Postharvest losses of fish reach 35 percent, nearly 25 million tons, of the world's fishing catch. The Food and Agriculture Organization of the United Nations (FAO) has estimated that in some developing countries, postharvest losses of fish exceed those of any other commodity, often surpassing 50 percent of the landed catch. The losses are highest in the countries whose populations have the lowest protein intake. Reducing these losses could increase protein availability, improve nutritional status, and eliminate some of the need to import food.
Postharvest losses of fish occur during the numerous steps from catch to market. The lack of appropriate methods to preserve the catch on board results in heavy losses. Additional losses occur in the period after docking and before marketing. During this period, exposure, inadequate processing, and insect infestation take their toll. The catch is further reduced by poor transport to market, unsatisfactory preservation, and further exposure during the marketing process.
This chapter will examine fish processing and preservation between catch and marketing. Some technologically simple preservation and processing methods, which could be adopted at the village level, are described.
PRIMARY PROCESSING-ON-BOARD HANDLING
Fresh fish are highly perishable and start to spoil as soon as they are landed. Concern for quality should begin on board the vessel. The first consideration should be to bring the fish aboard alive and in good condition. This is more likely, for example, if gill nets are set for six hours or less and trolling runs are two hours or less.
Fish should only come in contact with clean surfaces. It is important that bacterial contamination be kept low. Keeping the deck, hold, and storage boxes free of fish residues, dirt, and slime with the use of clean seawater and a scrubbing brush should be adequate for this purpose.
Fish should be handled with care. Kicking, trampling, or dumping the fish will increase the rate of spoilage. For high quality, fish should be chilled as quickly as possible to 0°C. Before fish are landed, hot decks should be cooled with clean seawater. Because high temperature is the single biggest cause of quality loss, fish should be moved promptly from the deck to cool storage.
It is most efficient to put the ice and fish together in a covered box or hold area. Flakes or small pieces of ice provide the most effective cooling. Large irregular pieces can damage the fish. Fish and ice should be packed in alternate layers. Dumping ice on a pile of fish will not give good results.
The sanitary quality of-the water used for producing the ice is also important. Both disease- and spoilage-causing microbes can survive in ice and contaminate the catch when the ice melts. Similarly, ice should not be reused. Once used for storing fish, it should not be recycled for cooling freshly caught fish.
For many fisheries, however, it is not practical to use ice- the vessel may be too small, or it may not be possible for the fisherman to recover the cost of ice through higher prices. Where the fisherman is at sea for only a short period, the use of ice may not be necessary. Delays in icing up to about six hours will still give reasonable quality for small pelagic fish, provided the fish are consumed promptly.
Fish can be kept cool by other methods. If possible, the fish can be kept in water in live wells until the boat lands. Water temperature is usually lower than ambient temperature. Fish kept shaded will be cooler than if they are exposed to the sun. Keeping the surface of the fish wet will help bring the temperature down (evaporation of the water absorbs heat from the fish). It is easier to keep the fish damp if materials such as wet seaweed, leaves, sacking, or sawdust are used as a light covering to increase the amount of water available for evaporation.
Whether the fish are stowed with ice or without, overfilling of containers should be avoided to prevent crushing them. Cool conditions and careful storage should continue through landing and marketing. Spoilage can never be prevented through chilling or cooling, but the cooler the fish are, the greater the reduction in bacterial and enzymatic degradation. For each 5°C increment in storage temperature above 0°C, there is a significant reduction in shelf life. Fish that can be stored for two weeks at 0°C may only last a day or two at 10°C.
In some areas, work has been done on ice-making machines that do not require gasoline, diesel oil, or electricity as an energy source.
A biomass-fueled ice-making machine has been developed at the Asian Institute of Technology in Thailand. It requires little maintenance and has no moving parts. Almost any waste biomass can serve as the fuel (figure 5.5). This ice maker uses an intermittent ammonia-water absorption cycle. These refrigeration systems produce their cooling effect through the heat absorbed when liquid ammonia is converted to gaseous ammonia. As liquid ammonia vaporizes, heat is extracted from its surroundings. When this change occurs in a closed container so that heat is extracted from water, ice is formed.
