DETERMINATION OF ALCOHOL AND SUGAR CONTENT IN WINE BY MEANS OF MICROWAVES

Moisture Determination in Food Substances by Means of Millimeter Waves

V.V. Meriakri, E.E. Chigrai. M.P. Parkhomenko

3rd Workshop “Water in Food”. 29-31 March, 2004. Lausanne, Switzerland.

Institute of Radioengineering and Electronics, Russian Academy of Sciences

Vvedenski sq. 1, Fryazino Moscow Reg. 141190, RUSSIA

Abstract

Microwave and millimeter wave methods are described for measuring the content of water and other polar substances (first of all alcohol and sugar) in liquids and powders. Examples of devices for such applications are designed. These methods and devices can provide accurate measurement in real time (including on line).

Keywords: water, alcohol, sugar, content, determination, microwaves, millimeter waves,

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 microwaves (MW) (frequencies f = 3 - 30 GHz) and relatively new millimeter (MM) waves to the alcohol and sugar content determination in water, juices, and wines. These waves can also be used for measuring the content of water and some other polar admixtures in powders and solids.

Presently, there is extensive literature devoted to the interaction of MW and MM waves with various liquids, water solutions, and emulsions [1 - 8]. The main results concerning 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 practically are 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 substances under test.

2. PROPERTIES OF WATER SOLUTIONS

The frequency dependence of e = e+ ie of water and water solutions in the MW and MM frequency ranges is well known [5, 2] and is usually described by the Debye–Cole-Cole law:

. (1)

Here, the complex permittivity e = e+ ie,= 2f, = (f ® 0), = (f ® ), is relaxation time, and » 1. All these values are functions of temperature t and chemical composition of a liquid. For water and alcohol, the values of are approximately equal to 80 and 25, respectively, the values of are approximately equal to 5.0 and 3.3, respectively, and » 10sec and 10sec, respectively (t = 15-25C).

It is important that, in the MW and MM ranges, the dielectric properties and losses (a, 1/cm) of water, as well as of alcohol and sugar solutions in water, are practically independent of small mineral and conducting admixtures.

Due to the difference in , , and , water and alcohol have essentially different behavior of () and losses () in the MW and MM wave ranges.

For example, at frequency of f » 10 GHz, losses in water are » 7 1/cm, while losses in alcohol are » 2 1/cm; at frequencies of f » 3 GHz, » 0.7 1/cm and » 1.5 1/cm [7]. 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 due to dependence of , , and on . Sugar (glucose) solutions have a similar behavior as s function of sugar content w, % weight.

Therefore, we measured the dielectric properties of water solutions of alcohol and sugar at frequencies in the regions of f= 7 –10 GHz and f= 2 – 3 GHz. The measurements were carried out at frequencies near f in a rectangular metal waveguide of cross section 2.3 1.0 cm and, at frequencies near f, in a waveguide of cross section 7.2 ´ 3.4 cm.

We have measured power transmission T and reflection R coefficients of alcohol and sugar solutions in these waveguides:

T = (1 – R)e. (2)

Here, L is the thickness of the liquid layer.

From (1), we can find the losses and also using the well-known relations between , T, and R [9].

R = , (3)

= -, (4)

= - (5)

Here, c is the velocity of light and a is the size of the wide wall of the waveguide.

For the alcohol and sugar content monitoring, the most suitable parameter is the transmission coefficient T of a liquid layer.

Tables 1-2 present the functions T() = T() – T(0) for the solution of alcohol in water at frequencies of f= 8,8 GHz and f= 3,4 GHz (t = 20 C).

Table1. (L = 0.5 cm, f = 8.8 GHz )

, %	0	1.6	3.3	5.0	6.8	8.5	10.2
T, dB	0	0.62	1.37	2.00	2.64	2.98	3.35

In this case, we have the sensitivity to alcohol about 0.35- 0.41 dB/1%. To obtain higher sensitivity, one should increase L.

Table 2. (L = 5 cm, f = 3.4 GHz)

, %	0	1	2	3	4	5	6	7	8	9	10
T, dB	0	0.25	0.48	0.68	0.89	1.10	1.25	1.42	1.56	1.68	1.8

For such L, we have sensitivity to alcohol of about 0.13– 0.21 dB/1%.

As for the sugar solutions in water, the results of measurements of T depending on the sugar concentration w, g/litre at frequencies of f and f are presented in Tables 3 and 4.

Table 3 (L = 0.5 cm, f = 9.4 GHz)

w, %	0	5	10	15	20	25
T, dB	0	0.65	1.35	1.95	2.50	2.60

From Tables 1 and 3, one can see that the sensitivity to sugar in this frequency interval is much less than the sensitivity to alcohol.

Table 4 ( f = 2.3 GHz, L = 5 cm)

W, %	0	1.3	2.6	3.9	5.3	6.6	7.9	9.2	10.5
T, dB	0	0.14	0.26	0.35	0.48	0.59	0.70	0.79	0.89

Here, the sensitivity to sugar is about 0.1 dB/1%.

The results of measurements of T(, w) at frequencies of f and f in alcohol--sugar aqueous solutionshave shown that it is possible to determine the alcohol and sugar content in water using measurements carried out at two different frequencies. For 20% and W 300 g/litre, the accuracy of such measurements can be practically the same as in the alcohol--water and sugar-in-water solutions.

