Accepted Manuscript

A low-frequency sensor for determination of honey electrical properties in vary-

ing temperature conditions

Bartosz Paszkowski, Andrzej Wilczek, Agnieszka Szypłowska, Anna

Nakonieczna, Wojciech Skierucha

PII: S0260-8774(14)00158-7

DOI: http://dx.doi.org/10.1016/j.jfoodeng.2014.04.004

Reference: JFOE 7775

To appear in: Journal of Food Engineering

Received Date: 16 September 2013

Revised Date: 4 April 2014

Accepted Date: 7 April 2014

Please cite this article as: Paszkowski, B., Wilczek, A., Szypłowska, A., Nakonieczna, A., Skierucha, W., A low-

frequency sensor for determination of honey electrical properties in varying temperature conditions, Journal of Food

Engineering (2014), doi: http://dx.doi.org/10.1016/j.jfoodeng.2014.04.004

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1

A low-frequency sensor for determination of honey electrical properties in varying temperature conditions

Bartosz Paszkowskia,*, Andrzej Wilczeka, Agnieszka Szypłowskaa, Anna Nakoniecznaa,b,Wojciech Skieruchaa

a Polish Academy of Sciences, Institute of Agrophysics, Department of Metrology and Modelling of Agrophysical Processes, Doświadczalna 4, 20-290 Lublin, Poland b Institute of Physics, Maria Curie-Skłodowska University, Pl. M. Curie-Skłodowskiej 1, 20-031 Lublin, Poland

*Corresponding author. E-mail: b.paszkowski@ipan.lublin.pl, phone no.: +48817445061 ext. 125.

Abstract

The aim of the research was to test a prototype sensor used to determine the electrical properties of acacia honey. The sensor, placed inside a measurement cell, was connected to a precise LCR meter, which collected the impedance readings in the frequency range from 20 Hz to 2 MHz. The measurement results were analyzed using the electrical equivalent circuit (EEC) approach. The values of parameters describing impedances of four EEC elements, which were a capacitor, a constant phase element and two resistors, obtained at various temperatures from -10oC to 35oC during honey heating and cooling processes, were presented, as well as dielectric permittivity spectra of honey. The paper discusses the temperature dependence of the EEC electrical parameters and the functionality of the sensor.

Keywords

Impedance spectroscopy, Acacia honey, Sensor, Non-destructive measurements

1. Introduction

The complex impedance of a sample-sensor setup describes the behaviour of the tested material in an alternating electric field. The impedance spectroscopy (IS) is an experimental technique relying on recording changes of the tested material impedance, which result from frequency variations of an applied voltage signal (Barsoukov et al., 2005). It is successfully applied in many fields of science such as electrochemistry and materials science (Ahmed et al., 2011), biophysics (Tura et al., 2007 ), pharmacology (Shervedani et al., 2006) and geophysics (Koelemeijer et al., 2012). In the case of sufficiently wide frequency range the impedance spectrum, which is a result of an IS measurement, incorporates the whole information about electric properties of an analysed system (Harrington et al., 2011).

The information contained in the frequency spectrum of impedance of the analysed sample can be correlated with its physical and chemical parameters, which may serve as indicators of the material quality (Nelson, 2010). An interest in these aspects enhanced the development of rapid and non-destructive quality assessment dielectric measurement techniques, such as impedance spectroscopy, time-domain reflectometry (TDR) and frequency-domain reflectometry (FDR) (Jha et al., 2011; Skierucha et al., 2012). Nelson et al. investigated the correlations between a set of two quantities, i.e. the dielectric permittivity and the loss factor, and the pulp density along with the dissolved solids content of a honeydew melon (Nelson et al., 2006). These correlations formed a basis for the qualitative analysis of fruits for consumption. An attempt to differentiate honeys by measuring their conductivity, the dielectric permittivity and the loss factor was also made (Łuczycka, 2010). Moreover, the

2

effort to distinguish fake and genuine honeys using electrical parameters measurements was presented (Łuczycka et al., 2012). It was also shown that impedance spectroscopy can be applied as an alternative method for the determination of floral origin of unifloral honey (Scandurra et al., 2013).

