chocolate lecture abingdon school-2010
DESCRIPTION
This presentation on the cold extrusion of chocolate was given to Abingdon School in 2010TRANSCRIPT
New ideas in shape-forming
chocolate
Science Innovation
Imagination
The ingredients and structure of chocolate
Processing chocolate to form shapes
DISCOVERY
INVENTIO
N
SC
IENCE ENGINEERI N G
TECHNOLO
GY
Melting, casting,
extruding
Flexible Chocolate!
By Prof Malcolm Mackley
Fellow of Robinson College
Professor of Process InnovationDepartment of Chemical Engineering and BiotechnologyUniversity of Cambridge
1965 O Levels; Gravesend Technical High School1967 A Levels; Bath Technical College1969 BSc Physics; Leicester University 1972PhD; Polymer Physics; Bristol University1976Lecturer in Material Science; University of Sussex1979Department of Chemical Engineering, University of Cambridge1994 Chocolate
The scientific mind
LB R
2 Y B
w
1
X2 1
The inventive mind
The Patent Self-Operating Napkin
The imaginative mind
Cocoa plantation
Harvesting cocoa
Cocoa pods
Structure of chocolate
about 0.1 mm
Cocoa butter
Sugar crystals
Cocoa solids
Milk solids
Cocoa butter - melted
Cocoa butter - solid
0
20
40
60
80
100
-10 0 10 20 30 40 50
Melting Chocolate
Proportion of fat that is
liquid (percent)
Temperature of chocolate
(degrees.C)
hard
solid
soft
solid
liquid
mixin
de-moulding
cooling tunnels from 300C to 180C
heating to about 400C
eating!
de-moulding
p
m i x i n g
The chocolate must be heated to melt the fat so that it can be poured into moulds. It must then be cooled to about 18oC before it can be removed from the mould. The whole process takes about thirty minutes.
Traditional chocolate forming
Temperature
50 100 150 200
C0
H
Cold Extrusion
Melt Processing
Polyethylene
Temperature
10 20 30 40
C0
H
Cold Extrusion
Melt Processing
Chocolate
Extrusion processing
Extrusion process
Go to movie on chocolate cold extrusion process
knotting
Go to movie on Chocolate cold extrusion knotting
moulding
Go to movie on chocolate cold extrusion moulding
Extruding a tube
extrusion "torpedo"
Axisymmetric ram experiments
D0 / 2
D / 2L
U
pressure
time
end ofpiston motion
pressure transducer
Cold extrusion of semi solid chocolate
Typical Pressure v/s Time trace
20
40
60
80
100
120
140
160
0 5 10 15 20 25 30 35
P (bar)
t (s)
Compaction
(no flow)
Yield pressure
A
Extrusion pressureB
C
Piston/(ram)
Chocolate feed
Die
Pressure transducer
Milk choc at 20 oC
0
5
10
15
10 15 20 25 30
Extrusion Pressure MPa
Temperature
0 C
Mechanism.Rate of doing work = Rate of heat generation
= pressure x area = force = velocity = density
= Specific heat = temperature increase
P A
Cp T
T = P
Cp
T = 100 105
1,200 1,200 = 6.9 0C
T C x A = x A p P
x
Temperature doesn’t change!
So where does the work go?
Another calculation
Rate of work done/kg =
Work done/kg in melting mass fraction of cocoa butter
= latent heat of melting / kg
P
P
107
1,200x150x103 = 5.5%
Equate
P
= x Ax A
P
Temperature / Centigrade
% Liquid Fat
100
50
0
20 3010
adiabatic heating
Microstructure change
1: Before flow
Sugar crystals
Liquid fat in isolated sub- micron regions
The mechanism
Some fat migrates to walls, reducing friction and easing flow
Sub-micron internal “slip planes” allow plastic flow. Some crystals melt
2: As flow begins
The mechanism
Liquid fat slip planes make the chocolate flexible
The mechanism3: During flow
The mechanism
4: Some time later....
As liquid fat reverts to its original regions and cocoa butter recrytallises, so the chocolate hardens
The scientific mind
LB R
2 Y B
w
1
X2 1
The inventive mind
The Patent Self-Operating Napkin
The imaginative mind