healing process and reduces scar formation.

Is eating fish guts a delicacy or a risk?

Usage of a fish’s internal organs is a controversial matter and it is viewed differently depending on cultural background. In Japan, Korea and other Asian countries the liver, eyes or intestines of some fish species are considered a delicacy. This is usually not the case in the western world although it must be said that in some regions it was, or is, not rare that fish heads, for example, are eaten. In the course of the “tip-to-tail” philosophy that rejects wastage of any edible parts of an animal this tradition is currently undergoing a revival among more courageous, curious or adventurous consumers. Critics and the US health authorities such as the FDA and EPA sooner advise against consumption of such foods because harmful chemical substances and toxins such as dioxins, PCBs, and mercury as well as toxins from cyanobacteria and ciguatera accumulate mainly in organs such as the eyes, brain, liver, kidney, spleen and intestine. Eating the green-grey tomalley of lobster and other crustaceans was also deemed risky, although this is particularly appreciated by lobster lovers. The tomalley organ serves a lot of crustaceans as liver and pancreas which can store toxic substances.


This doesn’t seem to worry consumers in South-East Asia, and in Korea there are very strict regulations for the human consumption of fish heads, fish guts (“visceral by-products”) and squid glands. In Japan there is a centuries-old tradition of eating raw fish and seafood. Not only as sushi and sashimi, which have long been part of the gastronomic standard throughout the world, but also in the form of more questionable products such as “ika ikizukuri”, fragmented live squid, whose tentacles attach themselves to the tongue and palate in the mouth with still functioning suction cups or, no less bizarre, “odori ebi”, a sashimi preparation made of live baby shrimps. The translation “dancing shrimps” is probably better understood as a euphemistic variation. Even more doubtful is the ceremonial consumption of the pufferfish fugu, some of whose body parts contain the deadly tetrodotoxin against which there is as yet no antidote. According to legend, fugu gives health, power, and virility to anyone who survives the risky meal unscathed. This “game” can therefore to be understood as a combination of Russian roulette and Viagra. In keeping with this, in Japanese literature death from fugu is romanticized as a particularly stylish and delicate suicide method.

Enzymes and hormones from fish waste

Enzymes, and above all proteases, lipases, oxidases and transglutaminases as well as bioactive peptides that can be extracted from fish waste are of great commercial significance. They were used for centuries based on long-standing experience but today experts are looking for rational and controlled usage options to make implementation of enzymatic processes less expensive, more efficient and more environmentally friendly. The fields of biocatalysis and biosensors are becoming more and more important. Enzymes play a decisive role as biocatalysts in numerous biotechnological processes, including fish and seafood processing. Of particular interest are fish enzymes from cold water species that display high activity at low temperatures. This enables gentle processing without thermal influences. Proteases are for example used for decalcifying or curing seafood products, they serve as agents for tenderising the fish fillets, or help remove the skin without damaging the meat.

The great progress that has in the meantime been made in the field of fish reproduction in aquaculture would hardly be conceivable without the use of hormones. Some fish species that do not readily reproduce in aquaculture on account of unsuitable environmental, feeding or farming conditions can today, through the use of sex and maturity hormones, reproduce more or less regularly – even outside the spawning cycles that are typical of the species.

Roe and caviar: popular and expensive

Fish roe is a good example of the fact that, carefully processed, the internal organs of fishes can be enjoyed as a delicacy. Fish roe is eaten both in its natural casing, the roe sac, and in the form of individual eggs, as caviar. A well-known roe product in the Mediterranean region is “bottarga” (Ital.), also called “poutargue” or “boutargue” (French). Bottarga is traditionally made from the roe of common grey mullet (Mugil cephalus) but also of tuna or sword fish. To make it, the roe is first salted, pressed, dried in the sun and then sprayed with a thin layer of wax. Bottarga has a piquant smoky flavour and it is enjoyed grated over pasta or in thin slices as a starter.

Consumption of roe is particularly popular in Japan where, depending on the fish species, various products are differentiated. “Sujiko” is the name of the roe of salmon and trout, “kazunoko” is pickled herring roe, “mentaiko” are the reproduction organs of Alaska pollock. One of the substances found in the orange to pale yellow coloured sea urchin roe, called “uni” in Japan, is cannabinoid anandamide and for that reason this product is considered an aphrodisiac. It is very popular in Chile where it is called “erizo de mar”.

Much better-known and usually more highly appreciated are the individual eggs of the roe, commonly known as “caviar”. The classic “genuine” caviar varieties Beluga, Sevruga and Ossetra all come without exception from sturgeon species. In addition, however, other types of fish and marine animals are also used as “egg providers”, although these products are not everywhere permitted to bear the name caviar. Pacific salmon (“keta-caviar”), trout, lumpfish (“German caviar”), capelin (masago), pike and bowfins, pollan, herring and flying fish (tobiko) are of most commercial importance. Taramá (“taramas”) is produced from the eggs of carp in Greece. Among the relatively rare marine delicacies is the roe of some shrimp and other types of crustacean, which is enjoyed in Japan under the name “ebiko”.

Fish’s milt and liver take some getting used to

In contrast to eating fish eggs and roe (which hardly meet with any reservations) the consumption of the male sexual organs with semen, usually called “milt” or soft roe, is less common. However there are some cultures in which consumers have hardly any reservations about this product which is then eaten fried, salted or pickled. In Russia, for example, the milt (“moloka”) of herring or capelin is eaten, in Japan the male reproduction organ of cod (“shirako” which translated means “white children”), and in Sicilian cuisine milt of tuna (“lattume”) is sometimes used as a topping for pasta dishes. A lot of Rumanians like fried “lapti” (derived from the Latin word for milk = lacteus) of carp and other freshwater fishes.

One of the commercially most important internal organs of a lot of fish species is the liver. It can be prepared and eaten directly or a thin liquid, the vitamin rich and easily digestible liver oil, can be extracted from it. Cod liver oil, for example, is very rich in omega 3 fatty acids, iodine, phosphor and the vitamins E, A, and D. The high Vitamin D content makes liver oil one of the effective natural medicines that prevents rickets.

The livers of some Gadidae species, for example cod, haddock, ling or blue ling, are suitable for direct consumption. The traditional fish dish “mölje” which is eaten on the Norwegian Lofoten Islands consists of poached cod, cod tongue, roe and a sauce made of cod liver. On account of its fine flavour and tender consistency the soft fatty liver of anglerfish (Lophius piscatorius) is sometimes called “marine foie gras”. It can simply be fried briefly like any other liver or cooked in foil and served with different sauces. In the days of ancient Rome the fish sauce “garum” (also called “liquamen”) was made from the livers and other entrails of fishes. Cultivated “high society” Romans
had a great liking for fish guts such as roe, milt and liver anyway. Perhaps it’s time for today’s haute cuisine to be inspired by such forgotten delicacies…


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