1) The document summarizes research on the rheology and microstructure of chocolate and cocoa butter during semi-solid processing like extrusion. 2) Experiments using a multi-pass rheometer showed that successive extrusions softened the materials through work-induced melting of triglyceride crystals, which then partially re-crystallized when processing stopped. 3) In-situ x-ray analysis confirmed processing caused melting of triglyceride crystals in both chocolate and cocoa butter, with crystals then recovering when materials were allowed to rest.

1. SEMI-SOLIDPROCESSING OF CHOCOLATE
AND COCOA BUTTER
:
Rheology and Microstructure
Anas Ejaz Shaikh
13FET1006
Department of Food Engineering &
Technology
Institute of ChemicalTechnology

2. Introduction
Chocolate
Products
Moulded Chocolate Products
[Viscous with yield stress and
mild shear thinning
characteristics]
Cooling Demoulding
Packaging
30⁰C
Chocolate
Products
Cold Extruded Chocolate
Products
Convergent die
5 to 25 ⁰ C
Isothermal
[No detectable
temperature change]

3. Example of the development of barrel pressure during a ram extrusion of
chocolate, measured in the MultiPass Rheometer just upstream of extrusion
die.
Solid line: barrel pressure. Broken line: piston position.

4. Microstructure
of Chocolate
Cocoa butter /
milk fat
Sucrose Cocoa
particles
Milk
particles
Surface
active
ingredients
[e.g.,
lecithin]
Flavours
[e.g.,
vanillin]
Crystals :
Assumed to
remain
unchanged
Semi-
crystalline fat
matrix ofTGs
Plastic Flow
and Mouthfeel
of Chocolate

5. Fraction of
crystal phase
triglycerides
liquid state
work dissipation
(cold extrusion)
Flow &
flexible
response
Work by Beckett et al., 1994
Work by Mulji et al.,2003
NMR techniques
Liquid fraction of cocoa
butter decreased
Indicating that:
it had increased during extrusion
Literature Survey

6. Inference from Literature
Survey
Increase in liquid
phase content
Recovery to the
original crystal
fraction
Post-extrusion
During-extrusion
This forms the basis of the present study

7. Objectives of Current Study
• To subject chocolate or cocoa butter to successive multiple
extrusions using a double-piston Multi Pass Rheometer
• To establish how the material responds to different process
histories
• To probe the crystal fraction of the fat matrix present at any
time using X-ray diffraction technique from which the liquid
fraction can be obtained

8. Scope of the Study
•The coupled data (MPR + XRD) gives
insight into the way processing
influences both :
1) The rheology of the materials.
2) The microstructure of the materials.

9. Materials and Experimental
Techniques
• Materials :
1) Chocolate
2) Cocoa Butter
• Multi-Pass Rheometer
• Extrusion Loading & Operation Protocol

10. Chocolate
Solid Granules (x < 5mm) of Milk Chocolate
[Nestle ProductTechnology CentreYork (York, UK)]

11. Cocoa butter

12. Multi-Pass Rheometer
Repeated processing of samples at a
con-trolled temperature and mean
hydrostatic pressure that can be fixed
by moving the pistons towards each
other
Two pressure transducers enable
time-dependent measurements of
differential pressure between them
In addition to these process
rheology measurements, it is possible
to use the MPR3 together with an X-
ray scattering facility
Beryllium capillary (to allow
sufficient transmission of X-rays)
Simultaneous measurements of
microstructure and process rheology

13. Extrusion Loading
Chocolate and Cocoa butter were in granular/powder form at the
temperature of interest
Aim :To maintain the particular polymorphic form of
the crystalline triglyceride molecules
Melting the chocolate
and solidifying it in the
instrument
Requires a well-
controlled ‘tempering’
Granules compacted in a medical
syringe made of plastic (from which the
front part had been sawn off)
Fed from this syringe into the bottom
and top sections of the MPR and into
the wide parts of the capillary section

14. Operation Protocol
1) Compaction Phase
2) Extrusion sequences
The temperature of the barrel system was kept constant at
20 +/- 1⁰C

15. Results and Discussion
Experimental Observations of 4 Interrelated effects
Mechanical
Softening
Re-
hardening
Melting of
TGs
Re-
crystallisation
ofTGs
Studied through Rheological
changes by differential
pressure measurements
Multi pass
Extrusion
Single pass
Extrusion
Studied through
Structural changes
by in situ XRD
experiments

16. Extrusion Softening
High Stress (Viscous fluid but softer
than the reference)
Low Stresses
(Rigid)

17. 1) The initial extrusion shows the highest extrusion pressure and
subsequent extrusion strokes show a progressive decay.
2) After approximately 20 extrusions, no substantial further change of
the extrusion pressure profile occurred.

