Utilizing all the waste

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There are three main categories of co-products that utilize all, or almost all, seafood processing wastes. These are compost, meals, and hydrolysates or digests. These products are often produced by independent entrepreneurs, who have the time and interest to learn the technology and to develop markets that, for the most part, have nothing to do with seafood. We will define each technology, explain how to get started on a small scale, and discuss the pros and cons of each.


Composting is the controlled, microorganism-mediated breakdown of organic materials containing both carbon and nitrogen, to produce a humuslike material that can be used as a fertilizer and soil amendment. Composting can be aerobic (requiring oxygen) or anaerobic (without oxygen). Aerobic composting reduces odors and improves nitrogen retention, compared with the anaerobic process.

The biggest advantage of composting is that it is the only process that accommodates all organic waste generated by fish processing: spilled or excess breading, crustacean shells, rotten material, smoked scraps, etc. Hard shells, such as those of clams and mussels, are not broken down by composting but they are cleaned by it and can be screened out later, and may have value as driveway fill. Composting will sometimes even break down toxic contaminants, but this must be tested for each individual contaminant. Where contaminants are not broken down, they will at least be diluted. It should be noted, however, that there is a real danger that the breakdown products of known contaminants will be unknown or lesser known contaminants, which may be difficult to test for. The disadvantage of composting for our purposes is that seafood waste cannot be composted by itself. Seafood is an excellent source of nitrogen but to compost it, a source of carbon must be added. This means that composters negotiate for and handle very large quantities of autumn leaves, sawdust, grain hulls, peat, shredded paper - whatever carbon source is available. To prevent odor, the carbon source must be available as soon as the seafood waste comes in to be composted, and so it usually needs to be stored onsite. Thus composting seafood wastes requires a lot of space, along with logistics and material handling expertise. There are in-vessel systems that greatly reduce the space requirements, but they are much more expensive and often require greater skills to operate.

Composting is a slow process, and may take anywhere from a few weeks to several months, after which the compost must 'cure' before it can be sold. Curing generally means that it must be stored, sometimes for as long as a year. The economic and space consequences of this are evident.

Although home composting is simple, running a commercial composting operation is not. Professional composters make about half of their income from selling compost and the other half from fees paid by businesses disposing of compostable waste. Composting might be a good solution for seafood processors who generate a relatively small amount of waste, and who have access to carbon sources and land. It is worth noting that crustacean shells may make the finished compost particularly valuable since they contain chitin, which provides slow-release nitrogen.

Another method for those generating a relatively small amount of waste is vermicomposting, or worm composting. Although called composting, this process actually comprises feeding worms with the waste and harvesting their excrement (professionally known as 'castings'). Worm castings are a particularly valuable fertilizer (due mostly to the microorganisms they contain rather than the amount of nitrogen and other nutrients), and worm composting has some real advantages over the more conventional process. First, it is much quicker. Second, unlike real compost, which has to cure prior to sale, worm castings have a limited shelf-life so the sooner they are sold, the better. Third, the worms are housed indoors, they do not take up huge amounts of room and multi-tiered housing can be constructed cheaply. Fourth, there is no odor and therefore the issues of land, permits, and litigation are minimized. Finally, there are no corrosive chemicals or high temperatures and pressures to deal with, no dangerous (and expensive) equipment, and the process is safe for humans. Unfortunately, the latter is not true for standard composting, where employees must wear appropriate protective breathing equipment to prevent the inhalation of mold and bacterial spores.

Unlike meal manufacture or hydrolysis, there is an enormous body of literature on composting, and a surprisingly large sub-section of this is on seafood composting. However, like all seafood by-product ventures, a fair fraction of seafood composting operations start out looking profitable but run into problems. Successful ventures in the literature should be researched, not least to see if they are still operating, and, since composting is rarely done in a proprietary way, operators are usually willing to share their problems and expertise. There is also a lot of information on vermicomposting, much of which is online. A number of references are included at the end of the chapter, but it should be noted that while the publications for beginners may have titles that sound more cute than serious - Worms Eat My Garbage (Appelhof, 2001) and Compost This Book (Christopher and Asher, 1994) - they tend to be the best places to start, since they explain everything a small-scale startup needs to know, use non-technical language, and are inexpensive.

