Rheological properties of stirred fermented milk products
1998
Dejmek, P. (Lund Univ. (Sweden). Dept. of Food Engineering) | Cuvelier, G.
Observed rheological properties of fermented milk products show wide variation. Some of the causes of the variation are: stratification: during storage, the stirred gel network deforms under its own weight,settles; even samples which do not present a whey layer on top can have a significant vertical protein concentration gradient; restructuring: loading a sample into a rheometer necessarily disturbs its structure in an unpredictable way. The structure recovers on resting, initially rather fast, and later with a constant temperature dependent rate of increase of dynamic moduli of the order promille to percent per minute. Wall slip: at stresses below a few tens of N and low strains, the gel behaves as a classical viscoelastic solid, with a viscosity of the order of 1000000 N per square m. With smooth measuring elements, wall slip may easily occur, and lead to apparent quasiconstant non-Newtonian viscosities of the order of 1 N per square m. Structure breakdown: there appears to be no limiting value of viscosity for a given level of shear rate. We have reached values as low as 0.010 N per square m on extensive shearing of yoghurt.
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