Drying Characteristics And Quality Of Red Jujube With Hot Air

Red jujube has a long history in China and has great value for both nutrition and medicine. As a new army of the red industry in the south Xinjiang, red jujube has now become one of the pillar industries in this autonomous province and the corps. Drying is the most important link of processing red jujube. In this paper, the drying characteristics and drying model was studied under different temperature and wind speed by combining the hot-air drying technology. The red jujube circulation hot-air drier was developed combining industrial PLC. The red jujube deep bed experiments were conducted under different drying temperatures, wind speeds and relative humidities. The change regulation of total sugar content, retention rate of VC, color, volume shrinkage, rehydration rate, and drying time of the dried jujube were studies, and the color of the dried jujube was detected and analyzed.The research on hot air drying characteristics of jujube suggested that drying temperature and wind speed have significant effect on the drying time of jujube. The drying time decreases as the temperature and wind speed increase. The hot-air drying contains two stages which are accelerated drying and decelerated drying. According to Fick’s second law, the effective diffusion coefficient of jujube’s moisture during the hot-air drying process was obtained, which increases with the drying temperature and wind speed increasing. And the activation energy(aE) of the dried jujube is calculated by using Arrhenius equation, which is 40.14kJ/mol.Mathematical models of jujube hot-air drying are reviewed. The fitting between experimental data and ten models, which are commonly used to describe the thin-layer drying of agricultural products, was assessed and analyzed by using the quadratic determination coefficient, chi-square test value and root-mean-square error. We find that the Weibull distribution model fits best with experimental data, so it can well describe the changes in the moisture content during the hot-air drying process.By combining the hot-air drying technology with the industrial PLC, an automation technology, the circulation hot-air drier was designed, which is an energy-saving and environment-protective machine that has high heat transfer coefficient and large loading capacity in unit volume, where materials can be heated evenly. The drier is designed with the ideas of thermal insulation, cyclic hot-air utilization and intermittent cycle of the material to be heated. Its control system is used to monitoring and control the drying parameters, including drying temperature, wind speed, relative humidity and cyclic rotation speed.Based on this circular hot-air drier, the effects of drying temperature, wind speed and relative humidity on the dried jujube’s quality indexes, namely, the total sugar content and VC retention rate, have been investigated. With designing the experiment by using the quadratic orthogonal rotation, the mathematical model of the effects of drying temperature, wind speed and relative humidity on total sugar content and VC retention rate was obtained, and the response surface analysis was performed by using this model. As shown by the experimental results, the combination of the optimal technological parameters for processing red jujube is that drying temperature is 57.99℃, air speed is 1.54m/s and relative humidity is 39.14%. And when the red jujube reaches its safe moisture content, the maximum total sugar content is 70.73%, while the maximum VC retention rate is 7.49%.Next, the shrinkage of the red jujube during the hot-air drying process has been studied. With designing the experiment by using the quadratic regression central combination, the mathematical model of the effects of drying temperature, wind speed and relative humidity on volume shrinkage, rehydration rate and drying time was obtained, and the response surface analysis was performed by using this model. As shown by the experimental results, the combination of the optimal technological parameters is that drying temperature is 60.04℃, air speed is 1.42m/s and relative humidity is 39.89%. And when the red jujube reaches its safe moisture content, the optimal volume shrinkage is 80.27%, the optimal rehydration rate is 1.45 and the optimal drying time is 24.33 h.The analysis of the changes in jujube color that is detected by using a colorimeter shows that the brightness L* of the dried red jujube decreases with increasing of the drying temperature, and the effect on L* becomes more significant as the temperature increases, and thus indicates that a lower drying temperature will lead to a larger value of L*. The green-red value a* of the dried red jujube decreases, and the effect on a* becomes more significant as the temperature increases, and thus indicates that a lower drying temperature will lead to a larger green-red value.The blue-yellow value b* of the dried red jujube decreases, and the effect on b* becomes more significant as the temperature increases, and thus indicates that a lower drying temperature will lead to a larger blue-yellow value. The saturation C* of the dried red jujube decreases, the color angle value H of the dried red jujube decreases, while the color ratio increases, and the color differenceΔE* of the dried red jujube increases, and thus indicated that a lower drying temperature helps to reduce the value ofΔE*.