Biosystems Engineering (2004) 88 (3), 343–349doi:10.1016/j.biosystemseng.2004.04.008

Available online at www.sciencedirect.com

PH}Postharvest Technology

Dielectric Characteristics of Grape Juice and Wine

A. Garc!ııa1; J.L. Torres1; M. De Blas1; A. De Francisco2; R. Illanes2

1Department of Projects and Rural Engineering, Universidad P !uublica de Navarra, 31006 Pamplona, Spain;e-mail of corresponding author: almudena@unavarra.es

2ETSI de Montes, Ciudad Universitaria, 28040 Madrid, Spain; e-mail:rillanes@montes.upm.es

(Received 4 July 2003; accepted in revised form 15 April 2004; published online 15 June 2004)

This study was carried out on samples of red grape juice and red wine at 208C made from red varieties ofgrapes of the denomination of origin Navarre. The relative dielectric constant e0r and the relative loss factor e00rwere measured in the frequency range from 200MHz to 3GHz with an open-ended coaxial probe technique.The dielectric parameters that best differentiate red grape juice from the red wine are the values of thedielectric constant at 200MHz and 3GHz. The presence of CO2 during alcoholic fermentation of the mustmay give unreliable measurements.# 2004 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

1. Introduction

Maxwell field equations describe the relationshipbetween time-varying electric and magnetic fields. Asboth fields may involve storage or dissipation of energyin the material, two pairs of parameters are required tocharacterise a dielectric property of a material. Inthe frequency range of interest, reference is made tothe complex permittivity e* and permeability m* as thefundamental parameters for the macroscopic descrip-tion of the dielectric property exposed to sinusoidalfields.

In practice, except for ferromagnetic materials,magnetic polarisation is generally very weak, such thatthe complex permeability m* can be replaced by themagnetic permeability of free space m0. For the case ofinterest, most biological and agricultural products arediamagnetic materials with a very low response tomagnetic fields (Nelson, 1973).

For a given dielectric material, the real part ofpermittivity or the relative dielectric constant e0r isdefined as

e0r ¼e0

e0ð1Þ

where: e0 is the absolute dielectric constant of thedielectric material and e0 is that for free space. Inaddition to the storage of energy, there are losses

represented by a loss current and it is customary torepresent this current by the complex relative permittiv-ity, given by

e�r �e�e0

¼ e0r � ie00r ð2Þ

where, e�r ; e0r and e00r are the complex relative permittivity,

the relative dielectric constant and the relative lossfactor, respectively. These terms refer to the permittivityof free space and, consequently, are dimensionless. Thereal part is a measure of the material property to storeenergy whereas the imaginary part refers to thedissipation of energy in the form of losses; i.e. a materialproperty to absorb energy from a electromagnetic fieldand to convert it in the form of heat (von Hippel, 1995;Engelder & Buffler, 1991; Nelson, 1989; Tinga & Nelson,1973). At microwave frequencies, the relative dielectricconstant e0r is large when the dipole moment can beorientated itself with an alternating electric field(Roebuck & Goldblith, 1972).

Generally, the dielectric characteristics of food pro-ducts are influenced by the inherent characteristics of thesubstance including water content, composition andtemperature. On the other hand, Tinga and Nelson(1973) considered frequency of the applied field to be themost dominant. Torres et al. (1998) briefly reviewedthe dielectric properties of fruits. Their studies relatedthe density, water content, dielectric properties of the

1537-5110/$30.00 343 # 2004 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

products and variations with frequency of the appliedfield to a quality index of the product and, in addition,the studies suggested equations for predicting dielectricproperties.

Garc!ııa et al. (2001) measured the dielectric propertiesof grape juice. The variation of the dielectric constantand the variation of loss factor with frequencies rangingfrom 0�2 to 3GHz turned out to be similar to thoseobtained by other authors for various fruits. While thedielectric constant showed a monotonically decreasingtrend with increasing frequencies, the total loss factordecreased and reached a minimum before increasingagain. In the range of the lowest frequencies measured,an important contribution of the ionic conductivity tothe value of the total losses was observed.

The objective of this study is the characterisation ofthe dielectric properties of red grape juices and redwines. The variation of the dielectric properties, relativedielectric constant and relative loss factor for frequen-cies in the range of 200MHz to 3GHz was obtainedfrom the time the grape juice entered the fermentationtank until the alcoholic fermentation was completed. Inaddition, the differences between the dielectric proper-ties of the grape juices and the wines were determinedand examined to establish which were more significant.