FIGURE 5.5 This refrigerator can make 225 kg of ice in about 12 hours using biomass as fuel. It has been successfully field tested in Khau Yai, a remote rural island off southern Thailand.
As seen in figure 5.5, fuel is burned in the stove (A) to heat water, which is then circulated through the generator (B) that contains a mixture of ammonia and water. The ammonia is distilled out of the water mixture, passes through the liquid seal (C), and is cooled to liquid ammonia in the condenser coil (D). The liquid ammonia is held in the ammonia receiver (E). To make ice, the liquid ammonia is released into the ice box (F) where it reverts to gaseous ammonia and converts containers of water to ice. The gaseous ammonia is then redissolved in water in the generator (B) and the cycle can start over. The complete cycle takes about 12 hours and produces about 225 kg of ice. The ice maker was built in Thailand at a cost of about US$3,000.
A compact solar refrigeration system that uses the same technology as the biomass-fueled ice-making machine has also been developed (figure 5.6). In this case, the ammonia-water solution is heated in the pipes of a solar collector.
FIGURE 5.6 A solar-powered icemaker has also been tested in Thailand. During the day the sun's heat is used to produce liquid ammonia, At night the liquid ammonia is used to produce ice-about 40 kg in a 24-hour cycle.
The purpose of secondary processing is to convert the raw fish into a form that is still acceptable to the consumer and that has a longer shelf life. However, to ensure a high-quality finished product, it is necessary to begin with a high-quality raw product. This, once again, accentuates the importance of primary processes.
Whether an end in itself, or as part of a smoking or drying process, salting has been used for thousands of years to preserve marine products. Salting has no adverse effect on the value of fish protein. Bacterial growth can be significantly retarded by the presence of sufficient quantities of common salt (sodium chloride). When fish is placed in a brine solution, the salt penetrates the fish, and water is extracted from the tissues by osmosis. At a salt concentration of 6-10 percent in the fish, the activity of most bacteria that cause spoilage will be inhibited. Since fish contain 70-80 percent water, the amount of brine used must be adjusted accordingly. The higher the salt concentration in the fish, the longer its storage life. Several methods of salting are commonly used: dry salting, kench salting, brine salting, and pickle salting.
Dry salting is the simplest method and is used primarily for fish with high water content. Granular salt is rubbed onto the outer and inner surfaces of the fish. Kench salting is a similar method that involves stacking split fish and layers of salt. The pickle or liquid formed is allowed to drain. In Brazil and India, sardines are preserved by pressing and salting. Avoiding air exposure is almost impossible in these dry-salting processes. However, wrapping the product in a plastic bag reduces contact with the air.
The wet-salting methods (brine and pickle) are recommended for tropical applications, especially with fatty fish. In brine salting, the entire or split fish is immersed in an aqueous salt solution. An 80-100 percent saturated brine solution (270-360 g of salt per liter of water) is preferred. For strongly cured fish, about 30 g of salt per 100 g of fish is needed. During processing, the brine solution will become diluted as water is drawn from the fish, and additional salt will be needed. Plastic or wooden barrels can be used for brine treatment. The largest fish should be able to lie flat in the container. A wooden lid, which can be weighted, should be employed to ensure that the fish are submerged in the brine solution.
Another wet cure is pickle salting. The fish are covered with salt and placed in layers with salt between the layers. Since a watertight container is used, the brine that is formed begins to cover the fish. If the fish are not completely covered within 3-4 hours, saturated brine is added to cover them. A lid is placed over the fish to ensure that they are completely submerged in the liquid. At least 10-24 days are required for complete curing.
Halophilic or salt-tolerant bacteria or molds may grow on incompletely dried salted fish or on dry salted fish that have become moist. However, pickle-cured fish are free of growths of halophiles, because these organisms are aerobic, and the brine of pickle-cured fish does not contain sufficient oxygen to support their growth. This oxygen-poor environment also reduces rancidity in fatty fish.
