Influence of Variety, Moisture Content, Kernel Size and Applied Current Frequency on the Electric Properties of Wheat Grain

Abstract

The work aimed at determination of electric properties of wheat grain in dependence on its variety, moisture, geometrical features of kernels and applied current frequency. Wheat grain of 4 Polish winter varieties: Korweta, Juma, Mikon and Kobra from harvest 2001 were used as the material for study. Grain was sized into 3 fractions: (1) > 2,8mm, (2) 2,5–2,8mm, and (3) 2,2–2,5mm. Basic geometrical features were determined for not sorted grain (control sample) and its three fractions by the use of digital image analysis. Electric properties of grain (at 11% and 15% moisture content) have been performed with the Hewlett Packard 4263B meter. Measurements of impedance, resistance, admittance, conductance, as well as equivalent parallel capacitance and equivalent series capacitance were made. Obtained results were subjected to statistical analysis with the use of Statistica™ programme. Changes in electric properties of grain significantly depended on all of studied factors. Most of all significant correlations appeared between geometrical features and studied electric properties of grain of 15% moisture. Statistical analysis of the results proved significant linear correlations between electric properties of kernels and their length, perimeter and circularity coefficient R_C2 at higher measurement frequencies.

Keywords:

• Wheat grain
• Variety
• Grain moisture content
• Geometrical features of kernels
• Digital image analysis
• Electric properties of grain
• Current frequency
• Correlations

INTRODUCTION

Grain of cereals owing to its cellular structure and chemical constitution reveals properties of electric current conductors and dielectrics, as well.[1,2,3] Its ability to conduct electric current results from specific properties of kernels cellular solution (properties of electrolyte), in which water is a solvent of some chemical constituents and also a medium of ionic migration.[3] Intercellular structural elements of kernels tissue and some constituents of cells are semiconductors, but they also reveal properties of dielectrics. Likewise, water included in kernels tissue reveals dielectric properties. Cereal grains from the view-point of physics of dielectrics belong to group of dielectrics with weak polarizability (heterogeneous dielectrics).[4,5,6] The quantity of electric charge accumulated in kernels is the factor deciding about behaviour of grain in the electric field.[2] If we put grain in unionized electric field, it undergoes to polarization. This phenomenon lies in appearance of electric charges on the surface of grain. They are called “induced charges”. They generate in grain (dielectric) a field with direction opposite to the external field, called “depolarization field”. Grain becomes electric dipole which interferes external field.[2,5] If grain mass is subjected to polarization in identical conditions, then electric charge accumulated by individual kernels will be closely depended on their geometrical features, morphology, surface texture, mass, and chemical constitution. Next, these physical properties are closely related with grain species and even with its variety. Important factors influencing on the electric properties of grain are also parameters of measurement current and other experimental conditions, for example temperature of measurement, kind, and arrangement of used electrodes.[2,7]

The electric properties of foods and biomaterials have been measured for over 50 years.[4] Generally, the electric properties of food materials may be determined by various measuring techniques, in frequency intervals from direct current to so-called optical frequencies (frequency ranges from 10¹¹ to 10¹⁷ Hz, corresponded to invisible and visible rays). Lumped circuit methods may be used to determine permittivity over the frequency range from zero to approximately 2 × 10⁸ Hz. At frequencies from 10⁰ to 10⁷ Hz, the capacitance and dissipation factor of samples are measured by a capacitance bridge. At frequencies from 10⁴ to 10⁸ Hz, resonant circuits with fixed inductors and variable capacitors may be used, with resonance indicated by voltmeter deflection. Resonance methods also measure the material's capacitance.[8] Until now, there are no elaborated standard measuring methods the aim of which has been to determine univocally the electric properties of cereal grain. Thus, these physical properties of cereal grain are relatively little recognized.[4,7]

The dielectric properties of grain are of interest mainly because of their correlation with grain moisture content and their usefulness for rapid moisture content determination.[9,10,11] These electric properties of grain have long been utilized by electric and electronic grain moisture meters, which sense the electric conductivity or dielectric properties of grain and are calibrated to read moisture content. The knowledge about electric properties of grain is also useful in designing of equipment for electrotreatment of grain, i.e., dielectric drying, electrostatic cleaning and sorting, disinsectization, and also electrical stimulation.[2,4,5,12,13]

Taking the above into consideration, the aim of investigations was to determine some electric properties of winter wheat grain in dependence on its variety, moisture, geometrical features of kernels, and applied current frequency.

MATERIALS AND METHODS

The research material was wheat grain of 4 Polish winter varieties: Korweta, Juma Mikon and Kobra derived from goods production (harvest from year 2001). Grain was cleaned and then mechanically sized into 3 fractions: (1) >2.8 mm, (2) 2.5–2.8 mm, and (3) 2.2–2.5 mm, according to grain thickness criterion, by the use of Steinecker-Vogel sieves. Control sample (not sorted grain) was also separated for further investigations. Afterwards grain was conditioned to two levels of moisture: 11 ± 0.2% and 15 ± 0.2%.

Before conditioning, moisture of grain was determined, according to PN-ISO 712:2002 (routine reference method). Next, two sets (2 × 16) of 1500 g grain samples with required two levels of moisture were prepared separately. The quantity of water needed to add to grain for its conditioning to the required level of moisture was calculated from the initial level of grain moisture and grain mass. After addition of water, grain samples were carefully mixed and placed in glass jars tightly closed by ground-in glass stoppers. After 48 h conditioning (in room temperature) of such prepared grain samples, moisture of grain was again determined for checking results (accuracy) of this operation. During the experimental process, proper moisture level of conditioned grain samples (preserved in closed glass jars) was maintained without of grain moisture losses. Glass jars with grain samples were opened only in moments of taking grain for measurement of geometrical features of kernels and for filling special glass container with electrodes.