The physicochemical properties of honey are temperature sensitive. As temperature increases, the amount of 5-hydroxymethylfurfural raises and the diastase number decreases. The diastase number is connected with the decline in the honey enzymes activity. The enzymatic activity ceases when honey is overheated, what takes place at temperatures above 40°C. The overheating changes electrical characteristics of honey, which is an important factor that may allow for distinguishing between overheated and not overheated honey. Additionally, the diastase inactivity in honey may indicate its adulteration with sugar syrup, which for the above reasons may be also detected with the use of electrical measurement techniques.

The purpose of this study was to test a prototype sensor for measuring the electrical parameters of honey and to determine the temperature effect on honey electrical parameters.

2. Materials and methods

The acacia variety of honey was selected for the research because it sustains its liquid phase for a long period of time and crystallizes slowly due to high fructose content (Popek, 2002).

The examined honey was collected in the Lublin area in the year 2011. The results of analyses of physicochemical properties of the material under test, which were made by the Bee Products Quality Testing Laboratory in Puławy, are presented in Table 1.

[INSERT TABLE 1 HERE]

A – the method accredited by the Polish Centre for Accreditation; PN-88/A-77626 „Miód pszczeli” (‘Bee honey’) – Polish Norm; PB-02, PB-04, PB-06, PB-07 – test procedures in Bee Products Quality Testing Laboratory; E-03, E-05 – edition number; HMF – 5-Hydroxymethylfurfural. *one unit of diastase is equivalent to the activity of enzymes that are contained in 1 gram of honey, capable of hydrolyzing 0.01 g of starch during 1 hour at 40°C (Waś E. et al., 2011).

According to the obtained assessment, in the honey sediment a high total number of pollen grains was found, which indicates the secondary enrichment of honey by pollen from the bee bread. The pollen analysis cannot determine the variety of honey in this case. The decisive factor for the classification is the glucose to fructose ratio, which in the studied case is equal to 1.39. This value confirms that the tested sample is the acacia honey. According to the International Commission for Honey, the norm which classifies the product to the acacia honey variety is the glucose to fructose ratio of about 1.4 (Council Directive, 2001). The parameters such as the 5-hydroxymethylfurfural content and the diastase number provide information that the honey was stored properly and it was not overheated during the packaging process (Council Directive, 2001). Moreover, the water content equal to 16.1% is in accordance with the standards, as it does not exceed 20% (Council Directive, 2001).

The schematic diagram of the sensor of custom design which was placed in the analyzed sample of honey is shown in Fig. 1. The sensor consisted of a 3.75 mm thick frame made of

3

polycarbonate with two silver-plated wire closed loops of the diameter equal to 0.5 mm separated by a 6 mm distance and wound around the polycarbonate frame through the precisely carved notches.

[INSERT FIG. 1 HERE]

Fig. 1. The scheme of the sensor. The dashed black lines represent the electrodes, while the solid ones – the polycarbonate frame.

The wire loops acted as electrodes since they were connected through coaxial cables to the BNC terminals of the LCR meter (Agilent E4980A). The scheme of the sensor-measuring device connection is depicted in Fig. 2.

[INSERT FIG. 2 HERE]

Fig. 2. The connection scheme of the sensor and the LCR meter. The shortcuts Hcur, Hpot, Lpot and Lcur denote the respective terminals on the front panel of the device.

The use of a silver-plated copper wire guarantees the reduction of measurement errors that could appear due to polarization effects on the surfaces of the electrodes (Barsoukov et al., 2005). The frame with the electrodes was placed in a polycarbonate disc (Fig. 2), in order to maintain the steady position of the sensor during the measurements. The sensor was immersed in a sample of the tested material to the 36 mm depth. The investigated samples consisted of 100 ml of honey placed in a plastic container.

The complex impedance of the sensor-honey system is described by the following relation:

ciXRZ += (1)

where Z denotes the impedance [Ω], R stands for resistance [Ω], Xc is reactance [Ω] and i is the imaginary unit.

The collected data were processed using the Frequency Response Analysis (FRA) method (Metrohm Autolab), i.e. an analysis of the frequency response in the frequency range 20 Hz – 2 MHz (201 measurement points, logarithmic scale of frequency) was performed. The amplitude of the voltage signal generated by the LCR meter was equal to 100 mV. The collection of data from the LCR meter was controlled by the PC compatible computer with the use of a custom written application.

