November | December 2012 Extrusion technology for the production of micro-aquatic feeds and shrimp feeds
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The International magazine for the aquaculture feed feed industry industr y
FEATURE
Extrusion technology for the production of micro-aquatic feeds and shrimp feeds
Image courtesy of Wenger Manufacturing, USA
by Mian N. Riaz, Ph.D, Head of Extrusion Technology Program, Food Protein R&D Center, Texas A&M University, USA
N
owadays we often hear startling news such as, ‘seven billionth baby born’ or ‘world population may reach 9.2 billion by 2050’ (world news, msnbc.com). Hunger is the world’s number one health risk, it kills more people than AIDS every year, one in seven people in the world will go to bed hungry tonight.
To overcome these issues, farmers must produce 70 percent more food by 2050 to feed the population. But the impending crisis is that the earth may run out of food by 2050. 2.4 billion extra people, no more land, how will we feed the world in 2050? At the same time we also hear in the news that global fish consumption has hit a record high. We have seen the commercial fishing trend is declining whereas aquaculture farming is growing rapidly all over the world. Is this supply enough to feed the future population? May be not, but fish demand is growing every day all over the world. To maintain baseline consumption in every coun try, 159 million tons of fish is needed to feed the world population in 2030. This demand is driven by population and income growth. If a country’s aquaculture production follows the recent trend, the expected aquaculture growth rate will need a four percent increase annually. To feed a growing world population, the required aquaculture growth rate is 5.6 percent annually. Some of the main challenges to achieve these goals are proper and large-scale feed production systems for micro aquatic feed. Recently extruder manufacturers came up with new technologies which can solve some of the aquaculture issues related to large
and commercial-scale feed, which is the key production of micro-aquatic feeds. The raw for growth of aquaculture industry. The fun- recipe is very finely ground or pulverized damental components of extrusion systems and does not possess good flow properties. have consisted of the following items for a The feed system must be able to handle number of years: these thes e finely fine ly ground grou nd formu lations latio ns and avoi d 1) Feed Feed delivery system bridging and non-uniform metering of the 2) Preconditioning feed. 3) Extruder Automated feed delivery systems with 4) Die and knife assemblies PLC control are the norm. Slurry tanks and Although existing extrusion systems were liquid feeding devices (pumps) are utilized to able to produce a wide range of good quality accomplish uniform metering of liquid ingreaquatic feeds (both floating and sinking), small dients. The slurry tanks are often jacketed diameter pellet sizes were difficult to produce for heating or cooking and are equipped with at reasonable or cost-effective throughputs. agitators as required. Positive displacement Recent innovations in the basic hardware metering pumps deliver metered liquids at components permit smaller diameter feeds at constant rates by varying length of stroke or attractive production throughputs. speed of rotation. Slurries or liquids can be premixed with dry ingredients but are preferably injected into preconditioning devices or Feed delivery system Hoppers or bins are an integral part of the extruder barrel. The nutrient profile of larval feeds is critical and the precise metering a feeding device and are used to hold the ensures correct formulations. dry ingredients above the feeders. The feed delivery system must be able to uniformly feed both a dry and/or liquid ingredient or a Preconditioning blend of ingredients. The dry portion of the feed and the Generally, when the added fat content of a liquid portion are separately introduced into raw formulation exceeds 12 percent, the pora preconditioning device where they are tion of fat above the 12 percent level should continuously mixed, heated, and moisturised be introduced into the extrusion system in by the injection of hot water and/or steam. a separate ingredient stream. The dry feed The intense mixing of water and steam added portion is delivered to the extrusion system to the dry feed and the ability to extend the through a specialised metering device capable retention time during the preconditioning of providing uniform flow at any desired phase allows the moisture level to be mainextrusion rate. tained at an optimum. Dry ingredients are usually free flowing, This ability to maintain optimum moisture and there are a number of capable feeding distribution not only initiates proper cooking devices which vary in their relative cost but also is reported as a significant factor in and complexity. However, gravimetric or the reduction of extruder barrel wear and loss-in-weight systems are necessary for the extruder shaft power per ton of product stable, precise metering of dry feed for the processed. The higher mixing mixing intensity of new
