Environmental Engineering Reference
In-Depth Information
4.4
Dielectric Spectroscopy of Edible Liquids: The Vegetable
Oils Case-Study
As previously discussed, the in situ evaluation of the Cole-Cole parameters is not
an easy task, especially when reference data are missing. This is the case, for exam-
ple, with vegetable oils, for whose dielectric characteristics only scarce reference
data are available [19, 23]. In fact, the complexity of the intrinsic characteristics of
vegetable oils, such as density and viscosity, makes the investigation of this kind
of material rather challenging, nonetheless interesting. Additionally, since quality
control of vegetable oils is becoming more stringent (especially for avoiding adul-
teration), the estimation of these dielectric parameters may be used as a reliable
indicator for certifying the product quality.
It is worth pointing out that although highly sophisticated methods for the anal-
ysis of edible oils are available, many of them are not easily applicable for routine
or continuous monitoring (e.g., gas chromatography [20] and liquid chromatogra-
phy [9]), whereas others (e.g., Fourier transform infrared (FTIR) [25], Raman spec-
troscopy [14], and nuclear magnetic resonance (NMR) [24]) are rather complex and
highly expensive. On the other hand, other simpler methodologies that are com-
monly adopted for routine analysis (such as those based on chemical titrations),
do not provide much information. Additionally, these methods of analysis require
skilled operators, and, most importantly, they cannot provide any information about
the possible presence of adulterants. As a matter of fact, all of aforementioned meth-
ods are affected by some limitations: in particular, they cannot be performed on the
process line and they are very laborious.
On such bases, not only does the presented case study aim at providing a prelim-
inary assessment of the use of microwave dielectric spectroscopy for the characteri-
zation of vegetable oils, but it also investigates the possibility of adopting a suitable
measurement procedure for a rapid and reliable evaluation of dielectric parameters.
This is done in view of possible practical applications, which may be useful, for
example, in the on-line monitoring of the characteristics of oils throughout a pro-
duction process.
The approach proposed for the accurate evaluation of the Cole-Cole parameters
is based on a procedure similar to the one described in the previous section, and it
comprises three major steps:
First, the TDR waveforms are acquired and processed in order to obtain the cor-
responding reflection scattering parameter, S 11 (
f
)
;
Secondly, the measurement cell (i.e., probe and filling liquid) is accurately mod-
eled as a TL, in which the dielectric characteristics of the considered liquid are
parameterized through the Cole-Cole formula;
Finally, the Cole-Cole parameters of the LUT are evaluated by minimizing
the deviations between 1) the measured reflection scattering parameter, S 11 (
f
)
,
 
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