Foreword
Aquaculture is the fastest growing food industry in the world. According to the 1998 report of the Food and Agriculture Organization of the United Nations (FAO), the People's Republic of China is the world's largest producer of aquatic products (Table 1). To make aquatic animals grow faster, you must first understand their nutritional needs and learn about feed production techniques. Of course, genetics, physiology, biochemistry and farming techniques are also important. Aquatic feed technology has progressed rapidly over the past 20 years. For example, aquafeed production has almost completely changed from steam granulation to extrusion curing. One of the many strengths of extrusion is the ability to make a floating feed so that the farmer can estimate how much feed the fish eats based on the amount of feed remaining on the surface. Overfeeding not only causes waste, increases production costs, but also pollutes the environment. Although extrusion will reduce the utilization of certain nutrients in the feed, it is still the first choice for farms. Farmers can see the feeding situation of the fish with their own eyes, so that they can better feed and master the fish. This article describes the aquafeed production process and makes some suggestions.
Source: FAO (FAO)
1 Aquatic feed plant design
Figure 1 is a flow chart of a full-process production line aquatic feed processing plant (Enterline, 1994). The feed ingredients are stored in the crushing shop silo or raw material warehouse through the receiving system. Feed ingredients can be comminuted separately and then batch-blended with vitamin premixes, mineral premixes, feed additives and liquid materials (this pre-mixing comminution system is common in North America). The system requires more storage bins, and the transportation and storage are not as good as the raw materials before the crushing. The raw materials can also be pulverized in batches (this post-crushing system is common in Europe), and then the vitamin premix, the mineral premix, the feed additive and the liquid raw materials are batch-mixed and stirred together. The post-grinding system is ideal for handling high-oil raw materials such as fishmeal and poultry lower feet. Such raw materials can also be mixed with cereal raw materials such as wheat and solvent-extracted oil mash, which can alleviate the trouble of pulverizing the high-oil-containing raw materials. The post-grinding system is provided with a buffer chamber for storing the pulverized material, sent from the buffer chamber to the mixer, and then formed. This system is used by most aquaculture feed mills because the feed mill uses a variety of by-products as feed ingredients, and it is not as easy to smash the various raw materials as the crushed mix. After batching, the raw materials of the batch are pulverized, stirred, and then sent to a silo in the workshop for granulation or extrusion.
When processing shrimp feed, the pulverized raw material discharged from the silo of the workshop is subjected to a two- or three-layer pre-modulator, the purpose of which is to lengthen the residence time of the material and add steam before granulation. The steam contains heat and water, and the heat raises the temperature to the temperature conditions required for the binder such as starch and wheat gluten to function. Without proper time, temperature and moisture, starch cannot be gelatinized and gluten cannot be formed. Shrimp is slowly fed at the bottom of the water, so the shrimp feed should be as stable as possible and stay in the water for a long time without disintegration. Figure 2 shows the effect of shrimp feed immersion time on water stability index (WSI) (Cheng Zongjia et al., 2002). The longer the feed is immersed, the greater the loss of feed. Figure 3 shows the effect of material moisture and process water temperature on shrimp feed WSI made with a meat grinder (Cheng Zongjia et al., 2002). It is clear from Fig. 3 that the shrimp feed prepared by the ground meat mechanism at 100 ° C and 40% moisture has the highest WSI.
The raw material processed by the pre-modulator is pressed into pellets in a granulator and then passed through a granulation cooker where more steam reaches the surface of the shrimp feed to enhance ripening. Finally the granules are dried by a dryer/cooler to a suitable moisture (less than 10%) for safe storage.
