Study on inversion temperature in low pressure superheated steam drying of green turnip slice

Abstract:Low pressure superheated steam drying is an innovative drying technology by utilizing steam beyond its boiling point as a drying medium in a dryer to remove excess water from the material. Low pressure superheated steam drying is commonly used for drying of heat sensitive products or to prevent degradation of volatile components and micronutrients (such as ascorbic acid and β-carotene). Compared with traditional drying methods, low pressure superheated steam drying can preserve the nutrients in food material better during the drying process. However, only when drying temperature is above the inversion temperature, superheated steam drying has an advantage in drying efficiency. There is no constant rate drying period for food material drying in low pressure superheated steam and whether there is an inversion temperature has not been reported in the literatures. Green turnip slice is rich in nutrients and has medicinal value, which is popular with consumers. Dried green turnip slice has been used as a resource of food seasoning and pickled food. In this paper, green turnip slice was selected as experimental material to study the inversion temperature based on the average drying rate of the first falling rate drying period and the whole drying period. Because the inversion temperature is usually higher, which may cause more loss of nutrients in the material during the drying process, the retention rates of vitamin C in the dried green turnip slices dried by the low pressure superheated steam and vacuum drying below and above the inversion temperature were investigated. Through analyzing the factors influencing the inversion temperature of green turnip slices, lower inversion temperature could be obtained and lower drying temperature for green turnip slice by low pressure superheated steam drying could be selected to reduce the damage of nutrients at high drying temperature. The results showed that there were 2 inversion temperatures based on the 2 drying rates, and the inversion temperature calculated by the whole drying stage was higher than that by the first falling rate drying period. The reason was that with the rise of drying temperature, the temperature difference changes between the material surface and the drying medium by low pressure superheated steam drying were larger than that by vacuum drying. Hence more heat was transferred to the material, and the drying rate was changed more significantly. According to the rehydration characteristics of the product, changes in the internal structure of the material affected moisture diffusion in the late drying period. Thus, the inversion temperature calculated by the whole drying stage was higher than that calculated by the first falling rate drying period. The inversion temperature calculated by the first falling rate drying period increased with the raise of drying pressure, and the influence of material diameter and thickness could be ignored. The inversion temperature calculated by the whole drying period increased with the raise of drying pressure and material thickness, while the influence of material diameter could be ignored. A lower inversion temperature could be obtained by reducing the drying pressure and the thickness of the material within the allowable range of the equipment and material. When the drying temperature was above the inversion temperature, low pressure superheated steam drying had not only a higher drying efficiency than vacuum drying, but also a higher vitamin C retention rate.