The Adaptability And Drying Characteristics Of Pulsed Vacuum Drying Technology For Poria Cocos

Drying is the important the key part in the primary production processing of poria cocos cubes.The current drying methods have the drawbacks of longer drying time and higher cracking rate.In order to find a better drying technology,the adaptability evaluation model of Chinese traditional herb was first constructed to evaluate the adaptability between the poria cocos and different drying methods.The pulsed vacuum infrared drying technology(PVID)was selected.The drying characteristics of poria cocos cubes under different PVID drying conditions were systematically studied.The influence on drying process,the change rules of volume shrinkage and texture of cubes were analyzed.The drying process was fitting by the Weibull drying model and Dincer drying model.The BP neural network model was constructed to predict the drying time and moisture content.Finally,a pilot PVID drying experiment was carried out to verify the feasibility of this new drying technology.Based on the extension theory,the adaptability evaluation index system and evaluation model of Chinese traditional herb was constructed,the weight of different index was obtained by analytic hierarchy process(AHP).The initial parameters of the misture content and shape size were unified.The 21 drying conditions under 7 drying methods of natural drying,microwave dring,hot air drying,jetting gas drying,temperature and humidiy control drying,infrared drying and pulsed vacuum drying were evaluated.And the drying efficiency,cracking rate,water-soluble polysaccharides content,energy consumption were compared.The results show that the water-soluble sugar content under infrared drying highest,which changed from 3.69 mg·g-1 to 4.07 mg·g-1.The cracking rate of pulse vacuum drying the lowest with the range of 3.28%～3.51%.So a new combined drying technology of PVID was proposed.The PVID drying experiments was carried out by usine carbon fiber infrared board as an heating source.Combine with the results of pre-test,the fore scientific hypotheses and its corresponding verification schedule were proposed.The effects of different drying temperatures,vacuum retention time,atmospheric retention time,loadint capacity and ambient air temperature on drying process were studied.The dynamic shrinkage of poria cocos cubes was quantified by image amalsis methods and Bigot’s curves.The dynamic changes of texture parameters were also measured by a texture analyzer.The results showed that the PVID drying time of the poria cocos cubes changed from 330min to 560min.The drying time can be shorted signigicantly by increasing the drying temperature and ambient air temperature.When the changing rate of the relative humidity inside the drying chamber was lower than 0.05RH/s,the drying efficiendy could be improved by swithing the pressure state of drying chamber.The volume shrinkage of poria cocos cubes was mainly related to the moisture ratio.When the mositure ratio is 0.7～1.0,the volumetric shrinkage coefficient has a linear relationship with the mositue ratio.When the water ratio is 0-0.3,the volumetric shrinkage coefficient is 0.According to the variation rule of texture parameters and moisure ratio,the drying process of poria cocos cubes can be divided into plastic stage,shape stage and forming stage.The maximum load of the single cube was nearly 500N when the moisture ration is 0.3,which can overcome the drawbacks of high cracking ratio.The range of water-soluble polysaccharides was 22.20～34.34 mg·g-1.Base on the systematic PVID drying experiment,the effective diffusion coefficient(Deff)was estimated by the a(scale parameter)and β(shape parameter)with the weibull model.The mass transfer coefficient(k)and the Bi number was calculated by the G(lag factor)and the S(drying coefficient)with the Dincer model.The regression relationship between the model parameters and the drying conditions was constructed.The prediction results of single hidden layer and double hidden layer BP neural network are compared.The results show that the double hidden layer BP neural network model can predict the drying time and the moisture ratio of the drying process with higher accuracy.The polit drying experiments of PVID and hot air dtying were conducted and compared,the results showed that the suitable drying temperature and longer vacuum retention time can help to maintain a higher water-soluble polysaccharides content of poria cocos.The drying uniformity,drying efficiency,daily processing capacity,water-soluble polysaccharides and product grade of PVID were significantly higher than those of hot air drying,but the energy consumption per unit was also higher than hot air.