Basic geometrical features of kernels from grain control sample and its 3 fractions (at 2 levels of moisture) were determined using digital image analysis according to previously described method.[14] The experimental stand consisted of a digital camera Olympus Z3040, illumination chamber (75×100×60 cm in height, width and depth respectively) with glow scattered light and computer with MultiScan Base ver. 11.06 (Computer Scanning System Ltd) software. During all measurements the camera was fixed 65 cm above the sample. Wheat kernels (n=200) were placed on the black surface in endosperm cavity downwards position, in parallel to the length axis. Afterwards they were uniformly illuminated from the top by the white light. All images taken were saved into a computer disk under uncompressed TIF format, 1024×768 pixels resolution and 24-bits colour depth. Basic geometrical features of kernels i.e., length, width, perimeter and surface area were determined and three shape coefficients were calculated—shape coefficient R_S and circularity coefficients R_C1, R_C2, described by formulas 1–3:[14,15]

(1)

(2)

(3)

where: L: kernel perimeter (mm) and S: kernel surface area (mm²). These three shape coefficients were chosen for calculations intentionally from others, tested previously in another investigations.[14] Shape coefficient R_S and circularity coefficients R_C1, R_C2, were useful because of their high sensibility to changes in size and shape of studied wheat grain kernels. They are many shape coefficients, but first of all they should effectively diversify geometric figures with various shape.[15]

The electric properties of grain (of 11% and 15% moisture) were measured by a Hewlett Packard 4263B meter (Hewlett Packard, USA). Applying parameters were: a sinusoidal voltage of 250 mV, current frequencies of 0.1; 1; 10 and 100 kHz. Before measurement of electric properties, wheat grain samples (in closed glass jars) were placed in the air washer (Memmert) in order to reach the measurement temperature of 20 ± 0.1°C. After reaching by grain the required temperature, 200 cm³ grain samples were taken successively from glass jars for filling home made glass container (60×80×42 mm in height, width, and depth, respectively). Plate electrodes, made from acid-proof steel, were attached adjacent to two opposite walls of container (those of the bigger area). Active surface of the electrodes was 0.0048 m². The following values were measured: impedance (Z), resistance (R), admittance (Y), conductance (G), as well as equivalent parallel capacitance (C_p), and equivalent series capacitance (C_s). Measurements were repeated 6 times according to the electric model previously described.[16,17]

Next, on the basis of the measurements obtained, specific: impedance (Z_sp), resistance (R_sp), admittance (Y_sp) and conductance (G_sp) were calculated using formulas 4–7:[17]

(4)

(5)

(6)

(7)

where: l is distance between the electrodes (m), and s is active surface of electrodes (m²). The results of investigations were statistically analyzed using the Statistica™ 6.1 programme. The significance of differences between the mean values of parameters describing electric properties of grain was determined by analysis of variance (Duncan's test). Linear correlations analysis was performed (p < 0.05) to investigate the relationship among geometrical features of studied wheat grain and parameters describing its electric properties.

RESULTS AND DISCUSSION

The results of geometric features measurements of kernels of studied wheat varieties (only for grain of 15% moisture) are placed in Table 1. They were chosen for presentation, because at this moisture level occurred significant changes in electric properties of grain closely related to geometrical features of kernels.

Table 1 Geometrical features of kernels of analyzed wheat varieties (moisture of grain: 15%)

