Kinetics Study On The Main Nutrients Of Chinese Jujube During The Drying Process

Chinese jujube is the fruits of Ziziphus jujube Mill. which is a thorny Rhamnaceous plant widely distributed in China. Chinese jujube is indigenous to China with a history of over 4000 years. It has been commonly used as a traditional Chinese medicine and also commonly used as food for thousands of years. The Chinese jujube production and cultivation area account for more than 98% of the world's total. Fresh Chinese jujube has characteristics of rich nutrition and high water content, and it decays quickly in natural conditions. Much of the fresh Chinese jujube has been dried for the purpose of long term preservation. In this work, Chinese jujube was dried in a laboratory scale dryer in order to study the change rules of the contents of main nutrients in Chinese jujube during the drying process. This research investigated the effects of drying conditions on the contents of water, sugar and vitamin C in Chinese jujubes, and selected mathematical models were fitted to experimental data in order to establish its kinetic model. The main research results are as follows:1. The moisture ratio of Chinese jujube decreased continuously with drying time. The moisture loss rate was fast at the beginning, then it decreased slowly and gradually turned to be stable. The drying process could be divided into two periods: accelerating period and falling rate period. The constant rate period was absent from the drying curve. The drying process took place in the falling rate period except a very short accelerating period at the beginning. Seven selected mathematical drying models for thin-layer drying were fitted to experimental data using non-linear regression analysis techniques. Among these models, the Page model was found to be the most suitable for describing the drying curve. It could adequately describe the change regulation of moisture ratio of Chinese jujube during the drying process.2. The concentration of ascorbic acid (AA) in Chinese jujube decreased continuously during the drying process. The degradation of AA is considerably affected by the drying conditions. Increasing drying air temperature increases the degradation rate of AA in Chinese jujube during the drying process. The Weibull model provided a good description of the kinetics of degradation of AA at different drying conditions.3. The concentration of dehydroascorbic acid(DHAA) in Chinese jujube increased at the early time, and then it decreased later .The kinetic model of DHAA well fitted its degradation in Chinese jujube. The degradation rate of DHAA was lower than that of AA and its activation energy was higher than that of AA, these results indicated that DHAA in Chinese jujubes was more stable than AA in thermodynamics.4. The concentration of reducing sugar and total sugar in Chinese jujube increased continuously during the drying process. It can be seen that for higher drying temperature the rising speed of sugar content is also higher. The exponential model provided a good description of the regularity for changes of reducing sugar and total sugar during the drying process.5. In this paper, data of moisture content and sugar content during the drying process had been analysed. Correlation analysis of these data indicated that moisture content and sugar content showed very notable negative correlation. For the further study on the relationship between moisture content and sugar content, the dry basis concentrations of sugar in Chinese jujube was calculated. The results showed that the concentration of total sugar (on dry basis) almost maintained stable, whereas the concentration of reducing sugar (on dry basis) increased about 15% at the early stage and then gradually turned to be stable. These results proved that the increasing of total sugar content was mainly caused by the decreasing of moisture content during the drying process.