The research was conducted in two series of cooling and heating of the honey samples, which were performed in the climatic chamber (Weiss WKL100) with adjustable temperature stabilized with accuracy equal to ±0.1oC. Stable thermal conditions were maintained to minimize the temperature change of honey electrical parameters during the readings.

The first measurement series included the temperature change from 21°C to -10°C, while the second one was performed from -10°C to 35°C. The temperature range was chosen to include storage temperatures of food in warehouses and households. The temperature was changed in steps by one degree every three hours during the first series and five degrees every five hours in the second series. The measurements were taken every ten minutes during the experiment. The honey heats up and cools down slowly, therefore the time interval appropriate for reaching the temperature equilibrium was carefully selected. After the sample temperature was stabilized, ten results were taken for elaboration and interpretation.

4

The results obtained during the experiment were analyzed using the computer software, specifically the 'ZSimDemo3.30 Demo' (EChem Software, 2001) and the 'EIS analyzer' (Bondarenko et al., 2008). The analysis was based on matching the impedance spectrum of an electrical equivalent circuit to the impedance spectrum of the honey sample obtained from the experimental data. The equivalent circuit appropriately constructed for interpreting the collected honey impedance frequency spectra consisted of two separate parts connected in series. These were the pairs of elements C-R1 and CPE-R2, each of which had a parallel connection. The scheme of the selected electrical equivalent circuit is presented in Fig. 3.

[INSERT FIG. 3 HERE]

Fig. 3. The electrical equivalent circuit, whose impedance spectrum was fitted to the experimental data. Its elements are: C – capacitor, CPE – constant phase element, R1 and R2 – resistors.

The constant phase element represents the imperfection of the standard RLC element. Its impedance depends on two parameters, that is Q measured in units -1

sn and dimensionless n.It is defined by the formula (Barsoukov et al., 2005)

ZCPE = 1/(Q(i2πf)^n), (2)

where f is the frequency of the electric field.

The universal physical interpretation of the constant phase element is not possible, because it depends on the exponent n. For n = 1 this circuit component can be regarded as an ideal capacitor, for n = 0 as an ideal resistor, and for n = -1 as an ideal inductor. For values of nclose to 1 the constant phase element describes the lossy capacitor. The values of the parameter n obtained in the experiment varied from 0.74 at the temperature equal to -10°C to 0.53 at the temperature equal to 35°C. Thus the CPE can be interpreted as a lossy capacitor at low temperatures, and as a Warburg impedance element at high temperatures.

3. Results and Discussion

The impedance spectra of the acacia honey-sensor system obtained during the experiments conducted within the investigated temperature range, i.e. from -10oC to 35oC, are presented in Fig. 4. They were presented on Nyquist diagrams, which illustrate the relation between the real and imaginary parts of impedance.

[INSERT FIG. 4 HERE]

Fig. 4. The Nyquist diagrams of the experimental impedance spectra of the acacia honey-sensor system collected at various temperatures. On the bottom plot: the magnification for the temperatures above 10°C.

Selected Nyquist diagrams presenting the measured impedance spectra in the complex plane and the corresponding impedance spectra of the equivalent electrical circuit, which was fitted to the experimental data, are presented in Fig. 5. As may be inferred from the plots, the chosen electrical equivalent circuit was appropriate for modeling the electrical properties of the examined sensor-liquid setup.

[INSERT FIG. 5 LEFT & RIGHT HERE]

5

Fig. 5. Nyquist diagrams of the impedance spectra collected at -10oC (left panel) and 35oC(right panel). The points represent the measured experimental spectra and the solid lines illustrate the impedance spectra calculated from the determined electrical equivalent circuit parameters.

The values of the four EEC parameters (C, Q, R1, R2) which were obtained from electrical equivalent circuit analysis as functions of temperature are shown on the graphs presented in Figs. 6-9. The points marked with black circles represent cooling from 21°C to -10°C, while the ones with red diamonds correspond to the heating process from -10°C to 35°C.

[INSERT FIG. 6 HERE]

Fig. 6. The temperature dependence of the capacitance C of the selected EEC.

