Chemistry Reference
In-Depth Information
S
: ram speed
H
D
0
Barrel
P
0
Pressure transducer
L
D
Die
P
Fig. 2.7
Schematic representation of a capillary system.
specifies final product quality, and which is affected by processing.
The rheological behaviour of many paste materials has not been fully
characterised, due to their complexity, both in terms of bulk behaviour
and of microstructure (Barnes, 1995). For characterising soft solid foods,
both capillary and squeeze flow rheometers are useful. Nonetheless,
these rheometers are less commonly used in food research and as such
they are only briefly described here.
2.7.1
Capillary rheometer
Capillary rheometers can perform both flow and elasticity experiments
(such as stress relaxation) by using a ram to force material through a
die of known dimensions and measuring the pressure drop across the
die by a pressure transducer. Fig. 2.7 schematically shows a capillary
rheometer, where the material being measured is contained in a barrel
with diameter
D
0
and is forced through a die of diameter
D
and length
L
by using a ram, moving with a linear speed of
S
.
As the material enters from the barrel to the die, the cross-section
decreases and as a result the sample extends and elongates along
the flow direction. The apparent wall shear rate ( ˙
γ
w
)andthewall
