Comparison of techniques for measuring the rheological properties of labneh (concentrated yogurt)

The effectiveness of conventional rheological dechniques (destructive) and the oscillatory dynamic test (non-destructive) for the study of the physical properties of concentrated yogurt (labneh)was studied. Six different types of labneh (control (cloth bag method), ultrafiltrated (UF) after and -before fermentation, reverse osmosis (RO) -after and -before fermentation and direct reconstitution from whole milk powder) were examined. Dynamic rheological studies revealed that labneh is a viscoelastic system in which its elastic characteristic is more dominant than its viscous properties. The elastic and viscous attributes of the control labneh were significantly different from the rest of the test samples. In general, the samples with low protein content (RO-after and -before fermentation and direct reconstitution labneh) produced weaker gel structures than their UF counterparts. The penetrometer and viscometer (destructive techniques) failed to reveal expected differences between the samples, and the results did not correlate with the oscillatory dynamic tests. In the light of these results, it could be suggested that dynamic studies are much more reliable than the destructive rheological techniques for the study of the physical properties of labneh.