Chemistry Reference
In-Depth Information
7.2
FLUID MILK
Milk is a colloidal suspension in which the solid components dissolve
or disperse in a continuous water phase. From a rheological perspec-
tive, milk can be categorised as a typical fluid because it deforms and
flows immediately when subjected to a stress. Knowledge of the flow be-
haviour of milk is important to the design and operation of dairy process-
ing equipments involved in mixing, storage and pumping. Rheological
properties of milk closely correlate with sensory perception of texture.
7.2.1
Rheological properties of milk
The rheological characterisation of a fluid can be described using the
relationship between shear stress (
σ
γ
) in a steady
laminar flow. A Newtonian fluid displays a linear relationship between
shear stress and shear rate, independently of time, and has no yield stress.
Because the major component of milk is water, normal milk shows
similar rheological properties as water - a Newtonian flow behaviour
(Equation 7.1):
) and shear rate ( ˙
Newtonian model
σ
=
µ
γ
˙
(7.1)
where
is the Newtonian viscosity, which is a measurement of the
resistance of fluid to flow due to the 'internal friction'. Newtonian vis-
cosity can be affected by temperature and composition of the fluid. The
Newtonian viscosity of water is 1.0 mPa
µ
sat20
◦
C, while representative
values for milk and milk fractions at 20
◦
Care2.0mPa
·
·
s (whole milk),
1.5 mPa
s (cheese whey) (Sherbon,
1999). From these values, it is evident that the casein micelles and the fat
globules in milk can contribute to its viscosity. The Newtonian viscosi-
ties of milks and creams of different fat contents increase with decreas-
ing temperature (Bakshi and Smith, 1984). van Vliet and Walstra (1980)
indicated that milk fat globules can undergo a crystallisation transition
and cold agglutination at temperatures below 40
◦
C, corresponding to a
deviation from the Newtonian behaviour of milk. When milk is concen-
trated, the particle-to-particle interactions can contribute significantly to
viscosity. A transition from Newtonian to non-Newtonian flow occurs
when the solid concentration of milk reaches a certain level (Walstra and
Jenness, 1984; Prentice, 1992), which is attributed to a decrease of free
volume of the solid particles with removal of water. Power law model
(Equation 7.2) is frequently used to describe a non-Newtonian fluid
when the flow behaviour is time-independent and shows no yield stress:
·
s (non-fat milk), and 1.2 mPa
·
n
Power law model
σ
=
K
˙
γ
(7.2)