18. The behaviour of cocoa butter is very similar to that of chocolate
A progressive decay in extrusion pressure with increasing number of passes
and a final plateau where the extrusion pressure is essentially independent
on the amount of further processing cycles
Softening is a consequence of changes in triglyceride matrix of chocolate

19. Re-hardening
Principle:
Generally, work softening of a material can be fully or partially reversible (thixotropic
materials) or irreversible, depending on whether the material can return to an
equilibrium state within the time scale of the observation
If the work softening were an irreversible effect then it would be expected that the
delay time would have no influence on the observed extrusion pressure evolution
No change in Pextrusion =
Irreversible work softening

20. The data shows for both chocolate and cocoa butter that if the delay time is short, the extrusion pressure does not
recover, however if the delay time is of order 40 min, near complete recovery of the extrusion pressure is reached.
For the case of chocolate ‘re-hardened’ chocolate with delay times tdelay >2 min required an even higher peak
pressure to initiate flow than for unprocessed chocolate, presumably due to the compaction of the chocolate during
extrusion.

21. Both the multi-pass experiments
with no time delay
with finite time delay
show that the PROCESS BEHAVIOUR of the
semi-solid chocolate and cocoa butter is sensitive
(i) to
the amount of work that has been applied
and
(ii) to
the time between application of this work and further
processing.
Inference

22. Microstructure Changes: Melting and Re-crystallization
Problem (Effect)
A difficulty in
applying X-ray
diffraction to study
triglyceride
crystallization in
chocolate
Problem (Cause)
Presence of sugar crystals as
their strong diffraction peaks
mask some of the
characteristic diffraction
peaks of the triglyceride
crystals
Solution
Focussing on the
‘long spacings’ of
triglyceride crystals, where
sugar crystals
do not contribute to the
diffraction pattern
β1 -3
‘Form VI’
β2 -3
‘Form V’
1) No other long spacings were
observed in our experiments,
indicating that no significant
crystallization other triglyceride
polymorphs occurred.
2) Furthermore, in all our experiments
the changes of these two peaks
were proportional to each other,
indicating that they corresponded
to the same crystal form.

23. Principle :
If we assume that the diffraction peaks
correspond to a single crystal form,
the integrated intensity, i.e.,
the area under all the peaks,
should be proportional to the crystal content.

24. Processing-induced Melting :
(i) Not a function of processing flow rate (piston velocity)
(ii) A function of Work dissipated during extrusion due to
extrusion pressure

25. Recovery in the crystal fraction of the triglycerides after extrusion a series of X-
ray data was obtained at different times after the extrusion had stopped
Recovery time for:
1) Chocolate : 40 min
2) Cocoa butter : 10 min

26. Conclusions
• Systematic extrusion sequences using a multi-pass rheometer
have shown that milk chocolate at ambient temperature is
softened considerably by extrusion processing.
• In situ X-ray characterization of the triglyceride matrix in milk
chocolate during and after extrusion showed a processing-
induced melting of triglyceride crystals which re-crystallized
during the ‘rest period’ after an extrusion.

27. Contribution to the Society
• A step ahead in the Semi-solid processing of
Chocolate and Cocoa butter
• Study of kinetics for formation of extruded
chocolate products with better mouthfeel
• The Experimental Correlation of Process
Rheology with Microstructure

28. Future Scope
• Characterization of the cold extrusion of
chocolate using a perfect plastic model with a
constant plastic yield stress is insufficient to
completely describe the processing
behaviour.
• An extended model that includes :
1) Effect of work-induced phase changes and
their effect on the rheological properties of
chocolate
2) Prediction of the local solid and liquid phase
contents of the triglycerides