2. Materials and methods

2.1. Samples

Four wineries took part in this study. All are locatedin the middle of the Navarre region.

The wineries received the grapes either from their ownvineyards or from independent grape growers located inthe area. All the grape juice analysed was from thespecies Vitis vinifera, particularly red wine grapes, whichwere dominant over white grapes in the area of study.The most common red wine grapes for ageing were thevarieties Cabernet Sauvignon, Tempranillo, and Merlot.All the sampled juice and wine belonged to one of thesevarieties and were perfectly identified.

The grape picking was done either by hand ormechanically, usually with self-propelled grape harvest-ers. The grapes were transported to the winery in tractor

trailers lined with a watertight canvas cover. At thewinery the grapes were fed into a de-stemmer/crusherwhere leaves and stems were removed and grapes werecrushed and pumped into a fermentation tank.

Test samples were collected into 500ml glass bottles,directly from the sampling tap of the tank, either as thegrapes arrived or only a few hours later. The sampleswere placed in an ice chest at about 108C and brought tothe laboratory immediately after the sample collection.The measurements were performed on the same day ofsample collection.

The wines used for characterisation of the dielectricproperties were those from the fermentation process ofthe grape juices tested as described above and werealways taken from the same batches. The number ofsamples from each winery was variable.

2.2. Physical methods

2.2.1. Measurement of dielectric properties at high

frequency

A network analyser was programmed to measure thedielectric properties at different frequencies of theproducts in the range from 200MHz to 3GHz.

The system for measuring dielectric properties wasbased on an open-ended coaxial probe, manufacturedby Hewlett-Packard1 (currently Agilent Technolo-gies1), catalogued as HP 85070M which is recom-mended for materials with losses. This system (Fig. 1)consisted of the HP 8753C vectorial network analyser,the HP 85070B dielectric probe and a personalcomputer. The HP 85071B materials measurementsoftware for the dielectric probe set controlled thenetwork analyser to measure the complex reflectioncoefficient of the material being tested and converting itto complex permittivity.

2.2.2. Temperature monitoring and control

The samples were placed in an 8 l methacrylate basin.A DIGITERM-100 immersion thermostat (manufac-tured by Selecta1) was used to control the temperature.The thermostat has a digital readout and a built-inPt100 thermoresistance temperature sensor with aresolution of 0.18C and can be regulated in the range

Notation

f frequency, Hze* complex permittivity, Fm�1

e0 absolute dielectric constant, Fm�1

e0 dielectric constant for free space, Fm�1

e�r complex relative permittivity, dimensionless

e0r relative dielectric constant, dimensionlesse00r relative loss factor, dimensionlessm* complex permeability, dimensionlessm0 permeability of free space, Hm�1

A. GARCIA ET AL.344

of +58C to 99�98C, with an accuracy of 0�028C. Theliquid used in the bath was water.

2.3. Operation procedure

At the laboratory, the network analyser was calibratedwith air, short-circuit and deionised water standards at208C. The subsamples of grape juice and wine, taken foreach test directly from the tank, were dispensed into 50ml

glasses and placed in the temperature-controlled bathuntil they reached 208C. For each glass corresponding toeach labelled sample both the relative dielectric constant e0rand the loss factor e00r were measured in the frequencyrange from 200MHz to 3GHz. The dielectric propertieswere monitored daily from onset to the end of fermenta-tion of the must and plotted in a graphic presentation.The subsamples were discarded after the measurements.

Descriptive statistics were performed on grape juicesand wines. Using T-test and analysis of variance(ANOVA) to compare the mean values of the dielectricparameters of grape juice and wine in order to discoverthe variables that most influenced the variability of theset. In addition, a discriminant analysis was performedto identify which of the dielectric parameters contrib-uted to a greater degree to the dielectric differentiationbetween grape juices and wines.

2.4. Descriptive statistics of grape juices and wines

The descriptive statistics used in the characterisationof grape juice and wine at 200MHz and 3GHz and at atemperature of 208C was based on commonly usedstatistics and applied on the measured parameters:

relative dielectric constant e0r, and total loss factor e00r .The calculated values were the mean, the 95% confidenceinterval for the mean (with the upper and lower limits),the standard error of the mean, the standard deviation,the coefficient of variation, the range and the maximumand minimum values of the obtained dielectric para-meters for the grape juice set. The coefficient of variationwas considered to be of special importance and wasobtained by dividing the standard deviation by thesample mean. This meant that the values obtained hadthe same order of magnitude and consequently permittedcomparing the different variables.

