Chemistry Reference
In-Depth Information
proposed that the individual protein strands (controlling the fracture in
parallel direction) were much stronger than interface bonds between
constituents (responsible for bond breaking when pulled in perpendic-
ular direction). Reduced fat mozzarella had higher fracture stress and
strain compared to low moisture, part-skim mozzarella. Other important
testing parameters include deformation rate and testing temperature. In-
creasing deformation rate increases fracture strain but decreases fracture
stress and deformability modulus (Ak
et al
., 1993). Raising the testing
temperature from 10 to 40
◦
C increases fracture strain but decreases
fracture stress and deformability modulus.
Tensile testing has been used to study the effects of aging on rheo-
logical properties of cheeses. During the first 2 weeks of refrigerated
storage, mozzarella showed an increase in fracture strain due to fusion
of the curd strands, which enhanced cohesiveness and strain (Ak
et al
.,
1993). At 28 days, fracture strain remained unchanged but fracture stress
significantly decreased, corresponding to an increase in proteolysis of
casein. The effectiveness of different large strain tests in determining the
cohesion of Gruyere-type cheese was compared (Pesenti and Luginbuhl,
1999). Cheeses with either strong or weak cohesion were prepared by
two different processes. Rheological tests performed on the cheeses in-
cluded uniaxial compression, uniaxial tension, 3-point bending, cutting,
stress relaxation and creep tests. Fracture parameters are especially suit-
able to quantify cohesion, since they reflect the macroscopic effect of
breaking bonds. Tensile testing proved to be the most powerful test, al-
lowing rapid differentiation of the cohesion of the two cheeses as shown
by the values of all parameters (fracture stress, fracture strain, work to
fracture and modulus of deformability).
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3
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2
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3
Torsion test
In torsion testing, the material is twisted about its longitudinal axis to
fracture. The specimen is formed to a capstan shape so that the fracture
takes place at the narrow centre of the specimen. The disk-shaped ends
allow the specimen to be gripped by the testing apparatus or glued
to plastic disks for attachment to the testing devices. During sample
twisting, torque and degree of rotation required to break the sample are
monitored and stress and strain at fracture can be calculated. Torsion
produces a condition of pure shear on the material, generating equal
distribution of stress and shear through the sample. The specimen shape
is maintained during the test, avoiding geometric considerations. The
material can fracture in shear, tension, compression or a combination
mode. The test is ideal for the materials that may exude water or fat under
applied stress. The disadvantages of torsion are the sample preparation
and that it is not suitable for soft materials.
