The Micromechanics of Chocolate Cold Extrusion
The extrusion of chocolate at temperatures below its melting point was discovered by Prof Mackley in 1992 and was a focus of research in the Polymer/Fluids group for a number of years. The extruded product has a degree of flexibility that facilitates subsequent processing, although no significant change in temperature seems to occur during the extrusion.
Previous Ph.D. and undergraduate research projects investigated the influence of processing parameters such as temperature, chocolate composition, flow rate and die geometry on the extrusion pressure and on the mechanical properties of the product. The occurence and modelling of flow instabilities under certain processing conditions was a further research topic.
The aim of Jan's project was to gain an understanding of the microstructural processes which govern the extrusion and post-extrusion behaviour. It is surmised that partial disruption of the fat crystal network during and recrystallisation after the extrusion are responsible for the observed behaviour during and after extrusion and his aim was to achieve a quantitative description of how these phase changes affect the mechanical properties. From such an understanding, the design of the extrusion and the post-extrusion shaping processes could be improved, and the material behaviour at other processing conditions predicted. Further, it may be possible to extend the 'cold' extrusion process to other materials.
The Cambridge MultiPass Rheometer was used to study changes in mechanical and structural properties during the extrusion process, making use of in-situ X-ray powder diffraction techniques.
The project was sponsored by Nestle who acquired the patent for the cold extrusion process and investigated its applicability to the manufacture of new and existing products.