Rheological Monitoring of Structure Development and Rebodying of Set-Style Yoghurt

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.