Low-temperature Adsorption Drying Characteristics Of Fruit-Vegetables And Testing Platform Scheme

The fruit-vegetables are nutritious food. It is convenient for preservation and transportation that fruit-vegetables are processed as dehydrated ones. Fruit-vegetables' drying is a process technology which has widely applied field. Low-temperature adsorption drying is a new non-thermodynamic drying method that has distinct advantages on environmental protection, low energy consumption, good rehydration capability and nutritional ingredient holding. Especially to the heat-sensitive materiel which can not be dried by hot air drying, it is energy consumed less and easier for the industry application using the low-temperature adsorption drying than using the freeze drying.Nowadays, the low-temperature adsorption drying is defined as: firstly, remove the moisture from the air through the solid desiccant and the dried-air with some temperature is gained; then, decrease the temperature of the dried-air by compression refrigeration, so the air with low-temperature and low-humidity is obtained which is applied in the drying of the fruit-vegetables. The energy consumption using this drying process is lower than that of the freeze drying, but is higher than that of hot air drying. If the method with non-compression-freezing or the freeze using the low grade thermal energy is used in cooling the air, the energy consumption of the low-temperature adsorption drying can be decreased . Therefore, our task group proposes to develop "Adsorption Drying- Desiccant Cooling Coupling Technology of Low-temperature Adsorption Drying", and design the pattern machine. In order to get the basic parameters of the pattern machine, the potato and the carrot are taken as the study object in this paper. Using the low-temperature adsorption drying, the drying theory of them is studied and experiments are carried out, which include: measuring the relation of the drying characteristic, rehydration capability, nutritional ingredient with the condition of the drying process; establishing the drying model of the potato and the carrot; analysis apparent and interior tissue of the test sample using the optical microscope and scanning electron microscope; designing the drier of the testing platform scheme of fruit-vegetables' low-temperature adsorption drying. The main contents and results are as follows:1. Established low-temperature adsorption drying's experimental equipment for using in dehydration of fruit vegetables, ascertained the testing methods and gained the reliable data.2. The experiment compared effects of nutritional ingredients of protein, reducing sugar, polysaccharides, ascorbic content of the potato and the carrot with different drying methods such as low-temperature adsorption drying, hot air drying and freeze drying. The result shows that the lost ratio of ascorbic content of low-temperature adsorption drying is lower than else drying methods and hardly destroy other nutritional ingredients in potato.3. Compared the apparent, interior and near-surface tissues of the potato views before and after by low-temperature adsorption drying, hot air drying and freeze drying are contrasted. It shows that the potato quality of freeze drying is the best. After low-temperature adsorption drying, the potato can keep basically original color, show no browning, and better surface versus that of the hot air drying.4. The term studied the technological process of the potato and the carrot on the low-temperature adsorption drying. The experiment reviewed the effects of temperature, relative humidity, flow-speed of dried-air and granularity of raw material on drying characteristic of the potato and the carrot. The drying curves of potato and carrot are regressed and gained the expression of Page model which has approved by many experiments on different drying condition. According to the experimental results, the separate maximum of relative error is no more than 10% between calculation numbers and actual numbers.5. In order to investigate the effect of drying conditions on rehydration ratio of the potato, the symmetrical designed experiment was carried out. The reasonable process condition is: drying medium temperature is 19.3°C, the relative humidity is 2 %, the speed is 0.341 m/s, and the side length of raw cube material is 2 mm. Then, using the symmetrical designed experiment, the preferable process of ascorbic content of the potato is: drying medium temperature is 30°C, the relative humidity is 5 %, and the speed is 0.270 m/s. Under this condition, the potato can keep high ascorbic content.6. Testing platform scheme of fruit-vegetables' low-temperature adsorption drying has designed.