Chemistry Reference
In-Depth Information
10
0
10
0
10
5
Dilute solution
Gel
10
2
10
4
10
-1
10
1
Concentrated
solution
10
3
10
-2
10
0
10
2
10
-1
10
1
10
-3
10
-1
10
0
10
1
10
2
10
-2
10
-1
10
0
10
1
10
2
10
-2
10
-1
10
0
10
1
10
2
ω
(rad per second)
ω
(rad per second)
ω
(rad per second)
(a)
(b)
(c)
Fig. 7.2
Typical dynamic frequency sweep data for a dilute solution (a), a concentrated
solution (b) and a gel (c).
, G
◦
, G
. Reproduced from Steffe (1996).
•
55
◦
at pH 5.4 to
phase angle of milk concentrates decreased from
∼
10
◦
at pH 4.8, corresponding to an increase of the elastic modulus
(Karlsson
et al
., 2005). Since the pI value of casein is
∼
pH 4.6, the
interactions among caseins become more intensive as the net charge
decreases with pH decreasing from 5.4 to 4.8. The pH-induced gelation
of milk concentrates leads to the formation of a fine network which
exhibits both elastic and viscous properties. High pressure treatment on
milk concentrates also showed an enhancing effect on elastic modulus,
suggesting the formation of a more solid-like structure (Velez-Ruiz
et al
., 1998).
Frequency dependence of rheological properties are used to generate
mechanical spectra that provide characteristic patterns for dilute solu-
tions, concentrated solutions and gels (Fig. 7.2) (Steffe, 1996). With
dilute solutions,
G
is larger than
G
over the entire frequency range but
approach each other at higher frequencies (Fig. 7.2a). For a concentrated
solution,
G
is larger than
G
in the lower frequency range, showing more
liquid-like properties, and the
G
becomes lower than
G
in the higher
frequency range, exhibiting more solid-like properties (Fig. 7.2b).
G
is always greater than
G
throughout the whole frequency range for a
gel (Fig. 7.2c). Concentrated solution behaviour was observed for milk
beverages with the addition of Salep glycomannan or locust bean gum;
while gel-like behaviour was found for milk beverages with addition of
guar gum (Ya¸ar
et al
., 2009).
A complex viscosity (
∼
η
*
) can be calculated using complex modulus
(
G
*
) and frequency (
) (Equation 7.8). Cox-Merz rule states that the
complex viscosity is nearly equal to the steady shear viscosity when the
angular velocity is equal to the shear rate (Equation 7.9) (Steffe, 1996):
ω
G
∗
ω
η
∗
=
Complex viscosity
(7.8)
η
∗
=
η
a
|
γ
=
ω
Cox-Merz rule
(7.9)
