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Although the number of feed mills specialized in aquaculture feeds (figs 7.5 and 7.6) and the number of animal feed mills which also produce aquaculture feeds are steadily increasing, there are large areas of the world that are not yet served by commercial feed milling for aquaculture. Either the existing demand does not justify large-scale manufacture or the farmers have not yet recognized the economics of using artificial feeds. Because of these reasons, aquaculturists who wish to use such feeds will have to resort to on-farm production of feeds, as import of manufactured feeds is often difficult or impossible.
It is possible to prepare feeds on a small scale on the farm using inexpensive equipment and locally available ingredients (figs 7.7 and 7.8).
For example, Pascual (1982) describes the procedure for small-scale preparation of a feed for the tiger shrimp (Penaeus monodon) juveniles. The suggested equipment includes weighing scales, sieves, a mixer of 5 or 10 kg capacity, a meat grinder, corn meal or coffee grinder, a steamer or a big cauldron and bamboo basket for steaming, a saucepan for gelatinizing starch, and a drier. The ingredients in the relevant formula are finely ground and passed through a sieve of nylon mesh (420 m/cm2). Weighed ingredients are mixed thoroughly, oil is added and then they are mixed again for a few minutes.The starch, gelatinized with water, is added to the mixture and mixed well to form a dough which is passed through a grinder using a 1–3 mm die. The extruded feed is cut into small sizes (0.5cm) and then steamed to make water-stable pellets. The steamed product is dried overnight in an oven at 60°C. Such pellets can be made once a week to meet the needs of the farm.
The feed requirements in large-scale aquaculture may make this type of feed production unfeasible, and large-scale milling may become necessary. However, the basic processes
involved are essentially the same and consist of particle size reduction, premixing of micro-nutrients, mixing of all components of the diet, pelleting and cooling, and then sacking. These processes are illustrated in fig. 7.9. Coarse ingredients are ground in a hammer mill or other type of grinder such as an attrition roller or cutter mill. Besides reducing particle size and facilitating handling of ingredients, grinding improves feed digestibility, acceptability, mixing properties and pelletability.
The ground ingredients may be sieved to separate materials into the required sizes. For example, feeds sifted through 177 mm meshescan be used for making feeds for fry of certain species like the catfish. An excess of dust in the ground feed may be controlled by adding a spray of oil or a semi-moist ingredient, such as condensed fish solubles or fermentation solubles, on feeds entering the grinder.
The next process, namely mixing, is performed to achieve uniformity of composition in the whole feed material according to the required formula. This may involve both scattering of particles and blending. Mixing can be either a batch or continuous process. The best-suited technique for formula feeds appears to be continuous mixing of proportions by weight or volume. There are many types of mixers used in feed mills such as vertical mixers, continuous and non-continuous ribbon mixers and liquid mixers. Accurate mixing requires the addition of ingredients in a tested sequence from batch to batch. Usually, large-volume ingredients are added first, and then the smaller amounts. Total mixing time is based on the composition of the formula.
The general process of pelleting involves passing a feed mixture through a conditioning chamber where 4–6 per cent water (usually as steam) may be added. The water provides lubrication for compression and extrusion and in the presence of heat causes some gelatinization of raw starch present in ingredients of plant origin, resulting in adhesion. Within a few seconds of entering the pellet mill, the feed goes from an air-dry condition to 15–16 per cent moisture at 80–90°C. Due to friction, the feed temperature may increase further to nearly
92°C, during subsequent compression and extrusion through the die. Pellets discharged from a pelletizer to a screen belt of a horizontal tunnel drier or a vertical screened hopper are air–cooled in about 10 minutes and dried to below 13 per cent moisture.
According to Hastings and Higgs (1980), finished pellets contain practically all the nutrients in the ingredients as compounded. The loss of thermolabile vitamins can be compensated for by extra supplementation in the vitamin premix used.
The cooled pellets may be ground on corrugated rolls and sifted into various sizes of granules and crumbles (fig. 7.10). The crumbles are suitable for feeding small fish and are more easily consumed by sightfeeders. They are, of course, superior to meal rations and less expensive to manufacture than small-sized pellets.
Two qualities of the pellets are of special importance and have been referred to earlier: hardness and water stability. Moderately hard pellets are easily consumed by many species, but for some there is a danger of overfeeding with hard pellets, causing swelling and rupture of the stomach. The feed may not be digested properly and may cause fermentation and gas formation in the stomach; the fish may then float upside down.
Hardness of a pellet does not necessarily correlate with water stability. In order to prepare more water-stable pellets and thereby improve feed conversion, a number of measures can be adopted. Before pelleting, the mixed feed may be ground through a 2 mm screen to an effective size of about 125 mm. Organic flour such as rice dust, wheat endosperm or other binders may be added, replacing about 5 per cent of some non-essential ingredients, if the formula is deficient in binding material. The addition of sufficient dry steam to condition the soft feed to a temperature of 85–90°C would cause gelatinization of raw starch. The pellet mill should be operated at its optimum rated amperage for maximum compression and extrusion.