Most fish feeds are either floating or slow-feeding and are made by extrusion techniques. The extrusion process is also the same as granulation. The raw materials from the silo are placed in the extruder, and the suitable processing conditions are created by suitable casing, screw and shear lock configuration to make fish feed. After the material is extruded into a certain shape and cut into a certain size, it is sent to a drying/cooling machine for drying and cooling. The biggest benefit that farms can get from extrusion processing
It is to improve feed efficiency. Extrusion processed fish feed has better durability and water stability than steam granulated or cold extruded products. Extrusion is also beneficial for making fish feeds that add a lot of oil to some formulas, such as high-energy salmon and salmon feed. For this type of fish, high-energy feed can save diet protein and improve feed efficiency. The feed efficiency of the young rainbow trout feed can reach 1.0g feed/g weight gain, and the feed efficiency of the whole production cycle is about 1.2g feed/g weight gain. In the United States, the rainbow trout market weight is usually about 0.75kg.
After the feed is dried/cooled, the ground is removed by sieving. If it is necessary to apply grease to the granules, it should be sprayed hot after the granules are discharged from the dryer. Then, the pellet feed is sent to the bulk warehouse for shipment to the factory or loaded into the package for shipment.
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2 crush
Most of the raw materials used in the production of aquafeeds are crushed (reduced particle size). The purpose of the comminution is to obtain materials suitable for making high quality feeds and most suitable for animal digestion (Martin, 1983). Stevens (1961) pointed out that raw material pulverization has the following reasons: pulverization can increase the exposed surface area of ​​the material; make the material easier to digest; easy to transport during processing; pulverization can improve the uniformity of mixing, which is especially important for young animals with small feed intake Crushing also aids in granulation and extrusion.
The production of fish and shrimp feed, especially shrimp feed, requires the comminution of raw materials, so that the pelletized feed produced has better water stability and can improve animal performance. Palaniswamy and A1i (1991) conducted a study to reduce the particle size of the raw materials to 500, 420, 300, 250, 210, 50 μm (micron, the same below), and the results showed that the water stability of the obtained pellet feed was preferably 210 μm. Penaeus indicus (Penaeusindicus) has the fastest growth rate, the best feed conversion rate and the highest digestibility when fed with a diet with a raw material size of 210 μm. Obaldo et al. (1998) reduced the raw material granules of a shrimp diet from 603 μm to 586, 52 l, 408, 272, 124, 69 μm, and found that feeds made from 124 μm of raw material particles improved the water stability of the granules, granules. Persistence, starch gelatinization rate, live weight and weight gain of shrimp. Granular feeds with poor water stability produce a lot of debris, which not only causes waste, increases production costs, but also pollutes the environment. However, the crushing of raw materials increases power consumption and moisture and powder loss during production, which increases production costs.
The hammer mill and roller mill are used to coarsely grind the grain to make 300-1200 μm crumb, which is very efficient. When a high-fat or high-oil raw material is processed by a hammer mill, the sieve plate is easily clogged. When smashing high-fat raw materials such as fishmeal and poultry lowering feet, some grains can be pulverized better. When the pulverizer is processed, a sieve should be added to the pulverization process, and the coarse granules are returned to the pulverizer for further pulverization, so that the desired particle size can be stably obtained.
The ultrafine pulverizer is the most commonly used pulverizer for shrimp feed and fry feed. The ultrafine pulverizer, or air classification mill, can pulverize the raw material to below 100 μm (Sorenson and Phillips, 1994). The ultrafine pulverizer can pulverize the raw materials to various particle sizes by changing the gas flow rate, the feeding speed, the grinding speed or simultaneously changing these three factors. Figure 4 is a flow chart of an ultrafine pulverizer. This air classification superfine pulverizer does not have a sieve plate, which avoids the phenomenon that the grease generated by the hammer pulverizer blocks the sieve hole, thereby reducing downtime and achieving better quality control. However, high-fat raw materials may also clog the pulverization chambers of the cutter and the ultrafine pulverizer. Table 2 (Cheng Zongjia et al., 2001) shows the reduction in particle size of poultry lower foot powder after processing in an ultrafine pulverizer. The particle size of the feather powder treated by the ultrafine pulverizer decreased the most.
Fat has a negative effect on the comminution of the material because the fat will liquefy during the comminution process, interrupting the comminution process. The liquid also makes the raw material soft and difficult to pulverize, and as a result, the product has an excessive particle size. Cellulose to material
Crushing also has a negative impact.
