DETERMINATION OF ALCOHOL AND SUGAR
CONTENT IN WINE BY MEANS OF MICROWAVES
V. V. MERIAKRI, E. E. CHIGRAI
Institute of Radioengineering and Electronics,
Russian Academy of Sciences
Vvedenski sq. 1, Fryazino Moscow Reg. 141190,
RUSSIA
E-mail: meriakri@ms.ire.rssi.ru URL http://home.arcor.de/frank.vongratowski/ras.htm
1. INTRODUCTION
The monitoring of the composition of
materials (including water solutions) is an important problem of applied
spectroscopy in the optical, infrared, and microwave wavelength bands.
The aim of this paper is to examine the
specific features of the application of electromagnetic microwaves (MW)
(frequencies f = 3 - 30 GHz) and relatively new millimeter (MM) waves to the
alcohol and sugar content determination in water and wines.
Presently, there is extensive literature
devoted to the interaction between MW and MM waves and different liquids and
water solutions [1 - 8]. The main conclusions in relation to these
water-containing substances are as follows:
1. The sensitivity to the content of water and
other dipole liquids in different media increases with frequency (e. g., a free
space absorption of MM waves in water a > 40 1/cm is much greater than
that in all monitored host materials; as wavelength l decreases, the absorption in water
increases more rapidly than the absorption in these host materials).
2. Unlike low-frequency waves, MW and MM waves are practically not sensitive to conducting impurities in liquids.
3. MW and
MM waves can be used for testing materials that are opaque for optical and
infrared waves.
4. MW and
MM waves allow one to realize non-destructive, real-time, in-flow measurement
of the dielectric properties of media. These properties are closely related to
the chemical composition of a substance under test. 
2. PROPERTIES OF WATER SOLUTIONS
The MW and MM wave dielectric
properties (complex permittivity e = e
+ ie
,
losses a, 1/cm) of water, alcohol, and sugar
solutions in water (including wine) are practically independent of small
mineral and conducting admixtures. Our measurements of e and a at frequencies 3, 10, and 30 GHz
have shown that the dielectric properties of alcohol (C
H
O) and sugar in water and in wine are practically the same
(the difference is less than 0.05- 0.1%).
The dependence of e of water and water solutions in the MW and MM
regions on frequency is well known [ ] and is described by the Debye–Cole-Cole
law
.
Here,
= 2
f, 
= 
(f ® 0), 
= (f ® 
), 
- relaxation time, 
and » 1. All these values are functions
of temperature t and chemical composition of a liquid. For water and
alcohol, 
equals approximately
80 and 25, respectively, 
equals approximately
5.0 and 3.3,
respectively, 
» 10
sec, and 10
sec (t = 15-25C). Due to the difference in 
and 
for
water and alcohol, these liquids have
essentially different 
(
) and losses 
(
) in the MW and MM wave range. For example, at frequencies of
f
» 10 GHz, losses in water are 
» 7 1/cm, losses in alcohol 
are » 2 1/cm; at frequencies of f
» 3 GHz, 
» 0.7 1/cm and 
» 1.5 1/cm. Such difference in
dielectric properties may be used for the determination of the alcohol content 
% vol. in water.
However,
the dielectric properties (including losses) of alcohol in
water solutions versus the alcohol content 
do not obey a simple
additive law. Therefore, we carried out the measurements of 
(
) and 
(
) at frequencies of interest. The measurements carried out in
a rectangular metal waveguide of cross section 7.2 ´ 3.4 cm have shown that, at
frequency of f
, the transparency T of alcohol solution allows us
to determine 
20% with an accuracy
of 
< 0.05 % vol. for given temperature.
As
for the sugar solutions in water, we also carried out the measurements of 
and 
and T depending
on the sugar concentration W g/litre at frequencies of f
and f
. It was found that it is possible to measure W < 300 g/litre with an accuracy of 
W/W 
0.05 %.
The
results of the measurements of 
(
,W), 
(
,W), and T(
,W) at frequencies of f
and f
in alcohol--sugar aqueous solutions
have shown that it is possible to determine the alcohol
and sugar content in water for 
20% and W
300 g/litre with practically the same accuracy as in the
alcohol--water and sugar-in-water solutions if we measure T at two
frequencies.
The
next step was the application of above-mentioned investigations to the method
of determination of both alcohol and sugar content in wine.
3. A SET-UP FOR MEASURING ALCOHOL
AND SUGAR CONTENT IN WINE
An
experimental set-up for measuring the alcohol and sugar content in wine was
designed. The set-up consists of a metal coaxial line with a TEM mode operating
at frequency f
and a measuring cell (a section of a circular metal
waveguide supporting a TM
mode with liquid under test) of length 4 cm inserted into
this coaxial line. The cell has dielectric matching windows on its both ends to
avoid reflection from the boundaries. The cell has longitudinal slots in its
walls and is inserted into a bath with liquid under test. These slots do not
affect the propagation conditions for a TM
mode and, at the same time, allow a liquid to flow through
the cell. A section of a rectangular metal waveguide of cross section 2.3 ´ 0.5 cm supporting a TE
mode at frequency f
and with open ends and a matching window is inserted
into a bath with a liquid. A second similar section is situated in the liquid
along the same axis as the first section. The distance between these sections
is of 0.45 cm.
After
the calibration of the set-up, the alcohol content 
and sugar content W
are determined from two measurements:
a) The measurement of transparency T
at frequency f
allows one to find 
independently of W;
b) The measurement of transparency T
at frequency f
allows one to determine W using the known value
of 
.
The measurements were carried out with five
kinds of wines (white and red), 
= 9 – 18 % vol., W
30 g/litre. The accuracy of determining 
and W was
about 0.1 % vol. and 0.3 g/litre, respectively.
4. CONCLUSION
The measurements of the transparency of a wine
material at two frequencies (3 and 10 GHz) in the microwave region allow us to
realize nondestructive, in-flow, and real-time method for determining the
alcohol and sugar content in these liquids.
5. ACKNOWLEDGMENT
The authors wish to
thank Prof. Guram Sh. Kevenishvili, Georgian State Polytechnical University,
Tbilisi, Georgia, for his assistance in the measurements of wines.
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