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Title: The rheology and microstructure of chocolate during cold extrusion processing
Author: Engmann, J.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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This dissertation is concerned with physical processes occurring in chocolate at temperatures near 20°C during the "cold extrusion" forming process. The microstructure of chocolate at these temperatures consists of solid particles dispersed in a continuous fat matrix, which itself consists of liquid regions coexisting with fat crystals. The approximate volume fractions of the solid particles and the continuous matrix were calculated from chocolate recipes and the total volume fraction of the solid particles is close to 60%. The chocolate microstructure has some similarities with other highly concentrated suspensions and granular pastes, and certain phenomena observed for these materials, such as "wall slip", were recognised to be important in understanding the behaviour of chocolate during cold extrusion. However, the coexistence of solid and liquid phases that form the continuous matrix introduced other phenomena to the deformation, which were investigated experimentally and theoretically. The extrusion behaviour of a milk chocolate and a cocoa butter was characterised at temperatures of 20-24°C using a "MultiPass Rheometer". This allowed sequences of extrusions to be performed, thus the effect of processing history could be explored. The extrusion pressures of both the chocolate and the cocoa butter strongly depended on the processing history: each extrusion pass softened the material, first strongly, then by smaller amounts in subsequent extrusions. If a delay time was introduced between the extrusions, the flow pressures were larger than during a continuous extrusion sequence and increased with the length of the delay time, indicating a re-hardening effect. For delay times of more than 60 minutes, the flow pressure of milk chocolate was identical to the original flow pressure, while for cocoa butter, this was the case after only 10 minutes. The previously reported "post-extrusion flexibility" of cold extruded chocolate was therefore shown to have direct consequences for the extrusion processing itself. An in-situ X-ray investigation, using a combination of the MultiPass Rheometer with an X-ray diffraction system, was then carried out for the two materials. Changes of the triglyceride crystal structure and crystal content in these materials were monitored during processing. Clear evidence for a reduction of the crystal fat content during extrusion processing and for re-crystallisation thereafter was obtained, for both cocoa butter and chocolate. The results were consistent with previous observations of changes in the liquid phase content of cocoa butter and chocolate after extrusion processing. Some information about the kinetics of this re-crystallisation process was also obtained. These kinetics were similar to those of chocolate extrudate re-hardening and to the recovery of the extrusion pressure seen in the multi-pass extrusion experiments. The insight gained through the experimental program was used to develop an extended model for the deformation of chocolate during cold extrusion. Macroscopic energy balances and finite element analyses were used in this development. Firstly, the effects of wall friction on the extrusion pressure were studied. The effects of extrusion processing on the temperature and on the amounts of the solid and liquid phases of the fat matrix were then considered. Finally, it was estimated how these changes affect subsequent processing and the extrusion pressure itself. The new model was used to discuss the experimental results from this work and from previous experimental investigations. It provided a qualitative explanation of previously observed localised concentrations of high liquid fat content and suggests that, for typical extrusion geometries, the softening does not alter the extrusion pressure in a single extrusion strongly compared to a material with a constant yield stress.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available