Chemistry Reference
In-Depth Information
sample. They indicated that comparison of the 'absolute' hysteresis loop
areas among samples of different viscosity might not provide valid con-
clusions on the time-dependent structural breakdown. Therefore, the
percentage of the relative hysteresis area was calculated as the ratio
of the 'absolute' hysteresis area to the area under the ascending shear
curve and used for thixotropic characterisation (Tarrega
et al
., 2004).
Both the 'absolute' and relative hysteresis areas were greater at lower
temperatures, suggesting a more significant destructive effect of shear-
ing on the structure; while the temperature-induced differences in the
relative hysteresis areas were not as significant as those in the 'absolute'
hysteresis areas (Tarrega
et al
., 2004).
Flow behaviour models have been used to fit the flow curves of hys-
teresis loop. The upward shear-rate flow behaviour of yoghurt samples
could be described by the Herschel-Bulkley model, while the down-
ward shear-rate curves were essentially linear (Ramaswamy and Basak,
1991). Similarly, the power law model has been used to fit the rhe-
ological data of stirred yoghurt during increasing shear rate (Keogh
and O'Kennedy, 1998). Analysis of the parameters in these models for
semi-solids is the same as for fluids.
The viscosity of semi-solid dairy products increases with the total
solid concentration (Sodini
et al
., 2004). In this case, the contributors to
texture are not only fat globules and proteins, as in concentrated milks,
but also other ingredients such as polysaccharides, which are added into
the systems and modify the texture (Sodini
et al
., 2004). Ramaswamy
and Basak (1991) described the relationship between apparent viscosity
and temperature for both the up and down curves in three consecutive
shear rate cycles for a stirred yoghurt using the Arrhenius equation
(Equation 7.13). The activation energy
E
a
(kcal/mole) for the up curve
in the first cycle gradually decreased as shear rate increased, suggesting
a less significant temperature effect on the flow behaviour; while the
E
a
values for the up curves in the second and third cycles and the three
down curves varied only slightly with shear rate.
7
.
4
.
1
.
3
Shear stress decay
In a shear stress decay test, a constant shear rate is applied to the semi-
solid samples over a period of time and the shear stress is measured.
The stress (
, Pa) decay behaviour of semi-solid dairy products can be
expressed as a function of time (
t
, second), such as in the Weltmann
(1943) model (Equation 7.15) or the modified one (Equation 7.16):
σ
Weltmann model
σ
=
A
−
B
[lnt]
(7.15)
B
ln
t
t
m
σ
=
−
Modified Weltmann model
A
(7.16)
