双孢菇MVD过程水分跨膜运动的介电响应与“热点”形成作用机制

Microwave vacuum drying (MVD) of A. bisporus slices is superior to conventional drying methods owing to shorten of the drying time, reduction of energy consumption and keeping quality during the drying process. However, it is easy to produce "hot spot" during drying process, which leads to product coke paste and flavor deterioration. The common problem of “hot spot” in fruits and vegetable microwave processing field has almost been researched from the point of view that the role of macro heat and mass transfer process and temperature distribution, and the fruits and vegetables with cell structure were assumed as porous media in the past. Nevertheless, it is not clear that the mechanism of microscopic cell water movement and the formation of "hot spots". The aim of this project is to study the water distribution, the water network structure and binding energy, the micro dynamic characteristics and the dielectric distribution of cell water transmembrane movement of A. bisporus during MVD, to couple electromagnetic field distribution and heat conduction differential equation based on micro dynamic, and to clear the mechanism of temperature distribution in the drying process using the finite element method combined with infrared thermal image analysis. Further more, we will explore the relationship between movement distribution of bound water and free water and the distribution changes of loss factor ε'', and the mediated processes of ε'' for temperature agglomeration, thereby to clarify the mechanism of action between dielectric responses of water transmembrane movement and hot-spot formation during microwave vacuum drying of A. bisporus. The results could provide theoretical basis for systematically revealing the mechanism of formation and regulation for "hot spot" during MVD process of A. bisporus.

微波真空干燥（MVD）在双孢菇脱水加工中具有提效、节能和保质优势，但干燥过程易产生“热点”导致产品焦糊和风味劣变。国内外针对果蔬微波加工中“热点”形成这一共性问题，以往研究大多将具有细胞结构的果蔬假设为多孔介质，从物料宏观传热传质与温度分布的作用角度进行分析，但对具有跨膜传输特征的微观细胞水分运动与“热点”形成的作用机制尚不明晰。本项目旨在探析双孢菇MVD细胞水分跨膜运动过程水分形态分布、网络结构与结合键能变化以及微观动力学和介电分布特性，并以水分跨膜传输动力学为基础耦合电磁场分布和热力学方程，采用有限元数值模拟法结合红外热像分析探明干燥过程温度分布变化机制。在此基础上探究束缚水和游离水的运动分布与损耗因子ε''分布变化的关系，以及ε''因子分布对温度集聚的介导作用过程，阐明水分跨膜运动的介电响应与“热点”形成作用机制。研究结果可为系统揭示双孢菇MVD“热点”形成与调控机制奠定理论基础。

微波真空干燥（MVD）在双孢菇脱水加工中具有高效、节能和保质优势，但干燥过程易产生“热点”导致产品焦糊和风味劣变。本项目针对这一难题，重点探索双孢菇MVD过程束缚水和游离水运动分布与损耗因子ε''分布变化关系，揭示双孢菇MVD过程“热点”形成和调控机制。研究发现，低频热超声处理降低双孢菇游离水分面积，并增大半结合水分面积，鲜样和低频热超声处理双孢菇片分别在含水率0.70和0.60 g/g (w.b.)出现水分转换点。低频热超声处理可以提高双孢菇片半结合水和结合水对介电参数影响，使双孢菇样品具有较高水分活度、较低介电损耗ε''和更大的穿透深度dp，特别是当含水率低于0.52 g/g (w.b.)时，水分分布均匀性显著提高。研究建立了低频热超声处理双孢菇片介电常数ε'和介电损耗因子ε''随温度和含水率变化回归预测方程，并基于膜渗透理论和植物生理学宏观测量方法，确定不同微波功率下水分跨细胞膜传输通量JV等参数变化规律，建立干燥过程细胞内水分跨膜传输模型。采用基于热特性分析仪的瞬时线性热源法，建立低频热超声处理双孢菇片的热传导系数κ、热扩散系数α等参数与温度和含水率关系的多元回归模型。在上述研究基础上，运用COMSOL仿真软件对双孢菇片MVD过程温度分布进行数值模拟，结合红外热像分析，研究提出“低频热超声处理→诱导细胞水分迁移→调控MVD后期细胞半结合水分布→增大细胞半结合水分活度→半结合水分均匀分布→降低介电损耗ε''→提高微波穿透深度dp→消解MVD过程温度集聚”的“热点”调控理论。本研究成果已发表SCI论文3篇，另投稿中SCI论文2篇，已授权国家发明专利1项、实用新型专利1项，新申请国家发明专利2项。本项目研究结论可为双孢菇等食用菌MVD脱水工艺优化和生产应用提供理论依据，项目取得的原创性发明成果及配套技术具有良好的应用价值。