Much of the fish in rural areas of the tropics is preserved by sun drying. While the cost of sun drying is low, there are significant losses due to spoilage, contamination by dust, and insect infestation, particularly when the fish are laid close to the ground. As a first step, raised structures would reduce contamination from some wastes and insects.
Solar fish driers are simple and inexpensive and can eliminate
much of the spoilage that occurs with traditional drying methods. These driers
usually have a wood or bamboo-frame table, covered with plastic or glass to
produce an enclosed chamber (figure 5.7). The surface of the table can be
covered with black plastic or paint to absorb the sun's heat. With openings at
the top and bottom of the drier, air will be heated and flow around the fish.
Fish exposed to this flow of heated air will rapidly lose moisture, reducing
drying time by as much as half over open-air drying. Similar driers have been
constructed in Bangladesh, Indonesia, Rwanda, the Philippines, and Papua New
Guinea. Solar driers have a number of advantages over traditional drying
methods. They exclude rain, insects, animals, and dirt, and can
temperatures high enough to reduce the possibility of mold or bacteria spoilage.
A wide variety of designs for solar driers has been developed. Most require only inexpensive, readily available materials. In addition to plastic film and bamboo, discarded oil drums, scrap wood, thin metal sheeting, and even mud may be used.
An oil drum solar drier has a creative design (figure 5.8). The ends of the drum are removed and three rectangular ports are cut in the side. The drum is mounted on a wooden frame that includes air vents and access doors on both ends of the drum. Two sheets of clear plastic enclose the drum. These allow sunlight to heat the drum and, because of the air space between the two sheets, provide insulation to retain the heat. The outside of the drum is painted black to absorb solar radiation and the inside is painted white. Cool air enters the base of this unit and is heated as it passes between the exterior of the drum and the plastic sheets. This heated air then enters the drum through the rectangular ports and passes over the trays of fish in the drum and out through the vents at the ends of the drier.
FIGURE 5.8 A solar drier can also be made using an oil drum with a wooden frame and plastic sheeting.
A mud wall solar drier has been developed in Tanzania. A rectangle of clay walls is constructed and, while the mud is wet, bamboo tube vents are inserted at 50-cm intervals from side wall to side wall (figure 5.9). The bamboo tubes have a number of small holes so that air can flow into the drier. Fish trays are placed on top of the bamboo tubes and openings are cut into the top edge of the wall to exhaust the heated air. The inside walls and bottom of the drier are plastered with mud that is mixed with charcoal powder to absorb the heat. In addition, a layer of dark-colored stones can be placed in the bottom of the dryer to provide heat storage. The roof can be a transparent plastic sheet or film.
FIGURE 5.9 In Tanzania, a solar drier is constructed from mud and bamboo. When in use, fish are placed on the bamboo supports arid transparent plastic covers the top.
A solar dome dryer (figure 5.10) has been developed and tested in Aden. Designed on the basis of results with solar tent driers, this large unit has a capacity of about 1 ton of prepared fish. In drying tests with local fish, moisture contents of 20-25 percent were obtained. Typical sun-dried fish in this area had a moisture content of 25-35 percent. Use of the solar dome dryer also significantly reduced contamination from insects, animals, and dust.
FIGURE 5.10 In Aden, this large solar drier can hold about a ton of fish.
A solar collector can also be attached to a cabinet drier (figure 5.11). The solar plate collector uses black coated, corrugated metal to absorb the radiation. This is covered by a double panel of glass or plastic to insulate the warm air inside the collector. Air passes on both sides of the metal collector, becomes heated, and flows into the upright cabinet drier and through the trays. The optimum angle for the collector panel must be determined by experiment.
FIGURE 5.11 A solar can be attached to a cabinet drier to provide a flow of warm air for drying fish
The solar agrowaste-fueled drier was designed and constructed in the Philippines. It has the advantage of utilizing alternative energy resources in the absence of solar heat. The drier has both a solar booster and furnace. It has a capacity of 170-200 kg of fresh fish.
