A b s t r a c t. Electrical properties of food materials can give information about the inner structure and physiological state of biological tissues. Generally, the process of drying of fruits and vegetables is followed by weight loss. The aim of this study was to measure the impedance spectra of carrot slices during drying and to correlate impedance parameters to moisture content in different drying periods. Cylindrical slices were cut out from the carrot root along the axis. The slices were dried in a Venticell 111 air oven at 50°C. The weight of the slices was measured with a Denver SI-603 electronic analytical and precision balance. The weighing of the samples was performed every 30 min at the beginning of drying and every 60 min after the process. The moisture content of the samples was calculated on wet basis. The magnitude and phase angle of electrical impedance of the slices were measured with HP 4284A and 4285A precision LCR meters in the frequency range from 30 Hz to 1 MHz and from 75 kHz to 30 MHz, respectively, at voltage 1 V. The impedance measurement was performed after weighting. The change in the magnitude of impedance during drying showed a good correlation with the change in the moisture content.

Determination of electrical properties is utilized in a wide range of disciplines and industries. A brief compendium of agricultural materials and food electrical properties exploitation is presented in this paper. The measurement of electrical conductivity or resistivity can be utilized at investigation of cell membrane properties on microscopic level. Moreover the electrical conductivity have utilization at the salinity of soils and irrigation water determination. Biological material properties are determined from their leachates too. The conductivity measurement are applied for determination of various characteristics of agricultural materials and food, for example for determination of the frost sensitiveness, of chilling and freezing tolerance, of moisture content, of seeds germination, of mechanical stress, of pasteurization, of other properties of grains, seeds, meat, sugar, milk, wood, soil, fruit and vegetable, infected food, … The utilization of dielectric properties are also described; for example in agricultural materials and food quality sensing (moisture content, maturity of fruit, freshness of eggs, potential insect control in seeds, radio frequency heating, …). The classification of permittivity measurement techniques at the low frequencies is mentioned. If the time τ is approaching to 0, charging component is approaching to ratio U/R. The line current decreases with the time and its value is approaching to the conduction component. Electric measurements by direct current are very influenced by the polarization effects at high moisture content of the material.If the current passing through the material is unsteady, for density of electric current is validwhere: ε -permittivity of material (F/m).Complex value of current density is valid in the case of alternating electric field in the shape î = (σ + jωε) Ewhere: E -complex value of electric field intensity, j -imaginary unit (1), ω -angular frequency (1/s).Permittivity of moist material must be considered to be complex. It has a real part ε´ and imaginary part ε″ σ ε = ε´ -jε˝ = ε -j --= ε (1 -jtgδ ε ) ω where: δ ε -the loss angle (1) of the dielectric and σ tgδ ε = ---ωε where: tgδ ε -tangent of loss angle (1).The maximum of dielectric losses is to be found around 20 GHz. This corresponds to a wavelength in air of about 15 mm and in many materials of about 5 mm to 10 mm. So a good measuring effect could be seen at this frequency, but only slight penetration of the material would be possible. That is why moisture measurement at very high frequencies is not useful for many applications. A lot of experiments had shown, that the well known ISM frequency 2.45 GHz is quite a good choice, because wavelength is about 10 times larger than at 20 GHz and dielectric losses are still high enough to be measured. ε r˝ is interpreted to include the energy losses in the dielectric arising from all dielectric relaxation mechanisms and ionic conduction. Materials such as agricultural material and foods generally have significant loss factors, and the diele...

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Knowledge of bee products' physical properties has a decisive importance for the monitoring of their quality. Thermophysical parameters are very important properties. Thermal conductivity and thermal diffusivity of selected bee products (honey, bee pollen and perga) were measured by two different methods. For identification of thermal conductivity and thermal diffusivity transient methods were used: Hot Wire (HW) and Dynamic Plane Source (DPS) method with an instrument Isomet 2104. The principle of measuring process is based on the analysis of timetemperature relation. In the first series of measurements thermal conductivity and diffusivity at constant laboratory temperature of 20 °C were measured. The second series was focused on identification of the changes in the thermophysical parameters during temperature stabilisation in the temperature range of 5-25 °C. For samples with constant temperature standard deviations and probable errors in % were calculated. For relations of thermal parameters to temperature graphical dependencies were obtained. Two different thermophysical methods were used for improvement of data reliability and data statistics.

VALACH MICHAL, MAREČEK JAN, HLAVÁČOVÁ ZUZANA, TRÁVNÍČEK PETR, GLOS JOSEF:Determination of impurities in biofuels with use of particle size analyzer. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 2013, LXI, No. 3, pp. 813-817 One of the qualitative parameter of biofuel is purity. Measurement of purity of biofuels is very important. The reason is that the number and size of particles infl uence an engine life. However number and size of various particles have infl uence on the hydrodynamics and rheologic properties. In the fi rst part of the paper is introduced the description of tested fuels, the method of measuring of number particles and size particles with use particle size analyzer Laser-Net Fines-C (LNF-C). In the second part of this paper results of measurements are stated. The results are compared and evaluated. Two kinds of fuels was tested -biofuels and traditional fuels. Pure rapeseed methyl ester and various mixtures of traditional diesels and rapeseed methyl ester represented Biofuels. Bioethanol was the next tested biofuels. Traditional diesels were the second group of tested fuels. particle analyses, particle counter, purity of biofuels Impurities in biofuels have the origin in several sources. At fi rst it is mechanical impurities (dust, abrasion) or products of chemical degradation of biofuels. If biofuels have high content of impurities, combustion process in an engine has worse running. The change of hydrodynamic behaviour of fuels or the change of rheological properties can be consequence too. In case that this factors concatenate, failure or accident of an engine an arise soon. The state of cleanliness in process can be monitored by detailed, accurate and periodic analysis of impurities in biofuels. For this purpose must be used simple device, which is not demanding in operation or maintenance. And samples need not complicated preparation. Impurities in biofuels are all foreign substances such as gases, liquids or solid particles. Sources of impurities are separated to four basic groups. Various complicated and exact methods we use for demanding of impurities total content. These methods are gravimetric method (fi ltering and weighing on the membrane fi lter), curing method and subsequent weighing, method of counting particles, which are collected on the fi lter with using of an automated optical microscope or method with using automatic particle counter.Currently liquid biofuels are o en discussed topic. But most of professional papers discuss topics on the theme of emissions, which are generated during biofuels combustion, their impact on the environment (Gasparatos et al., 2011;Acquaye et al., 2012;Liaquat et al., 2010) or problematic of biofuels production (Nigama et al., 2011). Currently purity of biofuels is not o en discussed topic. The aim of this work is evaluation of purity and free water of selected liquid fuels sample with using of automatic laser particle counter and classifi er Laser-Net Fines-C (LNF-C) and their comparing. MATERIALS AND METH...