Chemistry Reference
In-Depth Information
Fig. 7.7
The typical hysteresis loops in the flow curves (shear stress vs. shear rate) of
a stirred yoghurt during three consecutive shear cycles. Adapted from Ramaswamy and
Basak (1991).
7
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4
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1
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2
Hysteresis loop
The time-dependency of the flow behaviours of semi-solid dairy prod-
ucts can be tested under a shear rate cycle. A hysteresis in the flow
curve (shear stress vs. shear rate) suggests that shearing at high-shear
rate modifies the material structure (Doublier and Durand, 2008). The
area of the hysteresis loop corresponds to the extent of structural break-
down during the shearing cycle (Halmos and Tiu, 1981). Ramaswamy
and Basak (1991) measured the flow behaviour of stirred yoghurts under
three consecutive shear rate cycles (Fig. 7.7).
The hysteresis loop area associated with the first cycle is larger than
those associated with the following cycles, suggesting that the first
shearing cycle caused the major degradation of sample structure (Fig.
7.7). The hysteresis loop areas associated with the second and third
shearing cycles are comparable to each other, indicating similar structure
changes. The shear stress of the second or third shearing cycle is lower
than its preceding cycle, indicating a continual breakdown of structure.
The overlapping of the ascending shear curve of the second or third
cycle with its preceding descending shear curves suggested that the
structural loss in the ascending shearing process might be partially
recovering during the descending shearing process (Ramaswamy and
Basak, 1991). To determine the recovery of the thixotropic structural
breakdown, Ramaswamy and Basak (1991) rested the yoghurt samples
for an hour after three consecutive shearing cycles and repeated the test.
They found that the shear stress of yoghurt in the repeating test cannot
reach the same level as in the first test and suggested that the thixotropic
structural breakdown during shearing was irreversible within one hour.
Tarrega
et al
. (2004) compared the hysteresis area for seven samples
of a semi-solid Spanish dairy dessert and observed a larger 'absolute'
hysteresis area (the area within the hysteresis cycle) for a more viscous
