Quality Control Of Chinese Yam Atmospheric Freeze Drying

Chinese yam, which was the first of famous "four huaiqing Chinese medicines ", was the dioscoreaceae perennial herbaceous vines. Chinese yam, which have dualpurpose of medicine and food, was rich in mucus polysaccharide, saponin, polyphenol oxidase, starch, allantoin, mucus protein and iron, calcium, zinc and other nutrients, known as the "Huai ginseng" saying. Because of the high content mucus polysaccharide, starch, moisture, Chinese yam was easy to perish during storage. Drying was an important method to preserved Chinese yam nutritional value. There are many problems include the dehumidification, dehumidifying effect, the product quality problems in traditional drying technology. To solve these problems, the project team to design and set up the test bench of atmospheric freeze drying based on Vortex Tube Refrigeration(including air compressor system, cold vortex control system, drying oven and PLC control system). The vortex tube was used as the dehumidification cold source of atmospheric freeze-drying, while the convective heat transfer fluid medium was also provided. The research and development of such a drying technique as would combine the advantages of both freeze-drying(highproduct quality) and convective-drying(low-process costs). In order to improve the quality of Chinese yam, solve processing and fresh problems, and promote the development of Chinese yam industry, in this paper, the method of atmospheric freezedrying of Chinese yam was used to researched the drying characteristics and study quality were explored. The main conclusions are as follows:1. A set of atmospheric freeze-drying experimental equipment used by vortex tube as refrigeration was established. Chinese yam was selected as experimental material. The inlet pressure of vortex tube, the size of material, the heating mode, the shape and variety of material were investigated in the experiment. Results show that the material properties and internal structure have a larger influence on the drying rate in the drying process. Drying rate can be improved through reducing the size of material and using the coupled heating mode by convection and radiation. When the temperature of drying oven was a little higher than that of material’s eutectic point, the atmospheric freeze-drying process can still manipulated. The best refrigeration efficiency for Chinese yam was obtained through adjusting the hot end of vortex tube, and the inlet pressure is 0.3 MPa when the cooling air ratio μ is between 0.4 and 0.5.2. To obtain high quality dried products of Chinese yam, the microwave vacuum freeze-drying technology was applied and the drying characteristics and drying qualities of Chinese yam were investigated. Using sample slice thickness, vacuum degree in drying chamber and microwave power as experimental factors. The rehydration ration, polysaccharide yield and L value were chosen as experimental indicators to study the effects of drying conditions on the qualities of dried products. Linear weighting method was used to analyze comprehensive multi-objective optimization and to determine the best parameter combination of drying technology. The results showed that the influences of various factors on the composite indicator decreased as follows: slice thickness, vacuum degree and microwave power. The optimal technological parameters acquired from microwave vacuum freeze-drying experiments were slice thickness of 5 mm, degree of vacuum of 100 Pa, and microwave power of 3.75 W and the drying time needed to achieve safe moisture content 0.12 g/g(d.b) was 150 min and drying energy consumption was 7 875.9 kJ/kg. Under the condition of the optimal technological drying processing, the product quality was excellent, good rehydration ratio(95.6%), pure color(the value of color difference was 92.90) and high polysaccharide yield(18.97%).3. In order to obtain high quality dried product for Chinese yam, the effects of microwave freeze drying(MFD), freeze drying(FD), vacuum drying(VD), heat-pump drying(HPD) and atmospheric freeze drying(AFD) on the drying quality were investigated. The dehydration rate, shrinkage, color, texture, polysaccharide yield, antioxidant properties and the energy consumption of Chinese yam were comparatively analyzed. The results indicated that the quality of MFD and AFD products was the most similar to FD, however the drying energy consumption was only 36.9% and 28.8% of FD. At the same time, FD, AFD and VD were required a long drying time to get better quality of dried products compared with MFD and HPD. It was concluded that the method of AFD can not only improve the drying rate of Chinese yam, but also reduce the energy consumption on the premise of drying quality assurance.4. The regression experiment of the second general rotation design was used, drying with atmospheric freeze-drying tests of Chinese yam was studied, and regression equation of polysaccharides rate and drying time were established, then significant test was carried out. Obtained the optimal processing parameters of control quality of recession was the polysaccharides yield was the highest(10.16 %) when the drying temperature, inlet pressure and slice thickness were-5 ℃,0.3 Mpa and 10 mm respectively. the saponins yield was the highest(0.53 %) when the drying temperature, inlet pressure and slice thickness were-10 ℃, 0.6 Mpa and 2 mm respectively; the rehydration rate was the highest(93.76%) when the drying temperature, inlet pressure and slice thickness were-5 ℃, 0.45 Mpa and 2 mm respectively.