Biomedical Engineering Reference
In-Depth Information
whose viscosity is an important quality parameter, the present method would be of great
applicability because the viscosity of the liquid could be assessed quickly without the
necessity of opening the can (i.e. non-destructive method). One example of such application
is testing cans of tomato products, which are widely produced and consumed in most part
of the world and the viscosity of these products is a key parameter associated to their
quality in terms of sensory evaluation and processing applications. Viscosity of tomato
products is evaluated using a number of rheological techniques that range from empirical to
more fundamental methods. One standard method is the use of the Brookfield
TR
viscometer.
This viscometer is based on the rotation of a particular element (spindle) inside a can
containing the product. The viscosity of the product is obtained basically from the resistance
offered by the product to the rotation of the spindle provided shear rates and shear stresses
can be accurately calculated. Since the geometries of the spindles often are not regular it is
difficult to estimate the rheological parameters, i.e. shear stress and shear rate, which enable
the calculation of the liquid viscosity. Tomato paste is a very viscous product that exhibits
an important structure that can be destroyed during testing. One approach to overcome this
problem is the use of a helical-path spindle, which through a helical movement
continuously is touching a fresh sample. Given the complicated rheology, results of this test
are considered empirical. The other approach is to use technically advanced rheological
equipment, which though the use of geometries such as parallel plates the sample can be
minimally disturbed. For these cases the true viscosity of these tomato purees can be
obtained. These materials are known as non-Newtonian to indicate that their viscosities are
a function of the shear rate. Most of the non-Newtonian liquids can be described by a
rheological model known as the power-law model which can be expressed by the following
equation:
n
1
(10)
where
is the apparent viscosity of the material, which is a function of the applied shear
rate
and
k
and
n
are rheological parameter known as the consistency index and the flow
behavior , respectively. The value of
k
is an indication of the product viscosity whereas
n
gives a relation between the dependence of the viscosity with the applied shear rates. For
tomato products
n
<1, and the smaller is the value of
n
the largest is the effect of the shear
rate on the viscosity of the liquid. To test the feasibility of vibration methods, cans of tomato
puree and dilutions ranging from 23% to 3.5% solids were tested in an apparatus similar to
the one shown in Figure 1. Results of the tests can be observed in Figure 6, where frequency
spectra resulting from some the tests are shown. Figure 7 shows possible correlations
between the quality
Q
measured from the frequency spectra data and the parameter
k
determined using a standard rheological technique and a parallel plate geometry.
As indicated in Figure 6 the frequency spectra peaks shift to lower frequencies when the
solid content of the concentrates increases. In the range of frequency tested the two peaks
are visible for concentrates with low solid content whereas the second peak at higher
frequencies disappears for concentrates with higher higher solid contents (23 Brix). The
amplitude of the Absolute value of the mobility is considerably decreased when the solid
content and the viscosity of the concentrate increase (Figure 6). Given the existence of two
peaks quality values can be extracted from the two peaks and relate them with the
measured rheological properties, in this case the value of the consistency index (Figure 7). It
