Rheological characterisation of almost intact and stirred yogurt by imperfect squeezing flow viscometry

Samples of plain yogurt of two national brands, almost intact and after stirring for 1 min by hand and with a domestic blender, were placed in a wide shallow TeflonR container. They were subsequently compressed with a wide TeflonR plate to induce imperfect lubricated ‘squeezing flow’. The recorded force versus height relationships were plotted on logarithmic co-ordinates. The resulting curves all had a clear linear part indicating the region where squeezing flow was dominant. Its slope was on the order of −1.2 to −1.3 in the almost intact specimens and −0.5 to −0.9 in the stirred samples. The test reproducibility, expressed in terms of the coefficient of variation (100σ/ x) was on the order of about 15% more than sufficient to detect textural differences between the two brands and to monitor textural changes caused by stirring. These were expressed in terms of the apparent compressive stress at two preselected heights (1 and 2 mm) and the residual stress after relaxation for two preselected times (60 and 120 s). The flowability of the stirred samples was also expressed in terms of apparent elongational viscosity versus biaxial elongational strain rate relationships. All these mechanical parameters had a modest dependence on the upper plate diameter but the latter had no effect on their sensitivity as measures of yogurt consistency.