392 Handbook of waste management and co-product recovery Meal manufacture

Fish meal (or, for that matter, shrimp or crab meal) is basically the result of cooking, dehydrating, partially de-oiling, and grinding the raw material. Since fish is typically 65-80% water (depending upon its oil content), this process provides a huge reduction in bulk and renders the material shelf stable, so that it can be bagged, stored, and shipped at ambient temperatures. It also supplies feed mills (by far the largest users of seafood meals) with fish in the dry granular form they prefer. This section focuses on the production of fish meal because it far outweighs the production of any other type of seafood meal.

Modern fish meal manufacturing usually consists of several processes: coarse grinding, cooking, pressing, drying, stabilizing with antioxidant, and fine milling. In addition, there is often a short curing period which allows the meal to cool down before it is packed into containers for shipment. The liquid that is pressed out of the cooked fish is separated into oil and water streams. The oil becomes a product and, as we will see, the treatment of the water stream (which contains significant amounts of protein) varies, although it is generally evaporated down to a thick paste.

In meal production, the fish is cooked for two reasons: to sterilize it and stop any decay processes, and to denature the protein and free-up bound water. Fish decays rapidly at even slightly elevated temperatures, so how the fish scraps are stored prior to cooking is as important as the cooking process.

One of the main reasons why the old rendering plants were such bad polluters was due to raw material handling and storage. These plants collected scraps, including poultry and meat as well as fish, from multiple sources, many of which did not bother to refrigerate what they saw as waste. Containers might be left out in the sun, and might be held overnight or even over a weekend. Upon arrival at the plant, they were rarely refrigerated and were often left outside. Raw material delivery was not timed to plant operation, nor was raw material quantity limited to the plant's capacity. Poultry and meat wastes come from animals whose body temperature is close to 100 °F/38 °C. If cooled to around 45 °F/7 °C, the process of decay slows dramatically. But fish body temperature is more likely to be around 45 °F/7 °C, and so storage at that temperature or a higher one will allow enzymes and bacteria to break down tissue rapidly and hasten decay. In fact, as discussed later, warming fish tissues causes them to liquefy, digested by endogenous enzymes.

For a rendering plant, improper (i.e. warm or extended) storage causes two problems. First, as enzymatic autolysis causes more liquefaction, the yield of solid meal is reduced. Second, advanced bacterial decomposition causes the formation of highly odiforous compounds, such as cadaverine and putrescine. Fish also contain significant quantities of an odorless compound called trimethylamine oxide (TMAO) which, upon death and decay, is broken down to trimethylamine (TMA). It is the strong and very unpleasant odor of TMA that is diagnostic of spoiled fish. Hence the old style of rendering plant caused serious odor-pollution in neighboring areas.

Modern meal plants rarely take in scraps from more than a few processors because it is too difficult to maintain quality control. Most meal plants no longer take in scraps at all; they work with dedicated fisheries of what are called 'industrial' fish, usually oily fish such as menhaden in the US Gulf, sand eel in the North Sea, capelin off Iceland, or anchovy off Peru. These are typically small fish, which are not in great demand as human food, and are available in large quantities. The mealplants that take them have enclosed refrigeration facilities to hold fish that have been delivered but cannot be processed immediately. It should be noted that industrial fish are major components of their ecosystem's food chains and whether problems within those ecosystems are due to the removal of large quantities of such fish is currently hotly debated. These concerns, plus increasing regulations banning other forms of disposal and demanding total utilization of the catch, all add to the push for adapting meal production to processing waste.

Meal plants that do use processing scraps have found ways to keep those scraps in good condition. In areas like Alaska, where single corporations run enormous plants, there are several good-sized fish meal plants operating on the waste from a single processing operation. This arrangement makes sizing the meal plant to the primary operation straightforward. Processors in other areas have formed cooperatives to operate meal plants as a group. Since the cooperative benefits from the meal plant, it is in each member's interest to keep their contribution fresh. Unlike the small boats that catch the industrial fish, fish plants usually have refrigerated holding facilities where they can store waste that cannot be utilized immediately.