Wheat variety / grain fraction	Length (mm)			Width (mm)			L (mm)			S (mm^2)			RS (−)			RC1 (−)			RC2 (−)
	Min-max	Mean	c.v. ∗	Min-max	Mean	c.v.	Min-max	Mean	c.v.	Min-max	Mean	c.v.	Min-max	Mean	c.v.	Min-max	Mean	c.v.	Min-max	Mean	c.v.
Korweta
Control sample	6.06–8.25	7.01	5.5	2.68–4.17	3.02	9.6	15.43–20.51	17.25	5.7	15.45–25.70	17.80	12.6	1.19–1.53	1.34	4.7	4.44–5.72	4.75	6.4	4.92–6.53	5.49	5.7
Fraction 1	6.37–7.84	7.30	4.3	2.35–3.80	3.26	7.3	15.79–18.72	18.13	4.1	13.68–21.93	19.93	8.4	1.22–1.55	1.32	4.2	4.18–5.28	5.03	4.2	5.03–5.96	5.77	4.1
Fraction 2	5.62–7.58	7.09	3.8	2.21–3.39	3.07	7.7	13.87–18.42	17.44	3.5	11.06–19.45	18.12	8.3	1.22–1.56	1.34	4.4	3.75–4.98	4.80	4.1	4.42–5.87	5.55	3.5
Fraction 3	5.76–7.97	6.72	5.1	2.22–3.90	2.74	9.1	14.12–20.16	16.29	4.8	11.91–25.23	15.39	10.9	1.20–1.55	1.38	5.0	3.90–5.67	4.42	5.5	4.50–6.42	5.19	4.8
Juma
Control sample	5.88–7.57	6.62	5.5	2.84–3.90	3.26	7,6	14.87–19.43	16.62	5.5	13.67–23.86	17.56	11.2	1.18–1.33	1.26	3.6	4.17–5.51	4.72	5.7	4.74–6.19	5.29	5.5
Fraction 1	5.47–7.31	6.78	4.1	2.48–3.64	3.38	5.6	13.73–18.66	17.10	4.0	12.02–21.84	18.71	8.1	1.18–1.42	1.25	2.7	3.91–5.27	4.88	4.1	4.37–5.94	5.45	4.0
Fraction 2	5.34–7.36	6.39	5.5	2.21–3.29	3.06	7.5	12.94–17.73	15.92	5.4	9.98–18.13	15.87	10.9	1.19–1.58	1.28	3.2	3.56–4.81	4.49	5.5	4.12–5.65	5.07	5.4
Fraction 3	5.61–7.60	6.25	5.9	2.34–3.77	2.83	7.7	14.19–18.90	15.37	5.4	11.83–22.26	14.40	10.3	1.17–1.49	1.31	4.7	3.88–5.33	4.28	5.2	4.52–6.02	4.89	5.4
Mikon
Control sample	5.66–8.32	7.01	6.3	2.60–4.08	3.35	9.2	14.38–19.78	17.51	6.1	13.63–24.74	19.32	12.6	1.16–1.68	1.27	4.6	4.17–5.61	4.95	6.4	4.58–6.30	5.58	6.1
Fraction 1	6.38–8.39	7.21	5.1	2.86–4.08	3.53	5.7	16.14–20.92	18.11	4.5	16.46–26.39	20.89	8.3	1.18–1.38	1.25	3.2	4.58–5.80	5.16	4.2	5.14–6.66	5.77	4.5
Fraction 2	5.88–7.67	6.89	4.7	2.48–3.91	3.23	7.6	15.04–18.91	17.10	4.4	13.67–22.60	18.24	9.1	1.17–1.45	1.28	3.9	4.17–5.37	4.82	4.6	4.79–6.02	5.45	4.4
Fraction 3	5.46–7.86	6.58	6.7	2.08–3.67	2.98	9.0	13.25–18.91	16.17	6.3	10.77–21.20	15.98	12.9	1.16–1.54	1.31	4.7	3.70–5.20	4.50	6.4	4.22–6.02	5.15	6.3
Kobra
Control sample	6.11–7.96	7.07	6.1	2.73–3.90	3.25	8.8	15.43–19.90	17.48	6.2	14.81–24.92	18.82	13.0	1.19–1.44	1.30	4.1	4.34–5.63	4.89	6.6	4.91–6.34	5.57	6.2
Fraction 1	6.12–8.22	7.17	5.1	2.73–3.93	3.36	6.9	15.50–19.92	17.85	4.5	15.69–24.45	19.81	9.1	1.19–1.49	1.28	4.1	4.47–5.58	5.02	4.6	4.94–6.34	5.69	4.5
Fraction 2	5.75–7.79	7.14	5.4	2.47–3.90	3.32	6.0	14.34–18.76	17.74	4.9	12.37–22.68	19.52	9.4	1.18–1.55	1.30	3.5	3.97–5.38	4.98	4.7	4.57–5.97	5.65	4.9
Fraction 3	5.73–8.32	6.80	5.4	2.47–4.08	3.17	9.6	14.29–20.25	16.82	5.6	12.66–24.79	17.42	12.7	1.19–1.50	1.30	4.5	4.02–5.62	4.70	6.4	4.55–6.45	5.36	5.6
∗c.v. : coefficient of variation (%).

Kernels of wheat varieties used in this study showed differences in geometrical and shape related parameters depended on wheat grain variety and size. It was concluded, that the highest values of coefficients of variation (c.v.%) for geometrical features of grain of all studied wheat varieties were occurred in the case of surface area (S) and width of kernels, the lower ones—in the case of their shape coefficient R_S, length, and perimeter (L) and also circularity coefficient R_C2, calculated from grain perimeter. Sorting of wheat grain into fractions significantly lowered values of variation coefficients of all geometrical features of kernels from grain fractions 1 (>2.8 mm) and 2 (2.5–2.8 mm) comparing to c.v.% values of these features for kernels from grain control sample (not sorted) and fraction 3 (2.2–2.5 mm). Wheat grain of variety Korweta, which belonged to the highest class of technological quality (A), was characterized by the highest values of kernel shape coefficient R_S, in the range of 1.32–1.38, depending on grain fraction. Higher values of R_S testify to more elongated shape of kernels of this wheat variety. As regards to circularity coefficients R_C1, R_C2, the highest values of these geometrical features were found in the case of the most shapely kernels from grain fraction 1 (>2.8 mm) of all studied wheat varieties.

Substantial work dealing with the use of different morphological (size and shape) features for classification of different cereal grains and varieties has been reported in the literature.[14,18] Out of a great number of morphological features evaluated, kernel length and perimeter (in our investigations characterized by low variability) were of utmost importance for this classification and for mathematical description of kernel morphological model.[18] Moreover, the results of the studies of wheat grain technological quality parameters in relation to its geometrical features showed that kernel length and shape coefficient R_S significantly correlated with considerably number of investigated quality determinants (some physical properties of grain, its milling quality, quantity and quality of starch, protein, and dough rheological properties).[14]

Changes in the electric properties of studied wheat grain are presented in 2–5. As the frequency of applied current increased, the specific resistivity of grain (Z_sp and R_sp) and its capacitance (C_p and C_s) decreased. However, decreasing of values C_p and C_s followed considerably slower comparing with fast drop in values of grain specific resistivity. Moreover, increase of current frequency caused increasing of the specific conductivity of grain (Y_sp and G_sp). Such tendencies of changes of analyzed electric properties occurred in case of grain of 11% moisture and of 15% moisture, as well.

