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

Abstract

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. © 1999 Society of Chemical Industry

Elongational flow studies of set yogurt

“…Besides, the validity of the measurements, i.e. whether they are consistent or not, can be verified by comparing results of tests performed with containers of different diameters (Suwonsichon & Peleg, 1999). Fig.…”

“…Therefore, they are not in position to know how much their data deviate from those of the original yogurt. Moreover, there is another problem which affects the rheological measurements and this is slip (Suwonsichon & Peleg, 1999). Slip appears even when the measurement of the samples takes place in situ, i.e.…”

“…peanut butter , including dairy products such as American process cheese (Campannela, Popplewell, Rosenau, & Peleg, 1987), butter (Shukla, Rizvi, & Bartsch, 1995), and Mozzarella cheese (Ak & Gunasekaran, 1995). Suwonsichon and Peleg (1999) studied the rheological characteristics of almost intact and stirred commercial yogurt samples using the so called imperfect squeezing flow viscometry. That is, the bottom plate was replaced by a shallow container with a diameter bigger than that of the upper plate.…”

“…However, even by using this geometry there are problems which affect the measured forces such as entry effects, annular flow and buoyancy. Nevertheless, it is claimed (Suwonsichon & Peleg, 1999) that despite these drawbacks, the method is remarkably sensitive and useful for rheological characterization of viscous foods with slip problems and fragile structure.…”

“…Since, Suwonsichon and Peleg (1999) studied commercial yogurt samples, whose structure might have been disrupted to a certain extent during loading in the measuring unit, it was decided to prepare yogurt samples under controlled conditions and to study them in situ, using the squeeze flow method.…”