Lubricated squeezing flow of semi-liquid foods
The rheological characterization of semi-liquid foods has encountered two major difficulties: (1) the separation of a layer of fluid of different viscosity at the wall of the rheometer (slippage) and (2) the disruption of the internal structure of the specimen when inserted into the equipment.
The increasing utilization of lubricated squeezing flow viscometry in food industry has not only been determined by its relative simplicity but mainly by its ability to overcome the above mentioned problems.
Although the mathematical models to describe the lubricated squeezing flow of fluids have been extensively considered, several aspects and possible applications of this methodology remain unstudied.
Therefore, this research was focused on evaluating the effect of test conditions, especially the displacement rate, on the measured rheological parameters of several semi-liquid food products. A considerable discrepancy was noted between the observed rate effects and those predicted using a pseudoplastic (power law) model. Consequently, a alternative model was proposed and the effect of the deformation rate was described using the following empirical relationship (Fv1− FR)/(Fv2 − FR) = (V1/V2)m.
Since lubricated squeezing flow viscometry barely produces a significant disruption of the sample, it was used to quantify the structural damage caused to a specimen subjected to several disruption mechanisms such as compression and stirring. As expected, compression of the specimen prior to its testing had very little effect on the specimen consistency which supports the view that there was little friction between the Teflon® coated plates and the samples. On the other hand, shearing caused a measurable loss of consistency.
In order to study the recovery of the sample consistency after disruption a rest period was introduced and the rheological parameters of the treated samples were determined. Different levels of recovery were observed for different specimens, however in none of the cases a full recovery of the original consistency was observed in the time span considered (1–3 hours).
Finally, a relation between the instrumental value of consistency obtained using lubricated squeezing flow and sensory perception was estimated. It was established that squeezing flow is more sensitive to textural changes sensory evaluation and that the methodology was appropriate to determine thresholds for sensory detection.