Articles From Extru-Tech
* Introduction
The fish farming industry has undergone dramatic world wide growth in the last ten years. As the industry has matured, a more sophisticated understanding of efficiency and nutrition is placed on the fish feed manufacturers. Customers view critically the total quality package offered by the supplier of feed, stretching the capability of a poorly designed feed manufacturing system. Today's expectations are that the fish feed supplier can produce a product that hits precise quality targets in terms of size, durability, density and nutritional composition at a highly competitive price. The following article describes a few of the ways that Extru-Tech, Inc. has tackled these issues to keep our customers at the cutting edge of their market.
* Product Consistency
A typical feed system, in an extrusion plant, consists of a feed bin with bottom discharge into a screw conveyor for metering of dry feed or mixed grist. This volumetric dry feed addition system typically varies in dry feed rate by up to 15% over or under the target rate. This wide swing in feed rate causes dramatic changes in product quality as the extruder tries to cope with being starved of feed, then overfed. Degree of cook, product density and proportion of added materials such as fish oil can all swing wildly as the feed rate varies.
To address this issue we have developed a gravimetric feed system which weighs the feed bin and contents continuously to adjust the feed screw rate to accurately control feed rate. We have designed, built and tested the system that can control within 0.5% of control dry feed rate, this being achieved by a combination of sound mechanical design and advanced electronics which continuously learns. Other added ingredients such as fish oil, water and steam can then be controlled at a proportional rate, giving a highly consistent product. Our customers enjoy a definite edge in their field with this feed system.
* Product Density
It is hard to cook a product thoroughly and then form a dense pellet all in the same extruder barrel, especially if the feed is high in oil and low in starch. We realized some years ago that it is extremely difficult to ask an extruder to both cook and then form a dense pellet. This led to the development of the Product Densifying Unit or PDU (A Patented Process). This system uses an extruder to cook a rope of material which is then fed into the PDU. The extruder runs at high power and high speed (rpm), it’s operating parameters being set to optimally cook the product. The PDU runs under lower power at lower speed while precisely controlling the product's compression and temperature to give the required density. As the composition of the dry feed changes for different products or as ingredients vary the extruder and PDU can be independently adjusted to maintain product quality. The production rates that can be achieved with this combination are highly competitive with total capital and energy costs significantly less than an equivalent twin screw extruder system.
* Pellet Quality
A sinking fish feed pellet should be of uniform size and shape with minimum deformation of shape and no roughness of surface. If the surface of the pellet appears to be rough it is highly likely that there is too much drag through the die and the product is being cooked and expanded in an uncontrolled way as it leaves the forming machine. This can reduce pellet density and create more fines as the pellet is handled. We have developed a series of dies using a combination of materials that create the ultimate in pellet appearance. The drag as the pellet travels through the die is reduced to a minimum and the pellets travel easily from the machine in a uniform manner.
The way that the pellets are cut and handled as they exit the machine is very important for control of breakage and appearance and we are continually working to improve cutting and handling systems.
* Starch Cooking
A fish feed pellet is held together by a matrix of cooked starch. Water and steam are added to the dry feed to a total moisture content of between 22% and 32%, depending upon individual process requirements. As the moisture content rises it is easier to cook the starch, but other components of the feed compete with the starch for water, especially protein. The lower amount of water available to the starch means that more energy has to be put into the starch to cook it. This extra energy can have anti-nutritional effects on the rest of the dry feed and can increase energy costs.
We have developed the Enhanced Hydration System (A Patented Process) that first separates the starch component from the dry feed, then hydrates the starch and finally adds it back to the dry feed. The starch cooks much more quickly at relatively low temperatures, allowing not only energy savings and reduction of anti-nutritional effects, while using lower levels of starch.
Note that many fish feed manufacturers try to add as much oil as possible before extrusion. This added oil can have an ‘encapsulation’ effect on the starch, making it more difficult to cook the starch and to obtain a durable pellet. The Enhanced Hydration System allows higher levels of oil to be added before extrusion because it avoids this encapsulation problem.
* Oil Coating
Some feeds need oil contents as high as 35%. Typical maximum oil levels before the extruder can rise as high as 18%; but levels higher than this cause product quality problems with the cooked matrix not being able to hang together. This means that a significant proportion of oil has to be added after the drying system. A coating system that operates at ambient pressure can add up to 10-12% oil to a product, depending upon product expansion and density. A vacuum coating system is required for addition levels higher than this. Note that the product will need to be cooked in a different way if very high levels of post-drying oil are to be added, a more open matrix being needed to soak up and hold the extra oil.
* Pigment Loss
Canthaxanthin pigment is fed to salmon species to give the pink flesh color required by the consumer. This pigment is extremely expensive and is sensitive to heat. Much higher quantities of the pigment are added to the feed than are required by the fish to allow for heat destruction. We are actively working in this area to reduce the amount of pigment loss and are looking for industrial partners to take some of our more exciting developments through to full commercial application.
* Specific Density
A product characterized only in terms of bulk density will not necessarily predict whether it will sink quickly or slowly in water or brine. We typically make a sinking fish food with a wet bulk density off the extruder of about 42 pounds per cubic foot, knowing the dry bulk density will be 1 to 2 pounds per cubic foot below this. A more precise way of characterizing the product is by weight under water, measuring the displacement volume of water. This allows us to calculate the specific density of the product, a property that is independent of product size and shape.
The density of 4% brine is about 1040 grams per liter (64.9 pounds cubic foot). A sinking fish food should have a finished specific density of at least 1055 grams per liter (65.9 pounds per cubic foot). Remember that post-coating with fish oil will cause displacement of air in the product, thus increasing the specific density.
* Conclusion
It can be seen that Extru-Tech, Inc. is actively working on many issues in the area of sinking fish feed manufacture. We haven’t solved all the problems yet but feel we’ve made significant progress in many areas, as evidenced by some of our customers who are competing very effectively in the world market.
We are willing to work with anyone who can help us all to stride forward in this very important commercial area and will be pleased to talk to anybody who either wants more detailed information or would like to work with us on specific projects.
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