THE EXPERIMENTAL-OBSERVATION AND MODELING OF AN OVAICI NECKLACE AND STICK-SPURT INSTABILITY ARISING DURING THE COLD-EXTRUSION OF CHOCOLATE

This paper reports experimental results and modeling relating to the a mbient temperature, cold extrusion of chocolate. In particular, extrus ion conditions are given where two different modes of instability are described and modeled. In the first case, at low flow rates, a stick-s purt instability is observed, which leads to large amplitude fluctuati ons in the extrusion pressure and extrudate velocity. This instability is modeled as a modified relaxation oscillation process in which the combined nonlinear plastic flow behavior of the chocolate and the comp ressibility of the material in the reservoir control the instability m echanism. The second mode of instability, observed for a specific ther mal history, concerns the unusual extrusion of a ''necklace'' structur e consisting of beads regularly spaced on a narrow extrudate core. Thi s instability is explained in terms of the steady core-annular coextru sion of two Bingham-plastic chocolate components having different yiel d Stresses. (C) 1998 The Society of Rheology.