Table 2 Electric properties of wheat grain depending on wheat variety, grain fraction and moisture. f = 0.1 kHz

	Specific electric resistivity				Specific electric conductivity				Electric capacitance
Wheat variety / grain fraction	Zsp (MΩm)		Rsp (MΩm)		Ysp (μS/m)		Gsp (μS/m)		Cp (pF)		Cs (pF)
	Moisture of grain				Moisture of grain				Moisture of grain
	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%
Korweta
Control sample	20.88 ^cd	5.77 ^bd	4.90	2.47 ^cd	0.044 ^cd	0.175 ^bd	0.009 ^d	0.079 ^bd	8.5 ^cd	29.5 ^bd	9.1 ^cd	37.3 ^bd
Fraction 1	21.28 ^cd	5.11 ^acd	4.85	2.47 ^cd	0.044 ^cd	0.201 ^acd	0.009 ^d	0.114 ^acd	8.4 ^cd	32.8 ^acd	8.7 ^cd	45.7 ^acd
Fraction 2	19.17 ^abd	5.77 ^bd	4.58	2.57 ^abd	0.053 ^abd	0.175 ^bd	0.009 ^d	0.088 ^bd	9.2 ^abd	29.2 ^bd	9.8 ^abd	37.9 ^bd
Fraction 3	17.23 ^abc	7.01 ^abc	4.33	2.16 ^abc	0.061 ^abc	0.149 ^abc	0.018 ^ab	0.053 ^abc	10.2 ^abc	25.5 ^abc	10.9 ^abc	28.8 ^abc
Juma
Control sample	18.75 ^bcd	3.62 ^cd	4.07	2.69 ^c	0.053 ^bcd	0.289 ^cd	0.009	0.236 ^cd	9.4 ^bcd	33.3 ^cd	9.9 ^bcd	90.8 ^cd
Fraction 1	21.23 ^acd	3.55 ^cd	4.34	2.72 ^c	0.044 ^a	0.289 ^cd	0.009	0.245 ^cd	8.3 ^ac	33.9 ^cd	8.8 ^acd	93.2 ^cd
Fraction 2	22.32 ^ab	4.77 ^abd	4.14	3.12 ^abd	0.044 ^a	0.219 ^abd	0.009	0.158 ^abd	7.9 ^acd	29.3 ^abd	8.3 ^ab	59.6 ^abd
Fraction 3	22.13 ^ab	5.45 ^abc	4.01	2.63 ^c	0.044 ^a	0.184 ^abc	0.009	0.131 ^abc	8.4 ^ac	25.2 ^abc	8.4 ^ab	42.5 ^abc
Mikon
Control sample	20.43	6.07 ^bcd	4.91	2.74 ^bd	0.053	0.166 ^bcd	0.009	0.079 ^bcd	8.6	27.5 ^bcd	9.1	36.4 ^bcd
Fraction 1	20.61	2.83 ^acd	4.45	2.22 ^ac	0.053	0.376 ^acd	0.009	0.341 ^acd	8.8	37.1 ^acd	9.4	47.2 ^acd
Fraction 2	19.79	7.18 ^abd	4.67	2.66 ^bd	0.053	0.140 ^abd	0.009	0.053 ^abd	8.7	24.1 ^abd	9.1	28.6 ^ab
Fraction 3	19.54	7.79 ^abc	4.59	2.24 ^ac	0.053	0.131 ^abc	0.009	0.035 ^abc	9.0	22.9 ^abc	9.2	25.1 ^ab
Kobra
Control sample	20.72 ^bc	5.44 ^bcd	5.07	2.77 ^cd	0.044 ^bcd	0.184 ^bcd	0.009	0.105 ^bc	8.7	29.7 ^bcd	9.0 ^c	42.0 ^bcd
Fraction 1	19.34 ^ad	3.99 ^acd	4.51	2.69 ^cd	0.053 ^a	0.263 ^acd	0.009	0.201 ^acd	9.1	34.5 ^acd	9.6 ^d	74.7 ^acd
Fraction 2	19.29 ^ad	4.78 ^abd	4.42	2.89 ^abd	0.053 ^a	0.219 ^abd	0.009	0.140 ^abd	9.1	31.2 ^abd	9.9 ^ad	53.8 ^abd
Fraction 3	21.05 ^bc	6.10 ^abc	5.04	3.07 ^abc	0.053 ^a	0.166 ^abc	0.009	0.088 ^bc	8.8	26.7 ^abc	8.9 ^bc	37.6 ^abc
^a–dMeans put in the same columns and marked with different letters differ significantly at p < 0.05.

Table 3 Electric properties of wheat grain depending on wheat variety, grain fraction and moisture. f = 1 kHz

