Numerical simulation and analysis of the vacuum freeze drying kinetics of Lycium barbarum

Abstract: Lycium barbarum is one of the most popular herbs in China. The high medicinal value can serve as a functional food rich in nutrients (such as polysaccharides, plant flavonoids, and carotenoids), further promoting glucose and lipid metabolism for better immune system, and mental efficiency, and even preventing neurodegeneration. The berry fruits with high water content can be dried to improve their shelf life using vacuum freeze drying. In this study, a numerical simulation was carried out to explore the heat and mass transfer, stress, and strain of Lycium barbarum during freeze-vacuum drying. Among them, the heat and mass transfer was a benefit to exploring the drying mechanism of Lycium barbarum freeze during vacuum drying. The stress and strain was to explore the cellular changes of Lycium barbarum freeze-vacuum during drying. The temperature experiment was carried out by vacuum freeze dryer. The pre-freezing stage was set at-40 ℃ for about 4 hours, then the vacuum was drawn at two heating rates (5, and 10 ℃/h). The constant temperature was maintained when the heating plate temperature reached 40 ℃. The cell change experiment was carried out by FDCS196 hot and cold table. The whole process was combined with the microscope and CCD camera, in order to obtain the real-time image of berry cells. The heat-mass-stress coupling of the drying was dominated by the high quality of the products. The heat-mass-structure coupling model during vacuum freeze drying was established to use binarization processing using the experimental image of fresh berry slices. The temperature, moisture, as well as internal stress, and strain of the fresh berry sample, were analyzed and then verified by experiments during vacuum freeze drying. The results showed that the length shrinkage (Sl) and equivalent diameter contraction (Sd) of berry tissue cells were always negative during the pre-freezing process, indicating the ever-expanding cells. By contrast, the cell shape coefficient (Sf) was increasing all the time. At the same time, the maximum stress was found on the cells, when the cells were completely frozen. The moisture of the sample during the simulation was fitted better with the WangandSingh and HendersonandPabis models, indicating the more suitable WangandSingh model(R2 = 0.983). The experimental results show that the temperature of samples during the simulation was highly consistent with the experiment, indicating the better performance of the model. In addition, the cell shape was gradually altered around in the varying position with the decrease of sample temperature in the process of the pre-freezing experiment(R2 = 0.857). A better consistency was found with the analysis data of stress and strain of sample cells during simulation.The research results can provide reference for the optimization of vacuum freeze drying system and the scientific formulation of process parameters.