20 | IntnatInal AquAFeed | november-December 2012
FEATURE
preconditioner designs improves hydration and cooking, helping to capture the steam in the raw material. Excess steam can escape the preconditioner and create fugitive dust which creates housekeeping concerns in the plant environments. Better cooking with new preconditioners gives lower product viscosities which
increase. The mechanism of shear does not begin to play a dominant role until the screw flow channel is full. This full flow channel condition begins in the kneading zone. The flow channel fills, first, with loose granular material which is compressed and worked by shear as it passes through the kneading zone. It is in the kneading zone
desired final product texture, density, color, and functional properties. Twin-screw systems are preferred for extrusion of aquatic feeds smaller than 2 mm diameter due to their positive transport and self-wiping characteristics which prevents significant product build-up in the extruder barrel which could later dislodge and plug the Image courtesy of Wenger Manufacturing, USA
where the discrete particles of material begin improves extrudate flow through small die orifices. orifices. The result is smaller pellets to agglomerate because of their temperature and more uniform pellet size. The higher increase resulting from conduction, direct mixing intensities in new preconditioner steam injection, and viscous energy dissipadesigns is the result of unique beater tion. Here, the discrete particles begin to form designs and more beater contacts per a more integral flowing dough mass. At the discharge retention time. end of the kneading zone, the extruExtrusion date most typically Extruders are generally classified as either being reaches its maxia single or twin-screw design. In both designs, the mum compaction. impact of final product characteristics are affected The shear in this by screw and barrel profile, screw speed, processarea of the extruder ing conditions (temperature, moisture, etc.), raw barrel is moderate material characteristics, and die/knife selection. and the extrudate The feeding zone of the extruder is that area where the low-density discrete particles temperature begins The of raw material are transported into the to increas e. final cooking zone extruder barrel inlet. This low-density, often is that area where preconditioned, material is then transported amorphousizing and into the interior of the extrusion processing risi ng occur. occ ur. chamber. The flow channel of the screw is textu rising typically not filled in this zone due to the Temperature and pressure typically air entrapped in the incoming material. The incoming material is compressed slightly in this increase most rapidly in this region zone with the air being expelled. Water, an excellent excellent plasticizer, plasticizer, is typically typically as shear rates are highest because of injected into the barrel in the feeding zone to facilitate textural development, development, viscosity the extruder screw development, and to enhance conductive heat configuration and transfer. transfer. The kneading zone of the cooking maximum compression of the extruextruder continues the compression started in the feeding zone, and the flow channels of date. The pressure, the extruder extruder screw screw have a higher degree degree of fill. fill. tempe ratu re, and resulting fluid viscosAs the degree of screw fill increases and pressure begins to develop in the extruder ity are such that the extrudate will expel barrel, leakage flow (flow over the outside from the extruder diameter of the screw in a direction toward die to form the the extruder inlet) and pressure flow both
small die orifices. orifices. The C²TX system is a corotating system that includes a tapered screw diameter which de-aerates the extrudate and makes it easier to create high density feeds for good sinking characteristics without the need for vented barrels, pressurized density control
november-December 2012 | IntnatInal AquAFeed | 21
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FEATURE