2.4.1. Comparison of the means of grape juice and wine

Two procedures were carried out to ensure thevalidity of the results, the T-test for two independentsamples and the one-way ANOVA.

The prior requirements for ANOVA were theverification of the normality hypotheses (Kolmogorov-Smirnov and Shapiro-Wilk tests) and the homogeneityof variances (Levene’s test) of the data. The normalityand equality of variances for the T-test were alsochecked. A significance level of 0�05 was used.

2.4.2. Principal component analysis

One of the purposes of this technique was to provide abetter understanding of the interrelationships among aset of variables and was based on the transformation ofthe original variables into a new set of variables denotedby principal components. The new set had a fewernumber of variables and it was formed by a linearcombination of one or more of the original variablesused for the analysis. The statistical package SPSS wasused to carry out the analysis.

Computer

Software

Coaxial cable

Network analyser

8753C

BIAS INPUT

Transmission-reflection test set

Dielectric probe

HP-IB Interface card

Fig. 1. Dielectric probe system for measuring the permittivity complex with the open-ended coaxial probe technique

DIELECTRIC CHARACTERISTICS OF GRAPE JUICE AND WINE 345

2.4.3. Discriminant analysis

This analysis was used to identify the variables that bestdiscriminate between grape juice and wine. The proceduregenerated a discriminant function based on linearcombinations of the variables: relative dielectric constantat 200MHz, at 3GHz, total loss factor at 200MHz and at3GHz. The generated function was obtained from themeasured parameters of grape juice and wine.

3. Results

3.1. Variation of the dielectric properties with time

of change of state from grape juice to wine

The variation of the dielectric constant and the lossfactor are shown in Fig. 2 on a logarithmic scale offrequency. These presentations were for one sample, butthe same tendencies were observed also for other samples of

grape juice. The reference of the measurements corre-sponded to the day on which the grape juice was collected(day 1) and the variation of the dielectric parameters for theother days are shown in Fig. 2 until day 49.

Initially, it was believed that the change in thevariation of the parameters was very significant as timeprogresses. At the beginning of the fermentation and astime progressed, the fermentation became more inten-sive until a stage was reached where the fermentationstarted to decrease and gradually the process stopped.

However, inspection of the surface of the proberevealed bubbles of CO2 adhering to the probe and thusdistorting the measured data.

3.2. Analysis of dielectric properties of grape juice

and wine at 208C

The following analysis pertains to 12 juices and 13wines, of each there are 12 must-obtained wine pairs

18

28

38

48

58

68

78

8.3 8.5 8.7 8.9 9.1 9.3 9.5

Rel

ativ

e di

elec

tric

con

stan

t

(a)

(b)

0

10

20

30

40

Rel

ativ

e lo

ss f

acto

r

Log frequency

8.3 8.5 8.7 8.9 9.1 9.3 9.5

Log frequency

Fig. 2. Variation with time of change of state from grape juice to wine of the (a) relative dielectric constant; (b) relative loss factor:&, day 1; }, day 3; *, day 4; *, day 15; n, day 49

A. GARCIA ET AL.346

(wines resulting from the alcoholic fermentation). Theybelong to different varieties but our objective is todifferentiate between must and wine, not betweenvarieties. Previous work of Garc!ııa et al. (2001) did notconclude that musts of different varieties show differentdielectric properties.

The results of the analysis are summarised in Table 1.The mean of the dielectric constant at 200MHz isgreater in grape juices than in wines, whereas theopposite is true at 3GHz. The confidence interval forthe mean of the dielectric constant at 3GHz and at200MHz and the loss factor at 3GHz is very small, lessthan one for both the grape juice and for the wine.Furthermore, the confidence interval of the loss factor at3GHz and at 200MHz of both materials overlap eachother. In contrast, the loss factor at 200MHz for bothmaterials has the greatest confidence interval for themean.

Comparing these values with those obtained fordeionised water at 200MHz (Tables 1 and 2), it is seenthat the mean value of the dielectric constant of wine islower than that for grape juice and the dielectricconstant of grape juice is lower than that for water. At3GHz, the loss factor of grape juice and wine are stillhigher than that for water, but the difference is not largeas compared to the loss factor at 200MHz. The biggerdifference is due to the ionic conductivity of both grapejuices and wines at 200MHz.

For both the grape juice and the wine, the coefficientof variation of the dielectric constants at both frequen-cies is very similar and is about 1%.

As for the loss factor at 200MHz, the mean value ofthe grape juice is greater than the mean value of the

wine. The mean values are practically the same at thehigher frequency of 3GHz.