As mentioned earlier, feeds may be pelleted by the extrusion process to form floating pellets, which are suited for certain types of culture. One advantage of this type of pellet is that the fish can be observed while feeding andthe amount of feed regulated according to the amount eaten. The processing of floating pellets consists of (i) conditioning the feed which is in a meal form to contain 25–30 per cent water, (ii) conveying it by auger into a pressure cylinder, (iii) injecting steam to increase gelatinization of raw starch, and (iv) then extruding to atmospheric pressure, almost exploding the material through holes in the die plate at the end of the cylinder. The extruding ribbon is cut by a rotating knife outside the die plate and the pellets are then dried at about 120°C to a mois-ture content suitable for storage. Following oven drying, a standard pellet cooler is used to lower product temperature after internal moisture becomes less than 13 per cent. The previous high temperatures may partially destroy heat-labile vitamins and decrease the availability of some amino acids. Instead of over-fortifying the formula, as for hard pelleting, the necessary additional additives may be sprayed on to expanded pellets after extrusion.
The expansion process is more expensive and there is some evidence to show that fish fed on floating pellets contain relatively more liver and body fat (Hastings and Higgs, 1980), probably because of increased digestibility of the carbohydrates in the ration.
Therapy and chemoprophylaxis of a great majority of bacterial diseases and many endoparasitic invasions can be achieved efficiently through the use of medicated feeds. For this purpose antibiotics, sulphonamides, nitrofuran and antiparasitic compounds, as well as some of the disinfectants, can be added to the feed. In order to avoid misuse of such diets and hazards to human and animal health, it is necessary to follow proper standards and ensure that only permitted compounds are used.
Astaxanthin and canthaxanthin are the pigments commonly used to give attractive coloration to salmon and trout. They are generally added as water-dispersible gelatine beadlets in concentrations of 100 mg carotenoid per kg. A high dietary lipid content in the diet is known to improve the utilization of carotenoids.
Carotenoids are labile compounds and are prone to degradation by heat, acids, alkalis and oxidation. They can be stabilized somewhat by the use of antioxidants.
Except for off-flavours that may develop due to environmental conditions, there is as yet no consensus regarding the organoleptic qualities of cultured aquatic animals as against wild ones. It has to be remembered that the commonly used feed formulations have been developed to provide optimum growth and feed conversion and not texture or flavour.
The treatments involved in feed processing generally help in increasing the nutritional value of feeds. Heat treatment, for example, improves the nutritional value of soybean meal by destroying the trypsin inhibitor present and by increasing the utilization of the essential elements. Digestibility is improved by partial cooking. Similarly, heat treatment increases the nutritional value of cereal grains by gelatinizing starches and improving digestibility. Grinding also increases the nutritional value by reducing the particle size and thereby facilitating digestion. Pelleting improves palatability and steam conditioning improves digestibility. The heat produced during compaction of the pellet may also destroy thermolabile toxic factors that occur in some plant products. Table 7.25 gives details of the effect of processing on toxins and inhibitors in a number of feedstuffs.
Salmonella in feedstuffs such as meat meal is killed by pelleting, but aflatoxins, produced by Aspergillus flavus, are not inactivated by normal pelleting procedures. It is necessary to prevent mould growth. Materials susceptible to contamination by aflatoxins, such as corn, peanut meal, cottonseed meal, copra and fish meal, should be monitored routinely for the presence of aflatoxins.
Storage of both raw materials and processed feeds needs special care as both may undergo deteriorative changes during storage. Loss and deterioration of raw materials often occurs as a result of insect infestation, which is greatly facilitated by high ambient temperature, relative humidity and moisture content of the feed ingredient. Most feed materials undergo some chemical changes that alter their flavour and nutritive value. Besides eating the feedstuffs, insects also accelerate these changes by secretion of enzymes such as lipase. Fats in feedstuffs often break down during storage. Recontamination of feedstuffs by adventitious micro-organisms is another major hazard of be avoided. Fungi grow at moisture contents of 15–20 per cent and cause spoilage of feedstuffs in storage. They produce mycotoxin, raise the temperature and moisture content, and cause mustiness. The highly toxic and carcinogenic aflotoxins produced by A. flavus, are perhaps the most important mycotoxins contaminating feedstuffs.
Rancidity is another important type of deterioration in storage and is caused mainly by oxidation of lipids, besides some hydrolysis and ketone formation. Lipid oxidation can be inhibited by the addition of antioxidants such as ethoxyquin, butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA). The permitted levels are 150 ppm ethoxyquin or 200 ppm BHT or BHA.
In a feed mill, special care has to be taken to store both feed materials and processed feeds in as cool and dry conditions as possible, raised off the ground on pallets. Warehouses or silos should be constructed in such a way that the interior can be kept cool and dry with adequate ventilation. Large silos are used for storage of pellets and some of these are equipped with conveyors or worm screws to move feed conveniently (fig. 7.11). Large silos are equipped with dust collectors to collect the fines which accumulate from repeated storage of pellets. These fines can be repelleted or used for feeding mixed with fresh pellets.
Most commonly, pellets are packed in poly-ethylenelined sacks made of multilayered paper or other material. Feed can be stored right on the edge of the farm in hoppers, so that outlet chutes can be swung over the water to discharge pellets directly into a boat for distribution. It is advisable for the farmer to buy feeds to last only a few months at a time, as long-term storage inevitably results in some deterioration. Properly stored, the feed will keep well for one to three months in summer and two to four months in winter. It is necessary to protect the feed from moisture and dampness, and feeding hoppers should be placed at sufficient distances from water to avoid splashed water entering them.
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