Experiment Study On Technology Parameters For Freeze-Drying Of Ginseng Slice

Vacuum freeze-drying is a comprehensive technology integrated with vacuum science, cryogenicsengineering, fluid technique, control engineering, heat and mass transfer and dynamic engineering, whichhas been applied widely in food engineering, medicine engineering and biological product engineering. Thefreeze-drying is a procedure of heat and mass transfer under lower temperature and high vacuum, thedrying process needs long time and much energy because of lower drying velocity, so the production costsare relative high. The objectives of the study were to investigate the impact of vacuum freeze-dryingprocess, and to provide practical guidance for application of vacuum freeze-drying for ginseng processing.On the basis of summary of research results and analysis of vacuum freeze-drying, a comprehensivenew tow-dimension model had been established, and the theoretical values of the model was close identicalwith experiment data, which provided theoretical references for thermal control of vacuum freeze-drying.The eutectic point temperature and melting point temperature of ginseng were measured respectivelyusing differential scanning calorimetry (DSC). The DSC analysis indicated that the eutectic temperatureand melting point temperature were-16℃and-0.05℃respectively at heating and cooling rates of10℃/min. The eutectic point temperature of ginseng was-15℃measured by electric resistivity method,which was equal to DSC method. The measured results could be used for determination of technologicalparameters for vacuum freeze-drying procedure.Using FLUENT software the temperature field of the space between heating plate and the materialupper surface was simulated in the drying chamber, and the material could obtain much heating energy as itwas nearer to the heating plate, which led to a shorter drying time. As the temperature gradient of the fringeis higher than that of the center, so the material in the fringe section can get more energy and be dried morequickly.The procedure and state of thermal radiation between two parallel plates was studied, and calculationformula for effective and maximum effective transmission efficiency of heat receiving by plates duringradiation procedure was developed. The results showed that optimum temperature of receiving plate wasnot affected by material surface properties during the steady heat transfer process, and the temperaturebetween two parallel plates was nonconformity, the temperature of the receiver center was higher than that of the edge, the temperature gradient increased with the decrease of the radiation distance. Therefore, inorder to improve the uniformity of temperature within the material, the distance of the two plates should beenlarged.The change of temperature in ginseng in the frozen process was analyzed with ANSYS software, andthe results showed that simulation temperature field consistent with experimental data which provided ofaccurate judgment for frozen ending time.The temperature change in each point in Ginseng slice with time was analyzed in four differentheating modes by means of the ANSYS software; the heating modes were separable heating function,parabolic heating function, and exponential heating function, power heating function. The results showedthat separable heating function made the ginseng slice warming fastest. The result provided theoreticalreferences for increasing drying velocity and saving energy for freeze-drying process.Using quadratic regression experiment with general rotary combination design, regression equationsdescribing relations of criteria with ginseng slice, heating temperature and drying cabinet pressure, dryingvelocity, productivity and the area contraction ratio were developed. Based on regression equations andusing the method of nonlinear optimization, three optimal drying parameters were obtained as follows: Thehighest drying velocity was1.56h^-1with ginseng slice thickness4.8mm, heating temperature50.4℃anddrying cabinet pressure48.7Pa; the maximum drying productivity was111.7g/h·m²with ginseng slicethickness3mm, heating temperature60℃and drying cabinet pressure80Pa; the minimum area contractionratio was9﹪with ginseng slice thickness12mm, heating temperature40℃and drying cabinet pressure40Pa.Using BP neural network, vacuum freeze-drying process was simulated. The result showed that the BPneural network could precisely simulate vacuum freeze-drying process. After training of BP neural networkwith results of ginseng slice in orthogonal experiment, drying process conditions was predicted andoptimized, and the simulation results fitted well with experiment data. The results showed that the BPneural network had higher accuracy for prediction of drying procedure.