Real-time monitoring of moisture content and kinetics analysis of apple drying process by impedance measurement

Abstract: Apple is one of the most competitive agricultural exports in China, accounting for about 30% of the total production of fruits. Unsuitable preservation or storage methods can lead to the loss, which may amount for 30% of the total. To reduce the loss of apples after harvesting, deep processing is an appropriate method. As a kind of snack food, dried apple slice is produced by a drying process and the natural flavor of apple can be retained. During the drying process, the drying temperature and time have a great influence on the apple slice quality. To develop a new method to monitor and characterize the drying process of apple slices as well as evaluate the influence of the drying temperature and time on this process, the electrical impedance and moisture content of apple tissues were measured during the drying process under different conditions. The hot air temperature was set to 40, 50, 60 and 70℃, and the hot air speed was set to 0.5 and 1.0 m/s, respectively. The 6 same apple samples were placed in the drying chamber for drying test. One sample was to detect the change of the impedance characteristics, and the other samples were used to detect the change of the moisture content. The interval measurement time was 20 min. Until the mass difference of the 2 successive measurements for one sample was less than 0.01 g, the experiment was stopped. Results showed that the electrical impedance of apple slices increased with the increase in the drying time, and the increase rate was slow in the early stages and fast in the latter part of the drying process. The moisture content decreased during the drying process, and the decrease was fast at the beginning and slow in the latter part. Thus, the variation tendency of moisture content was contrary to that of electrical impedance during the process. The electrical impedance of apple slices showed a negative linear correlation with the moisture content when the moisture content was more than 20%. So electrical impedance measurement could provide a simple and rapid approach for predicting the moisture content, and furthermore it was capable of monitoring and evaluating the drying process of apple slices. The curves of the normalized electrical impedance and moisture content with the drying time could be approximated with the logarithmic drying model which could describe the characteristics of the drying process. The rate constant of the model at various temperatures was estimated by the normalized impedance and moisture content. The rate constant increased with the increasing of drying temperature. Then to analyze the effect of temperature on the drying rate constant, the rate constants at different temperatures were formulated by the Arrhenius equation. Based on the obtained rate constants at different drying temperature, the activation energy was calculated. It was found that when the speed of drying hot air was 0.5 and 1.0 m/s, the activation energy calculated from the normalized impedance was 33.925 and 28.912 kJ/mol, respectively; the calculated activation energy from the moisture content was 27.320 and 22.947 kJ/mol, respectively. The activation energy decreased with the increasing of hot air speed, which indicated that the higher the hot air speed, the faster the drying of apple slices. The findings are useful to monitor the drying process of apple slices and have the potential applications in the control of fruit drying process.