There are many factors to consider when choosing a crushing device. Power consumption is one of the most important factors. Power consumption can be calculated using the following equation:
Power consumption (kwh/ton) = √ 3 × A × V × EFF × PF / (ton / hr × 1000)
Among them, kwh/ton=kWh per ton:
A = amp; V = volt; EFF = efficiency factor;
PF = power factor; ton / hr = tons per hour.
As an example, Table 3 lists the electricity consumption of some poultry lower foot powder and feather meal by an ultrafine pulverizer (Cheng Zongjia et al., 2001). The power consumption (70.9-97.7 kwh/ton) of these animal protein powders pulverized by an ultrafine pulverizer is larger than that of pulverizing the grain by a roll mill. Heimann (1983) reported that corn was crushed to a particle size of 432 μm and 649 μm by roller mill, and the power consumption was 26.32 and 11.98 kwh/ton, respectively; the corn was crushed to a particle size of 399 μm, 548 μm, and 647 μm by a hammer mill, and the power consumption was 31.25, respectively. , 16.39, 16.26kwh/ton. Smashing also causes moisture loss. McEllhiney (1980) reported that corn was crushed with a hammer mill and the average moisture loss was 1.1% to 1.5%. Remen (1976) showed that whole-grain corn with 15% or more moisture lost an average of 1.2% of water in a pneumatic conveying system, while whole-grain corn with a moisture content of 14% or less lost an average of 0.81%. Wolfe (1982) treated the corn with a hammer mill. During the four-month period, the corn fine pulverization (3.2 mm sieve, mechanical transport) lost 1.1% of water, and the coarse pulverization (4.7 mm sieve, mechanical transport) lost 1.05% of water. . The power consumption for fine pulverization with an ultrafine pulverizer is larger than that with a hammer mill or a roller mill.
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3 granulation
The feed processing industry has undergone tremendous changes in the past 20 years. Expanded processing of poultry feed and piglet feed, as well as extrusion processing of fish feed, are generally accepted and adopted. However, shrimp feed production still uses granulation technology because granulation costs are lower than expansion and extrusion, while shrimp is fed under water. As long as feed manufacturers can find ways to improve the stability of granule water, they are not necessarily produced by extrusion. Shrimp feed.
Granulation is defined by the use of mechanical means of combining heat, water and pressure to compact the feed material and force it through the die orifice (Dominy et al., 1994). Both heat and water are present in the form of steam, which softens proteins and starch, denatures proteins, and gelatinizes starch. The denatured protein and the gelatinized starch can be bonded together to make the granulated feed more durable and more stable in water. After the raw materials are granulated, the feed efficiency is improved, and the various raw materials are more fused, which can also reduce the wind throwing loss and increase the bulk density.
The granulator consists of a feeder, a modulator and a granulation chamber. Figure 5 is a flow chart of granulation. The feeder delivers the material to the modulator and then to the granulation chamber. There are devices that prevent the modulator's steam from retrogressing into the feed hopper. The modulator provides heat and water to soften proteins and starch, denatures proteins, gelatinizes starch, and reduces anti-nutritional factors such as soybean anti-trypsin, reduces bacterial and mold counts, and improves palatability. Single, double and triple layer modulators are commercially available. The three-layer modulator has the longest residence time. Typically a residence time of 90 seconds is sufficient to allow heat and water to pass through the material. The modulator should use 1-2 kg/cm2 of saturated steam. A temperature of 90 ° C or higher is usually sufficient for the binder to function. Generally speaking, the moisture of the material should be 16-18% before reaching the granulation mold. Although more moisture will make the material bond better, too much moisture will block the granulation die and cause more downtime, which will cost more heat and increase the drying cost. Steam modulation usually increases the moisture by 2%, and the raw material moisture is usually 10-12%. Therefore, about 2% of the water can be added during the stirring to make the material reach 16-18% moisture before reaching the granulation ring mold. High quality, long lasting, high water stable feed. After granulation, the grease can be sprayed, and the granule moisture should be dried to less than 12% for safe storage.