A different set of operating conditions exists on factory trawlers, which often run on-board meal plants. These on-board plants operate differently from the land-based ones, and will be discussed later.

Modern meal plants are usually totally enclosed, so that odors cannot escape. Some plants go to the extreme of having negative air pressure inside the plant, so that when the doors are opened, air can enter but not escape. However, with fresh raw material and enclosed processing machinery, this is of secondary importance. The most likely stage for the escape of odors is from the process itself, and plants today have built-in mechanisms for eliminating those odors, usually via condensation of the vapor followed by incineration at very high temperatures, which oxidizes the volatile odor compounds to their odorless products.

As with all food plants, good housekeeping is the key to both good products and good relations with neighbors. Even a small amount of fish left to rot can cause unpleasant odors. Unlike most food plants, industrial fish meal plants are designed to operate continuously for the entire season.

The processing machinery is not easily accessible for cleaning after each shift, and each stage of the process is continuous with the preceding and subsequent stages. While some parts, such as the holding tanks, can be cleaned quickly and easily, other parts, such as the dryer, must be dismantled before they can be cleaned. This is a painstaking and time-consuming job. Because the dryer works continuously, it is often constructed in such a way that when it is turned off, wet fish is left in the first section. Since it is assumed that the dryer will only be turned off at the end of a processing season, when all the machinery is taken apart for cleaning, this is not as unreasonable as it first appears. But it does mean that if processing stops for a week or two, because of storms or work stoppages, the operators will often decide to keep the burner running to dry what is left in the machine while the plant is down, rather than pull everything apart for cleaning or deal with odor problems when the plant is re-started and the rotten material in the dryer heats up again.

In addition to fish meal, this type of processing plant produces fish oil and a product called fish solubles, or concentrated stickwater. After the coarsely ground fish has been cooked, it is pressed to remove as much oil and water as possible. This liquid is decanted to separate the aqueous and oily fractions. In some cases, a triple decanter is used, which separates the aqueous stream into high and low solids fractions. The resulting oil will be discussed below. The pressed meal goes into a dryer, while the aqueous stream, known as stickwater or presswater, is taken to an evaporator, where its water content is reduced from over 90% to about 50%. The evaporator removes water far more efficiently than the dryer, particularly modern multi-stage evaporators (Fig. 15.1).

On-board meal plants do not have evaporators. Their press stream goes through a double decanter, the presswater is pumped overboard, and the oil is added to the ship's fuel after polishing to remove any last bits of water and solids. At least 20% of the diesel fuel can be replaced by fish oil without any problems or modifications. By modifying the burner, fish oil can replace most or all of the diesel (see below). Unlike factory trawlers, land-based plants cannot dump their presswater unless they have the appropriate discharge permits to release it into local waters, although some plants have hired vessels to carry it out to sea and discharge it.

When the fish is pressed, the soluble proteins come out in the presswater. Prior to evaporation, the protein content of the presswater is about 6-7%, a figure that can increase greatly as the raw material ages. When the concentrated solubles emerge from the evaporator, they are thick and gluey, with the consistency of tomato paste. Traditionally, these were added to the dry meal, which was then re-circulated to the dryer. Such a product was called a 'full meal'. More recently, the prices for full meals have dropped and producers have found other markets for the concentrated solubles, usually the fertilizer market, where fish solubles compete with hydrolysates.

Raw fish: 1000 kg

Oil Water Solids 120kg 700kg 180kg iCOoKERi

Cooked fish: 1000kg Oil Water Solids 120kg 700kg 180kg

Cooked fish: 1000kg Oil Water Solids 120kg 700kg 180kg

Press liquor: 680kg Oil Water Solids 110kg 530kg 40kg

Oil 10kg

Water 170kg

Press liquor: 680kg Oil Water Solids 110kg 530kg 40kg

Oil 10kg

Water 170kg

Solids 140kg


Water 510kg

Solids 30kg

Solids 140kg




Oil 108kg

Stickwater: 542kg Oil Water Solids 2kg 510kg 30kg

Press cake (2): 350kg

Oil 10kg

Water Solids 190kg 150kg i evaporator]

Water removed

In evaporator 450kg

Oil 2kg

Concentrated stickwater: 92kg

Water 60kg

Solids 30kg

Drier input: 442kg

Oil Water Solids 12kg 250kg 180kg

Water removed in drier 230

Fish meal: 212kg Oil Water Solids 12kg 20kg 180kg

Composition of fish material during the process


Water (%)

Solids (%)

Fat (%)

Raw fish




Press cake




Press liquor




Dilute stickwater




Concentrated stickwater




Fish meal




Fig. 15.1 Composition of fish material during the process (from Windsor and

Barlow, 1981).

