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My project looks at the extrusion processing of a confectionery food paste which is formed by combining powdered and liquid ingredients along a co-rotating twin screw extruder. As with many food industry products, the process setup for the food paste concerned has been developed very much on a trial and error basis with little quantitative understanding of how variations in processing conditions affect the end product. Indeed, in our process, there are currently no fundamental criteria for what constitutes a good quality paste other than operator judgement. The bulk of my work is therefore based on developing quantitative indicators of paste quality that allow us to characterise the material with respect to fundamental parameters. Namely, I use a variety of techniques to visualise and assess paste homogeneity (how well mixed the ingredients are) as well as variations in paste microstructure and how they correspond with the viscosity and flow properties of the paste. Obtaining data to characterise the viscosity and flow properties of the paste is a significant challenge in itself as samples removed from the process behave differently from the material as it is being processed. This has lead to further work on developing on-line techniques for characterising pastes of this consistency. With sufficient quantitative tools in place, it will shortly be possible to conduct a parametric study of the process and determine the factors that most significantly affect paste properties and the trends of ingredient dispersion that are required for a well mixed product. It is also hoped to track changes in paste properties along the length of the screw profile, thereby facilitating the quantitative comparison of how efficiently different screw profile designs perform.
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