The objective of this thesis was to gain an understanding of the cold extrusion of chocolate, both physically and mathematically. This was achieved by quantifying the effect of pertinent process variables upon the pressure difference across a die as a chocolate flowed through it. Changes in the chocolate's cross-sectional area were found to have a large effect upon the extrusion pressure, whereas increases in its flowrate did not. It was also observed that the pressure on cessation of extrusion remained at a finite value. In addition, the boundary condition during cold extrusion was directly visualised and it was concluded that slip was occurring at the wall. This slippage was quantified by a low Coulomb friction coefficient. Therefore, it was concluded that during cold extrusion chocolate was deforming as a near-perfect plastic material. This characteristic allowed the successful use of a relatively simple theory to model the extrusion of chocolate through axisymmetric dies. The extension of this modelling by finite element techniques was investigated with the use of a commercial code (<I>ABAQUS/Standard</I>). The pressure difference across a die was found to be very sensitive to the temperature of the chocolate. The relationship between the pressure and the reciprocal of the temperature was fitted with a straight line. This relationship was extended to other chocolate compositions by scaling the chocolate's temperature by its nominal melting point. Across most of the flowrate range studied the flow pressure difference was either independent of, or increased slightly with, increases in the flowrate. However, at low flowrates and temperatures, the flow pressure was seen to decrease on increasing the flowrate. For dies with long capillaries, extruding in this region resulted in a flow instability. This instability showed a stick-spurt extrudate motion which was accompanied by large pressure oscillations, from which the chocolate's compressibility was calculated to be approximately the same as for some common polymers. The instability was successfully modelled by the combination of two simultaneous differential equations and a non-monotonic pressure-flowrate curve.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598171 |
| Date | January 1997 |
| Creators | Crook, S. J. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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