Wheat variety / grain fraction	Specific electric resistivity				Specific electric conductivity				Electric capacitance
	Zsp (MΩm)		Rsp (MΩm)		Ysp (μS/m)		Gsp (μS/m)		Cp (pF)		Cs (pF)
	Moisture of grain				Moisture of grain				Moisture of grain
	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%
Korweta
Control sample	2.70 ^cd	0.617 ^bcd	0.309 ^cd	0.183 ^bcd	0.368	1.26 ^bd	0.053 ^bcd	0.306 ^bd	6.7 ^cd	22.3 ^bd	6.8 ^cd	23.8 ^bd
Fraction 1	2.67 ^cd	0.743 ^ad	0.320 ^cd	0.206 ^acd	0.376	1.36 ^acd	0.044 ^acd	0.394 ^acd	6.7 ^cd	23.7 ^acd	6.8 ^cd	25.9 ^acd
Fraction 2	2.56 ^abd	0.800 ^a	0.366 ^abd	0.194 ^abd	0.394	1.24 ^bd	0.061 ^abd	0.315 ^bd	7.0 ^abd	22.0 ^bd	7.2 ^abd	23.5 ^bd
Fraction 3	2.41 ^abc	0.869 ^a	0.434 ^abc	0.171 ^abc	0.411	1.16 ^abc	0.079 ^abc	0.236 ^abc	7.4 ^abc	20.6 ^abc	7.6 ^abc	21.5 ^abc
Juma
Control sample	2.43 ^bcd	0.754 ^d	0.343 ^bcd	0.309 ^bcd	0.411 ^b	1.32 ^cd	0.061 ^bcd	0.551 ^cd	7.3 ^bcd	22.0 ^cd	7.5 ^bcd	26.6 ^cd
Fraction 1	2.62 ^ac	0.766 ^d	0.297 ^a	0.320 ^acd	0.324 ^a	1.31 ^cd	0.044 ^acd	0.569 ^cd	6.9 ^acd	21.7 ^cd	7.0 ^acd	26.6 ^cd
Fraction 2	2.70 ^abd	0.834 ^d	0.263 ^a	0.297 ^abd	0.368	1.19 ^abd	0.035 ^ab	0.420 ^abd	6.7 ^ab	20.3 ^abd	6.7 ^ab	23.2 ^abd
Fraction 3	2.63 ^ac	0.937 ^bc	0.274 ^a	0.274 ^abc	0.376	1.11 ^abc	0.035 ^ab	0.306 ^abc	6.7 ^ab	19.5 ^abc	6.7 ^ab	21.3 ^abc
Mikon
Control sample	2.65	0.846 ^bd	0.343 ^d	0.217 ^bcd	0.376	1.19 ^bcd	0.053 ^d	0.306 ^bcd	6.8 ^bc	20.9 ^bcd	6.9	22.4 ^bcd
Fraction 1	2.59	0.686 ^acd	0.343 ^d	0.320 ^acd	0.385	1.46 ^acd	0.053 ^d	0.700 ^acd	6.9 ^ad	23.5 ^acd	7.0	30.6 ^acd
vFraction 2	2.62	0.949 ^b	0.354	0.229 ^ab	0.385	1.05 ^abd	0.053 ^d	0.263 ^abd	6.9 ^ad	18.6 ^abd	7.0	19.8 ^abd
Fraction 3	2.64	0.994 ^ab	0.389 ^ab	0.229 ^ab	0.376	1.01 ^abc	0.061 ^abc	0.236 ^abc	6.8 ^bc	17.9 ^abc	6.9	19.0 ^abc
Kobra
Control sample	2.64 ^bc	0.811	0.331	0.206 ^bc	0.324 ^bcd	1.23 ^bd	0.044 ^bcd	0.324 ^bcd	6.8 ^bc	21.8 ^bd	6.9 ^bc	23.4 ^bcd
Fraction 1	2.53 ^ad	0.731 ^d	0.331	0.263 ^acd	0.394 ^a	1.37 ^acd	0.053 ^a	0.499 ^acd	7.1 ^ad	23.2 ^acd	7.2 ^ad	26.9 ^acd
Fraction 2	2.53 ^ad	0.789	0.343	0.240 ^abd	0.394 ^a	1.26 ^bd	0.053 ^a	0.385 ^abd	7.1 ^ad	21.9 ^bd	7.2 ^ad	24.3 ^abd
Fraction 3	2.64 ^bc	0.869 ^b	0.343	0.217 ^bc	0.385 ^a	1.15 ^abc	0.053 ^a	0.289 ^abc	6.8 ^bc	20.3 ^abc	7.0 ^bc	21.6 ^abc
^a–dMeans put in the same columns and marked with different letters differ significantly at p < 0.05.

Table 4 Electric properties of wheat grain depending on wheat variety, grain fraction and moisture. f = 10 kHz

