介电特性作用下的果蔬微波冷冻干燥行为及机理

Freeze drying (FD) is a very widely used processing technology of fruits and vegetables, but it costs too high energy and leads to high product price. Microwave freeze drying (MFD) can greatly reduce the drying time and decrease the energy consumption. Otherwise it also easily leads to plasma discharge and instable product quality with bad appearance because of low pressure condition. It was found that there were relationships between probability of occurrence of corona and materials, which showed a dynamic change characteristic. As a result, a thermoelectric coupling theory under the dielectric property effects of materials was brought up. In this project, based on the dielectric property change mechanism, the drying behavior change mechanism during MFD will be investigated. Further more, the distribution of electromagnetic field in the microwave resonance cavity under the dielectric property effect also will be clarified, and the mechanism of plasma discharge phenomenon during MFD process also will be explained. Besides, accurate heat and mass transfer equations will be set up based on consideration of the dielectric property changes in order to forecast the MFD process. Additionally, low frequency microwave will be introduced to reduce the probability of occurrence of corona, and then the dryng behavior and mechanism of the low frequency microwave freeze drying can be found. Finally, this project will supply basis theory and experiment data for promoting application of MFD in high quality fruits and vegetables processing, which has a significant sense for solving the industry problems of FD food field.

冷冻干燥是应用极为广泛的高品质果蔬加工技术，但能耗大，成本高。微波冷冻干燥技术可大幅度缩短冷冻干燥时间，降低能耗，但在低压下极易出现放电现象，产品质量不稳定，外观品质差。前期研究发现放电现象的发生概率和原料有关，且在干燥过程中呈动态变化，因此提出原料介电特性作用下的冻干过程热电耦合理论。本课题从果蔬原料在干燥过程中的介电特性变化机理出发，研究其介电特性引起的干燥行为变化机制，进一步阐明介电行为影响下的微波谐振腔电磁场分布规律，揭示微波冻干过程中发生放电现象的机理；在考虑介电特性变化的基础上建立精确的传热传质方程，对干燥过程进行正确预测；利用低频段微波加热来进一步降低发生放电现象的可能，揭示低频微波冻干过程的干燥行为与机制。本研究为进一步推动微波冻干技术在高品质冻干果蔬加工中的实际应用提供基础理论与试验数据，对解决困扰冻干食品领域的行业难题具有实际意义。

冷冻干燥（FD）是公认的能最大限度保持食品原料外观和营养品质的干燥方式，其利用升华作用的除水方式较传统的蒸发方式有着明显的优势，目前FD果蔬基本占据了国内外的高端脱水果蔬市场。但我国冻干果蔬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。微波冷冻干燥（MFD）技术通常被认为具有更高的干燥效率，具有替代传统FD技术的潜力，但其一直未能得到推广应用，其突出问题（低压放电）始终未能得到有效解决。本项目在此背景下展开，采用苹果、双孢菇（白蘑菇）、荔枝等为试验原料，进行了相关研究。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，同时发现了苹果的介电特性在干燥过程中的变化规律，从而可根据其介电特性变化确定微波加载方案；利用光纤测温技术对荔枝的微波真空干燥过程中的物料温度变化进行了跟踪，同时测定了荔枝在不同温度下的介电特性，建立了介电特性和温度分布的相关性模型，从而从温度变化角度来阐明其品质变化规律；通过对白蘑菇的微波冻干、常压冻干和普通冻干的温度与水分变化规律进行对比分析，阐明了各种冻干方式物料水分升华量与干燥品质的相关性，发现了微波冻干对冰晶的去除方式与其他冻干方式的区别；通过对白蘑菇微波冻干中放电现象的研究，发现固定的微波加载策略在降低放电问题的风险时，其干燥效率较低，且有时容易引起品质劣变，而基于白蘑菇玻璃化转变温度的动态微波加载策略能够明显的提高产品质量并缩短干燥时间；通过矢量网络分析仪对白蘑菇介电特性进行精确测定，得出白蘑菇介电损损耗因子相对其温度和水分含量的回归方程，在此基础上利用较为通用的升华-冷凝模型对微波冻干过程质热传递进行了数值模拟研究，考虑了介电特性的微波冻干质热传递模型可对物料温度分布进行较准确的预测。