The coefficients of variation of grape juice and wineare about the same for both frequencies. In addition, itis important to emphasise the fact that the coefficient ofvariation of the loss factor of grape juice and wine at200MHz are about 20%. These values are considerablylarger than the loss factor at 3GHz.

In addition to the univariate descriptive statisticsperformed a principal component analysis was used asan exploratory technique useful in gaining a betterunderstanding of the interrelationships among thevariables. The determinant of the correlation matrixwas very close to zero both for the grape juices and forthe wines, which indicates that the analysis is applicableto our data because there exists a high degree ofcorrelation among the variables. From the extractedmain components, two were chosen for the analysis forboth grape juice and wine. These two principalcomponents explain the 92�3% of variance for grapejuice and the 86�8% of variance for wine and indicatethat in general terms the analysis is reasonable.

Table 1Statistics of dispersion and the tendency of the mean of the dielectric constant e0r and loss factor e00r at 200MHz (subscript, 02) and at

3GHz (subscript, 3G) of 12 juices and 13 wines

Variable Mean Confidence interval forthe mean at 95%

Standarderror of

the mean

Standarddeviation

Coeff. ofvariation,

%

Range Minimum Maximum

Upperlimit

Lowerlimit

Grape juicee0r; 02 76�015 76�444 75�587 0�195 0�674 0�887 2�532 74�448 76�980e0r;3G 65�358 65�817 64�899 0�208 0�722 1�105 2�571 64�134 66�705e00r; 02 28�337 32�080 24�593 1�701 5�892 20�794 17�154 20�560 37�714e00r;3G 19�976 20�283 19�670 0�139 0�483 2�416 1�529 19�402 20�931

Winee0r; 02 74�587 75�039 74�135 0�207 0�748 1�00 2�458 73�279 75�737e0r;3G 66�631 66�989 66�273 0�164 0�593 0�89 1�835 65�487 67�322e00r; 02 26�598 29�788 23�409 1�464 5�278 19�84 17�133 19�927 37�060e00r;3G 19�867 20�158 19�577 0�133 0�480 2�42 1�328 19�280 20�608

Table 2

Relative dielectric constant e0r and loss factor e00r of deionisedwater at 208C, at 200MHz and 3GHz

Frequency e0r e00r

200MHz 80�361 0�718

3GHz 78�186 13�341

DIELECTRIC CHARACTERISTICS OF GRAPE JUICE AND WINE 347

For grape juice, the factor matrix reveals that thedielectric constant at 200MHz and both loss factors for200MHz and 3GHz are the parameters that present thehighest correlation with the first principal component,and the dielectric constant at 3GHz correlates with thesecond principal component. For wine, the dielectricconstant and the loss factor at 200MHz and thedielectric constant at 3GHz correlate more stronglywith the first principal component, whereas the lossfactor at 3GHz associates basically with the secondprincipal component.

A table of communalities was obtained and shows theproportion of the variance of the original variables asexplained by the principal component and the coeffi-cients are very high for wine and grape juice with anexception for the loss factor at 200MHz for wines,although the correlation is still good (0�633).

The rotated factor pattern (Table 3) of grape juicereveals that the first principal component (first factor) iscomposed of the loss factors at 200MHz and 3GHz.Furthermore, the variables that have the highest factorloadings in the first component have minimum factorloadings in the second component. In the case ofwine, the dielectric constant at 3GHz and the loss

factor at 200 MHz are the component variables ofthe first component and the loss factor at 3GHz andthe dielectric constant at 200MHz, of the secondcomponent.

3.3. Differentiation of the dielectric properties of

grape juices and wines

Figure 3 is a graphic representation of the dielectricproperties of each sample of the grape juices and theresulting wine. Each vertical line contains (a) thedielectric constants and (b) the loss factors of grapejuice and the resulting wine.

The dielectric parameters at each of the frequencies ofstudy of the grape juices and their corresponding wines,as well as the grape juices globally and the winesglobally do not, at first, present clear differences,although certain tendencies are observed.

3.3.1. Comparison of the means of grape juice and wine

For comparing the means of grape juice and wine, thenormality of the samples and the homogeneity of thevariances were first studied. The hypothesis of normalityin all cases was verified as well as the Levene’s test forequality of variances.

By applying the T-test and ANOVA, both thedielectric constant at 200MHz and at 3GHz weresignificantly different for grape juice and wine, whereasthe loss factors at these frequencies were not found to besignificantly different.

3.3.2. Discriminant analysis for grape juice and wine

The discrimination was based on a linear combinationof the dielectric properties. The set of variables waschosen by applying the method that generated the bestdiscriminant function for differentiating between grapejuice and wine.