	Specific electric resistivity				Specific electric conductivity				Electric capacitance
	Zsp (MΩm)		Rsp (MΩm)		Ysp (μS/m)		Gsp (μS/m)		Cp (pF)		Cs (pF)
Wheat variety / grain fraction	Moisture of grain				Moisture of grain				Moisture of grain
	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%
Korweta
Control sample	0.309 ^bcd	0.114 ^bd	0.018 ^bcd	0.034 ^b	3.29 ^bcd	8.77 ^b	0.201 ^bcd	2.68 ^bd	5.9 ^bcd	15.2 ^d	5.8 ^bcd	16.8 ^bd
Fraction 1	0.297 ^acd	0.103 ^acd	0.016 ^acd	0.037 ^acd	3.36 ^acd	6.79 ^acd	0.175 ^acd	2.96 ^acd	6.1 ^acd	15.4 ^cd	6.1 ^ad	17.3 ^acd
Fraction 2	0.286 ^ab	0.114 ^bd	0.019 ^abd	0.034 ^b	3.44 ^ab	8.70 ^b	0.219 ^abd	2.62 ^bd	6.2 ^ab	15.1 ^bd	6.2 ^a	16.6 ^bd
Fraction 3	0.286 ^ab	0.126 ^abc	0.024 ^abc	0.035 ^b	3.47 ^ab	8.23 ^b	0.289 ^abc	2.40 ^abc	6.3 ^ab	14.3 ^abc	6.3 ^ab	15.6 ^abc
Juma
Control sample	0.274 ^bcd	0.114 ^bcd	0.019 ^bcd	0.040 ^b	3.56 ^bcd	8.17	0.236 ^bcd	2.67 ^cd	6.4 ^bcd	14.0 ^bcd	6.4 ^bcd	15.7 ^bcd
Fraction 1	0.286 ^acd	0.126 ^acd	0.016 ^acd	0.042 ^acd	3.39 ^a	7.93	0.184 ^acd	2.63 ^cd	6.2 ^acd	13.6 ^ad	6.2 ^ad	15.3 ^acd
Fraction 2	0.297 ^abd	0.137 ^abd	0.014 ^abd	0.040 ^b	3.37 ^ad	7.74	0.158 ^ab	2.35 ^abd	6.1 ^abd	13.4 ^ad	6.1 ^ad	14.7 ^abd
Fraction 3	0.309 ^abc	0.149 ^abc	0.015 ^abc	0.040 ^b	3.40 ^ac	7.83	0,158 ^ab	2.21 ^abc	5.9 ^abc	12.9 ^abc	6.0 ^abc	14.0 ^abc
Mikon
Control sample	0.297 ^d	0.126 ^bcd	0.018 ^bd	0.039 ^bcd	3.35	8.05	0.193 ^bcd	2.51 ^bcd	6.1 ^d	13.9 ^bcd	6.1	15.4 ^bcd
Fraction 1	0.297 ^d	0.114 ^acd	0.017 ^acd	0.040 ^acd	3.40 ^d	8.92 ^d	0.201 ^ad	3.15 ^acd	6.1 ^d	15.2 ^acd	6.2 ^d	17.3 ^acd
Fraction 2	0.297 ^d	0.137 ^abd	0.018 ^bd	0.042 ^abd	3.38 ^d	7.07	0.201 ^ad	2.14 ^abd	6.1 ^d	12.3 ^abd	6.1	13.5 ^abd
Fraction 3	0.309 ^abc	0.149 ^abc	0.021 ^abc	0.043 ^abc	3.30 ^bc	6.81 ^b	0.219 ^abc	2.02 ^abc	6.0 ^abc	11.8 ^abc	6.0 ^b	13.0 ^abc
Kobra
Control sample	0.297 ^bc	0.114 ^bd	0.016 ^bcd	0.033 ^bd	3.38 ^bc	8.89	0.193 ^bcd	2.59 ^bd	6.1 ^bc	15.4 ^d	6.1 ^b	16.8 ^bd
Fraction 1	0.286 ^ad	0.103 ^acd	0.017 ^acd	0.035 ^acd	3.52 ^ad	9.08	0.210 ^ad	2.91 ^acd	6.3 ^ad	15.7 ^d	6.4 ^ad	17.4 ^acd
Fraction 2	0.286 ^ad	0.114 ^bd	0.018 ^abd	0.033 ^bd	3.47 ^ad	8.90	0.210 ^ad	2.62 ^bd	6.3 ^ad	15.5 ^d	6.3	16.9 ^bd
Fraction 3	0.297 ^bc	0.126 ^abc	0.019 ^abc	0.034 ^abc	3.40 ^bc	8.27	0.201 ^abc	2.35 ^abc	6.1 ^bc	14.4 ^abc	6.1 ^b	15.7 ^abc
^a–dMeans put in the same columns and marked with different letters differ significantly at p < 0.05.

Table 5 Electric properties of wheat grain depending on wheat variety, grain fraction and moisture. f = 100 kHz

Wheat variety / grain fraction	Specific electric resistivity				Specific electric conductivity				Electric capacitance
	Zsp (MΩm)		Rsp (MΩm)		Ysp (μS/m)		Gsp (μS/m)		Cp (pF)		Cs (pF)
	Moisture of grain				Moisture of grain				Moisture of grain
	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%	11%	15%
Korweta
Control sample	0.032 ^bcd	0.019 ^bd	0.0011 ^bd	0.0053 ^c	31.06 ^bcd	52.59 ^d	1.06 ^bcd	14.73 ^bd	5.6 ^bcd	9.2 ^d	5.6 ^bcd	10.0 ^d
Fraction 1	0.031 ^a	0.018 ^acd	0.0010 ^acd	0.0053 ^c	31.94 ^a	53.20 ^d	1.02 ^acd	12.90 ^acd	5.8 ^a	9.3 ^d	5.8 ^a	10.0 ^d
Fraction 2	0.031 ^a	0.019 ^bd	0.0011 ^bd	0.0052 ^abd	32.38 ^a	52.59 ^d	1.14 ^abd	14.51 ^bd	5.9 ^a	9.2 ^d	5.9 ^a	9.9 ^d
Fraction 3	0.031 ^a	0.021 ^abc	0.0012 ^abc	0.0053 ^c	32.03 ^a	50.23 ^abc	1.31 ^abc	13.42 ^abc	5.9 ^a	8.8 ^abc	5.8 ^a	9.5 ^abc
Juma
Control sample	0.030 ^bcd	0.018 ^bcd	0.0010 ^bcd	0.0046 ^d	33.51 ^bcd	52.15 ^bcd	1.16 ^bcd	12.43 ^bcd	6.1 ^bcd	9.2 ^bcd	6.0 ^bcd	9.7 ^bcd
Fraction 1	0.031 ^a	0.019 ^acd	0.0009 ^ad	0.0046 ^d	32.64 ^ad	50.84 ^ad	0.99 ^a	11.74 ^a	5.9 ^acd	9.0 ^ad	5.9 ^acd	9.5 ^ad
Fraction 2	0.031 ^a	0.021 ^abd	0.0009 ^ad	0.0046 ^d	32.11 ^ad	50.14 ^ad	0.96 ^a	11.45 ^a	5.8 ^abd	8.9 ^ad	5.8 ^abd	9.4 ^ad
Fraction 3	0.031 ^a	0.022 ^abc	0.0011 ^abc	0.0047 ^abc	30.80 ^abc	48.56 ^abc	0.98 ^a	11.36 ^a	5.7 ^abc	8.7 ^abc	5.6 ^abc	9.1 ^abc
Mikon
Control sample	0.031 ^d	0.021 ^bcd	0.0010 ^d	0.0052 ^bcd	31.59 ^bd	48.91 ^bcd	1.05 ^d	12.66 ^bcd	5.7 ^d	8.6 ^bcd	5.7 ^d	9.2 ^bcd
Fraction 1	0.031 ^d	0.019 ^acd	0.0010 ^d	0.0053 ^acd	32.20 ^ad	52.94 ^acd	1.06 ^d	15.15 ^acd	5.8 ^d	9.2 ^acd	5.8 ^d	10.0 ^acd
Fraction 2	0.031 ^d	0.022 ^abd	0.0010 ^d	0.0049 ^abd	31.85 ^d	45.06 ^abd	1.06 ^d	9.87 ^ab	5.8 ^d	8.0 ^abd	5.8 ^d	8.4 ^abd
Fraction 3	0.032 ^abc	0.023 ^abc	0.0011 ^abc	0.0050 ^abc	30.98 ^abc	43.58 ^abc	1.10 ^abc	9.57 ^ab	5.6 ^abc	7.7 ^abc	5.6 ^abc	8.1 ^abc
Kobra
Control sample	0.031 ^b	0.018 ^bd	0.0009 ^cd	0.0051 ^c	32.03 ^bc	53.73 ^bd	1.02 ^bcd	14.89 ^d	5.8 ^b	9.4 ^bd	5.8 ^bc	10.2 ^bd
Fraction 1	0.030 ^acd	0.017 ^acd	0.0009 ^cd	0.0051 ^c	33.25 ^acd	54.69 ^ad	1.10 ^ad	15.23 ^d	6.0 ^acd	9.5 ^ad	6.0 ^acd	10.3 ^ad
Fraction 2	0.031 ^b	0.018 ^bd	0.0010 ^ab	0.0050 ^abd	32.73 ^abd	54.34 ^d	1.11 ^ad	15.00 ^d	5.9 ^bd	9.5 ^d	5.9 ^abd	10.3 ^d
Fraction 3	0.031 ^b	0.019 ^abc	0.0010 ^ab	0.0051 ^c	31.94 ^bc	51.10 ^abc	1.07 ^abc	13.36 ^abc	5.8 ^bc	9.0 ^abc	5.8 ^bc	9.6 ^abc
^a–dMeans put in the same columns and marked with different letters differ significantly at p < 0.05.

