射频杀菌对预包装多孔性低水分活度食品的传热均匀性机制及改善方法研究

Microbial contamination of food with low water activity has become a new problem in food safety. Because of its excellent penetration depth, radio frequency (RF) has significant advantage in the sterilization of porous food with low water activity, which difficult to be treated by the traditional heating methods. However, the porous food with low water activity is heterogeneous composite system, and its dielectric constant is not uniform, which can cause RF heating uneven, resulting in sterilization is not complete. In order to avoid the second pollution of packaging process, pre-packaged food is always used for RF sterilization, so the electromagnetic properties of packaging have also become an important factor affecting the bactericidal effect. From the sterilization effect of RF and quality change of low water activity food, the food material, air in pore and packaging materials with different dielectric properties will be as research objects in this project, to studies following content mainly: .analyzing the dielectric properties and thermal properties of food components and the effect of pore size and porosity on the electromagnetic properties;.analyzing the influence of packaging structure on the dielectric properties of food materials;.providing a uniform distribution of the RF field in food through the simulation of temperature field design, auxiliary heating and metallized packaging structure..The project will rovide the theoretical basis for the further promotion of RF in food sterilization.

低水分活度食品的微生物污染已开始成为新的食品安全问题，由于射频具有较大穿透深度，将其应用于传统加热方式难以处理的低水分活度多孔食品的杀菌过程具有显著优势。但是低水分活度多孔性食品是一种多元非均匀复合相的物质，其介电常数呈非均匀状态，会引起射频加热不均，导致杀菌不彻底。同时为了避免包装工序的二次污染，射频杀菌多采用预包装形式进行处理，包装容器的电磁性质也成为影响食品杀菌效果的重要因素。因此，本课题从射频加热过程的灭菌效应以及对低水分活度食品品质的影响出发，分别对食品物料、孔隙空气、包装材料三者的介电特性及能量转化机理进行研究，分析食品组分的介电性质和热物性、研究孔径及孔隙率对其电磁特性的影响，建立“夹心”/“包覆”理论模型，研究包装结构对于食品物料介电特性的影响，以期通过温度场仿真设计、辅助加热和金属化包装结构处理，提供食品中均匀的射频场分布，为射频技术在食品杀菌领域的深入推广提供理论依据。

射频（radio frequency，RF）是频率范围3 kHz～300 MHz的一种高频交流电磁波，食品置于射频场中时，会成为一种交变电磁场中的有耗介质而被整体加热。射频加热相比其他的加热方式具有其特殊优势，尤其射频能够快速加热低水分食品物料。本项目以低水分活度食品物料花椒粉和大米为例，探究了介电特性与射频加热之间的关系，探究了薄膜类包装材料在射频处理前后外观、微观和力学性能上的变化，探究了对加热速率和加热均匀性有影响的因素，包括周围介质、设备参数、物料参数、包装结构等。研究结果表明，液体物料与固体物料的介电特性随温度呈现出不一样的变化趋势：液体的介电常数随温度升高而降低，但是固体的介电常数随温度的升高而略微增加；花椒粉射频杀菌的最佳温度为80℃，此时能够使菌落总数降低大约2个数量级；设备可调范围内，极板间距越低则物料升温速率越大；物料水分含量越高具有越大的加热速率；不同厚度和种类的周围介质会对加热均匀性产生影响；颗粒型分散结构应用于液体射频加热时可以将加热速率提升5倍以上；孔型分散结构应用于固体射频加热可以提高升温速率，将其应用于杀虫能够大大提高虫致死率，处理过后的物料在储存4个月后菌落总数处于低水平。研究结果对食品物料的工艺射频加工有很强的参考价值，为射频杀菌以及改善射频加热均匀性的方法提供了可行的思路。