Extrusion processing technology for fish feed (1)
Extrusion technology can provide special feeds for fisheries and aquaculture. This kind of fish feed can be customized for sinking, buoyant materials, and only the appropriate equipment is needed, and the appropriate extrusion procedures in production. In the past five years, the commercial aquaculture development as human food consumption has developed extremely rapidly. To adapt to this change, the feed industry that provides special feeds for this application has been rapidly developed. Many extrusion techniques applied in the processing of other animal feeds can also be applied to the production of fish feeds. Extrusion technology has proven to be an extremely effective way of cooking and granulating raw materials into special-purpose feeds. The main condition affecting feed characteristics is the temperature and humidity of cooking in the extruder. These conditions affect the cooking of starch and protein. Because starch paste cooking gelatinizes the raw materials, the degree of starch cooking has a great influence on the properties of the final product. In the raw material state, the cereal starch forms small particles that are insoluble in water. When mixed with water, they behave like sand. When the starch is cooked, its particles start to swell and then burst, and the starch molecules flow out of the tightly packed particles to form a loose state. This happens very quickly during the cooking process of the extruder, where the starch forms a gel and can absorb up to 10 times its own weight in water. This gel has the viscosity to bind together all other solid particles in the feed component, and the gel body also swells. When the cooking effect is enhanced, the starch gel properties change, the water resistance thereof decreases, and the water solubility of starch greatly increases. As with starch, certain proteins also form a gel, but if the protein is overcooked, the protein will escape from the gel and return to its original solid form. Many of the characteristics required in animal feed are directly related to starch cooking. - The animal feed should be conducive to digestion. Normally, the starch should be cooked to the state of broken particles. - The feed should be accurately formed into granules (treated by an extruder head cutter) without the presence of powder. - The particles should have the required density, usually the internal structure should be porous. - Particles should be able to absorb water and maintain their shape, if necessary, should be able to sink or float in water. In a typical extruder process, all or most of the starch cooking occurs within the extruder barrel. Raw, unwetted and heated feed components are fed into the extruder and then water is injected into the extruder to increase the humidity. Steam contains heat, which in turn increases the temperature of the material being processed by the frictional heat generated by the screw rotating the material in the extruder. The screw shaft presses the material toward the outlet end of the barrel, and the pressure applied to the material increases. A small number of small holes are opened in the template at the outlet end, which is hot and humid, and the cooked (pressurized) material is released. The pressurization in the extruder causes all the water in the extruder to be in liquid form, and even the vapor is compressed into a liquid form. When the product is released into the atmosphere, the water will immediately evaporate. If the starch is fully gelatinized, it will also swell to form thousands of small holes, which will reduce the product density. Because of the high pressure applied by the extruder, all of this is done in a relatively short period of time. The total squeeze residence time, from the entry of ingredients to the final expansion, the formation of cooking products is only 20 seconds. We can operate the extruder to cook the material so that the starch forms a high water-affinity gel-like material. This degree of cooking can usually be performed by operating the extruder at a suitable temperature of 240 to 280 degrees Fahrenheit, a very high moisture content of 27 to 33% to get it. The extruder can provide a greater degree of cooking, in which case some starch molecules begin to decompose into dextrin molecules and other smaller molecules. The partially gelatinized starch still forms a gel, but it does not absorb as much water as the ungelatinized starch. Some gelatinized starches can be dissolved in cold water. Therefore, gelatinization or, more precisely, gelatinized and starch-mixed fish feeds have different characteristics. Feed manufacturers can take full advantage of this by operating the extruder to obtain only minimal gelatinization. If necessary, or in the case of starch gelatinization to varying degrees, the degree of gelatinization can be enhanced by increasing the temperature (350 degrees Fahrenheit) and decreasing the humidity (20-30%). In this case, there will be a large amount of starch conversion. Dextrin. The factors affecting humidity and temperature are internal interactions to some extent. High temperatures are usually generated by screw friction materials, but if the humidity is high, the temperature is difficult to increase. So if a high extrusion temperature is required, this high temperature usually requires a lower humidity to obtain. The pretreatment of the feed processing will affect the cooking function of the extruder, especially the grinding degree (grain size) of the material. The grinded feed composition will be more difficult to cook than the finely ground components. It is more difficult to obtain uniform cooking of coarsely ground material. The central part of the larger particles is often difficult to be fully cooked. The larger the particles, the easier this is. To control the degree of cooking, you must make sure that the starch of the feed ingredients should be properly ground. Ideally, all particles should be milled to 20 mesh (U.S. Standard), and grinding to 40 mesh gives excellent results. However, feeds do not need to be completely ground, and no feed needs to be crushed below 100 mesh. The other feed ingredients in the formula, those containing no starch, do not need to be crushed to an excellent state. However, if the components themselves are too large, they will appear as particles in the pellet feed and they may be separated later. If you do not want this to happen, the entire formulation component should be crushed below 20 mesh. The fat in the feed formulation components reduces swelling and tends to form high-density pellets. How to add fat can determine whether the pellets float or sink. Adding fat before or directly into the extruder will increase feed pellet density and help ensure feed sinking. On the other hand, if the fat is sprayed onto the surface of the extruded feed particles, the swelling effect will not be affected, which will make the expanded granules heavier, but the interior still has a porous structure. If a feed formula requires more fat, but requires a floating function, adding fat after puffing rather than before puffing can be of great benefit. Although the extruded state has an effect on whether a product is floating or sinking. However, to a large extent, the feed component itself has a more major effect on this, and a high fat content will ensure that the product sinks. The extra gelling proteins in the starch or components will give the product a floating ability. The source of starch has a great influence on whether a product can float or not. There are indications that potato starch and some other starches are easier to float than corn or wheat starch. When extruding the pellets, it is important to control the amount of expansion so that the product does not exceed the desired degree of expansion, or it is distorted due to the instability of the bulking. Another important point is that the product should have a structure that fits the taste of the animal. This is affected not only by the cooking pressure of the extruder but also by the humidity and the work of the cutter when the pellets are extruded. If the pellets are squeezed out at high humidity (30%), the moisture inside the pellets (30%) will lose part of the moisture if the pellets are evaporated. Therefore, the resulting 27% water-containing particles will be too soft and broken. The crushing part forms a shell on the surface of the particle and makes the porous structure pores inside the particle smaller. After drying, the granules will have a tough, hard texture and a sufficient density to sink. On the other hand, extrusion at 20 to 25% humidity can cause the particles to harden after volatilization, so that only minimal fragmentation occurs. The pellets are then surface porous, with a larger internal pore structure and a thinner surrounding pore wall. This is suitable for a softer mouth feel and maintains a lower density of the product, so that the product is more likely to float. Extruded particles are cut into pellets by a rotary cutter attached to the surface of the template. The cutter should be set up so that the product can be cut neatly without any distortion. The best time to finish cutting a product is before the product is released from the die without the opportunity to expand. After cutting, the particles will expand when they overflow from the cutter area. The cutting blade should be very thin and parallel to the surface of the template to reduce the possibility of distorting the particles.
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