Simultaneously, as the moisture of grain increased, its electric resistivity decreased (except of specific resistance R_sp at measurement frequencies 10kHz and 100kHz – probably as a result of appearing polarization barrier), whereas its specific electric conductivity and electric capacitance increased in the range of all applied measurement frequencies. These changes were caused by passing kernels with higher moisture to the semiconductors state.[2] With increase of grain moisture, also its electric conductivity increased as a result of changes to which water included in grain is subjected reacting with its components, and changes to which are subjected these components as a result of hydration.[3]

In both cases of grain of 11% moisture as of 15% moisture, variety Juma differed from remaining studied wheat varieties by significantly lower specific impedance Z_sp at measurement frequency of 0.1kHz. Moreover, at measurement frequencies 0.1kHz and 1kHz, grain of variety Juma of 15% moisture was characterized by significantly higher specific admittance Y_sp and specific conductance G_sp than grain of remaining wheat varieties with this same level of moisture. The α–amylase activity in grain of this variety was significantly higher (falling number 177s), than in grain of remaining varieties (falling number in the range 317–421 s). This quality parameter was determined according to PN-ISO 3093 standard method, in grain of the same four wheat varieties, but in another previously conducted investigations.[14] Different biochemical state of grain of variety Juma was probably responsible for a little different electric properties, measured by above mentioned electric parameters. One ought to add, that in our another piloting investigations they were found significant correlations between specific electric resistance (R_sp) of grain of 9 winter wheat varieties and grain falling number.[19]

However, it is difficult to conclude univocally, because only 4 wheat grain varieties were investigated in this work, the electric property which the best of all differentiated analyzed grain varieties and fractions was specific electric conductance (G_sp). It was confirmed in our last piloting investigations concerning electric properties of grain of 17 varieties of winter and spring wheat.[19]

Significant and univocal changes in the electric properties of kernels related to their size were occurred in case of grain of 15% moisture. With decrease of kernels dimensions of all studied wheat varieties their specific electric resistivity increased which was expressed by Z_sp and decreased their specific electric conductivity (Y_sp and G_sp) and electric capacitance (C_p and C_s). It was confirmed by the results of analysis of linear correlations between geometrical features of studied wheat grain and its electric properties. These results are presented in Tables 6 and 7.

Table 6 Significant linear correlations∗ between geometrical features of wheat kernels and its electric properties. Moisture of grain: 11%

	Specific electric resistivity				Specific electric conductivity				Electric capacitance
	Zsp (MΩm)		Rsp (MΩm)		Ysp (μS/m)		Gsp (μS/m)		Cp (pF)		Cs (pF)
Geometrical features of wheat kernels	f				f				f
	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz
Length (mm)	—	—	—	—	—	—	—	—	—	—	—	—
Width (mm)	—	—	—	—	—	—	—	—	—	—	—	—
Perimeter (mm)	—	—	—	—	—	—	—	—	—	—	—	—
Surface area (mm^2)	—	—	—	—	—	—	—	—	—	—	—	—
Shape coefficient RS (—)	—	—	—	—	—	—	0.63	0.56	—	—	—	—
Circularity coefficient RC1 (—)	—	—	—	—	—	—	—	—	—	—	—	—
Circularity coefficient RC2 (—)	—	—	—	—	—	—	—	—	—	—	—	—
	f				f				f
	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz
Length (mm)	—	—	—	—	—	—	—	—	—	—	—	—
Width (mm)	—	—	−0.62	−0.72	—	—	—	−0.53	—	—	—	—
Perimeter (mm)	—	—	—	—	—	—	—	—	—	—	—	—
Surface area (mm^2)	—	—	—	—	—	—	—	—	—	—	—	—
Shape coefficient RS (—)	—	—	0.63	0.76	—	—	0.52	0.59	—	—	—	—
Circularity coefficient RC1 (—)	—	—	—	—	—	—	—	—	—	—	—	—
Circularity coefficient RC2 (—)	—	—	—	—	—	—	—	—	—	—	—	—
∗Critical coefficient of linear correlation r = 0.51.

