Yoghurt consistency and viscosity are two of the major factors involved in product quality and acceptance. Thus this paper presents a comparative study of the rheology of commercial yoghurts in Rio de Janeiro, Brazil, whose formulations included different thickeners. Three major market brands were used and the thickeners included guar gum, xanthan gum, carrageenan gum, locust bean gum and carboxymethylcellulose. Before the rheological tests, samples of the yoghurts were submitted to the determination of pH, acidity (expressed as lactic acid) and moisture content. The flow and viscosity curves were obtained using a rotational Thermo Haake Mars rheometer with plate/plate geometry (diameter 35 mm), varying the shear rate from 0.02 to 100 s-1 (rising curve) and from 100 to 0.02 s-1 (falling curve) in a total time of 20 minutes. Hysteresis was determined as the area between the curves and fitted to the models of Bingham, Casson, Herschel-Bulkley and the Power Law. Thixotropy tests were also carried out by measuring the viscosity versus time at a constant rate of 100 s-1 for 10 minutes. These were fitted to the Weltman model. Moreover, the changes in the rheological behaviour as a function of temperature (4-24 ºC) were observed, and the results evaluated using the Arrhenius equation. All the yoghurt samples showed pseudoplastic and thixotropic behaviour and all the models fitted the flow curves well, with the exception of the Weltman model, which did not represent the thixotropy tests well.
Yoghurt; Thickeners; Rheology; Viscosity