The body of the drier is trapezoidal and is made of wood or aluminum frames covered with polyacetate film. Black film covers the bottom of the drier. The side walls have up to 4 doors that allow 14 trays (114 x 53 cm) to slide into the drier. Woven nylon screen is used to line the bottom of the trays. The top portion of the back wall has several screened warm-air outlets.
A small stove can be connected to the drier. Charcoal or agricultural waste materials can be used as fuels in the stove furnace. Warm air from the stove is regulated with a shutter so that only heat enters the unit while fumes are excluded.
Various drying schemes are possible. The solar system may be used alone or simultaneously with the stove. Alternatively, the agrowaste system may be operated alone in the absence of solar heat or solar heat can be used during the day and agrowaste drying at night.
Other models of low-cost fish driers use only agrowaste as fuel. The agrowaste fish drier (figure 5.13), developed by the College of Fisheries of the University of the Philippines in Visayas, burns coconut husks, firewood, or rice hulls. The trays in the drier can hold up to 100 kg of fresh fish.
FIGURE 5.13 In this drier, the smoke from the burning fuel is diverted outside the unit while the heat passes up through the trays of fish.
At the base of the structure is a furnace made of hollow blocks, with inside dimensions of 0.90 x 1.20 x 0.85 m. An exhaust tube carries smoke and gases outside the system. The furnace and drying chamber are separated by a corrugated, galvanized iron heat guard. This guard has a 20-cm opening through which the hot air can pass into the drying chamber that contains 25 trays. Air vents are adjusted to ensure an optimum drying temperature of 55°-60°C.
Smoking is another traditional preservation technique that is used to prepare fish products with long storage lives. Smoke contains substances that kill bacteria, thus helping to preserve the product. The heat also dries the fish. Often fish are salted before they are smoked. In tropical countries, fish are generally heavily smoked at relatively high temperatures so that they are also cooked.
In hot smoking, temperatures may remain between 60°-110°C for 4-12 hours. This is usually long enough to eliminate the nonsporulating spoilage bacteria. However, the spores of Bacillus subtilis and B. mesentericus survive even with longer periods of smoking. The bactericidal action of the smoke is considerably increased by the presence of salt in the fish.
In simple smokers, fish are laid on trays or hung in the column of smoky air above the fire. The traditional Ghanaian mud oven is cylindrical, with a thatched cover. The oven consists of layers of mud about 2.5 m high and 10 cm thick. Grill bars are installed at about 1 m off the ground. The fish, placed on the grill bars, must be regularly turned to encourage even drying and smoking. A stokehole is located in the base wall.
A variation of the mud stove involves using 250-gal steel drums instead of mud for the construction of the cylinder. The drums are cut along their length and rejoined to form a larger cylinder. Fish are smoked on grills within this cylinder.
The Ivory Coast kiln (figure 5.16) is efficient and simple and has had a degree of acceptance, even though it deviates from traditional designs. The base of the kiln is 2 x 2 m and is about 1 m high. The sides are sheet metal or corrugated zinc, nailed to wooden support posts in the four corners. A steel drum, with a hole cut about one-third of the way down the side, is laid horizontally through one of the kiln walls. The fish trays are stacked on top of the oven.
FIGURE 5.16 The Ivory Coast kiln is simple and efficient but the construction materials are costly.
Another improved oven is an agrowaste fish drier and smoke chamber, developed in the Philippines (figure 5.17). The chamber is made of sheet metal and has three doors in the front where trays can be inserted. Charcoal or agrowastes are burned in the combustion chamber in the back of the smoker. After circulating through the drier, the smoke exits through three exhaust valves into the top of the structure.
FIGURE 5.17 In the Philippines, an agrowaste fish drier and smoker has been developed. This unit is made of hollow concrete blocks with sheet metal doors and chimneys.
