FBA Issue 23: November / December 2008
Wet or dry? Single screw or twin screw?
by KOH Hui Meng
Booming global aquaculture is creating a dynamic new international market for feed companies. Aquafeed now constitutes the feed industry's fastest growing sector. Over one hundred companies worldwide are manufacturing aquaculture feeds. In addition, companies that now produce feeds for conventional farm animals are increasingly also entering the aquaculture market.
Hence, while aquaculture is a small sector of the overall feed industry, its rapid expansion provides an opportunity for companies to gain a competitive edge in an otherwise entrenched and moderate growth industry. The exponential growth in aquaculture feeds is attracting companies from diverse industries, even ones not previously associated with aquaculture.
In order to meet rapidly growing for aquafeed, feed companies have to take a close look at a very efficient processing technology. It needs to produce high quality feed at a reasonable cost and given aquaculture's long-term declining rate of return, provide a sufficiently wide profit margin. As extrusion is one of the most versatile and energy efficient processes for making feed, it remains relevant to today's aquaculture industry.
Principles of Extrusion
Extrusion, which involves forcing feed mash to flow under a variety of controlled conditions and then to pass through a shaped hole or slot at a predetermined rate, has become one of the most important processes in feed production. It is a unique system, providing conditions of high temperature and pressure. These create physical and chemical modifications in feedstuffs at an accelerated rate. The result is an amazing process that combines mixing, shearing, kneading and cooking into one complete step.
Improves nutritional value of aquafeed
As efficient as extrusion is, there is often concern about the effects pelleting processes have on nutritional properties of feed materials. Many kinds of heat treatment of feed cause chemical modifications or results in the loss of nutritional properties. Hence, conditions often have to be chosen to obtain an acceptable compromise between nutritional value and other quality properties. It is recognized that products with high nutritional value can be provided by extrusion cooking, provided that processing conditions are properly chosen and controlled. Nutritional improvements arising from the extrusion process include the following:
Effect of extrusion on starch: The extrusion process, followed by rapid expansion upon expulsion from the extruder barrel, causes expansion and gelatinization of the starch granules with an immediate opening-up of molecular chains. Gelatinization is important in animal nutrition as it increases digestibility.
Effect of extrusion on protein: Inactivation of protease inhibitors and other anti-nutritional factors via heat treatment may increase the feed's protein value. Extrusion cooking has been demonstrated to have this effect, particularily on nutritional feeds based on leguminous seeds. This can go great lengths to meeting the global fishmeal deficit, which cries out for a solution based on vegetable protein sources.
Protein is denatured by cooking; an effect related to the mash cooking temperature. In general, there is little loss of protein nutritional value in the extrusion process. The primary bonds holding amino acids together are unaffected by extrusion, although the secondary bonds holding protein molecules in their respective positions are impacted. This explains why it is possible to denature enzymes (such as urease and trypsin inhibitor) without affecting the essential amino acids.
Effect of extrusion on fats and oils: The extrusion process, by reason of the high degree of friction imposed in the extruder barrel, ruptures cell walls and releases oils and fats. Nutritionally, the release of oil by extrusion processing is measurable. The digestibility of oil in full fat soya processed by certain extrusion processes is better compared with roasted full fat soya. Nevertheless, this must be qualified by the fact that not all commercial extrusion has the effect of improving oil availability.
Effect of extrusion on fiber: Very little documented research investigates the effect of extrusion on naturally occurring fiber in feed. In laboratory and research conditions, the dietary fiber content of extruded flour increased by a good percentage relative to its dry weight.
Effect of extrusion on vitamins: One group of nutrients that is easily damaged or destroyed during extrusion is vitamins. Being delicate enzymes, vitamins are always damaged or destroyed during extreme conditions involving moisture, heat or oxidation. As extrusion involves all of these to some extent, there is nothing in the process that can be changed or adapted to enhance vitamin retention. Vitamins A, C, E and B1 are most vulnerable to damage during extrusion while the water-soluble B-Complex is the most stable under such conditions.
Dry Extrusion vs. Wet Extrusion
Dry extrusion is a method of extrusion processing using dry extruder technology. While dehydrating the mash, it cooks, sterilizes, dries, expands, gelatinizes and stabilizes it into a high quality, highly digestible aquaculture feed.
Dry extrusion technology utilizes the principle of friction as a means of generating heat, steam and pressure, whereas wet extrusion technology which steam and or other forms of preconditioning. In the dry extrusion process, heat and pressure are generated by passing the mash through a barrel by means of a screw with increasing restrictions, then discharging the product into the atmosphere.
Due to the forces of friction and pressure within the barrel, the product is cooked to a pre-selected temperature in less than 30 seconds. Upon exiting the extruder, a rapid drop in pressure allows for expansion of the product and evaporation of 40 to 50 percent of the moisture from the originating material. By vaporizing moisture, starch cells are ruptured. The amount of expansion displaced by the product is dependent upon the starch content, processing temperature, pressure and moisture level in the material to be extruded.
Because it is easy to operate and maintain and its initial capital cost is low, dry extrusion is often recognized as a cost effective extrusion process for feed ingredients like corn or full fat soya. Due to its dehydration effect, the dry extrusion process can be used to produce storable fish feed with no drying involved. Coupled with the fact that no steam is needed for fish feed production, the dry extrusion process offers an economical, cost-effective technology for the aquaculture industry.
Originally developed for use in the plastics industry, the single-screw extruder has been adopted for widespread use in the production of diverse feed products. It relies entirely on friction between the materials being processed and the barrel wall to convey materials. The screw is the key element of the single-screw extruder, as its geometry influences the unit operation of the extruder.
It consists of a helical flight wound around a metal shaft enclosed within a cylindrical barrel. As the material is moved along the screw in extrusion cooking, it is transformed - under conditions of heat, pressure and mechanical shear. It enters the screw extruder in a granular state, then transforms into a viscoamorphic mass and finally, into a product with specific properties. Extruders can attain a wide range of temperatures ranging from 80ÂºC to 200ÂºC and their residence times can be varied from 10 seconds to 270 seconds.
Throughout the extrusion process, heat is inputted into the mash from several sources. These include steam injection into the extruder barrel, frictional heat developed at the barrel wall and in the die area during the screw's rotation, heat transfer from a steam jacket encasing the barrel, or form a steam quill in a hollow-cored screw.
By the end of the extrusion process, thermal processing conditions have produced changes in the physical properties, chemical properties or chemical composition of the ingredients.
Like their cousin with a single-screw, twin-screw extruders are a technology that was borrowed from the plastics industry. The two screws in this type of extruder are self-wiping, intermeshing, co-rotating or counter rotating.
Co-rotating extruders contain screws that turn in the same direction. These machines are noted for their high capacities and high shear rates. Such counter rotating extruders often serve as positive displacement fillers. Product mixing takes place only in individual chambers and slow screw speeds are often necessary in a counter rotating type of arrangement.
Single vs twin screw: Pros & Cons
With respect to functionality, the difference between single-screw and twin-screw extruders is rather distinct. Each one has its pros and cons.
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