Modelling the rapid cooling and casting of chocolate to predict phase behaviour

Abstract

In recent years, rapid cooling processes have been developed to manufacture complex, three-dimensional shapes for confectionery products (e.g. chocolate Easter eggs). These processes are similar to injection moulding and casting processes used in the metallurgic and polymer industries. Although these manufacturing routes are commonly used, they are still poorly understood and thus not optimised. In traditional chocolate processing, cocoa butter is cooled at a low rate (<2 °C/min) to obtain the desired polymorph (form V), and higher cooling rates lead to undesired products. It is thus a paradox that rapid cooling processes which generate very high cooling rates (>100 °C/min) are used to manufacture acceptable products. To obtain an understanding of the occurring phenomena, a mathematical model of the phase change of cocoa butter and heat transfer has been developed, solved using the finite element method and validated against X-Ray diffraction data. During this rapid cooling step, only a small fraction of the cocoa fats is solidified. This fraction increases the viscosity and the yield stress of the material locally. This allows maintaining the shape prescribed during the rapid cooling throughout the rest of the process. Numerical experiments over a range of processing conditions indicated that the Planck solution of the Stefan problem could be used to design processes. The numerical simulation shows that the rest of the material does not crystallise during the rapid cooling step but when subsequently passing through cooling tunnels. If a filling is injected at high temperature into the egg shells, local melting of the chocolate shells will occur. It was calculated using the model that for polycarbonate moulds a deposition at temperatures above 36 °C would compromise the structure of the chocolate shell. The model predicted that metal moulds of higher thermal conductivity would allow better cooling efficiency and raise the critical filling deposition temperature up to 45 °C.