Aquaculture is a developing activity around the world. As the industry grows, so does the aquafeed market. Aquafeeds are comprised of a number of ingredients that are mixed in various proportions to complement one another to form a nutritionally complete compound diet. According to the physical characteristics, they can be divided into powder, particle, pelleted and extrusion aquafeed. They can also be divided into sinking, slow sinking and floating aquafeed according to their buoyancy. On the basis of aquafeed energy, they can be divided into low energy, medium energy and high-energy aquafeeds.
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May | June 2013
Pelleting and extrusion in aquafeed technology
The International magazine for the aquaculture feed industry
Pelleting and extrusion in aquafeed technologyby Doris Du, sales representative, Allance Machinery, China
Aquacultureisadevelopingactivityaroundtheworld.Astheindustrygrows, so does the aquafeedmarket.Aquafeedsarecomprised
of a number of ingredients that are mixedin various proportions to complementoneanother to form a nutritionally completecompound diet. According to the physicalcharacteristics, they can be divided intopowder, particle, pelleted and extrusionaquafeed. They can also be divided intosinking, slow sinking and floating aquafeedaccording to their buoyancy. On the basisofaquafeedenergy,theycanbedividedintolowenergy,mediumenergyandhigh-energyaquafeeds.
Raw materials for aquafeed production
Common basic raw materials to themanufactureofmostaquafeedsaresimilartothoseemployedinthemanufactureofothertypes of animalfeeds.However,itshould be notedthat aquafeedsshould avoid theuse of high levelsof plant or veg-etable fibres andcarbohydrateswhich are poorlydigested and
The major components of aquafeeds areprotein (plant protein, animal protein andsingle cell protein), starch (sinking aquafeedconsistsmorethan10%andfloatingaquafeedconsistsmorethan20%),crudefat,rawfibre,crudeashandmicronutrients.Starchhasgreatgelatinization under high temperature andhigh humidity conditions. It can improve thestabilityofaquafeedsandextrusiondensityofextrudedaquafeeds.
Aquafeed processing technologyThemanufacture of aquafeeds ismore
complex than common feed manufacture.
Hardness, size, buoyancy, durability andwater stability must all be taken intoaccount. Aquafeed processing technologyisnotunique.Itreliesonthedevelopmentmadeinconventionalanimalfeedprocess-ing technology, but with some specificapplications. The main processing tech-nology for aquafeeds is pelleting (mainlyproduce sinking aquafeeds) and extrusion(can produce sinking or floating extrudedaquafeeds).
Aquafeed pelleting technology is stricterthancommonanimalpelletedfeed.Typically,sinkingaquafeedsaremostlymadewithfeedpellet mills. Pelleting is still the predominatemethod for sinking feed production. Theaquafeed pelleting processes is describedbelow:
target species salmon, trout seabass, turbot carp, tilapia
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homogeneous mixture. During the mixingprocess,feedribbonblenderordoublepad-dle feedmixer is themost commonlyusedequipment.
PelletingPelletingcanbedefinedastheagglomerationofsmallparticlesintoalargersolidwithagivenshapeandtexture,bymeansofamechanicalprocess in combination with moisture, heatand pressure. The main factors which affectpelletingare ingredientscharacteristics,mois-turizingorsteamingbeforepelleting,diethick-nessandbindersetc.Theringdieaquaticfeedpelletmillistheprofessionalpelletingmachineforaquafeedprocessing.
CoolingThe temperature of the aquafeed pelletssqueezed out the pellet mill is high. They
shouldbecooledbeforeotheroperations.Acounterflow pellet cooler is a high-efficiencycoolingmachine.
Coating Coating can improve upgrade the aquafeedquality. The physical property of the feed isimproved. The fat sprayed on the aquafeedpelletshascertainpermeability;itcanperme-ate into the pellets in subsequent crafts andtransportprocess.Thatcanreducethescatterof feed particles and dust. The fat sprayingmachine isalsoknownas feedpelletcoatingmachine.
CrumblingIt is a necessary process in aquafeed pelletprocessingas some fishneed smallerpellets.The roll type crumbler can break the aqua-feedpelletsintosmallerparticles.
