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Introduction to Extrusion Technology
Mian N. Riaz, Ph.D.Food Protein R&D Center
Texas A&M UniversityCollege Station, Texas 77843, USA
Tel: 979-845-2774; Fax: 979-845-2744E-Mail: mnriaz@tamu.edu
http://foodprotein.tamu.edu/extrusion
What is Extrusion?
Extrusion:– Process of Extruding
Extrude:– to force, press, or push out: to shape by
forcing through a die.
Extrusion
Food Extrusion is a process in whichfood/feed ingredients are forced to flow underone or several conditions of mixing, heatingand shear, through a die that forms and/orpuff-dries the ingredients
Definition of Extrusion Cooking
A continuous process bywhich moistened,expansile, starchy, and/orproteinaceous materialsare plasticized and cookedby a combination ofmoisture, pressure,temperature, andmechanical shear.
A tool used to introduce thermaland mechanical energy to foodand feed ingredients, forcing thebasic components of theingredients, such as starch andprotein, to undergo chemical andphysical changes. Meanwhile,forming a predetermined shape.
Definition of Extrusion Cooking
What is Extrusion?
Cooking under pressure, moisture & elevated temperature.
Functions That An Extruder Can Provide
Texture Alteration
Thermal Treatment
Partial Dehydration
Homogenization
Protein Denaturing
Gelatinization
Destruction of micro-organisms and some toxic compounds
Grinding
Hydration
Expansion
Shearing
Mixing
Shaping
Texture Alteration
Partial Dehydration
Thermal Treatment
─ Heat generated in conjunction withpressure and moisture achievescooking, deactivation of anti-nutritional factors, protein denaturingand starch gelatinization
Mixing & Homogenization
Gelatinization
Soybeans
Soybeans Extruded
Full Fat Soya
Grinding
Expansion
Hydration
Shaping
Advantages of ExtrusionCooking
Adaptability
Ample variety of foods arefeasible
Only changing the ingredients
Operation conditions of theextruder
Extrusion process is remarkablyadaptable to accommodate thedemand by consumers for newfood products
Product CharacteristicsA variety of shapes,texture, color andappearances can beproduced, which is noteasily formed usingother productionmethod
Energy EfficientExtruders operate atrelatively low moisturewhile cooking foodproducts, so less re-drying is required.
Low CostExtrusion has lowerprocessing cost than othercooking and formingprocesses. we can save 19%raw material, 14% labor, and44% capital investment.
Extrusion processing alsoneed less space per unit ofoperation than other cookingsystem.
New foods/feeds
Extrusion can modifyprotein (vegetable andanimal), starches (almostall sources), and otherfood material to producea variety of new andunique food products.
High Productivity &Automated Control
An extruder provides a continuoushigh throughput processing andwe can have a fully automatedcontrols for these extruders.
High Product Quality
Since extrusion is HTST heatingprocess, it minimizes degradation offood nutrients, while improving thedigestibility of proteins (by denaturing)and starches (by gelatinizing)
Quality -- Continue
Extrusion cooking at high temperaturealso destroy the antinutritionalcompound, i.e. trypsin inhibitors,gossypol, hemagglutinings, andundesirable enzymes, such as lipases,lipoxidases and microorganisms.
No Effluent
Very few process effluent are
produced.
Classification of Extruders
Single-Screw Extruders
Twin Screw Extruders
Steam
SteamWater
Water Oil, Meat, Slurries
Typical ProductsHi Energy Pet FoodsExpanded Pet FoodsSinking Aquatic FeedFloating Aquatic FeedPet Food TreatsFull Fat Soy
Single Screw Extruder CharacteristicsCapacity: up to 20 metric ton / hourScrew Speed: 300 - 700 rpmLength to Diameter Ratio: 10 - 14Optional Variable Speed DriveGrooved BarrelSegmented Screws, Steam Locks, & Mixing Lobes
Single Screw Extruder
Dry Recipe Delivery
ExtrudingCookingForming
Preconditioning
Shaping
Dry Recipe Delivery
Deliver uniform flow rate of dry raw materials to system– Volumetric
– Gravimetric
10 - 12% MC25° C
Preconditioning
Hydrate raw material particles
Heat raw material particles
Mix raw materials added in separate streams
10 - 12% MC25° C
10 - 30% MC25 - 95° C
Steam
Water
Extruding
Extruder barrelHeads
Screws
Die/knife
14 - 35% MC60 - 175° C
Steam
Water
10 - 12% MC25° C
Steam
Water
To Dryer
Single screw extrudersSegmentedJacketed
Categories of SSE
Cold Forming Extruder (Pasta-type)
High-Pressure Forming Extruder(Cereal and Fried Snack Foods)
Low-Shear Cooking Extruders (Meatlike Snack)
High Shear Cooking Extruders (PetFoods, RTE, TVP)
Collet Extruder: (Puffed Snacks)
Twin Screw Extruder
Steam
SteamWater
WaterOil, Meat, Slurries
Twin Screw Extruder CharacteristicsCapacity: up to 16 metric ton / hourScrew Speed: 300 - 1200 rpmLength to Diameter Ratio: 13 - 25Variable Speed DriveSmooth BarrelSegmented Screws and Mixing LobesSelf-Wiping Screw Elements
Typical ProductsHi Energy Pet FoodsExpanded Pet FoodsSinking Aquatic FeedFloating Aquatic FeedPet Food TreatsFull Fat Soy< 0.8 mm Diameter Aquatic Feed> 17% Internal Oil
Twin screw extruders
SegmentedJacketed
Modular Screws
Twin Screw Extruder Generally one and one-half time
more expensive than single screwextruder for the same capacity
Yet the degree of quality control andprocessing flexibility they offer canmake them attractive to food/feedindustries
Advantages of Twin Screw Extruder
They can handle:
viscous, oily, sticky or very wetmaterial and some other productswhich will slip in SSE, (It is possibleto add up to 25% fat in a TSE)
Advantages of Twin Screw Extruder
Less wear in smaller part of themachine than in SSE.
