Chemistry Reference
In-Depth Information
shear rate range. Rotational viscometers can be used to obtain the flow
behaviour of milk products at a controlled shear rate.
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Rotational viscometers
Rotational viscometers are useful to evaluate the rheological properties
of liquids, as they allow measurements over a range of shear rates. The
basic theory of rotational instruments is covered by many reviews (e.g.
Steffe, 1996; Malkin and Isayev, 2006). The rheological parameters -
Newtonian viscosity, consistency coefficient, flow behaviour index and
yield stress - can be calculated from the curve of shear stress vs. shear
rate and used for further analysis. Often a pre-shear step, usually at
a shear rate of
∼
100 per second, is conducted before measurement
to equilibrate the sample. Shear rate ranged from 0.132 to 13.2 per
second (Chang and Hartel, 1997) to 0 to 2700 per second (Solanki and
Rizvi, 2001) in studies on milk rheology. An advantage of rotational
viscometers is that flow behaviour of a fluid can be characterised at a
selected range of shear rates to coincide with a particular application.
Shear rates of oral processing and typical industrial operations of dairy
products are summarised by Steffe (1996).
Many rheometers are capable of operating in oscillatory mode as
well as rotational. Velez-Ruiz
et al
. (1998) determined the viscoelastic
properties of concentrated milks using a frequency sweep from 1 to
100 Hz at a strain of 0.2. They found that the effect of solid content
on the elastic modulus is more apparent at the lower frequency range
(1-10 Hz). The elastic moduli of all samples increased and became
similar at a higher frequency range (10-100 Hz).
7.2.3
Factors influencing milk rheological properties
The flow properties (bulk properties) of milk - a colloidal sol/emulsion -
are related to its microstructure, which is dependent on a number of fac-
tors: colloidal (i.e. particle-particle interactions), Brownian (the random
movement of particles), hydrodynamic (i.e. particle to fluid interac-
tions) and more complex interactions (Kyazze and Starov, 2004). The
major factors influencing milk rheological properties are its compo-
sition, especially the volume fraction of colloidal particles, and the
measurement or processing conditions, such as temperature and shear
rate.
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Milk composition
The bulk viscosity of milk is associated with the volume fraction of dis-
persed particles - fat globules, casein micelles, whey protein aggregates
