2. CONTENTS
1. Introduction and History
2. Principle
3. Factors Affecting Extrusion Cooking
4. Effect of processing variables on protein digestibility
5. Applications In Food Processing
6. Advantages & Disadvantages
3. What is Extrusion Cooking
• Modern (HTST) cooking process(
thermomechanical process) in which heat
transfer, mass transfer, pressure changes
and shear are combined to produce effects
• The food material is heated either by
external heat source or by heat produced
through dies
• Extruder is used to produce extrudate of
different shapes by forcing a specially
designed opening after a previous heating of
the material.
CookingTexturizing
Drying
Enrobing
Cooling
Puffing
Plasticizing
Mixing
4. • The main method of preservation of both hot and
cold extruded foods is by the low water activity of
product
Examples of extruded foods
5. WHY :
Properly designed convenience foods can make an
important contribution to nutrition
Social changes are altering traditional patterns of food
preparation
8. WORKING PRINCIPLE
• The raw materials are allowed into the extruder barrel and the screw(s) then convey the food
along it
• Further down the barrel, smaller flights restrict the volume and increase the resistance to
movement of the food
9. • As a result, it fills the barrel and the spaces between the screw flights and becomes compressed
• As it moves further along the barrel, the screw kneads the material into a semi-solid, plasticized
mass
10. • Here, frictional heat and any additional heating that is used cause the temperature to rise rapidly
• High temperature of operation in presence of water promotes gelatinization of starch components
and stretching of expandable components
11. • The food is then passed to the section of the barrel having the smallest flights, where pressure and
shearing is further increased
• Finally, it is forced through one or more restricted openings (dies) at the discharge end of the barrel
as the food emerges under pressure from the die, it expands to the final shape and cools rapidly
as moisture is flashed off as steam
12. CUTTING PRINCIPLE
• Must permit obtaining final products with
uniform size
• Product size is determined by the rotation
speed of the cutting blades
• Mechanism can be horizontal or vertical
13. 1) DRAG FLOW :- Flow of material inside the barrel due to the action of dragging
QD = 0.5 π D N Cos Φ (Wh1)FD
2) PRESSURE FLOW :- Reverse flow of the material in the screw channel
QP =
2 𝑊 𝑆𝑖𝑛Φ ℎ2−ℎ1 Δ 𝑃
12 𝐿
1
ℎ1
2−
1
ℎ2
2 μ
FP
Where, W =Width of screw channel, ΔP = pressure drop
h1 & h2 = Depths at the metering and feeding zone, respectively,
D = Barrel diameter, Φ = Helix angle,
N = screw rotation, μ = viscosity of material, and
FD & FP = drag & pressure flow correction factors, respectively.
Material flow in the barrel
14. 3) LEAKAGE FLOW :- Flow through the barrel clearance
GL =
π 𝐷ߜ3 Δ 𝑃
12
𝐿
𝑝
(e/CosΦ) μ𝒔
Where
δ = clearance between screw flight and barrel
ΔP = pressure drop
P = pitch of the screw, and
μ 𝒔 = viscosity of material flowing through channel
Φ = Helix angle
Net flow = Drag flow – Pressure flow – Leakage flow
QNET= QD- QP- QL
15. 1) Extrusion Drive:
Power supply in food extruder is done by Electric
motors
The size depends on the capacity of the extruder and
may be as large as 300 KW
The speed on food extruders is normally less than 500
rpm
The screw speed on extruder is a valuable control
parameter
EXTRUDER PARTS & COMPONENTS
16. 2) Feeder:
A device providing a uniform delivery of food ingredients which
are often sticky, non free-flowing substances
It will regulate rate/pressure of flow. Some types of feeders
commonly used are vibratory feeders, variable speed auger and
weigh belts
17. 3) Barrel or sleeves :
The barrel is divided into feeding, kneading
They are often jacketed to permit circulating of
steam or superheated oil for heating and water
or air for cooling
18. 4) SCREW :
Most important component
Conveys the materials
Diameter varies between 2-15 cm Length to
diameter ratio varies between 8-20 and helix
angle between 200 – 30o
19. 5) Die or nozzle:
Gives shape to the final product and promote
resistance
Increases internal pressure
Designed to be highly restrictive, giving
increased barrel fill, residence time and energy
input
24. • Structure forming
• Fillers
• Raising
• Plasticizing
Type of feed
materials
• 25 to 30%
• Affects die pressure
Moisture
content
• Cooking
zone 80-150° C
• Higher degree of
gelatinization
Temperatue
• Decides
residence time
• Extrudate
Characterstics
Screw speed
• Die material
Die
Characteristics
• Single screw
• Twin screw
Screw
design
Factors Affecting Extrusion Cooking
25. Effects On Quality of Extruded Products
Nutritional
Properties
Protein
Sugar
StarchDietary
Fibre
Lipids
Digestibility
Sugar losses as sucrose
--- glucose (reducing
sugars) undergo Millard
reaction with proteins
Subjected to
relatively high
pressure
up to 103 psi
Gel formation and
viscosity
Insignificant changes in
dietary fibre content
Lipid is released from
cells
Torque is decreased ---
lipid reduces slip within
the barrel
27. Functional
Properties
Expansion
Ratio
Bulk
Density
WAI
WSI
Hardness
Effect of the process
parameters was opposite
of expansion ratio
Most dependent on screw
speed and temperature
Increases with increased
Temp.
Decreases with increased
MC
HigherWSI with higher
screw speed
Increases with increase in
Temp.
Inverse relationship with
extrudate expansion
30. 2) Low processing Cost
Can save 19 per cent raw material,14 per
cent labor and 44 per cent capital
investment
Eliminates water treatment costs
1) Less space
Needs less space per unit of operation than
other cooking system.
Advantages of extrusion
3) Energy efficient
Operate at relatively low moisture while
cooking food products, so less re-drying is
required
31. 4) Adaptablity
Remarkably adaptable in being able to
accommodate the demand by consumer for
new product
5) High production rates
production rates of up to 315 kg/h for
snackfoods, 1200 kg/h for low-density cereals
and 9000 kg/h for dry expanded petfoods are
possible (Mans, 1982)
6) Automated production
Can operate continuously and have high
throughputs
32. 7) Product quality
Involves high temperatures applied for a short
time and the limited heat treatment retains
many heat sensitive components
8) No process effluents
Low-moisture process that does not produce
process effluents
Does not create problems of environmental pollution
33. Disdvantages
• Larger lot size required as tooling cost is high
• Variations in size/shape of final products possible
• High Initial cost
• May produce carcinogenic acrylamide