Chemistry Reference
In-Depth Information
tests is to understand how the structure and interactions of the cheese
network determine sensory texture, it becomes more complex because
a solid requires structural breakdown and lubrication to prepare for
swallowing. In order to use rheological tests to characterise the physi-
cal properties of cheese, the tests must measure the physical properties
associated with a human psychophysical interpretation of the tactile
and visual perceptions that occur when consuming cheese. Descriptive
sensory analysis is the tool for qualitatively and quantitatively differen-
tiating cheeses and for defining the relationship between sensory and
instrumental perception of cheese texture. Cheese texture terms have
been developed to define texture attributes of several different types of
cheeses (Drake
et al
., 1999; Lawlor and Delahunty, 2000; Gwartney
et al
., 2002; Adhikari
et al
., 2003). By using these attributes, the effects
of cheese making parameters as well as the differentiation between
cheese types can be determined.
7.3.1
Small amplitude oscillatory tests
The viscoelastic properties of cheese can be determined under small am-
plitude oscillatory shear conditions by applying small sinusoidal stresses
(force per unit area) or strains (deformation per unit length) at levels
that do not cause significant irreversible changes in the cheese, and
the responding strain or stress is measured. Small amplitude oscillatory
tests are implemented in the linear viscoelastic (LVE) region and are
suitable for probing material structure and structure development dur-
ing different processes. The measurements allow the determination of
(i) storage modulus (or elastic modulus,
G
) and (ii) loss modulus (or vis-
cous modulus,
G
) as a function of test frequency (
)intheLVEregion
of the test material (Gunasekaran and Ak, 2003). Another important
parameter is phase angle (
ω
) or the phase shift between the deformation
and response. Phase angle is often expressed as loss tangent (tan
δ
δ
=
the ratio of energy lost to energy stored or
G
/
G
), which describes the
relative degree of viscoelasticity. Phase angles of 0
◦
and 90
◦
indicate
purely elastic and viscous materials, respectively. A decrease in phase
angle indicates that the material is reacting to an external stress in a
relatively more elastic manner. When
G
=
G
(
45
◦
and tan
δ
=
1), the modulus value at this point is called the 'crossover modulus'
and the state of the material is crossing over from predominately solid
to liquid or vice versa (Gunasekaran and Ak, 2003). Different types of
small strain oscillatory tests can be set up by changing strain (or stress),
frequency, temperature, and time. Common tests are strain (or stress)
sweep, frequency sweep, temperature sweep and time sweep.
Frequency sweep is used to investigate time-dependent shear be-
haviour of cheese, since frequency is the inverse value of time. High
δ
=
