Chemistry Reference
In-Depth Information
Fig. 7.3
Activation energy as a function of % total solids concentration from different
studies. From Velez-Ruiz and Barbosa-C anovas (1998):
◦
, concentrated milk (week 1);
,
concentrated milk (week 4). From Chang and Hartel (1997):
×
, freeze-concentrated skim
milk. From Solanki and Rizvi (2001):
•
, microfiltrated concentrated skim milk (pH 6.0);
,
microfiltrated concentrated skim milk (pH 6.5).
temperature dependent) can be modeled with the Arrhenius equation:
A
0
exp
E
a
RT
Arrhenius equation
η
·
η
a
·
K
=
(7.13)
where
η
a
is the apparent viscosity,
K
is the
consistency coefficient,
A
0
is a constant,
E
a
is the activation energy for
flow,
R
is the universal gas constant and
T
is the absolute temperature.
Activation energy values estimated from different studies are in a similar
range and increase with the total solids concentrations (Fig. 7.3).
The activation energy increases in a faster rate in freeze-concentrated
skim milk than in the others (Fig. 7.3), possibly due to different structures
of the constituents, such as casein micelles, from various concentration
techniques. The activation energy slightly increases with the storage
time, which is interpreted as being due to changes occurring during
storage such as structural transformations in casein micelles and whey
proteins (Fig. 7.3; Velez-Ruiz and Barbosa-Canovas, 1998). The activa-
tion energy is greater for concentrated skim milks as pH increases (Fig.
7.3; Solanki and Rizvi, 2001). This, again, can be attributed to different
structures and voluminosities of the constitutes at various pHs (Solanki
and Rizvi, 2001).
η
is the Newtonian viscosity,
7.2.4
Correlating rheological properties of milk
to sensory perceptions
Viscosity as measured in rheometers reflects the resistance of a fluid
to the flow. This physical viscosity is a component of the perceived
