There are three reasons to utilize some portion of the waste generated in fish processing. First, the removal of one specific type of waste may significantly reduce the total bulk that must be disposed of. This can reduce tipping fees or can bring the processor below a regulatory cap for dumping at sea. Second, processing one potentially valuable organ may increase revenue. Finally, pulling particular organs out of the mass of waste may increase the value of the remainder. All of these cases will be considered below, on an organ by organ basis.
Skins are particularly interesting since they may fit all three of the reasons cited above. Skin often forms a significant fraction of the total waste, and the current trend for manufacturing value-added products at the point of origin means more skinless fillets are being produced, and so more skins are being added to the waste streams of plants, whereas previously the skins were shipped off as part of the product. Additionally, because skins have a high ash content, their inclusion can reduce the price of fish meal, although it increases the volume produced. Skins can actually be profitable, if conditions are right.
There are two major products for which fish skins are the raw material: gelatin and leather. Gelatin production is suited to very large volume processing plants and leather to plants with relatively small production. In both of these cases, the processing of the skins is likely to be by separate entities, due to the heavy capital investment in equipment and the expertise required for each of these ventures.
Fish gelatin has potentially large markets in the United States, because it offers food manufacturers the possibility of kosher and halal labeling. While the percentage of the population actually keeping kosher is small, a surprisingly large percentage of consumers prefer to purchase food products labeled as kosher in the belief that such products are superior. For this reason, food manufacturers try hard to achieve kosher certification. Not all fish are kosher; only those with removable scales. This eliminates sharks and other elasmobranchs, eels, swordfish, and catfish, to name the most common. Additionally, fish certified as kosher must be processed in a plant where there can be no admixture with non-kosher species. The non-kosher fish species can be used to make halal gelatins for the Muslim market, which comprises 1.3 billion people worldwide.
Fish gelatin is currently used mostly in refrigerated or frozen products. The major reason for this is that the melting point of many fish gelatins is lower than that of beef or pork gelatin, and therefore they tend to liquefy at room temperature. While this is undesirable in a product that should hold its shape on the table, it is highly desirable for a product designed to melt in the mouth. However, fish gelatins are surprisingly variable, with different species having different melting points as well as different gel strengths, often called 'bloom strength' (Choi & Regenstein, 2000; Gudmundsson, 2002; Gomez-Guillen et al, 2003). Fish gelatins therefore offer not only a range of gel strengths or firmness, but also the possibility of blending those from different species to achieve very specific melting points. This does however mean that some species' skins are unlikely to have commercial value; for example, salmon skins have so far yielded relatively small amounts of very low bloom strength gelatin.
In the past, industrially produced fish gelatin was mostly used for nonfood purposes, such as photoresists in printing. However, food and pharmaceuticals now represent the larger market for fish gelatin, and researchers are working to increase both yield and bloom strength. Gelatin is essentially denatured collagen, so fish skins are also a source of collagen - a product used in cosmetics, nutraceuticals, and pharmaceuticals (Johns & Courts, 1977; Nagai & Suzuki, 2000).
In addition to being a source of collagen and gelatin, fish skins make surprisingly strong and beautiful leathers. Salmon skins were traditionally used by northern natives to make boots, and many fish skins take dyes well and can be produced in a range of brilliant colors. The technology for tanning fish skins is essentially the same as that for tanning hides, although smaller equipment is needed.
Shark and ray skins are famous for their toughness and textures, and are occasionally used in expensive boots and accessories. Shagreen, a sharkskin product used to cover expensive decorative products in the nineteenth century, recently had a small revival. One unique property of sharkskin leathers is their unidirectional denticles, which make them valuable where a non-slip grip is required. The classic use for this has been sword handles. It has also been used for 'pickpocket-proof wallets', but the problem here is that the owner will have as much difficulty removing them from his pocket as the thief. When the denticles of certain sharks are polished, the skins can be used to make an extremely expensive and rare leather known as 'boroso' (Kreuzer & Ahmed, 1978). Barramundi skin from Australia, which has a unique texture, has been used in a variety of products ranging from barstool upholstery to bikinis. Eel skins (actually those of hagfish) are used in the manufacture of wallets, belts, etc., despite their narrow size. Although the scales must be removed from fish skins destined to make leather, the patterns that they leave are similar to scale patterns in endangered and costly snake and lizard skins, which they might be able to replace.