The old markets for fish meal were as ingredients in pig and chicken feeds. The current markets for fish meal are in aquaculture feeds, where they have high value, especially for carnivorous fish. This has resulted in fish meal prices fluctuating independently of soybean meal prices for the first time in fish meal's long history. However, the use of fish meal in fish farms has placed new demands on meal manufacturers.

Fish farms have big problems with water pollution, and so require fish meal that contains very few soluble proteins and has a low bone content. Bone contains significant amounts of phosphorus, much of which is indigestible. It passes through the fish and ends up in the water, where it acts as a fertilizer and can cause massive algal blooms. When these algae die, their decay uses up the oxygen in the water (the potential for this is called 'biological oxygen demand' or BOD), and causes local anoxia which can kill fish (and all other animals). In meal made from fish caught solely for that purpose, bone content is less of an issue since the meat to bone ratio of these small fish is relatively high. However, in meal plants using processing scraps, the bone level can be very high, as heads and frames may constitute much of the input. With aquafeed markets paying a premium for low ash (i.e. low bone) meals, many plants today screen or air-classify their meal after drying, to separate out the larger pieces of bone. This creates an additional product, fish bone meal, which can be used in fertilizers or feeds. Fish bone meal is currently fairly low in value, but the economics as a whole are affected by the fact that certain aquafeed markets see white fish meals (i.e. those made from scraps from food fish as opposed to the 'brown' meals made from industrial fish) as of special value. These markets also place a high value on freshness of raw material and digestibility of the finished product, both of which are routinely tested by buyers.

Historically, many meal plants were built to produce fish oil, with meal as a low-value by-product. Today, meal is the primary product and oil the by-product, despite the market for high-priced fish oil capsules and omega-3 fatty acids promoted as dietary supplements. High-priced consumer products containing fish oils are usually made in specialized refineries and not in meal plants, so meal plants, especially those in isolated areas like Alaska - where fuel prices are high and most coastal communities are off-road -burn the oil to run their boilers, rather than shipping it to a buyer. The oil is usually mixed 50 : 50 with diesel, but it can replace diesel completely, provided all solids are taken out and the boiler retro-fitted. Burning fish oil produces only 80% of the British thermal units (BTUs) from an equivalent amount of diesel, and requires a different burner, but currently the economics of burning fish oil are quite favorable. In addition, as the price of fuel rises, they will become more favorable and the use of fish oil for fuel ('biodiesel') may spread (Steigers, 2003). Markets and the prices they command fluctuate constantly, and if the value of fish oil, either as a human nutraceutical or as an aquafeed ingredient, goes up and the price of fuel comes down, the oil will go back on the market.

One big change in meal production over the last 50 years is the use of antioxidants to prevent oxidation. The long-chain polyunsaturated fatty acids that confer so many benefits on fish oil are extremely prone to oxidation when exposed to heat and oxygen. This causes oxidative rancidity, which makes the meal (or the oil) less appetizing and even harmful in feed. Extreme oxidation may actually cause meal to spontaneously combust. This has happened often enough to mean that high oil fish meal is still considered a hazardous cargo, and the manufacturer may have to prove to shippers that sufficient antioxidants have been added. Antioxidants are added to the meal as it emerges from the dryer and before milling and bagging. The oil may be protected with antioxidants as well, although most buyers prefer flushing with nitrogen or argon followed by air-tight packaging.

