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