马铃薯薄壁细胞组织一维等温干燥模型

One-dimensional isothermal drying model for parenchyma cell tissue of potato tuber

为真实描述植物薄壁细胞物料干燥过程的水分传输机理,该文基于组织生理结构、微观参数测量技术和细胞结构变化,提出了适用于整个低温对流干燥过程的薄壁细胞组织模型。模型假设组织由细胞聚集而成,细胞由细胞壁、细胞膜和细胞腔模型溶液组成。细胞壁中的水分为纯水,干燥过程中细胞壁仅变形,不收缩;细胞膜为理想半透膜,集总了真实细胞内所有的跨膜渗透效应;模型溶液中的水分扩散则代表了真实细胞内部所有的扩散效应;干燥过程中,细胞膜始终紧贴细胞壁,细胞失去膨压后,塌陷收缩。基于组合参数传输模型建模方法构建了考虑细胞和收缩的一维传质模型。模型中细胞尺度的水分传输为局部水势平衡假设下的细胞腔到细胞腔、细胞壁网络和细胞气相间隙传输,宏观传递系数直接由细胞传输特性推演获得。模拟和试验表明:平均干基含水率不低于1.0 kg/kg时,模型可准确预测马铃薯组织的干燥过程,相对误差不超过20%。模型分析揭示:马铃薯组织干燥过程水分传输途径的优先级为细胞腔到细胞腔>细胞壁网络>细胞间隙。

Drying is a widely used technique in processing plant materials, such as fruits and vegetables. The material structure significantly change before and after drying, an accurate description of this structural change, however, has not yet been seen. From the perspective of whole drying process, the relationship between cell water potential and its water content is no longer an approximate linear relationship;and the water diffusion resistance inside cells is no longer negligible compared with the cell membrane resistance. The known mass transfer models for evaporative water loss, including drying models, with considering the cellular structure, are based on an accurate description of the geometric structure of fresh tissue with positive cell turgor pressure, and are not fully applicable to entire drying process. In order to describe the moisture transport mechanism of entire drying process, it is necessary to propose a cell tissue model suitable for the structure change from the state of positive turgor pressure to the state of turgor pressure lost during drying, and to consider the characteristics of moisture transport at cell scale. Based on tissue physiological structure, micro-parameter measurement technology and cell structure change, a parenchyma cell tissue model for isothermal convection drying under low temperature is proposed. The drying temperature is lower than 50 °C, because higher temperature will damage cell membranes. The tissue is made up of cells that are composed of cell walls, cell membranes, and model solutions in the cell cavities. The water in the cell walls is pure water, and the cell walls only deform during drying and do not shrink. Smooth the subcellular structures in the cell cavities. A cell membrane is an ideal semi-permeable membrane, which lumps all the transmembrane effects in the real cell. The diffusion of water in a model solution represents all the diffusion effects inside the real cell. During drying process, the cell membranes always cling to the cell walls, and after turgor lost, the cells collapse and shrinkage. Based on the method of building composite parameter transport model, a one-dimensional mass transfer model was constructed, considering cells and shrinkage. The transfer coefficient is directly derived from cell transport properties by replacing plant tissue with regularly arranged cells. The cell-scale water transport is identified as the cell cavity to cell cavity, the cell wall network and the intercellular air space transports under the assumption of local water potential equilibrium. The diffusion effect in cell cavity and the nonlinear relationship between water potential and cell moisture content is included in the transfer coefficient. The composite parameter model does not depend on a precise description of the cellular structure of tissue, but only its representative parameters, it is expected to be useful in describing the drying process of plant cell tissue. Simulation and experiment results show that the model can predict the drying process of potato tuber tissue accurately when the average moisture content is not less than 1.0(d.b.). Model analysis reveals that the priority of water transport pathways in the drying process of potato tuber tissue is cell cavity to cell cavity > cell wall network > intercellular air space. However, this model cannot explain the vapor diffusion effect in the intercellular air space in the end period of drying. To solve this problem, the stop of transports from cell cavity to cell cavity and in cell wall network should be studied in the future research. To describe the drying process better, the influences of anisotropic shrinkage on the porosity and tortuosity tensor in drying also should be studied in the future.