If fish skin leather is so beautiful and useful, why is it not more popular? This is a hard question to answer. A number of ventures have been started to tan fish skins and to turn them into products, and many of these have failed. The tanning itself is not the critical element, but the small size of fish skins, especially when compared with hides, is often cited as a problem. Another issue is the difficulty of getting the processors to provide the skins in the desired form, which may necessitate re-training the workers or buying new and expensive skinning equipment. Based on internet searches, however, fish skin leathers are surprisingly popular, although often in small niche markets.
Johnson & Associates (2002) state that:
Pearl essence are crystals produced from fish scales through a process that removes, collects and purifies the crystals for use in paint pigments, cosmetics and a host of other products where unique luster is important. Soft, cloudy lotions and shampoos often contain pearl essence. Pearl essence pigments are also found in some high-end automotive paints.
It is difficult to quantify the market for fish scales. The leading US buyer of fish scales, Mearl Corporation, is now a part of Engelhard Corporation, a Fortune 500 company that develops, manufactures, and markets technology-based performance products and engineered materials for a wide spectrum of industrial customers. Mearl purchases fish scales from the North Atlantic herring fishery and produces pearl essence for paint pigmentation and consumer products. While the markets for these products are huge (the pearl-escent pigment market for automobiles is growing at a rate of 12% per year), it is difficult to pin down the value of the raw material, the fish scales.
New entrants may find it difficult to break into the fish scale market. The only major company known to purchase fish scales in the eastern United States is closely tied to the sardine industry in Maine. It is likely (although not confirmed) that this business is viable only when it is a part of a highvolume secondary processing operation (in this case the production of sardines). In effect the fish scales are produced as a by-product that is sold to a plant that converts the scales into a marketable material (pearl essence). The barriers to entry in this business would seem insurmountable for new players.
Fish scales are a good organic fertilizer, providing slow-release nitrogen. They must be dried to render them stable and acceptable to the marketplace. Although grinding is recommended, this may not be necessary. They would be an appropriate addition to fish bone meal fertilizer and would allow the processor to get rid of two by-products at once.
Almost all fish processing removes heads. Heads or portions of heads may be used in a variety of ways, depending on the species in question. In the case of cod, there are ethnic markets (largely in the Atlantic provinces of Canada) for tongues and cheeks. Heads are often sold as bait, both for sport and pot fishing. Salmon heads are an excellent source of oil, currently used in human food supplements. The heads of many species are also saleable as food in some areas. Fish heads as food would appear to be the most attractive option for most processors, but it is important to note, as mentioned earlier, that the heads for this market must be cut further back than is normal for headed and gutted or filleted fish, to include sufficient meat. The price obtained for the head must therefore compensate for the loss of high-value fillet meat, and may change the shape of the fillet in ways that are not acceptable to primary customers.
A number of fish with lean flesh use their livers as oil storage organs. This includes cod, haddock, pollock, ling, halibut, and sharks, among others. The livers of these species can be composed of 50% or more oil, and this oil can be easily rendered out. The authors have demonstrated (Regenstein et al., 2003) that the liver oil of Alaskan pollock can be separated out at room temperature if the livers are allowed to age, under refrigeration, for 24 h after death, at which point autolysis is occurring. Since livers used for oil storage tend to be quite large, they can be segregated during processing and handled separately.
Fish-liver oils contain fat-soluble vitamins as well as valuable omega-3 fatty acids. The first human health market for fish oils was due to their content of vitamin D and its ability to prevent rickets. A century or more ago, cod fishermen gutted the fish prior to salting, but saved the liver, which was thrown into a 'gurry butt' (see Rudyard Kipling's Captains Courageous for a surprisingly accurate description) and left to rot for the months-long voyage. Those old enough to remember early cod-liver oils will therefore recognize that the nauseating flavor was due to oxidation and impurities as a result of the poor handling. The discovery of effective and rapid methods of oil release and the use of antioxidants has completely changed fish oils, which today are mild-tasting and inoffensive (Stansby, 1990). For those wishing to access the lucrative human supplement market, it is important to recognize that before being enclosed in a capsule, the oil is refined, cleaned of pollutants such as polychlorinated biphenyls (PCBs) or heavy metals, and the omega-3s often concentrated.