Screening In order to improve aquafeed quality, finesfrom cooled aquafeed pellets should beremoved and the pellet size should be uni-form. Rotary feed pellet grading sieve is justthemachine.
Aquafeed extrusion technologyA significant application of the extrusion
process is in the production of aquafeeds.Extrusion has won some popularity and iswidely accepted. Compared to the pellet-ing method, extrusion can be defined as atechnological process, namely, forcing feedrawmaterialinoneormoreofthefollowingprocess conditions (such as mixing, heating,cuttingetc.)flowthroughthedie,makemate-rialformingoreruptiongasification.Extrusionallows buoyancy control to make floating,sinking or slow sinking feed by changing the
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Table 2: Starch contents of common energy feed raw materials
raw material Starch content (dry matter) raw material Starch content
extrusion condition including temperature,pressure, die aperture diameter and shearrateetc.
Dry or wet extrusionDifferent shapes and sizes of aquatic
feed can be produced using extrusiontechnology.Extrusionwithsteamprecon-ditioning is known as wet extrusion andextrusion without steam preconditioningis called dry extrusion. Dry extruderswere used in processing low moisture,highly expanded starch products and inprocessing whole soybeans to full-fat
soy flour for both the food and feedindustries.
Preconditioningisaveryimportantpartofthewetextrusionprocessforsteamorwaterinjection into either the conditioner and/ortheextruderbarrel.
Themostimportantaspectoftheprecon-ditioningsystemisthepotentialforadditionalmixingandtheretentiontime,whichisneces-saryforchemicalorphysicalreactionstotakeplace. The Allance aquafeed extruder forexample,canproducedryandwetaquafeeds.Dry feedpelletsarestable forrelatively longperiods, for convenient storage anddistribu-
tion.Thewet typeprocess-ing needs a boiler to makesteamtotheconditionerandripen the raw material inadvance.
Twin-screw extrusion is ideal for aquafeed
According to the opera-tion methods, extrusionequipmentgenerallyfallsintotwo categories: single-screwextruders and twin-screwextruders. Producing smallpellet aquafeed by single-
screwextrudersdirectlyhasmanyadvantages,suchaslowingredientcost,lessfinegenerat-ed,goodwaterstabilityandpleasantappear-ance.Twin-screwextruderswerefirstusedinthe 1950s for manufacturing thermoplastics.Theyweredevelopedonthebasisofsingle-screwextrusionmachine,withadoublescrewinstalled side by side in the barrel. A fullymeshed co-rotating (both screws turn samedirection)extrudergreatlyimprovedmaterialconveying,pumpingandmixingcharacteristics.Comparedtoasingle-screwextruder,aTSEhas a much wider operating range in termsof use of moisture, internal fat, mechanicalenergyinputandproductsize.
The domestic high quality twin-screwextruder has reached an advanced level intechnicalperformanceandmanufacturinglevelwithhighpropertyprice ratioand is thebestchoiceoftheaquaticfeedprocessingindustry.
The main advantages of twin extruderdescribedasfollows:• Twin-screwprocessmandatorydelivery
and self-cleaning feature, making thematerial stay in the barrel for a shorttimeanduniformity
• Astwoscrewskneadandrubeachother,extruder enables material mixed and rip-enedmorefullyinlowerenergyconsump-tion.Thewholeripeningdegreecanreachmore than 95percent,water stability lastfor24hours
• Thebiggestcharacteristics is that itcanmixallkindsofrawmaterialsregardlessofitsformation.Thiswillalsoenablefishfarmersuseawiderrangeofingredients
ConclusionThe production of aquafeed is the fast-
est growing sector in the global feed industry.Although a large amount of aquatic feed isstillproducedbyconventionalpelleting technol-ogy,extrusionisbeingutilizedtoalargerextentbecause ithasseveraladvantagesoverpelleting.So,pelletingorextrusion?It’saccordingtovariousfactorsthatdiscussedabove.
Table 3: extruded aquafeeds versus pelleted aquafeeds