This presentation on the cold extrusion of chocolate was given to Abingdon School in 2010

1. Fxle
leib Science
Innovation
EN GI N EE R I N Imagination
Ccla!
h ot
o e
G
E
NC
IE
SC Melting,
The ingredients casting,
and structure extruding
of chocolate
N
O
TI TEC
N E HN
IN V O
L
New ideas in
Processing shape-forming
O
chocolate to
G Y
chocolate
form shapes
RY
O VE B P fMlc lm ak y
y r a o Mcle
o
D I SC

2. Fellow of Robinson College

3. Professor of Process Innovation
Department of Chemical Engineering
and Biotechnology
University of Cambridge

4. 1965
O Levels; Gravesend Technical High School
1967
A Levels; Bath Technical College
1969
BSc Physics; Leicester University
1972
PhD; Polymer Physics; Bristol University
1976
Lecturer in Material Science; University of Sussex
1979
Department of Chemical Engineering, University of Cambridge
1994 Chocolate

5. The scientific mind
R  YB   1 
LB =    2 − 1
2  τw   X 

6. The inventive mind
The Patent Self-Operating Napkin

7. The
imaginative
mind

8. C ocoa plantation

9. Harvesting cocoa

10. C ocoa pods

12. Structure of chocolate
Cocoa
butter
Sugar
crystals
Cocoa solids
Milk solids about 0.1 mm

13. Cocoa butter - melted

14. Cocoa butter - solid

15. Melting
Chocolate
100
liquid
80
60
solid
soft
Proportion of 40
fat that is
liquid 20 hard
solid
( percent )
0
-10 0 10 20 30 40 50
Temperature of
chocolate
(degrees.C )

16. Traditional chocolate forming
i
mxing
ixin
heating to
about 400C
pouri cooling tunnels
from300C to 180C
ng
eating!
de-moulding
The chocolate
m be heated
ust
to m the fat so that it
elt
can be poured into moulds.
It m then be cooled to about 18oC
ust
before it can be removed fromthe mould.
The whole process takes about thirty minutes.

17. Extrusion processing
Polyethylene
Cold Melt
Extrusion Processing
∆H
50 100 150 200
Temperature 0C
Chocolate
Cold Melt
Extrusion Processing
∆H
10 20 30 40
0
Temperature C

19. Extrusion process
Go to movie on chocolate cold extrusion process

20. knotting
Go to movie on Chocolate cold extrusion knotting

21. moulding
Go to movie on chocolate cold extrusion moulding

22. Extruding a tube
extrusion
"torpedo"

27. Axisymmetric ram experiments
U
D0 / 2
pressure transducer
pressure
α end of
D/2 piston motion
L
time

28. Cold extrusion of semi solid chocolate
Typical Pressure v/s Time trace
Piston/(ram)
A Yield
pressure Chocolate feed
Pressure
transducer
Die
∆P
(bar)
B Extrusion pressure
C
Compaction
(no flow)
Milk choc at 20 oC t (s)

30. 15
Ex trusion 10
Pre ssure
M Pa
5
0
10 15 20 25 30
Te mp e rature 0C

31. Mechanism.
Rate of doing work = Rate of heat generation
P A x = ρ A x Cp ∆ T
 
PA = pressure x area = force
x
 = velocity
ρ = density
Cp = Specific heat
∆T = temperature increase
P
∆T =
ρ Cp
100 10 5
∆T = = 6.9 0C
1, 200 1,200

32. Temperature doesn’t change!
So where does the work go?

33. Another calculation
P Ax P

Rate of work done/kg = =
ρ Ax ρ

Work done/kg in melting mass fraction φ of cocoa butter
=φλ
λ = latent heat of melting / kg
Equate P
=φλ
ρ
P 10 7
φ= = 3 = 5.5%
ρλ 1,200x150x10

34. 100
%
Liquid
Fat
50
Microstructure
change
adiab atic heating
0
10 20 30
T e m pe ra ture Centigrade
/

35. The
mechanism
1: Before
flow
Liquid fat in
isolated sub-
micron
regions
Sugar
crystals

36. The
mechanism
2: As flow
begins Sub-micron
internal “slip
planes” allow
plastic flow.
Some crystals
melt
Some fat
migrates to
walls, reducing
friction and
easing flow

37. The
mechanism
3: During
flow
Liquid fat slip
planes make the
chocolate flexible

38. The
mechanism
4: Some time later....
As liquid fat reverts to
its original regions
and cocoa butter
recrytallises, so the
chocolate hardens

40. The scientific mind
R  YB   1 
LB =    2 − 1
2  τw   X 

41. The inventive mind
The Patent Self-Operating Napkin

42. The
imaginative
mind