Semi-Solid Processing of Chocolate and Cocoa Butter: Modelling Rheology and Microstructure Changes During Extrusion

Abstract

This paper describes the development and application of a model that is capable of describing the extrusion behaviour of semi-solid chocolate. The model is based on the flow of a perfect plastic material within an extrusion die. During the isothermal extrusion process, work is carried out and a model has been developed to correlate the work done with the transformation of part of the crystalline triglyceride molecules to the liquid state. Numerical results from finite element calculations are presented for the variation of liquid fat content as a function of position within the extrusion die for a single extrusion, and analytical results are presented for the variation of the average liquid fat content and of the extrusion pressure for sequences of extrusions. The model demonstrates that, in certain situations, changes of a microstructural variable, rather than temperature changes, are necessary in order to describe the dissipation of work during a forming process.