Both fish processing plants and meal plants will need to adapt to increase the utilization of processing wastes in meal. Few fish plants are large enough to justify their own meal manufacturing facility. This means that new meal plants will only be built in locations where there are sufficient fish processors within a workable radius to justify their construction. The fish processors will have to cooperate with the meal plant by keeping fish scraps refrigerated and delivering them to the meal plant as rapidly as possible. This is particularly important where viscera form a significant fraction of the raw material. The ideal situation is where all processors are sufficiently close to the plant that material can be pumped from one building to another. If the fish scraps have to be transported, the issues of noise, odor from the trucks, and leakage onto the roads must be resolved. In most communities, building permits will not be issued until the meal plant has guaranteed very high levels of hygiene and cleanliness.

A new meal plant must also assure itself that there is sufficient raw material available to make a profit. The profit may be less critical if the plant is run by a group of processors whose major concern is complying with waste disposal regulations, but independent owners will not operate at a loss. Meal plants are bulk businesses and need to take advantage of economies of scale, whereas fish processing can be profitable at almost any scale. Equipment in a large fish processing plant tends to be multiples of the same equipment items that a small plant would purchase singly. Small family businesses avoid the labor problems of large operations, and can make money by sticking to high-value species, providing customers with special services, and developing value-added products. This is not true of meal.

Meal plants sell into commodity markets, via brokers, and the most profitable plants are not necessarily those that produce the best product, but those that have a large capacity and operate up to that capacity as many days out of the year as possible. Since most fisheries have seasons, plants operating on waste are most efficient when the season for one fishery immediately succeeds another, so that processing is more or less continuous throughout the year. The most difficult situations in which to operate a profitable (or at least break-even) meal plant are those that occur, for instance, in wild salmon fishing, processing and canning communities, where the total season may be 4 months, but where peak production lasts for only about 6 weeks. It is extremely difficult to establish a profitable plant under these circumstances, although there are possibilities. One option is to aim for a more moderate level of production, and dispose of excess fish waste during the peak season externally, for instance via ocean- or land-dumping, paying a composter's tipping fees, or some other solution, depending on the individual situation. Another possibility is to source reasonably priced equipment that can handle peak loads, but which can also operate easily with smaller loads. It might be worth looking at rendering equipment, which can be both smaller in scale and cheaper than modern meal plant equipment. However, it is important to determine if the raw material to be processed matches the requirements of the machinery; for example, some rendering presses will not work unless the raw material contains 20% bones.

Bones can be a problem for fish meal producers. When whole fish are turned into meal, the ratio of meat to bones is high, and these small fish, targeted by the fishing fleets that service meal plants, have small light bones. However, scraps from processing plants, which may contain a high proportion of heads and frames, often contain large bones and have a high bone to flesh ratio. The larger the proportion of bones in the raw material, the higher the ash content of the finished meal will be. There are ways to reduce ash content in a finished meal, such as air-classification (similar to winnowing chaff from grain) and screening, which could allow entry into certain markets, that set limits on ash content (such as those for fish feed). Cat food manufacturers often specify low ash because high ash can cause urinary tract problems in neutered male cats and, although acidification of the feed can solve this problem, this has not been sufficiently marketed to cat owners.

Preferences for specialized meals for very specific markets bring up an important point. It was mentioned above that fish meal is a commodity, handled through commodity brokers. In general, the smaller meal manufacturer will not sell directly to the user but, rather, to a broker who is often also a blender. The broker will blend high ash meal with lower ash meals to achieve a saleable mix. While some customers will demand an unmixed meal that meets their specifications, most bulk customers will buy blended meals. The premium price for low ash white fish meal may not justify the extra costs of the machinery required to lower ash content. In addition, although removing ash may increase the price per ton of the meal, the volume produced and sold will fall. When all factors are taken into consideration, manufacturing a superior high-quality meal may not compensate for the added costs of production.


Hydrolysates are sometimes called 'digests', and they are also often referred to as fish protein hydrolysates (FPH), or as 'liquid fish', as in 'liquid fish fertilizer'. Unlike fish meal, which refers to a reasonably consistent process and product, hydrolysates are extremely variable - both in their processing and in the form, function and pricing of the final product. (Fish silages, a sub-group of hydrolysates, will be discussed later in this section.)

What unifies all of these products is that they use protein-digesting enzymes to liquefy the fish flesh, after which bones and other indigestibles can be screened out. The two main sources of variability are the enzymes and conditions under which digestion takes place, and whether the resulting liquefied fish is left as a liquid, concentrated to a paste, or dried. Unlike meal, which is almost always sold as a bulk commodity, hydrolysates offer the possibility of small-scale production, value-added processing, and individual marketing.