Certain fish livers, particularly those of sharks, are so rich in vitamin A that sharks were hunted during World War II specifically for their vitamin A content and its supposed role in improving night vision in fighter pilots. Luckily, the development of synthetic vitamins put an end to this before the shark populations were decimated. It should be noted that fish-liver oil is so rich in the fat-soluble vitamins that acute fish liver intoxication has been described in the medical literature and is assumed to be caused by hypervitaminosis A.
Where production is sufficient, liver oil can be burned to replace some or all diesel fuel, as discussed in the section on fish meal above.
The value of fish roe (eggs) is best exemplified by that of sturgeon, whose caviar is so highly valued that this ancient group of fishes is close to extinction. Luckily, sturgeon or black caviar is not equally highly valued in all cultures. Salmon or red caviar is very popular in Japan and is considered superior to black caviar by many Russians and Eastern Europeans. Middle Easterners process and eat 'butarga', the smoked or salted and dried eggs of grey mullet or cod. Alaskan processors ship whole unprocessed egg sacks and milts (sperm sacs) of cod and pollock to Asian markets, although far fewer milts are sold than roes. American processors throw flatfish roes and milts away as waste, but Korean buyers will pay higher prices for flatfish cut so that these organs remain inside (karimi, as mentioned in Section 15.2.1).
Gonads are the only part of the sea urchin that is eaten, and European customers pay top prices for scallops with the brilliant red roe sacs attached. Herring roes are shipped to Japan from North Pacific spawning grounds, with the highest prices often paid for herring roe on kelp, where the fish have spawned and glued their eggs to the kelp. To produce this latter product, either pre-spawning herring are captured alive and penned in enclosures containing harvested kelp, or harvested kelp fronds are hung from floating rafts in known spawning areas. Although this product is little known in Europe, the average North American harvest of roe on kelp is about 500 metric tons (http://www.caviarguide.com/fishroe/herring-on-kelp. htm). Herring roe on kelp is not a typical by-product, since there is no primary product - the herring swim away. On the other hand, the roe stripping of herring, where everything but the roe became waste, created such a disposal problem and public outcry that it has been outlawed in most of the United States. Those who want to take the roe must find a use for the rest of the herring, even if that use is only fish meal.
Most fish processing operations discard a significant amount of edible flesh. This may be in the form of pieces removed from fillets, to meet size or shape standards, or as cheeks, or as the meat left on the frame when the fish are turned into boneless fillets. The meat industry and, more recently, poultry processors, have increased yield and profitability by producing minced products from what is left after the high-value parts have been removed. Fish processing has been slower to take up this practice, perhaps because of the rapidity with which minced fish loses quality.
The most commonly used deboning machines, such as Baader and Bibun, work by pressing the fish parts, which are placed on a rubber belt, against a revolving perforated metal drum. The soft parts go through the holes, while skin, bones, eyeballs, etc. do not. The quality of the mince can be varied by altering the size of the perforations (generally between 1 and 10 mm, with 3-5 mm being most commonly used for fish) and by changing the belt tension.
Frozen fish mince is an accepted international commodity, sold in 35 lb (16 kg) blocks. It can be used to manufacture low-quality fish sticks or fish fingers or, at a set percentage, to fill spaces in frozen fillet blocks that are destined to be used in higher quality fish sticks/fingers or portions. There are both generic and specific issues that must be addressed by those wishing to produce mince. Generically, there is the balance between yield and quality. For example, trim mince is a higher quality product than frame mince, because frame mince may contain blood and other pigmented and strongly flavored materials. By increasing belt tension, the yield - especially of frame mince - will increase but the quality will decrease. More specifically, most mince is produced from the gadoid species, such as cod, haddock, Pollock, and hake. When gadoid flesh is frozen at temperatures common in the US seafood industry (e.g. above -22 °F/-30 °C), shelf-life is poor and the flesh tends to become rubbery and tough, due to an enzyme that is still active below freezing. Regenstein (personal observation, 1982) has shown that this gadoid reaction can be eliminated by initial freezing at a very cold temperature (-40 °F or -40°C). This appears to kill the enzyme and the mince can be made available to the market, and its higher freezer temperatures.