The next step is the application of the above-mentioned investigations to the design of a sensor for the determination of both alcohol and sugar content in wine.

3. A SENSOR FOR MEASURING ALCOHOL AND SUGAR CONTENT IN WINE

An experimental sensor for measuring the alcohol and sugar content in wine was designed. The device consists of a metal coaxial line with the TEM mode operating at frequency of f= 2,35 GHz and a 4-cm-long measuring cell (a section of a circular metal waveguide supporting the TM mode with the liquid under test) inserted into this coaxial line. The cell has dielectric matching windows at both ends to avoid reflection from the boundaries. The cell has longitudinal slots in its walls and is inserted into a bath with the liquid under test. These slots do not affect the propagation of the 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 the TE mode at frequency of f= 8.85 GHz and with open ends and a matching window is inserted into a bath with a liquid. A second similar section is placed in the liquid along the same axis as the first section. The distance L between these sections is of 0.45 cm. The complex permittivity of the matching windows is chosen so that to suppress the dependence of T on w for = 9– 18% and L = 0.45 cm.

After the calibration of the device, the alcohol content and sugar content W are determined from two measurements:

(a) The measurement of the transparency T at frequency f allows one to find independently of W;

(b) The measurement of the transparency T at frequency f allows one to determine W using the known value of .

The measurements were carried out with five sorts of wines (white and red), = 9 – 18 % vol., W 30 g/litre. The accuracy of determining and W was about 0.1 % vol. and 3 g/litre, respectively.

4. APPLICATION OF MILLIMETER WAVES TO THE DETERMINATION OF WATER CONTENT

As was pointed out in Introduction, the losses in water increase with frequency; hence, in the MM wave rangew, is very large (Table 5). The results of Table 5 are in a good agreement with [4]. Therefore, MM waves are very attractive the determination of low content of water in materials and media. Some results on the application of the MM aquametry to powders and liquids are presented in [3, 5-8].

Table 5. Dielectric properties of water

l, mm

T, C

a, dB/mm

23.5

26.7

29.8

31.9

33.1

33.7

15.5

15.3

15.1

6.3

6.5

6.8

8.8

9.7

10.5

41.1

43.7

46.1

5. CONCLUSION

The measurements of the transparency of a wine material at two frequencies (2.3 and 9 GHz) in the microwave range allow us to develop a nondestructive, in-flow, and real-time method for determining the alcohol and sugar content in these liquids. In the case of juices, it suffices to carry out the measurements at a single frequency point.

MW and MM waves can be used for measuring the water content in some other substances of interest for food industry. Our investigations of water emulsions in oil [8] have shown the possibility to measure the water content in such emulsions with accuracy of 0.05% of water. Moreover, MW and MM waves allow us to measure the water content in powders with accuracy of about 0.1- 0.2% or better [6].

5. ACKNOWLEDGMENT

The authors wish to thank Prof. G. Sh. Kevanishvili, Georgian State Polytechnical University, Tbilisi, Georgia, for his assistance in the measurements of wines, and to Dr. L.I. Pangonis and Dr. M.P. Parkhomenko for joint investigations of emulsions and powders.

REFERENCES

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IEEE Trans. MTT-39, Vol. 39, No. 5, 828-835, 1991.

[2]. Apletalin V.N., Garin B.M.,. Meriakri V.V.: Dielectric properties of liquids in the submillimeter band. Radio Engineering and Electronic Physics, (USSR), Vol. l28, No.1, 1-15, 1983.

[3]. Meriakri V.V., Chigrai E.E., Nikitin I.P.: Monitoring the water content of media and materials with millimeter waves. Radio and Communications Technology (Russia), Vol.1, No 2, 92-96, 1996

[4]. Liebe H.J., Hafford G.A., and Manabe T.: A model for the complex permittivity of water at frequencies below 1 THz. International Journal of Infrared and Millimeter Waves, Vol. 12, No 2, 92-96, 1991

[5] Meriakri V.V., Parkhomenko M.P.: Application of dielectric waveguide for water control in alcohol. Electromagnetic Waves and Electronic Systems, Vol. 5, No 1, 38-39, (in Russian), 2000.

[6] Meriakri V.V., Pangonis L.I., Microwave methods moisture testing into materials and nedia used in metallurgy. Abstracts of the 28-th October Mining and Metallurgy Conference, Bor, Yugoslavia, 1996.

[7] Meriakri V.V., Chigrai E.E.: Properties of materials for practical use at the MM and SMM wavelengths. Digest of the 18-th International. Conference on Infrared and Millimeter Waves, (Colchester, UK), 68-69,1996

[8] Meriakri V.V., Chigrai E.E., Parkhomenko M.P. : Millimeter waves for water content monitoring in materials and media. 11 Feuchtetag 2002, Vortrge, Weimar, Germany, 13-22, 2002.

[9] Born M., Wolf E. Principles of optics. Pergamom Press, Oxford-London-Edinburgh-New York-Paris-Frankfurt, 1968.

Moisture Determination in Food Substances by Means of Millimeter Waves

W, %