Chemistry Reference
In-Depth Information
Fig. 7.4
The relationship between the sensory scores and the relative viscosity for milks.
(a): milks with fat contents of 0.06, 0.5, 1.0, 1.5 and 2.0%. (b): milks of 0.06% fat with
addition of non-fat dry milk (NDM) powder of 0.0, 0.5, 1.0, 1.5 and 2.0%. Samples were
evaluated under red light to mask color. Data is from Phillips et al. (1995a, 1995b).
viscosity - 'thickness' - of a fluid, which can be evaluated by oral or
non-oral (vision and touch) methods (Christensen and Casper, 1987).
The perception of solution viscosity has been discussed by Christensen
(1987).
Phillips
et al
. (1995a, 1995b) found that the textural sensation (thick-
ness, mouth coating and residual mouth coating) of milk increased as
the relative viscosity was increased by addition of fat or nonfat dry milk
(NDM) powder (Fig. 7.4).
It should be remembered that drawing conclusions between mechan-
ical and sensory viscosity is complicated by a number of factors, in-
cluding testing conditions. For example, the addition of nonfat dry milk
(NDM) powder into milk containing 0.06% fat can achieve similar rel-
ative viscosity as milks containing higher fat amounts; however, the
sensory textural scores of the NDM-containing milks are still lower
(Fig. 7.4) (Phillips
et al
., 1995a, 1995b). It is likely that complex con-
ditions experienced in the mouth (various shear rates and increasing
temperature) could explain differences when milk is filled with fat as
compared to protein (i.e. NDM powder).
7.2.5
Process engineering calculation
The rheological properties of fluids are important for controlling the
flow during various manufacturing operations, such as pumping, spray
drying, mixing, homogenising, and thermal processing (Steffe, 1996;
Steffe and Daubert, 2006). Fluid viscosity is included in many equations
used for process engineering calculations. Some such equations and
examples are presented in food engineering topics (Steffe, 1996; Steffe
and Daubert, 2006). Examples of pumping milk and spray drying dairy
concentrates will be given in the following text.