Wide range of particle size (from finepowder to grains) may be used,whereas SSE is limited to a specificrange of particle size.
Advantages of Twin Screw Extruder
TSE produce a more uniform flow ofproduct through the barrel due to thepositive pumping action of the screwflights.
Because of the self wipingcharacteristics clean up is very easy.
Disadvantages of Twin Screw Extruder
Cost 1.5-2.0 X SS
Maintenance 2X
Types of TSE
1. Non-intermeshed, co-rotating
2. Non-intermeshed,counterrotating
3. Intermeshed, co-rotating
4. Intermeshed, counter rotating
Types of Extruders
Single screw
Twin screw
Wet extruders
Dry extruders
Wet Extrusion
Wet extrusion requires:
Steam injection
Drying after extrusion
Dry Extrusion
Dry extrusion capitalizes on the heatgenerated through friction as thesole source of heat to cook anddehydrate the product
Selection of an Extruder
The choice of an extruder type shoulddepend on;
The type of raw material to be used
The desired product
The processing rate
The investment cost
Operating cost
TerminologyFeedstock:
Material processed in extruder
TerminologyPreconditioner:
Moisture-heat equilibration unit ahead ofextruder - May precook
Terminology
Screw:
Member that conveys feedstock
Terminology
Interrupted, cut flight screw
Screw with flight section missing
Terminology
Shearlock, steamlock, shear ring:
Ring that locks worm segmentstogether, and also increasesproduct shear
Terminology
Vent
1/2 P - 6 unitsTwin-flight
3/4 P - 9 units
Twin-flight3/4 P - 9 units
Single-flight
1/2 P - 9 unitsTwin-flight
1/1 P - 9 unitsTwin-flight
3/4 P - 6 unitsTwin-flight
Cone-screwWasher
1/2 P -cut6 unitsTwin-flight
Paddles 0.75 units Paddles 1 unit
3/4 P4.5 units
Twin-flight
3/4 P - cone9 units
1/2 P - Rev1.5 units
1/2 P0.75 units
3/4 P - cone9 units - cut
Shaft Collar3 units
Disks 0.75 units
3/4 P - cut9 units
Twin-flight1/2 P - Rev0.75 units
1/2 P - Rev6 unitsTwin-flight
Elements for generating screw profiles and pressures
TerminologyBarrel:
Shell around screw
TerminologyShear:
Working, mixing action thathomogenizes and heats conveyedproduct
Terminology
L/D
Length to Diameter Ratio (1:1 to 1:25)
Terminology Compression Ratio C.R
It is the volume of the full flight of thescrew at the feed opening divided byvolume of the last full flight beforedischarge. Typical CR is 1:1 to 5:1
Extruder Dies
Provides resistance at discharge of extruder
Shapes product
Factors to Consider in Selecting an Extruder
Type of product(s) to bemade
Size of plant sustainable bymarket
Capital availability andrecovery period
Local backup expectationsand capabilities ofmanufacturer
FactorsRelative energy
requirements; local costof steam, electricity,other energy forms
Flexibility is costly
Hardened wearing partsfor sustained usage -screw tips, barrelsleeves, die inserts
FactorsExtruders don’t run well when
starved
Used extruders/plants areavailable
Extrusion “Rules of Thumb”
Formulation:
Sufficient starch/protein required to givedesired expansion, texturization, cohesion
Over 5-6% fat interferes withtexturization/expansion. Products preferablycoated after extrusion and cooling
Rules of Thumb
Addition of antioxidants, flavors, heat-labilevitamins, deferred until after extrusion andcooling
Size Reduction Largest particle less than 1/3 diameter of die
opening
Might be done less expensively inhammer/roller mills than in extruder
Rules of Thumb
Feeding the Extruder:
Might be a problem with extremelypowdery ingredients, heavy or sticky doughs.
Extruder Discharge:
Density depends on temperature, pressure,moisture content and formula
Rules of Thumb
Protein texturization need laminarflow before discharge
Approximately 4-7% moistureflash- off at die
Cooking
Might be done less expensively inpreconditioner (5-10 X holdingtime)
Rules of ThumbCoating and Enrobing
Preferred place for addition ofheat-sensitive material
Cleaning
Long runs favored, disassemblyreasonably of large machinesdifficult.
References
Books
ReferencesExtruders in Food Applications
M. N. Riaz, Technomic Publishing Co. 2000 (Bought by CRC Press)
References
Extrusion Cooking Technologies and ApplicationsR. Guy, Woodhead Publishing UK, , 2001
References
Extruders and Expanders in Pet Food, Aquatic and Livestock Feeds.
M. N. Riaz, Pub. Agri Media, Germany, 2007
References
Extrusion problems solved: Food, pet food and feed
M N Riaz and G J Rokey, 2012
Woodhead Publishing, UK
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