Experimental and modelling studies of the flow properties of concentrated yogurt as affected by the storage time

The effect of storage time on the flow properties of concentrated yogurt (labneh) was investigated using a rotational viscometer. Commercial labneh samples were stored at 8 °C. Experiments were performed at 25 °C after 0, 1, 2, 5, 8, 10 or 14 days after the packaging date. The apparent viscosity of fresh sample was then measured as a function of shear rate. For a given shear rate, the apparent viscosity was measured as a function of shearing time. The completely destructed labneh flow properties were also measured after subjecting the labneh samples to a high shear rate for 2 h. The increase in the apparent viscosity of fresh labneh with storage time is attributed to the further development of the gel structure occurred. It was also found that, labneh samples exhibited shear-thinning and thixotropic behaviours with different storage times. The shear-thinning behaviour is fitted well by the power-law model, and the effect of storage time on the power-law parameters was determined. The thixotropic behaviour of labneh was modelled using the structural kinetic approach. For the completely destructed labneh, the effect of storage time on the flow properties was insignificant, and the flow properties were modelled well by the power-law model.