Extruding specialty feeds

by Teresa Acklin
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The animal feed industry is moving toward extrusion for highly specialized feeds, such as aquaculture feeds.

   By Don Wenger, Wenger Manufacturing, Inc., Sabetha, Kansas, U.S. Wenger manufactures and markets complete processing systems for the animal feed industry worldwide.

   The processing and cooking of specialty feedstuffs by extrusion for piglets, calves, laboratory animals, zoo animals and pets as well as for cultivated aquatic species comprise a major new market opportunity for the feed industry.

   Extrusion cooking systems have been used the world over for the thermal processing of products for human consumption and for pet foods as well as for a variety of feed and nonfood applications. But, the utilization of extruded feeds for commercially cultivated aquatic species is relatively new. The industry is moving rapidly toward extrusion as the preferred technology for these highly specialized feeds. No other process offers so many advantages.

   Those in the aquaculture industry who have converted from meal or pelleted feeds to extrusion-cooked feeds report savings in feed quantity of 20% to 25%. Furthermore, many other important benefits are obtained from using this process, including the following:

   1 By adjusting processing conditions and/or formulation, pellet-bulk density may be regulated to permit the production of feeds that float for a predetermined time, sink rapidly or sink at a controlled rate as best suited for a particular species' feeding habits.

   2 Extrusion cooking produces uniform pellets that can be precisely sized to best feed the various aquatic species and age brackets within those species. Pellets as small as 0.8 mm may be extruded, eliminating the need to crumble larger pellets and the waste associated with crumbling operations.

   3 Extruded aquatic feeds are extremely stable in the dry state and can be stored for prolonged periods without nutrient degradation.

   4 The mechanical durability of the extruded pellet results in fewer fines during handling, transportation and feeding. This greatly reduces product loss between production and feeding.

   5 Extrusion-cooked feeds are extremely stable in water and will maintain their physical integrity for prolonged periods, as long as 24 hours, if desired. This allows more feed to be consumed while maintaining water quality.

   6 The high-temperature/short-time extrusion cooking process eliminates many toxic elements common to raw ingredients used in aquatic feeds. The feed is, therefore, a safer and more pathogen-free product.

   7 Each pellet contains all nutrients of the feed formula, and its long water stability reduces the loss by leaching of water-soluble nutrients.

   8 The extrusion process allows greater varieties of feed ingredients to be used including grains, soybeans, fish meal, shrimp meal, yeast, dehydrated alfalfa, rice bran, wheat bran and middlings, meat and bone meal, minerals and vitamins, to name a few. Feed ingredients may be selected on the basis of their nutritional value and cost, allowing feed producers to supply nutritious products while keeping the price of ingredients down.

   An overview of the extrusion process may be helpful in understanding the differences, similarities and advantages as compared with traditional processes.

   The properly ground feed ingredients, preselected for their nutritional value and cost, are fed to a batch mixer where the formulation is mixed. This mixture is then transported to the live-bottom holding bin over the extrusion cooker.

   The live-bottom holding bin assures a constant supply of raw materials, eliminating the possibility of interrupted material flow to the extruder. A variable-speed screw meters the mixed raw materials to a preconditioner where they are conditioned with steam and/or water for as long as four minutes under atmospheric conditions prior to being introduced into the extruder barrel. This preconditioning not only enhances finished product quality but improves the extruder efficiency and capacity and greatly reduces extruder wear and operating costs.

   The preconditioned meal enters the properly configured extruder barrel where it is cooked by heat and mechanical shear. As the extrudate passes through a sizing die at the end of the barrel, it is cut by a rotating knife to the desired length.

   The extrusion-cooked feed is then dried to the desired moisture content by means of a horizontal moving tray dryer/cooler. Today, it is particularly important that these dryer/coolers be designed to permit recirculation and exhaust air volumes to be adjustable for maximum dryer efficiency.

   The dried products may then be coated with animal fats, vitamins or marine oils in a fat/vitamin enrobing system. The process is similar whether using single-screw or twin-screw extruders.

   The highest quality products will be achieved by utilizing quality ingredients, proper operating conditions and by matching the extruder barrel components to the product(s) being produced. Automated PLC control packages are available. These packages regulate extruder throughput, process pressure, temperature and liquid additions, as well as monitor any post-extrusion process functions. These functions are required to optimize product quality, process efficiency, personnel safety and machine loading.

   The modern extrusion cooking process yields a very uniformly-sized, spherical feed with an internal matrix that does not easily disintegrate. The feed integrity, or its ability to hold its shape in water for extended periods, is particularly important for slow-bottom feeders. The thermal bonding of ingredients yields a water-stable product more sterile, sanitary and pathogen-free than pelleted feeds. This is especially important to aquatic species susceptible to pathogens, pesticides and other chemicals that may be carried through the processor's grain supply.

   Extrusion cookers — both single-screw and twin-screw — have improved dramatically in regard to energy efficiency and production capabilities. Unit capacities exceeding 15 tonnes per hour are now practical, permitting production costs and energy efficiency competitive with the traditional ring-die pelleting process.