Rheological Modeling of Cheese Extrusion

Sammanfattning

Understanding the physical properties and flow behaviour of foods is essential for designing reliable processing equipment. In this thesis, the rheological properties and flow behaviour of Pasta Filata cheese are investigated to improve the understanding of the flow-distribution inside a Tetra Pak Cheese Extruder. Uneven flow-rates have been observed during production, which lead to inconsistent dimensions of the final product. These variations are believed to be a consequence of non-Newtonian properties of the cheese. This thesis presents a theoretical and numerical investigation of the flow behaviour in a cheese extruder. The literature study showed that Pasta Filata is a shear- and temperature-thinning viscoelastic material. The viscosity was considered to be best modeled by Power-Law or Herschel-Bulkley models. Research also showed that viscoelastic flows can exhibit elastic instabilities even at low Reynolds numbers, potentially explaining transient behaviour of the flow-rates. Viscosity models, Power-law and Herschel-Bulkley, were fitted to data from the experimental study conducted by Muliawan and Hatzikiriakos [1] (2007). This was implemented in the Computational Fluid Dynamics (CFD) simulations in the software StarCCM+ to simulate the flow in the cheese extruder. Several thermal cases were simulated to reflect the combination of heated and uninsulated regions in the extruder. The simulations showed that the flow distribution was strongly affected by the temperature. Cases with lower inlet temperature compared to the heating jackets reproduced the steady-state phenomena of uneven flow-rates between the pipes observed during production. No transient behaviour of the flow could be captured by the simulations which supports the theory that these possible fluctuations are due to elastic properties, which were not included in the simulated rheological model. This thesis concludes that the flow-distribution of Pasta Filata in an extruder is strongly dependent on temperature. Improving thermal insulation in critical regions of the process could reduce the uneven flow