die/knife area. The small diameter feeds results in spillage in this area and will cause sanitation problems. b) For product separation. Floating feeds have a tendency to stick together when wet on belt or HVH conveyors and pneumatic conveying enhances separation. c) For separation of ‘tails’ from pellets. Pneumatic systems ‘scrub’ the product Process and remove tails for later separation guidelines A S U during sifting. Process , g n 7) Fluid bed dryers are recommended guidelines i r u t for products under 1.2 mm diameter c required for die a f u in size although horizontal dryers with holes smaller n a M than 1.2 mm polyester screens can work with some r e g products. diameter: n e 8) Final product sifting after dryer and 1) Recipe to W f o before coating. This sifting operation is contain ade y s e t critical for three reasons: r quate starch u o c a) To remove ‘overs’ (large tails and levels for bind e g a ‘doubles’) for regrind. ing (at least m I b) To remove ‘fines’ for regrind. This 25% starch for prevents a mess during coating step floating feeds). where the fines are also coated and 2) Maximum particle size of the recipe cause buildup. must be smaller than one third the die c) To separate good pellets into several hole sizes. different diameters depending on the 3) A spring-loaded knife blade is client criteria for size. The expectarecommended. tions from the industry will be for 4) All mass flow inputs must be free of tight specs on pellet size and this can material that is large enough to block easily be controlled at this point by or partially block the die openings and sifting product and producing several this includes the steam, water, fat, and different sizes at the same time and other liquid inputs. The water and steam setting the standard for the industry. lines going to the extruder system need The primary-sized product can be sent to be fitted with screen filters having on through the system for coating and 30 mesh (0.6 mm) openings and these into final product bins. The secondary should be adequate if maintained. The sizes can either be reworked or saved fat line (and fat source) also needs to separately in tote bags for coating and be filtered to remove debris larger than bagging later. 30 mesh (0.6 mm). All strainers 9) Production procedures. This small diamor filters must be easy to clean eter product requires a dedicated line, or they will get removed ‘in the strict startup and shutdown procedures heat of a run’ where liquid flows to avoid die plugging, and thorough are interrupted due to plugged cleanup techniques. The extruder and filters. It may be necessary to coater areas should be considered as have a dual filter set up for fish ‘wet areas’ for cleaning. The coater solubles and fat lines. With this may need to be cleaned between each installation, if one filter is plugged different pellet size to avoid cross you can close the valves to the contamination. primary filter for cleaning and By following these guidelines and using open the valves for the second newly innovative extruder parts micro-aquatic for continued operation. floating feed can be produced on a large 5) The dry feed must pass through a vibratscale basis. This micro-aquatic floating feed ing sifter after the grinder and before will be the foundation to start fish farming on the extruder live bin. This sifter must be commercial scale to fulfill the fish demand in sized to remove particles the same size the world. or larger than the die openings. High fishmeal diets plug vibrating sifter screens very easily and the industry often employs rotary sifters to avoid this bottleneck. More InforMatIon : 6) Pneumatic conveying is required from Email:
[email protected] the extruder die to the dryer inlet for several reasons: a) For product containment around the which minimised nutrient destruction. Floating products are possible by simply decreasing die open area.
devices, or double extrusion. By adding a BPV (Back Pressure Valve) after the extruder, the necessary restriction is provided to expand the product if floating pellets are desired.
Die assemblies The die is the most critical part of the complete system as it determines product shape and size, but also determines throu ghputs ghpu ts and buoyancy buoy ancy prope rties of the final fina l aqua tic feed. As pellet diamet ers became smaller, the die created more restriction and drastically reduced throughputs. One die assembly design that allowed an increase in throughputs by increasing die
"This micro-aquatic floating feed will be the foundation to start fish farming on commercial scale to fulfill the fish demand in the world" open area is the OTD (Oblique Tube Die). This die actually increased die open area (the number of orifices) by two to three times which maintained high throughputs even for small diameter products due to larger die hole populations. The tubes created longer reten tion times for improved cooking. Pressure drop in the tubes created a denser product so that micro-aquatic feeds could be cooked thoroughly but still maintain high densities for sinking characteristics. The process was still a short time/high temperature process,
22 | IntnatInal AquAFeed | november-December 2012
FEATURE
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VOLUME 15 ISSUE 6 2012
An overview of the UK fish vaccination industry Why check selenomethionine levels in selenium yeast? Extrusion technology for the production of micro-aquatic feeds and shrimp feeds EXPERT TOPIC –Salmon
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