Table 7 Significant linear correlations∗ between geometrical features of wheat kernels and its electric properties. Moisture of grain: 15%

	Specific electric resistivity				Specific electric conductivity				Electric capacitance
	Zsp (MΩm)		Rsp (MΩm)		Ysp (μS/m)		Gsp (μS/m)		Cp (pF)		Cs (pF)
Geometrical features of wheat kernels	f				f				f
	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz	0.1 kHz	1 kHz
Length (mm)	—	—	—	—	—	0.56	—	—	—	0.69	—	—
Width (mm)	−0.69	—	—	0.61	0.69	0.61	0.68	0.71	0.69	—	0.67	0.66
Perimeter (mm)	—	−0.57	—	—	—	0.66	—	—	0.6	0.73	—	0.58
Surface area (mm^2)	−0.58	−0.54	—	—	0.55	0.7	—	0.55	0.69	0.69	—	0.67
Shape coefficient RS (—)	0.62	—	—	−0.85	−0.65	—	−0.69	−0.69	—	—	−0.66	—
Circularity coefficient RC1 (—)	−0.58	−0.54	—	—	0.55	0.7	—	0.55	0.69	0.69	—	0.67
Circularity coefficient RC2 (—)	—	−0.57	—	—	—	0.66	—	—	0.6	0.73	—	0.58
	f				f				f
	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz	10 kHz	100 kHz
Length (mm)	−0.7	−0.54	—	0.69	—	0.57	0.69	0.76	0.73	0.52	0.75	0.61
Width (mm)	—	—	—	—	—	—	0.58	—	—	—	—	—
Perimeter (mm)	−0.68	−0.56	—	0.53	—	0.58	0.76	0.71	0.69	0.55	0.74	0.61
Surface area (mm^2)	−0.58	—	—	—	—	0.52	0.76	0.58	0.57	—	0.64	0.54
Shape coefficient RS (—)	—	—	—	0.55	—	—	—	—	—	—	—	—
Circularity coefficient RC1 (—)	−0.58	—	—	—	—	0.52	0.75	0.57	0.56	—	0.64	0.54
Circularity coefficient RC2 (—)	−0.68	−0.56	—	0.53	—	0.58	0.76	0.71	0.69	0.55	0.74	0.61
∗Critical coefficient of linear correlation r = 0.51.

The most significant correlations were found between electric properties of kernels of 15% moisture and their length, perimeter and circularity coefficient R_C2 at higher frequencies of current (10 kHz and 100 kHz). Specially remarkable were significant and relatively high correlations between length of kernels and their specific impedance Z_sp (r = –0.70) and between perimeter, surface area, circularity coefficients R_C1 and R_C2 of kernels and their specific conductance G_sp (r ≥ 0.75)—at current frequency of 10kHz, and also correlations between length, perimeter and circularity coefficient R_C2 of kernels and their specific conductance G_sp (adequately r = 0.76, r = 0.71 and r = 0.71)—at current frequency of 100kHz.

CONCLUSION

Changes in the electric properties of studied wheat grain significantly depended, first of all, on the applied frequency of current and grain moisture, next, on its geometrical features and wheat variety. With the increase of measurement frequency specific electric resistivity and electric capacitance of grain decreased, whereas increased its specific electric conductivity. With the increase of grain moisture, its specific electric resistivity also decreased and followed increasing of its specific electric conductivity and electric capacitance. Significant (p < 0.05) correlations occurred between electric properties of grain of 15% moisture and its geometrical features. The must significant linear correlations occurred between electric properties of kernels and their length, perimeter, and circularity coefficient R_C2 at higher measurement frequencies (10 kHz and 100 kHz). With the decrease of kernels dimensions (grain of 15% moisture) of all studied wheat varieties, specific impedance Z_sp increased and decreased specific electric conductivity (Y_sp and G_sp) and also electric capacitance (C_p and C_s). Observed changes in the conductive and capacitive electric properties of studied wheat grain in function of its variety, moisture, kernel geometrical features and applied current frequency make justifiable conducting further investigations in the range of quick evaluation and control of wheat grain quality with the use of its electric properties. Specially, that they were found significant correlations between geometrical features of wheat grain and its electric properties and these geometrical features significantly correlated with some technological quality parameters of wheat grain.

NOMENCLATURE

RS	=	Coefficient of shape
RC1	=	Coefficient of circularity
RC2	=	Coefficient of circularity
L	=	Perimeter of kernel, (mm)
S	=	Surface area of kernel, (mm2)
Z	=	Impedance, (MΩ)
R	=	Resistance, (MΩ)
Y	=	Admittance,(μS)
G	=	Conductance, (μS)
Cp	=	Equivalent parallel capacitance, (pF)
Cs	=	Equivalent series capacitance, (pF)
Zsp	=	Specific impedance, (MΩm)
Rsp	=	Specific resistance, (MΩm)
Ysp	=	Specific admittance, (μS/m)
Gsp	=	Specific conductance, (μS/m)
l	=	Distance between the electrodes, (m)
s	=	Active surface of electrodes, (m2)
f	=	Frequency of current, (kHz)

Notes

14. Majewska, K.M. Badania wyróżników jakości technologicznej ziarna pszenicy w aspekcie jego cech geometrycznych. [Studies of wheat grain technological quality parameters in the aspect of its geometrical features]. Thesis for the degree of associate professor. University of Warmia and Mazury Publishers, Olsztyn, Dissertations and monographs 2004, 98, 1–93. PL ISSN 1509–3018.