[INSERT FIG. 7 HERE]

Fig. 7. The temperature dependence of the parameter Q of the selected EEC.

[INSERT FIG. 8 HERE]

Fig. 8. The temperature dependence of the resistance R1 of the selected EEC.

[INSERT FIG. 9 HERE]

Fig. 9. The temperature dependence of the resistance R2 of the selected EEC.

A characteristic feature of the temperature dependence of the parameters C, R1 and R2 is the decline of their values when the temperature increases (the standard errors are less than 2%). The value of the parameter C changes linearly with temperature (Fig. 6). The resistance values R1 and R2 decrease exponentially as temperature increases (Figs. 8-9). The value of the parameter Q increases exponentially with temperature (the matching errors are less than 10%) (Fig. 7). The curve fitting parameters are shown in Table 2.

[INSERT TABLE 2 HERE]

The values of R1 and R2 obtained from fitting the electrical equivalent circuit spectrum to the experimental one were similar during both cooling and heating stages, what is confirmed by the parameters a and b of the approximated models. A small discrepancy exists for the parameter C. Significant differences between cooling and heating stages were noticed for the values of the parameter Q.

The dielectric properties of the material determine its interaction with the electric field. In order to describe the dielectric properties, the calibration of the measurement circuit are needed. The model which describes the circuit is defined by the formula

1/ZM = 1/Z0 + 1/ZS (3)

where ZM is the measured impedance, Z0 represents the impedance of the connector and the impedance of the part of the sensor which is never immersed in a sample, and ZS is the impedance of the measured sample. According to the application note (Agilent, 2013) describing the LCR meter performance, the impedance of the connecting wires can be neglected because of the four-wire measurement (four-terminal configuration).

6

The complex dielectric permittivity is obtained by dividing the impedance Zair in air of the part of sensor which is immersed in the sample by the impedance of the sample ZS obtained from equation (3), i.e.

ε* = Zair/ZS (4)

The values of Zair and Z0 were calculated together with the value of ZS using the system of equations (3) and (4), provided that the former was written both for air and transformer oil with known dielectric properties. The presented method for the measurement data processing was validated using materials of known dielectric permittivity, i.e. pure water, methanol, ethanol and isopropyl alcohol. The real and imaginary parts of complex dielectric permittivity as functions of frequency within the investigated range of ambient temperatures stabilized during the experiment are presented in Figs. 10 and 11. As can be noticed, in the low-frequency range the absolute values of the real and imaginary parts of honey dielectric permittivity were very high, as was shown also in another study (Łuczycka, 2012). This was due to the influence of electrical conductivity and the sample-electrode interface phenomena, visible in low frequencies.

[INSERT FIG. 10 HERE]

Fig. 10. The frequency dependence of the real part of dielectric permittivity at various temperatures.

[INSERT FIG. 11 HERE]

Fig. 11. The frequency dependence of the imaginary part of dielectric permittivity at various temperatures.

The electrical equivalent circuit method can be a more advantageous approach for the impedance measurements analysis in the frequency range of interest than calculating the dielectric permittivity spectrum, because it allows separate study of each electrical phenomenon occurring in the sample-sensor system.

The calibration of the prototype sensor in terms of electrical conductivity was made at 20°C in order to analyze the meaning of the equivalent circuit parameters. Four NaCl aqueous solutions of various concentrations not exceeding 0.05 mM were used. Conductivity of the solutions was measured using a conductometer (MeterLab CDM210). Next, the impedance spectra of the solutions were collected. An analysis of the obtained data was made similarly to the analysis for honey described above. A simplified version of the electrical equivalent circuit from Fig. 3 without resistor R2 was used, because the calibration solutions are less complex materials than honey. The electrical impedance spectrum of the electrical equivalent circuit was matched to the electrical impedance spectra obtained for the NaCl aqueous solutions. It was confirmed that the resistor R1 is related to electrical conductivity of the material, as was also noticed for one-component low conductivity aqueous solutions of salts in (Szypłowska et al., 2013). The direct measurements of honey using the conductometer confirmed that the relation between the reciprocal of R1 and electrical conductivity derived using the calibration solutions holds for honey. Therefore, the EEC parameters can be interpreted as follows. The parameter C is the capacity of the sensor-sample system, R1 is the electrical resistance of the sample, while R2 and CPE parameters could be related to interfacial electrical phenomena occurring at the electrode-sample phase boundary.

