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”

4

Types of Food Extruders

Single-screw extruder

Twin-screw extruder

Source : Heldman and Hartel (1997) and Riaz (2000)

Fig. 1 Types of food extruders

5

Food Extruder components

Fig. 2 Cross-section view of food extruder

Screw

Barrel

Die

Hopper and feed conveyor

Pre-conditioner

6

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

7

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)

8

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

9

The die characteristics

Qnet = K*ΔP /µapp

Where

K= Die conductance

ΔP= Die pressure drop

µapp = Apparent melt viscosity

10

The product expansion process

• The product expansion is influenced via

The die pressure drop

Rheology of melt

Degree of superheating

11

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

12

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

13

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

14

Elastic Effects In Extrusion

EXTRUDATE SHAPEDIE SHAPE

15

• 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