Worms (known as cod worms or seal worms) tend to accumulate in the flesh of gadoid fish. These must be removed from fillets individually, typically using light tables to make the worms visible and tweezers to pull them out, but this process would be uneconomical for mince. However, worms can pass intact through the mincer, and can be seen in the resulting mince. Although the worms represent harmless protein, they may not be tolerated by the end user. Reppond and Babbitt (1991) showed that worms could be eliminated by grinding, followed by passage through a Brown Finisher, a machine generally used in surimi production.
As with hamburger mince, fish mince has a surface to volume ratio approaching infinity, and this maximizes problems of contamination, oxidation, and spoilage. Frame mince - which contains blood, pigment, and mixed tissue types - is particularly challenging. Regenstein and Regenstein (1986) have suggested that one way to avoid some of these problems is to cook the mince, using such natural antioxidants as rosemary extract, prior to freezing. The saleability of the frozen cooked mince depends upon forming a collaboration with an end user, such as a company preparing institutional foods. Baker and Regenstein's work has shown that white fish mince can replace beef in many popular foods, such as spaghetti, chili, tacos, etc., and confers both economic and nutritional advantages (Regenstein, 1980). Surprisingly, consumers do not recognize the fish in such preparations, as the strong colors and flavors of the accompanying ingredients act as masking agents and the minced fish offers a meat-like mouthfeel. Frame mince, with its stronger flavors, is more difficult to mask, but offers the health advantages of more omega-3 oils and bioavailable iron, which is in short supply in white fish meat.
Mincing is rarely practical for smaller operators and rarely economical for fillets, which are minced only as a first step towards surimi manufacture. Mincing can make sense for larger processors looking for a way to utilize some of the flesh cut off the fillets during trimming, or left on the frame after filleting. On the other hand, smaller operators often make higher profits by selling small chunks of trim as 'chowder fish', or by developing one or a few specialty food items (such as pates, mousses, or spreads) that can be sold locally. The latter are especially attractive secondary products for smokeries.
Shellfish are disparate members of four different phyla: Arthropoda (which includes crustaceans such as crabs, shrimp, and lobster); Mollusca (which includes clams, oysters, whelks, etc. as well as the squids and octopods); and - as we enter more Asian markets - Echinodermata (which includes sea urchins and sea cucumbers); and Coelenterata (which includes jellyfish). What this means in practice is that their biochemistry, and thus the potential and treatment of their by-products, is incredibly varied.
Coelenterates and echinoderms are rarely processed in the United States or Western Europe. Processing sea urchins produces large quantities of waste (and waste water) of low value. The shell of the urchin is mostly calcium carbonate. Urchin waste, if not too salty from the inclusion of sea water, can be used as a fertilizer; particularly for crops requiring calcium. The problem is in transporting large quantities of wet, rapidly degrading material. It can be a reasonable addition to compost and so can the waste water. Again, salt content may be a barrier, but this is ameliorated by diluting the waste with large amounts of other materials.
Molluscan shells also consist of calcium minerals and can present a disposal problem where clams, oysters, or mussels are processed. Where permitted, dumping at sea may be the best alternative, and may provide good cultch (a hard surface for larval settlement) for the next generation of molluscs, especially where they are farmed. Cultch is sufficiently important that some oyster farmers who sell their catch in the shell have found it worthwhile to import shells from a sea clam processor and to dump those (Wellfleet Shellfish Department, 2005). Shellfish processors faced with huge piles of shells often hope that these will provide a useful and perhaps lucrative source of calcium for laying hens, because hens are given oyster shells as a calcium source; however, oyster shells for hens are mined from ancient, fossilized beds where the material is soft, abundant, unpolluted, and cheap.
A more interesting shellfish by-product could be made from some of the waste waters, particularly those with the highest organic load, which otherwise often create disposal problems. A good example of this is clam postgrind wash water. When large clams are processed, the pieces of meat are chopped and then washed. Because the chopping breaks so many cells, a large amount of protein comes out in the wash water. While this particular stream makes only a minor contribution to the plant's total waste water, it contributes a large proportion of the total BOD, which may cause problems in a wastewater treatment plant. However, the pollutant in this case is protein, and is actually a useful clam flavor. If post-grind wash water were used to pack the clam meats (rather than the plain water that is generally used), the waste stream would be cleaner, and the product would be superior. Note that this use of wash water is most appropriate in a product such as chopped clams which will be sterilized by cooking (S. Goldhor, personal observation, 1995-6).
