Chemistry Reference
In-Depth Information
(heat-treated milks) or total solids. Although dissolved carbohydrates
and salts can influence viscosity, the major factors determining milk
viscosity are the two dispersed particles - fat globules and colloidal
proteins (McCarthy and Singh, 2009).
In fluid milk, the milk fat is almost entirely in the form of globules,
ranging from 0.1 to 15 µm in diameter (Jenness, 1999). van Vliet and
Walstra (1980) indicated that the volume fraction and physical state,
e.g. crystallisation at
T
35
◦
C, of fat globules can influence the flow
behaviour of milks and creams. At higher temperatures (
T
<
40
◦
C),
the flow behaviour of milks and creams approached Newtonian flow
(Phipps, 1969). More non-Newtonian flow behaviour can be expected
as temperature decreases. This is partially explained by crystals inter-
acting with each other and/or with other milk constituents in a different
way during shearing, leading to shear-thinning behaviours (van Vliet
and Walstra, 1980). The viscosities (Newtonian or apparent) of milks
and creams increase with increasing volume fraction of fat globules,
due to more interactions among the solid constitutes (van Vliet and
Walstra, 1980). Kyazze and Starov (2004) indicated that fat globules
can form clusters with increasing volume fraction, changing the milk
microstructure. In this case, fat globules cannot be simply assumed as
discrete particles when evaluating their effects on milk viscosity; in-
stead, the cluster size distribution and the packing density of droplets
inside clusters become the major parameters (Kyazze and Starov, 2004).
Velez-Ruiz and Barbosa-Canovas (2000) observed the microstructure
of concentrated milks and found fat globules surrounded by a mem-
brane of proteins, which was thicker in concentrated milk than in fresh
milk. This is attributed to a variety of changes to proteins and fat glob-
ules under the effects of heat and water removal during concentration
steps. Note that different concentration steps - evaporation, freeze con-
centration, or microfiltration - result in variations in the compositions
of concentrated milks, corresponding to slightly dissimilar flow be-
haviours (Chang and Hartel, 1997; Velez-Ruiz and Barbosa-Canovas,
1998; Solanki and Rizvi, 2001).
The viscosity of skim milks can be described as a function of the
volume fraction occupied by macromolecular materials (proteins: ca-
sein, native whey protein, denatured whey protein) (Snoeren
et al
.,
1982). This volume fraction depends on weight concentrations and vo-
luminosities of the macromolecular materials. Increasing solid content
in skim milk corresponds to an increase of the volume fraction oc-
cupied by macromolecules, assuming unchanged voluminosities, lead-
ing to a higher apparent viscosity. Factors influencing the voluminosi-
ties of macromolecules, such as heat treatment, solvent modification
(pH adjustment, salt addition) and storage, will alter the viscosity of
milk.
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