Chemistry Reference
In-Depth Information
Fig. 7.1
The Herschel-Bulkley yield stresses, extrapolated from flow curves, for concen-
trated skim milks (48.6% total solids) as a function of storage time. The flow behaviour was
measured at the temperatures indicated in the figure. The concentrated skim milks were
stored at 2-4
◦
C. Data are from Velez-Ruiz and Barbosa-C anovas (1998).
where
σ
0
is the yield stress value at ˙
γ
=
0 per second and
η
pl
is the plastic
viscosity. The transition from low shear stress (
σ<σ
0
)tohighshear
stress (
σ>σ
0
) corresponds to the transition from solid-like to liquid-like
behaviour. Bienvenue
et al
. (2003) used the Bingham model to describe
the rheological properties of concentrated skim milk with a total solids
content of 45%. The yield stress for freshly concentrated milk was close
to zero but increased to
44 Pa after being stored at 50
◦
C for 11 hours.
The storage-induced increase of yield stress suggests flocculation due to
weak attraction among casein micelles and rearrangement of the three-
dimensional structure during storage. Velez-Ruiz and Barbosa-Canovas
(1998) observed yield stresses for concentrated milks with
∼
42.4% solid
contents at 5-25
◦
C after storing them for 1-4 weeks at 2-4
◦
Candthe
flow behaviour conformed to Herschel-Bulkley model (Fig. 7.1).
Chang and Hartel (1997) found that the Herschel-Bulkley model
better described the flow behaviour of freshly freeze-concentrated skim
milk (25-40% total solids) than power law model, suggesting the need
to account for yield stress. However, they also indicated that the pre-
dicted yield stresses were not significantly different from zero. This is
not surprising since the yield stresses for concentrated milks are close to
zero for the fresh samples and increase to more significant values dur-
ing storage (Velez-Ruiz and Barbosa-Canovas, 1998; Bienvenue
et al
.,
2003). Yield stress is an important parameter for engineering calcula-
tions, product quality and sensory assessment (Steffe, 1996) and a major
characteristic for the semi-solid dairy products.
The apparent viscosity of fluids cannot be predicted as a finite value
by using power law model for a shear-thinning fluid when shear rate (or
shear stress) is close to zero (Equation 7.3). There are two characteris-
tic types of flow behaviours of a shear-thinning material at low shear
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