枸杞真空冷冻干燥动力学的数值模拟及分析

为探究枸杞真空冷冻干燥过程中的热质迁移，克服应力应变现象不能直观获取的问题。该研究通过对鲜枸杞切片试验图像二值化处理，建立了鲜枸杞真空冷冻干燥的热-质-结构耦合的物理模型，对真空冷冻干燥过程中枸杞温度变化、水分变化以及其内部的应力应变进行了热-质-力耦合分析，并对分析结果进行试验验证。模拟分析结果表明，预冻结过程中，细胞始终在膨胀，当细胞完全冻结时细胞所受应力达到最大，而干燥阶段热质传递对应力影响较小。提高真空冷冻干燥过程中的升温速率，在一定程度上能够缩短冻干所需时间，并且真空冷冻干燥过程中的枸杞样本的含水率下降速率随干燥时间的增大而减小，该变化趋势与Wang and Singh模型更加贴合（R2为0.983）。同时试验验证结果表明，该研究建立的模型能够较好反映并预测真空冷冻干燥过程枸杞样本的温度及应力应变的变化趋势（R2为0.857）。研究结果可为真空冷冻干燥系统优化和工艺参数的科学制定提供参考。

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.