Chemistry Reference
In-Depth Information
Comparison of the dynamic rheological properties (
η
*
) and the steady
shear rheological properties (
) can provide insight on the structure of
the sample. For example, milk beverages with locust bean gum conform
to the Cox-Merz rule but those with guar gum added required a shift fac-
tor (
η
α
) of 0.18, (i.e. the complex viscosity was calculated at a frequency
of
)(Ya¸ar
et al
., 2009). The departures from the Cox-
Merz rule with the magnitudes of
αω
instead of
ω
a
was explained by
the structure decay due to the effect of the stress applied to the system,
which is low in oscillatory shear and high in steady shear (Ya¸ar
et al
.,
2009). Milk beverages with addition of guar gum, which showed more
solid-like properties than the samples containing locust bean gum, would
be more sensitive to the effect of stress deformation as it may be dis-
rupting a network structure.
η
greater than
η
*
7.2.2
Measurements of the rheological properties of milk
Three types of viscometers are frequently used to measure milk vis-
cosity: falling sphere method (an empirical method), U-tube capillary
viscometers and rotational viscometers. For foods that exhibit New-
tonian behaviour, viscometers that operate at a single shear rate (e.g.
falling sphere method and U-tube capillary viscometers) are acceptable.
However, for foods that exhibit non-Newtonian behaviour, data should
be obtained at several shear rates, and this is most typically done using
rotational viscometers.
7
.
2
.
2
.
1
Falling sphere method
Falling sphere method involves a vertical tube where a sphere of known
size and density is allowed to fall under a given force (e.g. gravity)
through a Newtonian fluid. The falling time of the sphere over a certain
distance or the falling velocity is determined by two forces - the sphere
gravity and the drag force (viscosity) of the fluid - and is used to
calculate the fluid viscosity according to Stokes' law. The fluid viscosity
is assumed to be independent of the shear rate (Newtonian behaviour)
when measured by this device. The flow behaviours of milks with
<
30%
40
◦
C are very close to Newtonian behaviour
solid content and at
T
>
with
n
0.9 flow behaviour index observed in many studies. The
falling sphere method is considered acceptable to determine the apparent
viscosity of milks in these cases (Fernandez-Martın, 1972).
A general discussion of falling sphere viscometers that presents con-
ditions for and limitations of their use is found in Weber (1956). The
falling sphere method has been frequently used in earlier studies but
rarely used recently. A more frequently used method to determine the
Newtonian viscosity is U-tube capillary viscometer.
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