果蔬微波冷冻干燥中的孔道演变对其电磁行为和干燥过程的影响机制

The industry of freeze drying (FD) fruits and vegetables has been greatly limited because the high energy consumption of FD leads to a high cost of FD products. Microwave freeze drying (MFD) has been proved to be a more efficiency sublimation drying method, which has a potential to replace traditional FD technology. However, microwave can lead to corona discharge under low pressure condition, which will result in products quality deterioration, and sometimes thermal runaway also will take place. It was found that there was relativity between corona discharge phenomenon and electromagnetic properties of materials which showed a dynamic change tendency with the pore structure formation during MFD. As a result, this project will exhibited mechanism of corona discharge in micro pore cannel by investigating the effect of pore structure evolution and properties of materials on the electromagnetic behaviors. By precisely testing different pore structure and electromagnetic properties in freeze and dry zones respectively, some characteristics of electromagnetic respond will be obtained. Then, the distribution of electromagnetic field of materials will be simulated in order to find how the pore structure effects on the distribution of electromagnetic field. On the premise that the electromagnetic field distribution regulation of materials is confirmed, a heat and mass transfer model will be set up. Further more how the MFD process can regulate and control the pore formation of products also will be clarified by study of sublimation interface and pore structure evolution during MFD process.

冷冻干燥能耗大、成本高的缺点严重制约了果蔬冻干产业的发展。微波冷冻干燥技术在果蔬干燥中已被证实可显著降低能耗，具有替代传统冷冻干燥技术的潜力。但微波在低压条件下易引起放电现象，导致产品质量劣变甚至热失速。前期研究发现放电现象与果蔬原料的电磁特性具有相关性，而物料的电磁特性在果蔬物料的微观孔道形成过程中呈动态变化趋势。因此本项目将通过研究果蔬微波冻干过程中的孔道演变规律和孔道特性对微波冻干中电磁行为的影响，来揭示微孔道内部发生放电现象的机理；通过对物料干区和冻区的孔隙结构和电磁特性进行精确测定，获得基于孔道信息的电磁响应特性，从而进一步对物料内部的电磁场分布进行模拟，阐明孔隙结构对电磁场分布的影响机理；在获得物料电磁场分布规律的前提下，建立微波冻干过程的传热传质模型，并通过观察升华界面移动和孔道演变来验证和揭示干燥过程对孔道形成的调控作用。

干燥加工是应用最为普遍的果蔬深加工技术之一，具有极为庞大的产业链。近年来国内外消费者对以真空冷冻干燥产品为代表的高品质脱水果蔬的需求越来越大。但我国FD 果蔬产业的规模还远远满足不了需求，限制FD 产业发展的根源在于真空冷冻干燥技术的高能耗导致高成本的缺点。所以，加快研究开发高效、节能的冻干方法已成为业内共识。微波冷冻干燥技术可比传统的FD 节约1/2 以上的干燥时间，可大幅度降低能耗。然而在高真空状态下，微波的加载会频繁引起放电现象，从而使物料产生烧焦等品质劣变。. 本项目研究了在苹果不同干燥压力下的低压放电和微波功率以及初始含湿量的关系，同时对苹果的微观结构和放电特性进行了相关性分析；研究了双孢菇微波冻干过程中的孔道演变规律，发现当双孢菇含水率在20±5%到15±3%范围内物料开放型孔道会转化为封闭型孔道；研究了怀山药微波冻干过程中的孔道变化与质构的耦合关系，结合干燥特性分析、电镜扫描分析、压汞仪测试及穿刺测试等方法，建立了微波冻干怀山药脆片干燥过程中的孔隙变化数学模型；通过对双孢菇微波冻干中放电现象的研究，发现双孢菇的放电现象与孔隙结构具有相关性；利用低场核磁共振技术研究了怀山药微波冷冻干燥过程内部水分扩散特性并建立了干燥模型。. 本项目的科学意义在于阐明了微波冻干果蔬孔道特性的演变对电磁特性和干燥过程的影响机理，这些基础数据对解决果蔬微波冷冻干燥技术的产业化应用具有重要意义。.