Chemistry Reference
In-Depth Information
Similar to uniaxial compression and tension, strain rate is an im-
portant test parameter in torsion testing. Fracture modulus (the ratio
between true shear stress and true shear strain) increases as strain rate
increases, indicating the viscoelastic, time-dependent nature of cheeses
(Brown
et al
., 2003). Comparing full-fat and low-fat commercial cheeses
(Monterey Jack, mild Cheddar, sharp Cheddar, and American cheeses),
Gwartney
et al
. (2002) found that fracture stress is correlated with sen-
sory firmness and springiness (first bite terms). Low-fat cheeses have
higher fracture stress and thus increased sensory firmness, springiness
and chewiness. Torsion testing has been used to study the effect of
aging on rheological properties of Monterey Jack cheeses from goat
milk (Van Hekken
et al
., 2004). Fracture stress and fracture modulus
decreased during the first 8 weeks of storage, while fracture strain sig-
nificantly increased over the first 4 weeks. A decrease in fracture stress
correlated with a decrease in native casein and an increase in peptides.
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Vane method
The vane method is an effective means to measure the yield stress using
the rotational viscometer without slipping effects (Barnes and Nguyen,
2001). The vane geometry is similar to the concentric cylinder system,
except that the inner cylinder is a spindle with 4-8 thin blades. The
vane is immersed in a sample and rotated slowly at a constant rate,
while the torque exerted on the vane blades is measured as a function of
time. Stress and strain at fracture can be calculated and used to describe
textural properties of foods. Key assumptions in vane test are that the
shearing stress is uniform over the cylindrical surface, the material
trapped between the blades is rotating as a rigid body and there are no
secondary flows between the blades (Barnes and Nguyen, 2001). These
assumptions are valid if the vane has at least four blades and rotates
at low speed. The advantages of the vane method are the following:
sample preparation is simple, weak samples can be tested, and the wall-
slip problem is avoided. Using vane method in testing cheddar cheese,
Truong
et al
. (2002) found good correlation between vane stress and
apparent strain with sensory firmness and cohesiveness, respectively.
When compared with torsion method in characterisation of cheeses
(process, cheddar and mozzarella), Truong and Daubert (2001) showed
similar texture maps of cheeses were generated by plotting stress and
strain or angular deformation values from the two testing methods.
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Cutting with wire
Wire cutting is a popular process used in the food industry during the
manufacture and testing of products. The process comprises fracture,