Hydrolysates left as liquids are acidified to prevent spoiling. The raw material to produce liquid fish hydrolysates is digested rapidly under optimal conditions, pasteurized to kill any microorganisms present and to inactivate the enzymes and stop the digestion, and then acidified. The acid(s) used are chosen according to cost, functionality, and flavor. For example, liquid fish fertilizer is invariably stabilized with phosphoric acid, which is cheap and adds phosphorus to a product where phosphorus content is a plus. However, phosphoric acid, like other inorganic acids, is ineffective as a mold retardant, so a small amount (e.g. 0.25%) of propionic acid or a propionate salt is added as well. Hydrolysates for the cat food market (where they are sprayed on to dry feed to enhance palatability) are also acidified with phosphoric acid because cats like this acid. Phosphoric acid is also a good acidifier for concentrated fish hydrolysates destined for the salmonid feed market, because salmon as well as some other species tested seem to prefer the taste of phosphoric or formic acid to other acids. Phosphoric acid is not only far cheaper than formic acid, it is less volatile and thus easier to work with - not just in the hydrolysing facility, but also in the feed plant, where heat is usually applied to form pellets. However, phosphoric acid should be limited or not used at all where water pollution with excess phosphorus could cause problems. The producer must balance the requirements for the market, the target organism, and product stability.

Determining the correct amount of acid needed to stabilize the product usually takes a few trials. This can be particularly tricky in products containing bits of bone, which dissolve over time and neutralize inorganic acids. Those just beginning production of liquid fish products, as well as most small-scale operators, typically produce fish fertilizer. This is always put into plastic containers because, if stabilization was incomplete, there is a possibility of gas build-up and consequent rupturing of the container. In extreme cases, the container might explode and a glass container would be much more dangerous than a plastic one.

Concentrated fish solubles, made as a by-product of fish meal, are invariably cheaper than purpose-made fertilizer, and offer the additional advantages of cheaper shipping and storage since they have a much lower water content. They are not the same as hydrolysates, since solubles are made of the soluble proteins expressed after cooking, while hydrolysates include all the proteins solubilized by digestion. However, there have been no studies to demonstrate any differences in quality between these two products, despite various claims, such as those from small producers of liquid fish fertilizer who suggest that the processing method makes their products superior. It is difficult to make objective judgments about different products because most studies are carried out using fertilizers of unknown composition, as producers tend to treat their manufacturing process as a trade secret. Scientific research in this field could be of benefit, but despite this lack of quantitative comparisons, the liquid fish fertilizer market remains buoyant.

There are two things that make fertilizer production attractive to beginners. One is that the product can be sold wet, thus greatly reducing capital costs. The other is that the choice of enzymes is a lot simpler than it is for products where flavor is the key point. The types of enzymes used to digest fish wastes are protein-digesting enzymes, or proteases. The simplest and cheapest way to digest seafood is to include the viscera, grind and mix, and then raise the temperature to around 140 °F/60 °C. This is the temperature at which the endogenous enzymes of even cold-water fish work fastest. The enzymes do not remain active at this temperature for very long, but long enough to liquefy the raw materials. This method will not work if the fish has been frozen or cooked, as both these processes denature the proteins. Using such an endogenous digestion gives specific flavors, which are useful in certain products such as fish feed ingredients. However, the use of viscera is not approved for human food use (nor would the flavors be appropriate) and it is too coarse a method for producing fertilizers, because fertilizers must not clog the nozzles of large-scale delivery systems.

Proteases are of two general types, endopeptidases and exopeptidases. Endopeptidases chop up proteins in the middle, while exopeptidases snip off single amino acids from the ends. Random endopeptidases cut proteins or peptides (pieces of proteins) in random places; others separate the protein at particular amino acids. When a large number of free amino acids are desirable, a combination of endo- and exopeptidase is used because the endopeptidase creates many more peptides for the exopeptidase to act on. At the outset, it is important to find out if these two enzymes can be used in combination or if they will digest and thus inactivate each other. If the latter, then the endopeptidase should be used first, and de-activated before the exopeptidase is introduced.

