In practice, there is a demand for quick characterization of rheological properties of food materials. The exact model calculation requires complex and long-term mathematical process. In this work, a simple, quick linearization method – the Peleg linearization – is discussed and is compared with the Prony series method. In the Peleg linearization only two constants are used, one of them gives the initial rate of relaxation or creep and the second one gives the equilibrium value of relaxing force or of creeping strain. The Prony series approach the relaxation and creep with the sum of two or more exponential functions and equilibrium values. Both methods give the same equilibrium values for both the relaxation and creep of wine gums and apple. The initial increasing rate of creep is higher by the Peleg linearization and lower by the Prony series. At relaxation the initial decreasing rate is lower by the Peleg linearization and higher by the Prony series.
The statement of overheating of honey during the processing is important in quality characterization of honey products. Four Hungarian acacia honeys were heated up to 35, 40, 50, 60, and 80 °C and held in water bath for 0.5, 4 and 24 h. The electrical impedance spectrum of honeys before and after heating at room temperature (22 °C) were measured with precision LCR meters in frequency range from 30 Hz up to 30 MHz at 1 V voltage with Ag/AgCl electrodes. The spectra after open-short correction were approached with a circuit model consisting of a serial connection of two distributed elements and a resistance. The model parameters were determined. One of the resistance parameters can be used for detecting the previous heating of honey after detailed investigation of the recrystallization process following the heating. The complex electrical permittivity also was determined in the frequency range from 1 MHz up to 3 GHz.
Generally the drying process of fruits is followed by weight loss. The weight loss characterizes only the global moisture content of fruits and does not give information about the inner state of tissue. Electrical impedance spectroscopy of biological tissues shows ab-dispersion band that is associated with membrane structures and is sensitive to their integrity and functionality. The aim of this study was to measure the impedance spectra of apple slices during drying and to correlate impedance parameters to moisture content in the different drying periods. The electrical impedance spectra of apple slices were determined during drying by an HP 4284A Precision LCR Meter in frequency range from 30 Hz up to 1 MHz. The measured spectra were approximated by Cole-impedance elements. Parameter values for the fitted curves that characterized the state of drying tissue showed good correlation with the moisture content calculated from weight loss in the two falling-rate drying periods.
In the case of fruit syrups, consumers prefer thicker, deep-coloured ones because they remind them of good old homemade ones. Physical properties – as viscosity, colour, refractive index, soluble solid content (SSC), and electrical impedance - of fruit syrups flavoured with raspberry or orange juice were determined. The change of the measured physical properties was determined in three different experiments: 1. evaluating the effect of sugar and sweetener content 2. evaluating the effect of fruit juice composition and 3. evaluating the effect of sugar content on syrups containing raspberry or orange juice only. Variations in fruit and sugar content had a definite effect on the physical properties of squashes. Viscosity, SSC value, refractive index, and impedance magnitude increased with increasing sugar content and decreased with replacement of sugar with sweeteners although colour characteristics were defined mostly by the colorant added to the squash. Changes in the fruit composition of syrups could influence viscosity, refractive index, and SSC values but the magnitude of impedance showed little changes for all samples. The characteristics of squashes containing only raspberry or orange juice and different sugar content did not follow any trends, which emphasized that additives had a significant effect on the physical properties.