Evaluation of new methods and measuring systems for characterisation of flow behaviour of complex foods

Conventional methods often impose difficulties when measuring viscosity of fluids containing particles or fibres, e.g. tomato paste or fruit preparations, mainly due to the small gaps of the viscometer. In order to solve these problems different geometries have been developed, e.g. different mixer systems and wide gap double concentric cylinders. These, however, can not be evaluated assuming constant shear rate throughout the gap. Instead an average shear rate must be determined and a quotient between torque and shear stress must be found. In this study, three different kinds of mixer systems have been studied; a small and a large paddle and a helix. In addition, a wide gap double concentric cylinder (DCC) was examined. When evaluating the mixer systems a method based on the Couette analogy has been used successfully for different fluids. The DCC has been evaluated using a numerical and an approximate method, both giving satisfying results. All systems had the limitation that no consistent results were obtained for tomato paste. However, when starch was added to the diluted tomato paste dispersion, satisfying results were once again reached Introduction When measuring the viscosity of a fluid containing particles the conventional types of viscometers are often not suitable, due to the narrow gaps of e.g. a concentric cylinder or a cone and plate system. One alternative is to use different kinds of mixer viscometers e.g. vanes or helical ribbons. These systems have the drawback that shear rate and shear stress can not easily be calculated due to the complexity of the geometry. Different methods have been evaluated in order to calibrate such mixer systems [1], [2], [3] but the problem has been to find constants that can be used for all kinds of fluids. In 2000 Choplin developed a new method based on the Couette analogy [4]. This method has been used successfully on fruit pulps [5] and on fluids with varying power law indexes [6]. Another type of viscometer with the ability to handle fluids containing particles is a wide gap double concentric cylinder (DCC). Also in this system, the shear rate can not be considered constant throughout the gap due to the size of the gaps. In order to find e.g. the power law parameters K and n with this system the shear stress and rate can be calculated throughout the gaps with a numerical method [7]. The procedure takes end effects into account and is absolute. In addition, an approximate method that ignores end effects has been developed [8]. In this study, three different mixer systems are calibrated for various products, including fluids containing particles up to 10 mm in size. The method used is based on the Couette analogy. In addition, a wide gap double concentric cylinder (DCC) is examined using both the numerical and the approximate method developed by James [7], [8]. Theory The method used in order to calibrate the mixer systems is based on the so called Couette analogy and developed by Choplin et al. [4] following the approach used by Bousmina et al. [9]. The torque calculated for a power law fluid in the evaluated geometry is equated to the one calculated with the same fluid in the case of a cylindrical bob rotating in a cup at the same rotational speed. An equivalent internal radius, Ri, can be found, which can then be used for calculating the shear rate and the torque at a certain position in the gap.