Table 3

Rotated factor pattern for the dielectric constant e0r andloss factor e00r at 200MHz (subscript, 02) and at 3GHz

(subscript, 3G)

Factor 1 Factor 2

Grape juice Wine Grape juice Wine

e0r; 02 0�390 0�644 0�866 0�733e00r; 02 0�937 0�726 0�126 0�325e0r;3G �0�157 0�944 0�956 �0�009e00r;3G 0�979 0�052 0�000 0�996

Rotation Method: Varimax.

62

64

66

68

70

72

74

76

78

Samples Samples

Rel

ativ

e di

elec

tric

con

stan

t

(a)

16

20

24

28

32

36

40

Rel

ativ

e lo

ss f

acto

r

(b)

Fig. 3. (a) Dielectric constant at 200 MHz and at 3 GHz of the must and the resulting wine; (b) loss factor at 200 MHz and at3 GHz of the must and the resulting wine; } and *, must and wine at 200 MHz, respectively; * and &, must and wine at 3 GHz,

respectively

A. GARCIA ET AL.348

The coefficients of the discriminant functions(Table 4) show that 100% of the samples tested arewell classified.

4. Conclusions

(1) The presence of CO2 during fermentation of themust may give unreliable measurements due to thebubbles that adhere to the edge surface of the probe butthis may be useful to determine the extent of fermentation.

(2) The dielectric constant e0r and the loss factor e00r ofgrape juice at 200MHz are greater than those for wine.There is a high variability in the loss factor at 200MHz.At 3GHz, the dielectric constant e0r of grape juice isslightly less than that of wine, whereas the loss factor e00rat 3GHz is practically the same for grape juice andwine.

(3) In statistical terms, the parameters that bestexplain the total variance are the loss factor e00r at200MHz and 3GHz for grape juice and the loss factore00r at 200MHz and the dielectric constant e0r at 3GHzfor wine.

(4) The dielectric constant e0r at 200MHz and 3GHzare the parameters that proved to be significantlydifferent for grape juice and wine.

(5) Generating a function which takes into accountthe dielectric parameters dielectric constant e0r at

200MHz and 3GHz and the loss factor e00r at 3GHzgives the best discrimination between grape juice andwine.

Acknowledgements

The authors wish to thank Bodegas Nekeas, BodegasFern!aandez de Arcaya, Bodegas Irache, Bodegas Castillode Monjard!ıın and Bodegas Se *nnor!ııo de Sarr!ııa fordonating the samples.

References

Engelder D S; Buffler C R (1991). Measuring dielectricproperties of food products at microwave frequencies.Microwave World, 12(2), 6–15

Garc!ııa A; Torres J L; Prieto E; de Blas M (2001). Dielectricproperties of grape juice at 0�2 and 3 GHz. Journal of FoodEngineering, 48 (2001), 203–211

Nelson S O (1973). Electrical properties of agriculturalproducts. A critical review. Transactions of the ASAE, 16,384–400

Nelson S O (1989). Dielectric properties of agriculturalmaterials and their use in agricultural engineering andtechnology. Fourth International Conference PhysicalProperties of Agricultural Materials and their Influence onTechnological Processes, pp 560–565

Roebuck B D; Goldblith S A (1972). Dielectric properties ofcarbohydrate–water mixtures at microwave frequencies.Journal of Food Science, 37, 199–204

Tinga W R; Nelson S O (1973). Dielectric properties ofmaterials for microwave processing}tabulated. Journal ofMicrowave Power, 8(1), 23–65

Torres J L; de Blas M; Garc!ııa A; Prieto E (1998). Propiedadesel!eectricas de frutas y verduras: una revisi !oon [Electricalproperties of fruits and vegetables: a revision.] Alimentaria,Revista de Tecnolog!ııa e Higiene de los Alimentos, 295,25–29

Von Hippel A R (1995). Dielectrics and Waves. MIT Press,Cambridge, MA, New York. (New ed., originally published,Wiley, New York, 1954)

Table 4Coefficients of the discriminant function e0r and e00r , dielectricconstant and loss factor, respectively, at 200MHz (subscirpt, 02)

and at 3GHz (subscript, 3G)

Variable Grape juice Wine

e0r; 02 96�070 61�838e0r;3G 81�731 111�013e00r;3G 6�891 39�653(Constant) �6391�804 �6399�243

DIELECTRIC CHARACTERISTICS OF GRAPE JUICE AND WINE 349