Chemistry Reference
In-Depth Information
Changes in viscosity of skim milk after heat treatment can be at-
tributed to denaturation of whey proteins. The viscosity of skim milk
does not change much after heat treatment at 60
◦
C, which is lower than
the denaturation temperature of whey protein (70
◦
C), but increases sig-
nificantly after heat treatment at 90
◦
C (Jeurnink and de Kruif, 1993). The
effect of heat treatment on skim milk viscosity becomes less significant
after removal of whey proteins, suggesting the enhancing effects are at-
tributed to heat-denatured whey proteins, especially β-lactoglobulin ag-
gregates (Jeurnink and de Kruif, 1993). When milk is heated, denatured
whey proteins can form whey protein aggregates as well as associate
with casein micelles. Interactions between whey proteins and casein
produce two different changes in casein micelles: (i) The micelle grows
in size, and (ii) the interaction between micelles is altered (Jeurnink and
de Kruif, 1993). Anema
et al
. (2004) indicated that viscosity changes in
reconstituted skim milk after heat treatment were related to a change in
volume fraction of the casein micelles, which was correlated with the
amount of denatured whey protein associated with casein micelles.
Karlsson
et al
. (2005) indicated that the voluminosity of casein mi-
celles was affected by addition of salts and changes in pH, corresponding
to variations in the apparent viscosity of skim milk concentrates. Addi-
tion of NaCl increased the voluminosity of casein micelles due to the
exchange of Ca
2
+
bound to casein micelles with Na
+
altering micelle
structure. The reduction in the voluminosity of casein micelles upon
lowering its pH from 6.5 to 6.0 (Solanki and Rizvi, 2001) corresponds
to a less significant enhancing effect of the solid content to the apparent
viscosity for microfiltrated skim milk retentates. Griffin
et al
. (1989)
found that addition of ethanol at concentrations below that required for
coagulation caused a reduction in the voluminosity of casein micelles,
corresponding to a reduction in the hydrodynamic radius of micelles
and a decrease of the apparent viscosity of concentrated skimmed milk.
The effect of storage on milk viscosity is attributed to the loosen-
ing of casein micelles, and thereby an increase of the voluminosity
(Snoeren
et al
., 1984). A higher storage temperature corresponds to
a faster loosening rate of casein micelles and a faster increase of the
apparent viscosity with storage time (Snoeren
et al
., 1984).
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.
2
.
3
.
2
Mathematical models
Various models, including polynomial, logarithmic, exponential and
power law, have been used to fit the relationship between viscosity and
solid concentrations for milks (Fernandez-Martın, 1972; Buckingham,
1978; Bloore and Boag, 1981; Langley and Temple, 1985). These mod-
els can provide empirical predictions but rarely are explained based on
milk microstructure and how it influences viscosity. The volume fraction
