Revealing the influence of temperature and moisture content on electrophysical parameters of raw apple materials

Abstract

Additional heating of fruit raw materials in the drying process by direct passage of alternating electric current makes it possible to intensify the dehydration process by 3-5 times. The amount of added thermal energy during direct electric heating depends, first of all, on the value of the specific electrical resistance of the raw material. Therefore, the list of necessary properties of the material must necessarily include the specific electrical resistance of the raw material as an integral indicator. During drying with additional direct electric heating, the concentration of dry soluble substances, which directly depends on the moisture content of the studied raw material, and its temperature change throughout the drying time. This indicates a change in specific electrical resistance during the dehydration process. It was determined that the initial resistivity of apples before drying at a temperature of 20 °C and a moisture content of 8 kg/kg is within 195–220 Ohm∙m, which indicates the high conductivity of the material. With a decrease in moisture content from 8 to 6 kg/kg, the specific electrical resistance of the raw material decreases by 25–30 % compared to the initial values. With a further decrease in moisture content below 6 kg/kg, the electrical resistivity of the raw material begins to gradually increase. At the same time as the temperature rises, there is a significant decrease in specific electrical resistance. When heating raw materials from 25 to 55 °C, the value of specific resistance decreases by 10–13 times. The equation of the dependence of the specific electrical resistance of apple raw material on its moisture content and temperature for the studied apple varieties was obtained. The obtained results of the change in the value of the specific resistance provide the necessary data for the development of an energy-saving technological device for drying fruits and the selection of optimal processing conditions in compliance with quality indicators.