Unlike the endogenous enzymes, which are of course free, other enzymes used to produce hydrolysates must be purchased. Exopeptidases tend to be particularly expensive. The enzyme of choice for fertilizer production is the cheapest, fastest-acting endopeptidase on the market. Papain is used when the only requirement is for a smooth, fine hydrolysate that will pass through a fertilizer delivery system, but the price of papain fluctuates according to the papaya crop. Where flavor is important - whether for fish, pets, or humans - the choice of enzymes is critical and a long period of research and development may be required to find the optimum mix.

Flavor - and the specific flavors that tell us whether we are eating fish or broccoli - is largely due to the presence of free amino acids in food. Digestion may increase the amount of free amino acids, thus intensifying flavor, but the flavor produced may be an inappropriate or unpleasant one. Exposing certain amino acids, particularly hydrophobic amino acids, by cutting the protein or the peptide at hydrophobic linkage sites, creates bitter flavors. While fish, as a substrate for hydrolysis, tends to generate less bitterness than, for example, soybeans or casein, this may still be an issue. For products such as fertilizers, bitterness is irrelevant. For the production of high-value palatability enhancers, bitterness in fish hydrolysates may be reduced by removing the gall bladder from the starting material (Dauksas et al., 2004). Hydrolysis, sometimes in combination with fermentations, has been widely used to produce flavors for pet foods (note the ubiquity of 'digests' in pet food ingredient lists), although these have commonly used poultry or beef parts as starting materials; hydrolysis has also been used to produce high-value flavorants for human foods (In, 1990).

Peptones or microbial growth media are another high-value class of products produced by enzymatic hydrolysis (Aspmo, 2005) As industrial fermentations increase in number and variety, the demand for growth media increases as well, and fish has been a satisfactory feedstock for peptone production. Additionally, fish-derived peptones may be uniquely suited to support the growth of marine organisms. Indeed, there are no limits to the creation of novel products from fish waste via controlled hydrolyses (Gildberg, 2004).

A typical protocol for small-scale liquid fish fertilizer production is shown in Fig. 15.2. The wastes are ground, typically to a quarter-inch or hamburger consistency, because a very fine grind may make it difficult to screen out the resulting small bone fragments which can clog nozzles. If the raw material has a high oil content, an antioxidant such as ethoxyquin may be added. The raw materials are then transferred to a jacketed kettle or other tank where they can be heated and continuously mixed. The temperature is based on the optimal operating temperature for the protease used, generally in the range of 140-160 °F/60-71 °C. The pH may have to be adjusted for some enzymes, but most of the random endopepti-dases work well in the neutral pH range of most fish. The viscous mass will turn to soup surprisingly quickly. The operator decides when to stop the digestion, and does so by raising the temperature to inactivate the enzyme molecules and to destroy any microorganisms (200 °F/94 °C for 10 min is typical). The digest is then screened, stabilized with acid(s), and cooled.

Fish silage is a very different product, in that acidification is the first step, rather than the last. In ensiling, the fish is ground, mixed with acid, stored,

^Incoming waste conveyor

Antioxidant tank and pump

Enzyme tank and pump


^Incoming waste conveyor

Hot water heater

Jacketed kettle for digestion and pasteurization

Storage silo_\

Antioxidant tank and pump

Enzyme tank and pump


Storage silo_\

Hot water heater

Fish Ensiling

Jacketed kettle for digestion and pasteurization

V^Outlet valve

Mixing tank

V^Outlet valve

Fig. 15.2 A process for simple fish protein hydrolysis (Goldhor, 1992).

and occasionally stirred, usually at ambient temperatures. Over time and after self-digesting, it separates into a watery top layer consisting of soluble proteins and peptides, and a bottom layer of sludge. This is a very old method and there are a number of variations.

