Dairy Systems - Practical Food Rheology - page 147

Pump selection requires an evaluation of the power requirements to
move fluid through a pipeline, which is calculated using the overall
mechanical energy balance equation (the modified Bernoulli equation).
The energy changes in a pipeline include (i) the pressure energy loss,
(ii) the kinetic energy loss, (iii) the potential energy loss and (iv) the
friction energy losses (Steffe and Daubert, 2006). The friction energy
loss in a straight pipe can be determined using a Fanning friction factor,
in which the viscosity of fluid is required. Bakshi and Smith (1984)
developed a regression equation to estimate the Newtonian viscosity
of milk with varying fat content (0.1-30%) and temperature (0-30 ◦ C),
which can be used to calculate the friction energy losses in a pipeline
and the power requirement during pumping.
The viscosity of fluids can also influence the droplet size and thereby
the powder particle size during spray drying. Keogh et al . (2003) used
ultrafiltrated milk samples from different sources to standardise their
fat contents prior to evaporation and drying. Concentrated milk samples
containing a variety of total solid contents were obtained after ultra-
filtration, corresponding to dissimilar apparent viscosities. A positive
linear relationship was established between the powder particle size and
the apparent viscosity (Keogh et al ., 2003).

7.3 SOLID CHEESE

Cheese rheology is a function of its composition, microstructure, the
physical chemical properties of its components, and the macrostructure
(the presence of heterogeneities such as cracks and fissures) (Fox et al .,
2000). Factors that influenced the physical properties of cheese include
initial cheese-milk composition, processing procedures, and the extent
of proteolysis during ripening. These are influenced by environmental
conditions such as pH, temperature, and ionic strength. In the first
ripening phase (7-14 days), the degradation of casein structure over
time due to proteases is the primary contributor to textural changes.
This proteolysis leads to the reduction in firmness and an increase in
deformability as cheese ages (Tunick et al ., 1990). More gradual changes
occur in the second phase where the rest of α s1 casein and other caseins
are broken down. Understanding the aggregation of caseins is vital in
understanding the physical and chemical properties of cheese (Lucey
et al ., 2003). The number, strength, and types of bonds among casein
molecules as well as the spatial arrangements of these bonds constitute
the basis of cheese rheology.
As with fluid milk, rheological measurements are used in a variety of
applications such as evaluating structure development and understand-
ing the physical basis for sensory texture. When the goal of rheological