| Apple slices retaining the original quality of apple is a kind of leisure food and generated by a series of processes, and drying is the key process in the production process of apple slices. We mainly rely on experience to judge when to stop the drying process. If the judgment is not accurate, the time that high temperature air in contact with the apple slices may be too long, and it is easy to result in a decline in the quality of products. Thus, real-time monitoring of apple slices drying process and accurately determining the end point of drying has great significance to improve product quality and reduce energy consumption. In the present study, the electrical impedance spectroscopy method and the finite element method were used as a research tool, Moisture content and electrical characteristics of apple slices during drying process were measured. The changes of apple in a hot air drying process were analyzed from both macro and micro aspects. This research analyzed the relationship between the moisture content and electrical impedance of apple slices from the macro point of view and studied the water changes in apple cell from the microscopic point of view. The temperature and moisture distribution in apple slices were simulated and validated. The main research contents and results of the full text are as follows:1) In the drying experiment process, the hot air temperature was set to 40℃,50℃,60℃ and 70℃, and the hot air speed was set to 0.5m/s and 1.0m/s, respectively. The moisture content and the electrical impedance of apple samples were measured, and the correlation between the two was analyzed; it was found that the electrical impedance of apple slices showed a linear correlation with the moisture content. It suggests that the electrical impedance can be used in place of the moisture content to online monitoring the drying process of apple slices.2) The curve of the normalized electrical impedance with the drying time could be approximated with the logarithmic model which can describe the characteristics of the drying process. The rate constant of the model at various temperatures was estimated by normalized impedance. As the temperature increases, the drying rate constant increases. To analyze the effect of temperature on the drying rate constant, the rate constants at different temperatures were formulated by Arrhenius equation. When the speed of drying hot air was 0.5 m/s and 1.0 m/s, the activation energy calculated from normalized impedance was 33.925 kJ/mol and 28.912 kJ/mol. The activation energy decreased with increasing hot air speed, which indicated that the hot air speed is higher, the drying of the apple slices is faster. It is found that the change rate of normalized impedance is two section, so the continuous first order reaction equation was used to fit the normalized impedance. The results show that the fitting is very good. The activation energy obtained from the fitting rate of continuous first order reaction equation is 24.044 kJ/mol and 23.186 kJ/mol. The fitting rate of the first stage is much smaller than that of the second section.3) The change of moisture content in cells of apple during the drying process was analyzed by the method of electrical impedance spectroscopy. The result shows that the combination of temperature and water loss result in the damage of cellular structure. When the moisture content(d.b.) is about 1, the cellular structure was completely destroyed.4) Establishing the heat and mass transfer mathematical model of the hot-air drying process to forecast internal moisture and temperature distribution of apple slices, using COMSOL software to solve the mathematical model. The test data measured in the experiment was used to verify the model. The results show that the change of temperature and moisture content is consistent with the simulated values, and the root mean square error is 6.453% and 1.400%, respectively. The finite element method provides a new method for the optimization of the drying process of apple slices. |
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