7

4. Conclusions

The impedance spectra obtained using the proposed sensor and modelled by the described electrical equivalent circuit impedance allows to obtain the EEC parameters with standard errors not exceeding 2% for C, R1 and R2. The values of parameter Q are determined with errors less than 10%.

The temperature change affects the electrical parameters of honey. The parameter C increases linearly, R1 and R2 decrease exponentially and Q increases exponentially when the temperature decreases.

The model of the equivalent circuit consisting of four elements, that is C, Q, R1, R2, worked properly within the specified temperature range. The C parameter can be interpreted as capacitance of the sensor-sample system, while R1 is related to the electrical conductivity of honey.

Since honey is a complex object, the physical interpretation of the elements CPE and R2 of the electrical equivalent circuit is not obvious. It is suspected that they are related to an electrical double layer and other phenomena occurring at the electrode-sample interface. However, in order to confirm this conclusion further study is required.

The proposed sensor can be calibrated in order to obtain complex dielectric permittivity spectrum. However, for the frequencies below 1 kHz the dielectric permittivity is strongly influenced by electrical conductivity and interfacial phenomena.

The applications of the sensor could be extended to liquid materials like thick syrups, thick juices and oily substances with electrical properties similar to honey.

AcknowledgmentsThe research was supported by the National Center for Research and Development (NCBR), grant no. PBS1/A9/12/2012.

References

Agilent Impedance Measurement Handbook, 2013. USA, 5950-3000.Ahmed, R., and Reifsnider, K., 2011. Study of influence of electrode geometry on impedance spectroscopy,

International Journal of Electrochemical Science 6, 1159–1174.

Barsoukov, E. J., Ross Macdonald, J., 2005. Impedance Spectroscopy Theory, Experiment, and Applications, Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Bondarenko, A. S. and Ragoisha, G. A. 2008., EIS Spectrum Analyser, http://www.abc.chemistry.bsu.by/vi/analyser/.

Council Directive 2001/110/WE Council of the European Union, annex II.

EChem Software, 2001. ZSimDemo 3.30, http://www.echemsw.com/. Harrington, D. A., van den Driessche, P., 2011. Mechanism and equivalent circuits in electrochemical

impedance spectroscopy, Electrochimica Acta 56, 8005–8013.

Jha, S. N., Narsaiah, K., Basediya, A. L., Sharma, R., Jasiwal, P., Kumar, R., Bhardwaj, R., 2011. Measurement techniques and application of electrical properties for non-destructive quality evaluation of foods – a review, Journal of Food Science and Technology 48, 387-411.

Koelemeijer, P. J., Peach, C. J., Spiers, C. J., 2012. Surface diffusivity of cleaved NaCl crystals as a function of humidity: Impedance spectroscopy measurements and implications for crack healing in rock salt, Journal of Geophysical Research 117, B01205.

Łuczycka, D., 2010. Dielectric Properties of Selected Honey Varietes, Inżynieria Rolnicza 5(123) (in Polish).

Łuczycka, D., Nowakowski, P., Szewczyk, A., Pruski, K., Howis, M., 2012. Electric Properties in Commodity-Science Evaluation of Honey, Acta Agrophysica, 19(4), 749-759.

8

Metrohm Autolab Application Note EIS03.

Nelson, S. O., 2010. Fundamentals of Dielectric Properties Measurements and Agricultural Applications, Journal of Microwave Power and Electromagnetic Energy, 44 (2), 98-113.

Nelson, S. O., Trabelsi, S., Kays, S. J., 2006. Dielectric Spectroscopy of Honeydew Melons from 10 MHz to 1.8 GHz for Quality Sensing, American Society of Agricultural and Biological Engineers ISSN 0001−2351, 49(6), 1977−1981.

Popek, S., 2002. A procedure to identify a honey type. Food Chemistry, 79(3), 401–406.

Scandurra, G., Tripodi, G., Verzera, A., 2013. Impedance spectroscopy for rapid determination of honey floral origin, Journal of Food Engineering 119, 738-743.

