Extrusion
Dr. Sirichai Songsermpong
Department of Food Science and Technology
Kasetsart University
2
Introduction
Extruder
3
Definition
Heldman and Hartel (1997)
“ food extrusion process: heating
and cooking of raw materials occur
as they are mixed and formed to
produce essentially finished product in a single operation”
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Types of Food Extruders
Single-screw extruder
Twin-screw extruder
Source : Heldman and Hartel (1997) and Riaz (2000)
Fig. 1 Types of food extruders
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Food Extruder components
Fig. 2 Cross-section view of food extruder
Screw
Barrel
Die
Hopper and feed conveyor
Pre-conditioner
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SYSTEM PARAMETERS
•SME
•Residence Time
•Thermal Energy
•Product Temperature
•Product Pressure
•Melt Viscosity
OPERATIONAL
VARIABLES
•Feed Rate
•Water Addition
•Screw Speed
•Screw Profile
•Cutter Speed
•Barrel temperature
RAW MATERIAL
•Composition: Oil, starch, protein, fiber
•Moisture
•Particle Size
•Additives
SYSTEM DISTURBANCES
•Screw Wear
•Ambient Conditions
DIE PROFILE
•Open Area
•Flow Resistance
SME: Specific Mechanical Energy
WAI: Water Absorption Index
WSI: Water Solubility
GELATINISATION
DEXTRINISATION (Molecular Degradation)
PRODUCT QUALITY
•Product Moisture
•Product Temperature
•Bulk Density or Expansion
•Morphology
Shape
Size
Uniformity
•Texture/Sensory
•WSI/WAI
•Gelatinisation
•Dextrinisation
Expansion
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Processing parameters
Material parameters Operating parameters
-Compositions
-Moisture contents
-Particle size
-Other ingredients
-Type of extruder
-Feed rate
-Die characteristics
-Screw configurations
-Screw speed
-Barrel temperature
Source : Chang (1992) and Riaz (2000)
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Flow components within the extruder system
Qnet= Qdrag- Qleakage- Qpressure
Where
Qnet= Net volumetric output of the system
Qdrag=max volumetric conveying capacity (Drag flow)
Qleakage=leakage flow of the flight gap
Qpressure=pressure flow in the chanel of the screw
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The die characteristics
Qnet = K*ΔP /µapp
Where
K= Die conductance
ΔP= Die pressure drop
µapp = Apparent melt viscosity
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The product expansion process
• The product expansion is influenced via
The die pressure drop
Rheology of melt
Degree of superheating
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Influence of rheology on the product
Rheology of the melt has a direct effect on
the product characteristics :
Control of the bubble growth rate
Control of the degree of expansion
Control of the degree of curvature
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Influence of rheology on the process
• The rheology of the melt has a direct effect
on the process design :
Control of the die pressure drop
Control of the screw flow dynamics
Control of the screw energy consumption
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H
W
xzy
SCREW
ND Vz = V cos
Vx = V sin
BARREL
e
Material Flow
H
ND
H
VZ
cos
Shear Rate
cosNDVZ
Stationary Plate
w
H
Velocity
Moving Plate
SHEAR FLOW IN THE EXTRUDER
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Elastic Effects In Extrusion
EXTRUDATE SHAPEDIE SHAPE
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• Ideal Liquid (Newtonian)
Extrusion Process
D
1.1D
D
O
Do
Do
32D
D
O
• Non-Ideal Liquid (Viscoelastic)
6/12
w12
2211
o 22
111.0
D
D
Prediction Equation
Normal Stresses
Shear Stress
Die Swell Phenomena