The granulation chamber shapes the material into granules which have two parts: a ring die and a press roll. There are three types of steel that can be used to make ring molds: carbon gradeallox, carburized stainless steel, and neutral hardened c hrome stainless steel. For the production of shrimp feed, a neutral hardened chromium stainless steel ring mold is preferred because the high chromium content prevents the feed from eroding the ring mold, thereby preventing a decrease in feed yield and improving the quality of the pellet (Dominy et al., 1994). The second choice is a carburized stainless steel ring mold. This ring mold has a high coefficient of friction due to carburizing, and can push the material at a faster speed, but its corrosion resistance is inferior to that of a neutral hardened chromium stainless steel ring mold. Carbon-grade alloy ring molds are rarely used in shrimp feed production because of their poor corrosion resistance and short cycle life. Shrimp feeds typically require a particle diameter of 2.0-2.5 mm and a ring mold thickness typically in the range of 45-50 mm. In addition to the ring mold, the granulation chamber has 2 or 3 press rolls. The press rolls should be reasonably designed and should have sufficient support to withstand the load pressure, prevent debris from entering the bearing, and have as much propulsion (powder) force as possible (Fairfield, 1994). There are three types of press roll faces: cemented carbide roll face, scored roll face and cogging roll face. The cogging roll surface is most commonly used in shrimp feed production. Its end can be either open or closed. Increasing the number of slots reduces the width of the slot to enhance propulsion and prevent blockage of the ring. The pressure roller controls the efficiency of the material passing through the ring mold, thus also controlling feed yield, and also controls the wear of the ring mold surface. The layer between the press roll and the ring die should be as thin as possible, and the press rolls should be inspected and adjusted every day. In order to achieve maximum production efficiency, the new ring mold must be equipped with a new pressure roller.
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4 drying / cooling
After the feed is granulated and extruded, it is dried or cooled. There are two types of dryers: vertical and horizontal. The horizontal dryer is suitable for drying/cooling high-moisture aquatic feed. After the granules are granulated or extruded, they are evenly spread on the moving conveyor belt of the dryer at a certain thickness. The dryer can be used with natural gas or steam, and the natural gas fuel dryer is more efficient than steam. The hot air temperature is in the range of 100-200 ° C, and the feed can stay at this temperature for 4-6 minutes. After that, the hot air temperature should be lowered to prevent the Maillard reaction (non-enzymatic browning) from affecting the nutritional value of the feed. When hot air passes through the particles, it absorbs moisture and causes the temperature of the particles to drop, partially achieving the purpose of drying/cooling. Part of the exhaust gas from the dryer can be recycled back to the dryer to reduce heating costs. There are single, two and three horizontal dryers in the city. Two-layer dryers are more common in the aquafeed processing industry. The particles move along the upper conveyor belt (particle thickness about 10 cm) in this dryer, and then fall to the lower conveyor belt (particle thickness is about 15-20 cm), and only about 3 percentage points of water can be lost in this indexing process. The granules are then discharged from the outlet end of the dryer. After drying, the granules should be cooled, usually by selecting a drying/cooling machine to do the job. As an example of a two-layer dryer, the lower conveyor belt can be extended outside the dryer and the particles are cooled through the particles at room temperature (Fairfield, 1994). If the particles need to be sprayed with grease, it is usually hot sprayed, ie immediately after drying and before cooling, which promotes the absorption of animal fats. After drying and cooling, the feed should be stored in a dry ventilated chamber to maintain quality. Appropriate drying/cooling makes the feed easier to store and is therefore an important part of feed processing.
(1, ASA Beijing Office; 2, Hagerman Fish Farm Experimental Station, University of Idaho, USA; 3. Aquatic Feed and Nutrition, American Institute of Oceanography; 4. Department of Grain Science and Technology, Kansas State University, USA)
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