Shervedani, R. K., Mehrjardi, A. H., Zamiri, N., 2006. A novel method for glucose determination based on electrochemical impedance spectroscopy using glucose oxidase self-assembled biosensor, Bioelectrochemistry 69, 201–208.

Skierucha, W., Wilczek, A., Szypłowska, A., 2012. Dielectric spectroscopy in agrophysics, International Agrophysics 26, 187-197.

Szypłowska, A., Nakonieczna, A., Wilczek, A., Paszkowski, B., Solecki, G., Skierucha, W., 2013. Application of a Coaxial-Like Sensor for Impedance Spectroscopy Measurements of Selected Low-Conductivity Liquids. Sensors 13, 13301-13317.

Tura, S. A., Sbrignadello, S., Barison, S., Conti, S., Pacini, G., 2007. Impedance spectroscopy of solutions at physiological glucose concentrations, Biophysical Chemistry 129, 235–241.

Waś, E., Rybak-Chmielewska, H., Szczęsna, T., Kachaniuk, K., Teper, D., 2011. Characteristics of Polish Unifloral Honeys. III. Heather Honey (Calluna Vulgaris L.), Journal of Apicultural Science 55 No. 1.

9

FIGURE CAPTIONS

Fig. 1. The scheme of the sensor. The dashed black lines represent the electrodes, while the solid ones – the polycarbonate frame

Fig. 2. The connection scheme of the sensor and the LCR meter. The shortcuts Hcur, Hpot, Lpot and Lcur denote the respective terminals on the front panel of the device.

Fig. 3. The electrical equivalent circuit, whose impedance spectrum was fitted to the experimental data. Its elements are: C – capacitor, CPE – constant phase element, R1 and R2 – resistors.

Fig. 4. The Nyquist diagrams of the experimental impedance spectra of the acacia honey-sensor system collected at various temperatures. On the bottom plot: the magnification for the temperatures above 10°C.

Fig. 5. Nyquist diagrams of the impedance spectra collected at -10oC (left panel) and 35oC(right panel). The points represent the measured experimental spectra and the solid lines illustrate the impedance spectra calculated from the determined electrical equivalent circuit parameters.

Fig. 6. The temperature dependence of the capacitance C of the selected EEC.

Fig. 7. The temperature dependence of the parameter Q of the selected EEC.

Fig. 8. The temperature dependence of the resistance R1 of the selected EEC.

Fig. 9. The temperature dependence of the resistance R2 of the selected EEC.

Fig. 10. The frequency dependence of the real part of dielectric permittivity at various temperatures.

Fig. 11. The frequency dependence of the imaginary part of dielectric permittivity at various temperatures.

0

2 M

4 M

6 M

8 M

10 M

12 M

14 M

16 M

0 5 M 10 M 15 M 20 M 25 M 30 M 35 M

-Im

Z [

Ω]

Re Z [Ω]

35oC

30oC

25oC

20oC

15oC

10oC

5oC

0oC

-5oC

-10oC

0

50 k

100 k

150 k

200 k

250 k

300 k

350 k

0 100 k 200 k 300 k 400 k 500 k 600 k 700 k

-Im

Z [

Ω]

Re Z [Ω]

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30 35

-Z”

[106

Ω]

Z’ [106 Ω]

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80

-Z”

[103

Ω]

Z’ [103 Ω]

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

3.1

-10 -5 0 5 10 15 20 25 30 35

C [

F 1

0-1

1]

T [oC]

f(T) = -1.58 ⋅ 10-13

T + 2.84 ⋅ 10-11

g(T) = -1.35 ⋅ 10-13

T + 2.80 ⋅ 10-11

cooling

heating

f(T)

g(T)

5.0 ⋅ 10-10

1.0 ⋅ 10-7

2.0 ⋅ 10-7

3.0 ⋅ 10-7

4.0 ⋅ 10-7

5.0 ⋅ 10-7

6.0 ⋅ 10-7

-10 -5 0 5 10 15 20 25 30 35

Q [

Ω-1

sn]

T [oC]

f(T) = 9.34 ⋅ 10-10 ⋅ 1.25

T

g(T) = 7.93 ⋅ 10-10 ⋅ 1.20

T

cooling

heating

f(T)

g(T)