The Chorkor smoker is gaining acceptance by West African women involved in traditional fish smoking. The design, based on traditional smokers, has a long life, low construction cost, and low firewood consumption. The capacity of this smoker is large; up to 18 kg of fish can be smoked on each tray, and as many as 15 trays can fit on an oven.
The ovens are rectangular and about twice as long as they are wide. There is a dividing wall in the middle, two stokeholes in the front, and a fire pit in each chamber. The walls are made of clay mud, cement, or clay blocks. The top of the walls must be level so that the wooden-framed trays can rest snugly against them. The oven should be low, but the fire ought to be at least 60 cm below the lowest tray. The wooden frames of the drying trays rest on the edge of the oven walls and therefore do not catch fire. These trays effectively form a chimney above the fire in which heat and smoke constantly circulate.
Small and medium fish may be smoked whole or split; large fish are cut into fillets. This smoking process yields a product with 10-15 percent moisture content and may require from 2 hours to 2 days. Fish smoked with the Chorkor smoker can be stored up to 9 months in the tropics if the trays are tightly covered with plastic, brown paper, or banana leaves. The fish should be resmoked every 2 months to eliminate mold, bacteria, and insect larvae.
Fish Paste Products
Kamaboko is a popular fish paste product made from surimi, a washed minced fish common in Japan since the fifteenth century. Codfish, croaker, lizard fish, and conger eel have the texture necessary to produce surimi.
To prepare surimi, the head and viscera are removed, the fish are cleaned in water, and the bones and skin are removed. Surimi, the minced meat, is then washed repeatedly with cold fresh water to produce a bland and functional meat. The surimi is then chopped in a cutter for 4 minutes while 30 g of salt per kg of fish are added. Next, potato or wheat starch is added (100-250 g per kg of fish), and the mixture is chopped for 10 minutes longer. Sugar (30-100 g per kg of meat) and chopped vegetables may be added before a final 5 minutes of chopping. The resulting paste is then shaped and cooked in a variety of ways.
Kamaboko is produced by shaping the surimi paste into half cylinders, like loaves of bread, on wooden blocks. The loaves are steamed at 85°-90°C for 40 minutes and then cooled for 2 hours in air. The products are packaged in cellophane and have a shelf life of 1 week in warm weather.
If the surimi paste is shaped into semicircles or squares, steamed at 90°-95°C for several minutes, and cooled on a grid, a product called hanpen is obtained. The surimi may also be shaped into a ball or cake and fried to produce satsuma-age. If it is shaped into a tube and steamed, it is called chikuwa.
In Taiwan, fish balls are made from fish paste. Shark, lizard fish, pike, eel, and marlin are the main species used. It is shaped by hand, made into balls, and then steamed.
One advantage of these fish paste products is that the raw materials are not recognizable. Therefore, low-priced fish or fresh species that are disliked can be utilized.
Boiled Fish Products
Boiling fish in water, as a method of short-term preservation, is accepted throughout Southeast Asia. This method may have applications in other tropical areas where high humidity and rainfall during part of the year make drying difficult. Boiling could allow distribution of the catch to market with low-cost equipment and facilities.
Boiling denatures the fish proteins and also eliminates many of the bacteria present. Therefore, such treatment may extend the shelf life of the product. Salt may also be added before, during, or after boiling to help retard spoilage.
In Indonesia, boiled fish products are known as pindang. Many species, including shark, may be used. The fish are gutted, washed, and arranged in clay pots or metal bowls, with alternating layers of salt and fish. A little water is added, and the fish are heated until nearly cooked. Most of the liquid is drained, more salt is added, and the fish are heated again until no free water remains. The top of the pot is sealed with leaves or paper. Shelf life may range from a few days to 3 months, depending on the amount of salt and the container seal.
In many Southeast Asian villages, rice and fish are the primary foods. Since both are relatively bland, a long tradition of preparing more flavorful products through fermentation of fish and shrimp has developed.
Since these products are generally derived through hydrolysis in the presence of high salt concentrations, they have good keeping qualities. The nutritive value of the fish or shrimp is retained and the processes are relatively simple. In some cases, the fish or shrimp retain their original form, but usually the end product is a liquid or paste.
