Band 10, Heft 2

Impregnation and infiltration of fibrous materials by LC copolyester melts and the blends with viscous thermo-plastic polysulphone were quantitatively described as unsteady and steady stages in terms of rheological characteristics of the melts using an equivalent capillary model. Addition of LC polyester to viscous thermoplastic melt noticeably enhances impregnation condition. Study of the melt flow through various capillaries demonstrated a development of the melt slippage at high shear rate along the capillary walls. Slippage phenomena may be important at the initial stage of impregnation but can be neglected for final stage due to low shear rate in the last case. On the other hand, at those conditions a yield stress of the melt may considerably complicate the impregnation process due to a drastic viscosity increase.

The present work was undertaken in order to evaluate the rheological changes of set-style yoghurt at different incubation times, during production. The study of rheological changes of milk destined to set-style yoghurt production, in different processing stages prior to fermentation was accomplished using steady and oscillatory shear, while the study of the influence of storage time was also carried out using steady shear. Set-style yoghurt samples with 2 and 3 hours of fermentation exhibited a major structural breakdown during upward shear rate sweep experiments as expected, since the protein network was initially intact. Both samples exhibited two points of sudden decay. For T2 the these points occured at shear stresses of 38 Pa and 77 Pa and for T3 at 47 Pa and 80-83 Pa. The first point corresponds to the main break-up of protein network and the second one to the disruption of primary-aggregates, breaking the portion of casein network that was associated with exopolyssacharides produced by lactic acid bacteria. Structural analysis performed by means of oscillatory measurements indicate that the protein network is already partially formed after 2 hours, albeit full development is only achieved after longer periods. Heat-treated milk, evaporated milk and set-style yoghurt samples with 0 and 1 hour incubation times showed an apparent yield stress value of approximately 0.4 Pa and a strong shear-thinning behaviour afterwards. At higher shear stresses (approximately 1.6 Pa), however, the behaviour changed to shear-thickening, this effect being attributed to flow-induced interactions between the milk components. For the latter sample, a further change in flow behaviour occurred at a stress of 8 Pa, which is probably due to the breakdown of the weak protein network that had formed during incubation.

A mixer viscometry procedure, coined the Matching Stress Method, was developed to use four-bladed vanes devices in applications requiring steady shear measurements. Based on the concept that vane tools shear fluids in a cylindrical pattern defined by their geometry, this technique determined a mixer viscometry constant to predict average shear rates for vanes. Three cylindrical bobs and three, four-bladed vanes were used to investigate the impact of attachment geometry on the mixer viscometry constant. Two Newtonian fluid standards and three carboxy-methyl-cellulose (CMC) solutions were used to examine fluids with varying rheological properties. Four cups of varying dimensions contained the sample fluids and provided a system of fluid gaps for comparison. A Bohlin VOR Rheometer collected torque and angular velocity data for vane and bob attachments. For the bob devices, torque and angular velocity measurements were converted into shear stress and shear rate rheograms. Torque responses collected using the vanes were converted into shear stress measurements, and the proposed method matched the vane rheograms with each bob of identical height, diameter, and corresponding system geometry. Variation of system geometry and flow properties revealed the mixer constant depended on fluid gap size when below a cup-to-vane diameter ratio of 2.1 and a flow behavior index of 0.86. This procedure enables vane tools to be used in steady shear applications, not limiting their employment to single point, yield stress determination.