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 sti rring for 1 min by hand and with a domestic blender, were placed in a wide shallow Teflon(R) container. They were subsequently compressed with a wide Teflon(R) plate to induce imperfect lubricated 'squeezing flow'. The record ed force versus height relationships were plotted on logarithmic co-ordinat es. 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 variatio n (100 sigma root 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 c ompressive stress at two preselected heights (1 and 2mm) and the residual s tress after relaxation for two preselected times (60 and 120s). The flowabi lity of the stirred samples was also expressed in terms of apparent elongat ional viscosity versus biaxial elongational strain rate relationships. All these mechanical parameters had a modest dependence on the upper plate diam eter but the latter had no effect on their sensitivity as measures of yogur t consistency. (C) 1999 Society of Chemical Industry.