Chemistry Reference
In-Depth Information
because the fat in the former, while contributing to the overall volume
fraction, is inert with respect to the age thickening process.
Snoeren et al. (1984) proposed the following explanation of age thicken-
ing. Concentration of milk causes an increase in ionic strength and a reduction
in pH. These changes reduce the amount of calcium bound to protein, espe-
cially to
-casein. This in turn leads to increased solubility of
-casein, the
consequence of which is a loosening of the casein micelle structure, increasing
cas
and thus
p
. Snoeren et al. (1984) pointed out that only a very small
increase in casein voluminosity is required to cause a substantial increase in
viscosity. The effective size of the loosened micelles is reduced by higher shear
rate, leading to shear thinning behaviour.
15.8.5.2.
Phenomenological Relationships for Describing
the Non-Newtonian Behaviour of Concentrates
As %TS increases, the rheological behaviour of freshly prepared milk
concentrates changes from Newtonian to time-independent shear thinning to
time-dependent shear thinning (Binh Trinh et al., 2007b).
Under conditions where time dependency is absent, flow behaviour
changes, as %TS increases, from Newtonian (Equation 21) to shear thinning
with no yield stress (power law behaviour; Equation 39) to shear thinning
with a yield stress (Bingham plastic behaviour; Equation 40) or Herschel-
Bulkley behaviour (Equation 41) (Randhahn, 1973, 1976; Hallstr om and
Dejmek, 1988a; Stepp and Smith, 1991; Horne, 1993); Sierzant and Smith,
1993; V´lez-Ruiz and Barbosa-C´ novas, 1997, 1998; Hinrichs, 1999; Bienvenue
et al., 2003a, b; Binh Trinh et al.,2007a,b).
In fact, because (as pointed out above) the Newtonian, power law and
Bingham equations are all special cases of the Herschel-Bulkley equation, this
last equation can be said to describe adequately the flow curves of time-
independent milk concentrates, or the instantaneous flow curves of time-
dependent concentrates, under all conditions of temperature, shear rate and
other variables. When t
0
¼
0andn
¼
1 in the Herschel-Bulkley equation, k
¼
,
the Newtonian coefficient of viscosity.
For example, V´lez-Ruiz and Barbosa-C´ novas (1997) demonstrated
that whole milk concentrates prepared by evaporation were Newtonian up to
approximately 20% TS, obeyed the power law between about 20 and 34% TS
and obeyed the Herschel-Bulkley equation
>
37% TS. Newtonian behaviour
persisted to higher %TS at higher temperature. For milk concentrates in
general, the precise %TS ranges over which Newtonian, power law and
Herschel-Bulkley behaviours are seen will depend on numerous factors