Bagoong is a Philippine fermented fish or shrimp paste. Bagoong na isda is the fish derivative, dark gray in color with a cheeselike flavor. Bagoong na alamang is a thick paste obtained through shrimp fermentation. Although pieces of the shrimp remain, the characteristic aroma of raw shrimp is no longer detectable.
Bagoong na isda is prepared by mixing three parts of fish with one part of salt and enclosing the mixture in a fermentation jar. With occasional stirring to keep the salt concentration uniform, the fermentation is complete in 60 90 days. The corresponding shrimp product is made in the same fashion, but the fermentation is complete in only 3 days.
Nuoc-mam is a clear brown liquid, rich in salt and soluble nitrogen compounds, with a distinctive odor and flavor. It is produced in most coastal regions of Vietnam from small sea fish. More recently, production of nuoc-mam from freshwater fish has increased greatly. Traditionally, the fish are kneaded, salted, and placed in earthenware pots that are tightly sealed and buried in the ground for several months. When opened, the supernatant liquid (nuoc-mam) is carefully decanted. Except for histidine, nuoc-mam contains good concentrations of the nine essential amino acids. Although not a good source of the B vitamins, it is a valuable supplement to cereal diets through its content of other vitamins and minerals. Similar methods are used to produce nam-pla in Thailand and patis in the Philippines.
In the Philippines, fermented rice and fish mixtures known as buros are popular. These are prepared by mixing cooked rice and fish or shrimp with salt and allowing the mixture to ferment for up to 7 days. The products become acidic due to the action of lactic acid bacteria and have a shelf life of about 2 weeks at ambient temperatures.
In Korea, the fermented fish product sik-hae is widely consumed. Flat fish are eviscerated, sliced, salted overnight, and then mixed with cooked millet, red pepper powder, and garlic and fermented at 20°C for 2-3 weeks. The pH of sik-hae drops quickly to 4.5 due to the organic acids formed from the millet by the lactobacillus. After fermentation, the product can be stored for up to 1 month at 5°C.
Indonesian trassi is a paste made from small shrimp. Interestingly, its production starts on shipboard. When caught, the shrimp are mixed on deck with about 10 percent salt. On shore, the mixture is respread and more salt is added. After exposure to air and sun for 1-3 days, the moisture content drops to about 50 percent and the foul odor disappears. The mass is kneaded and redried and red colorants are added. Trassi has excellent keeping qualities. It is often mixed with Spanish peppers to give a spicy product called sambal, which is consumed with rice. The Colombo method of curing is used in South Kanara, India, to ferment mackerel. The fresh fish are gutted, washed, and rubbed with salt (ratio 1:3). After the fish are put in cement tanks, the fruit of a small evergreen tree Garcinia cambogia, similar to tamarind, is added (8 kg of fruit per ton of fish). The fish are left for 2-4 months in the brine that forms and are exported in wooden barrels. They can be consumed for up to a year.
In Aden, the same mackerel species is similarly salted in cement tanks, but the brine is allowed to escape. The fish are sewn into palm leaf bags and exported to East Africa.
Fish silage has also been studied as a source of protein for poultry and swine. The starting materials are fish-processing wastes or trash fish and a carbohydrate such as starch or molasses. These are inoculated with a lactic acid bacteria and fermented for 4-7 days before being fed to pigs or chickens.
Ice is desirable for preserving fish and would enjoy more widespread application if its manufacture were economically feasible. A simple, efficient, and economical technology for ice production using nonpetroleum fuels needs to be developed. Alternatively, more efficient and economical driers and smokers must be developed for application in Third World coastal villages.
There is a continuing need for research in the development of new fish products that are acceptable to the local consumers and that will increase storage life. Underutilized fish, especially shrimp by-catch, are obvious targets for product development.
Country-specific studies are needed to provide more precise information on losses during the various stages from capture to marketing. This information could help reduce postharvest losses while increasing protein consumption without increasing the catch.
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