Chemistry Reference
In-Depth Information
7.3.2
Large strain rheological analysis
When deformed to a large extent, cheeses fracture: the phenomenon
that resembles what happens during biting and mastication of foods.
Large-strain and fracture methods are commonly used when the desire
is to correlate with sensory properties. By deforming a cheese sample
to the point of abrupt mechanical yield, fracture properties such as frac-
ture stress, fracture strain, and fracture modulus can be determined. The
measurements are performed in the non-linear regime and deformation
of the sample is permanent. One of the challenges of any rheological
testing, that is emphasised in large-strain tests, is that they are based
on the assumption that samples are homogeneous, isotropic and incom-
pressible, which may not strictly be the case with cheese. These factors
should be considered when evaluating results.
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3
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2
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1
Uniaxial compression
Uniaxial compression is the most popular test for determining the rheo-
logical properties of foods at fracture due to the simple sample prepara-
tion and execution. In a typical compression test, a specimen of known
shape (typically cylindrical shape) and size is placed between two par-
allel rigid plates of a Universal Testing Machine, and often the upper
plate is moved downward at a constant crosshead speed while the force
is recorded as a function of time. The responding force-time data can
be converted into stress and strain values. When the test is executed
correctly (i.e. cylindrical sample with flat and parallel ends and no sig-
nificant contribution of plate-cheese friction), the Young modulus (slope
from stress and strain plot) may be calculated from the initial slope. If
the goal is to only compare various cheeses, the maximum force can be
used. It must be realised, however, that only results expressed as true
stresses and true strains can be compared with those of other tests (van
Vliet, 1991). Attention must be given to assure that the test is performed
under conditions where there is no friction so that the deformation is
homogeneous and the sample retains its cylindrical shape. Friction be-
tween the sample and the loading plate will lead to an inhomogeneous
stress - strain state in the sample, which can be seen as bulging or a
barrel shape of the specimen during compression. The resulting strain
calculation becomes inaccurate or invalid (Gunasekaran and Ak, 2003)
and the sample appears stiffer (Charalambides
et al
., 2001). The effect
of friction can be reduced by lubricating the sample-plate interfaces or
increasing the sample height (Charalambides
et al
., 2001). Since cheese
is viscoelastic and therefore time-sensitive, another test parameter to
consider is strain rate or crosshead speed. Shama and Sherman (1973)
found that firmness of Gouda cheese depended on the compression speed
