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