Chemistry Reference
In-Depth Information
Fig. 7.8
The typical hysteresis loops in the flow curves (shear stress vs. shear rate) of a
stirred yoghurt (a) and a cream cheese (b). The displacement profile (dotted line) of strain
(deformation) vs. applied stress for a cream cheese sample (c). The shear stress as a function
of time measured using the vane geometry at a certain rotation rate for a custard model
(d). The assigned yield stresses are marked in figures. Adapted from Ramaswamy and Ak
(1991), Sanchez et al. (1994), Kealy (2006) and Doublier and Durand (2008).
sample to flow a short distance because of a large zero-shear viscosity
(
s) or a high resistance to deformation. Therefore, the
applied stress on the semi-solid samples needs to reach a certain level -
a 'yield stress' - to make the sample flow with a measurable viscosity.
Yield stress is an important parameter for the sensory quality and the
engineering calculations of semi-solid products. Kealy (2006) indicated
that cream cheeses with a larger yield stress corresponded to higher
sensory panel scores for hardness and adhesiveness. Breidinger and
Steffe (2001) established a texture map of yield stress vs. yield strain
for various cream cheese samples. This approach showed that a decrease
in yield stress corresponded to an increase in yield strain. Fat-free cream
cheese showed the highest yield stress (the lowest yield strain), while
whipped cream cheese exhibited the lowest yield stress (the highest
yield strain), corresponding to differences in spreadability (Breidinger
and Steffe, 2001). In the same study, it was suggested that a texture
map could be used for determining acceptable levels of spreadability
in quality control and product development applications. Yield stress is
involved in engineering calculation for semi-solid products, which has
been covered by several engineering topics and reviews (Steffe, 1996;
Steffe and Daubert, 2006).
>
10 000 Pa
·
7.4.3
Viscoelastic properties of semi-solid dairy products
Dynamic oscillatory testing can provide very useful information on the
gel formation process of semi-solid dairy products such as yoghurt. The
effects of processing variables, especially preheating of milks, on the for-
mation and properties of gels prepared by acidification with glucono-
δ
-lactone (GDL) have been extensively studied (van Vliet and Keetels,
1995; Lucey
et al
., 1997a, 1997b, 1999). Preheating milk resulted in a