While using selected process water streams as packing material is a viable option for the primary producer, such streams are generally too dilute for the flavor market and would require concentration by evaporation, co-drying with a carrier such as dextrose, or (more experimentally) through ultrafiltration. Flavor markets actually prefer dry powders which are shelf-stable and easy to handle. However, chefs and small industry users will often accept a frozen, concentrated flavor slush (rather like frozen orange juice concentrate), which can be used a spoonful at a time.
Flavors can be produced out of other wastes as well - some of the more intense and interesting flavors are produced from shellfish body parts; e.g. clam viscera (particularly from clams that are dug out of mud in deeper waters and are unaffected by red tides), brown crab meat, lobster bodies, etc. As more value-added shellfish is produced (with meat to be sold canned or frozen) and fewer are sold live and whole, more of these sorts of raw materials become available.
It should be noted that although some flavors are produced through a sort of tea-bag process, by simply leaching the smaller molecules out into a water soak, some flavors are intensified by enzymatic digestion of the materials. In fact, flavor production - whether for human or pet food use -is one of the most sophisticated sets of by-product processes, and may use digestions and/or fermentations. Thus, although a small processor could simply boil shells and body parts to make stocks and soup bases for sale to local chefs, large-scale industrial sales require greater capital expenditure and sophistication. Again, it is important to emphasize that viscera are not only rich in flavor, they are also rich in industrial pollutants such as PCBs and dioxins (deep-sea clams seem to be an exception), and in seasonal pollutants such as red tide toxins. A careful testing program should be part of any startup operation working with viscera.
Crustacean waste waters, such as shrimp peeling or cook waters, may contain not only flavors but also astaxanthin, the pigment that gives salmon its color. If this can be captured (usually in oil), it may be surprisingly valuable as a salmon feed additive (Meyers et al, 1990).
The shells of crustaceans (e.g. shrimp, crab, and lobster) are partly composed of calcium carbonate. They also contain significant quantities of protein and chitin. Roughly speaking, each of these components comprises a third of the shell composition. Chitin is a polysaccharide (a long-chain carbohydrate composed of many linked sugars) that is very similar to cellulose, except that chitin contains nitrogen. Cellulose is the most abundant organic molecule in the world; chitin is the second most abundant. Chitin is an extraordinary molecule with many different properties. It appears to be able to stop bleeding from deep wounds even when only applied to the surface of the skin, and the US Army and Navy have supported two separate chitin business ventures, both of which are now producing hemostatic pads. While the Navy's pads are made of material produced by biotechnology, the Army's are produced from shellfish waste.
Chitosan, a soluble form of chitin produced by deacetylation, can clean up waste water. Chitosan captures polluting biological molecules, such as fat and protein, but can also clean up PCBs and heavy metals. Chitosan pills are available as a food supplement to bind fat in the intestine as an aid to dieting. Chitin can also be converted into polyglucosamine, which is widely taken to halt arthritic degeneration.
Unlike proteins and many other carbohydrates, chitin does not arouse any immune responses in humans. Since it can be spun, woven, felted, etc., it has been made into items such as intraocular lenses, biodegradable sutures, and second skins for burn victims. For these types of uses, it is essential that all the protein, even that most tightly bound, is removed from the chitin because protein does cause an immune response.
Despite the promise of chitin and the large amounts of shellfish waste, few chitin-/chitosan-based businesses have succeeded in North America. They have, however, succeeded in Japan, where chitin is well known, highly regarded, and - perhaps most importantly - legally mandated by the Government for certain tasks, such as cleaning up food-plant waste waters. Such businesses have also succeeded in China, where labor is cheap and environmental monitoring minimal. In the United States there have been two barriers to success. One is the cost of the labor and chemicals required to turn shells into chitin, a process requiring treatment with both acid and alkali. The second is the natural variability of chitin itself, which has changing proportions of chitin and chitosan, and which differs depending upon the species and the developmental stage of the animals. Batch to batch variability makes it extremely difficult to sell products into markets that are used to uniform products adhering to strict standards (Muzzarelli & Pariser, 1978).
Crustacean shells are excellent raw materials for compost. Chitin provides not only slow-release nitrogen but is believed to act as a nematocide, and to provide some level of organic pest control - presumably by inducing chitinases in plants, which act against both insect and fungal pests. At least one Canadian company composting shrimp shells with peat was able to position its product as a very high end soil amendment with great success.
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