Ensiling fish is a primitive process, and the products of ensiling are usually primitive as well. Because the process is poorly controlled, digestion may proceed too far, leading to the breakdown of protein into ammonia and thus loss of feed value (Hardy et al., 1983; Stone & Hardy, 1986; Stone et al., 1989). Because the mix needs to be stored (and stirred at least occasionally) for weeks, it requires space. It is a useful method for less developed areas where the quantities of fish waste to be ensiled are not too large, and where livestock must be fed; it should be withheld from pigs and chickens prior to slaughter to avoid the meat tasting of fish. However, ensiling (or, more accurately, acidification of minced fish) is also sometimes used as a short-term method of preserving the material, prior to enzymatic hydrolysis. In situations where waste is to be collected from many generators for hydrolysis, grinding the fish and mixing it with acid is a reasonable way of ensuring that it will be in good condition when it arrives at the plant. This method is also used in Norway to preserve material on board vessels prior to delivering it to a land-based plant (Jangaard, 1987). Any operator planning to follow this method should consider the choice and amount of acid very carefully. It may be handled by unskilled workers, it will probably have to be at least partially neutralized prior to mixing the silage into a finished feed, and - if it is volatile - heating during hydrolysis may become a health issue in the plant or in the neighborhood.

When dairy farmers talk of silage, they are generally referring to that produced by the anaerobic fermentation of corn with endogenous or added sugars, which generates acid. The raw mix may be inoculated with cultures of specific microorganisms to ensure that the acid generated is the one that the farmers require. In making fish silage, acid is generally added directly. However, it is possible to carry out fermentations where at least some of the acid is generated by the fish materials, by adding a source of sugar and the appropriate microbial cultures and storing the mixture anaerobically, usually for about 48 h. The acid produced is generally lactic, but sometimes propionic (van Wyk & Heydenrych, 1985; Hassan & Heath, 1986; Giurca & Levin, 1992; Cao et al., 1997). It should be noted that in order to ensure that the acid penetrates the mass of fish and that no spoilage occurs inside the fish chunks in this process, it is usually necessary to carry out an enzymatic hydrolysis prior to fermentation. This thus becomes a fairly expensive process and one that is quite different from acid silage, which is cheap and well suited to primitive conditions; ensiling fish using this method is therefore only carried out under particular circumstances, such as when liquid acids are not available or are expensive to transport, or where lactic acid fermentation adds significant palatability. An early weaned pig feed might be an example of the latter. In fact, fish ensiling is as capable of being upgraded and used to generate sophisticated products as is enzymatic hydrolysis, of which it is a variant (Gildberg, 2004).

The earliest fish digest that we know of is the ancient Roman condiment called 'garum'. Worcestershire sauce contains a direct descendent of garum, and what is arguably the most popular fish digest in the world today - Asian fish sauce - has similarities. Known as 'nam pla' in Thailand, 'nuoc nam' in Vietnam, 'patis' in the Philippines, and 'tuk trey' in Cambodia, fish sauce provides a major portion of the protein and vitamin B, as well as flavor, in diets that consist largely of white rice. It also provides a big dose of salt. One major way in which fish sauce differs from silage is in using salt rather than acid as the preservative. It is made by combining whole fish and/or shellfish with at least 20% of their weight in salt, and then storing the mixture underground or in sealed containers for months while it ferments and digests. The fish sauce sold is the clear (usually amber colored) supernatant from this process, and the sediment is either discarded or re-used to make a lower grade of sauce. The strong flavors and aromas of fish sauce are due to the long fermentation, and the process may be compared with that of wine-making (Lopetcharat et al., 2001).

The market for fish sauce is enormous. Many of the traditional areas for its manufacture have experienced a dramatic decline in fisheries, while the

United States and Europe have seen big increases in populations from countries where fish sauce is popular. Surprisingly, manufacture of fish sauce has not moved easily to other countries, partly because of the need to suit traditional palates, and partly because of the need for a warm climate where the long storage and fermentation can occur. Western attitudes and regulations covering fish processing may also play a part.

The many ways in which digested and/or fermented fish products are manufactured are reflected in how many markets there are for such products. These markets include aquaculture feeds, livestock feeds, pet foods, human foods, fertilizers and foliar sprays, and microbial growth media. In the first four categories, it should be noted that digests are used largely as flavorants, attractants, and palatability enhancers, i.e. as minor ingredients playing major roles. Because of the immense plasticity of hydrolysis as a process, other specialty products are expected to emerge.

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