7.0 ⋅ 104

1.5 ⋅ 106

5.0 ⋅ 106

1.0 ⋅ 107

1.5 ⋅ 107

2.0 ⋅ 107

2.5 ⋅ 107

3.0 ⋅ 107

3.5 ⋅ 107

-10 -5 0 5 10 15 20 25 30 35

R1 [

Ω]

T [oC]

f(T) = 5.35 ⋅ 106 ⋅ 0.85

T

g(T) = 5.41 ⋅ 106 ⋅ 0.85

T

cooling

heating

f(T)

g(T)

2.0 ⋅ 103

1.0 ⋅ 105

2.0 ⋅ 105

4.0 ⋅ 105

6.0 ⋅ 105

8.0 ⋅ 105

1.0 ⋅ 106

1.2 ⋅ 106

1.4 ⋅ 106

-10 -5 0 5 10 15 20 25 30 35

R2 [

Ω]

T [oC]

f(T) = 13.58 ⋅ 104 ⋅ 0.81

T

g(T) = 12.41 ⋅ 104 ⋅ 0.79

T

cooling

heating

f(T)

g(T)

0

2000

4000

6000

8000

10000

12000

14000

101

102

103

104

105

106

107

ε'

f (Hz)

35oC

30oC

25oC

20oC

15oC

10oC

5oC

0oC

-5oC

-10oC

10

20

30

40

50

60

70

80

90

100

101

102

103

104

105

106

107

ε'

f (Hz)

0

50000

100000

150000

200000

250000

101

102

103

104

105

106

107

ε"

f (Hz)

35oC

30oC

25oC

20oC

15oC

10oC

5oC

0oC

-5oC

-10oC

4

6

8

10

12

14

16

18

20

102

103

104

105

106

107

ε"

f (Hz)

10

TABLES

Table 1. Physicochemical properties of the acacia honey tested in the Bee Products Quality Testing Laboratory in Puławy.

Property Norm / test procedure Unit Value

Water content PN-88/A-77626 ptk. 5.3.3. refractometric method (A)

%16.1

Fructose content PB-02: E 05 from 17.02.2012 r.

HPLC method g/100g

41.2

Glucose content 29.6

Sucrose content <0.5

HMF content PB-04: E 03 z 14.01.11 r. HPLC method (A)

mg/kg 10.9

Electricalconductivity

PN-88/A-77626 p. 5.3.10. conductivity method (A)

mS/cm0.17

Diastase number PB-06: E 03 from 14.01.11/ Phadebas method (A)

Schade*20.5

pH and free acids PB-07: E 03 from 14.01.11/Potentiometric method (A)

meq/kg

pH 4.22

free acids 15.3

11

Table 2. The curve fitting parameters referring to Figs. 6-9.

C - model: y = aT+b

cooling heating

a = -1.580 · 10-13 ± 0.006 · 10-13 a = -1.35 · 10-13 ± 0.02 · 10-13

b = 2.840 · 10-11 ± 0.001-11 b = 2.800 · 10-11 ± 0.004-11

R2 = 0.9947 R2 = 0.9716

R1 - model: y = abT

cooling heating

a = 5.35 · 106 ± 0.03 · 106 a = 5.41 · 106 ± 0.08 · 106

b = 0.850 ± 0.001 b = 0.850 ± 0.001

R2 = 0.9922 R2 = 0.9909

R2 - model: y = abT

cooling heating

a = 13.58 · 104 ± 0.12 · 104 a = 12.41 · 104 ± 0.29 · 104

b = 0.810 ± 0.001 b = 0.790 ± 0.002

R2 = 0.9920 R2 = 0.9909

Q - model: y = abT

cooling heating

a = 9.34 · 10-10 ± 1.22 · 10-10 a = 7.93 · 10-10 ± 1.58 · 10-10

b = 1.25 ± 0.01 b = 1.20 ± 0.01

R2 = 0.8919 R2 = 0.9732

12

Highlights• Applicability of an innovative impedance spectroscopy sensor for the measurement of

honey’s electrical parameters.

• Collection of electrical parameters of acacia honey by the prototype sensor in 20 Hz – 20 MHz frequency range.

• The temperature dependence of the electrical equivalent circuit (EEC) elements of the sensor-honey system.