基于大分子凝聚态理论的微波真空干燥莲子微观结构及介电特性演化机制研究

During the latter part of microwave vacuum drying of lotus seeds, the internal biological macromolecules condensed, and the openness and connectivity of the pore channels decreased. Meanwhile, the macromolecular condensation not only changed the original structure and distribution characteristics of functional groups, but also affected the dielectric properties of the lotus seeds, consequently resulting in a sharp decline in microwave absorbance and thermal conversion efficiency. Based on this finding, lotus seeds are chosen as the subject for this project. Modern spectrum technology will be combined with a molecular dynamics simulation to construct models of the condensed state of macromolecules and to accurately demarcate both open and enclosed channels inside lotus seeds. Electron microscopy three-dimensional recombinant technology will be applied to analyze topology features of pore channels. Fractal theory will be employed to build surface roughness models of the channels, and subsequently combined with macromolecular condensed state theory to elucidate the transition mechanism between structural elements and pore channels. A heat transfer model for microwave vacuum drying will be established to quantitatively describe the conversion mode of microwave energy inside lotus seeds, whereas a polynomial mathematical equation will be built to describe the relationship among condensed morphology of structural elements, pore channel characteristics and the dielectric properties of samples during microwave vacuum drying. Water migration and quality changes of the samples will be analyzed by combining microstructure and dielectric properties based on a BP neural network in order to reveal the essence of why a sharp decrease of heat transfer efficiency occurred during the latter drying period of lotus seeds. This study will promote a theoretical foundation related to microwave vacuum drying for starch-based foods, such as lotus seeds; it will also provide a systematic approach that can be used to optimize the technology and processes currently used for drying of foodstuffs with a new quality control perspective.

微波真空干燥后期莲子内部生物大分子凝聚，孔道开放性及连通性下降，原有官能团结构及分布特性发生改变，直接影响物料的介电特性，从而导致物料微波能吸收与热转化效率急剧下降。基于此，本项目以莲子为研究对象，利用现代光谱技术并借助分子动力学模拟手段构建大分子凝聚态模型，精确标定物料内部开放型和封闭型孔道，运用电镜三维重组技术解析孔道拓扑信息，利用分形理论构建孔道粗糙度模型，并结合大分子凝聚态理论阐明“骨架”和“孔道”间相互演变机制；构建微波传热模型定量阐述微波辐射能在莲子中的转化途径，拟合微波真空干燥过程中骨架凝聚形态和孔道特征与物料介电特性之间的多项式数学方程，最后耦合微观结构和介电特性构建BP神经网络模型解析物料水分迁移及品质变化规律，以期揭示微波真空干燥后期物料传热效率急剧下降的现象本质。本研究可提升以莲子为代表的高淀粉质食品的微波真空干燥理论水平，为工艺优化和产品品质控制提供科学依据。

莲子作为高级滋补食品，在传统中药和功能性膳食中都有应用。鲜莲子由于本身水分含量较高，内部的酶与环境微生物容易降解莲子中的有机物，不可避免地降低其营养价值和感官特性，同时产生多种有害物质。为保存莲子，通常采用干燥减少含水率来减缓莲子的质量退化，延长莲子的保质期。干燥方法除了对莲子干燥速率有影响，对其成品品质更是有着至关重要的影响。研究了不同干燥方法对莲子全粉的理化性质、微观结构和功能特性的影响，结果表明，USMVD处理的莲子全粉中淀粉和蛋白的红外光谱特征峰较强；MVD比HAD莲子全粉的结晶度更高，并改变了莲子全粉晶型，而USMVD减少了莲子全粉的结晶度；USMVD能够降低莲子全粉的糊化温度、糊化焓值和糊化特性参数，使莲子全粉内的物质大量聚集，并伴随着莲子淀粉表面出现凹陷，且随着超声波功率的增加聚集程度越高，除此外USMVD还能提高莲子全粉的溶胀度和溶解度。分析不同干燥技术处理后的莲子淀粉及蛋白的特性，结果发现，MVD能够提高莲子中淀粉的结晶度，而US+MVD和USMVD处理后莲子淀粉的结晶度降低，双螺旋结构含量降低，晶型结构由C型改为V+C型，同时这种现象随着超声波功率的增强而越发明显。USMVD能够降低莲子淀粉的糊化温度、糊化焓值和糊化特性参数。HAD莲子淀粉表面光滑与鲜莲子淀粉结构相似，MVD莲子淀粉的结构改变，并有不规则的片状，而US+MVD和USMVD致使莲子淀粉表面出现损伤，淀粉相互聚集形成团状，进而导致经过USMVD处理的莲子淀粉溶胀度和溶解度提高。MVD能够减缓莲子淀粉的消化速率，而USMVD的莲子淀粉消化速率更低，且抗性淀粉含量升高。USMVD处理后莲子蛋白的α-螺旋结构减少，而β-折叠和无规则卷曲等无序结构含量增加，并伴随着三级结构构象改变；蛋白结构的改变导致蛋白Zeta电位绝对值增加，粒径减少，同时变性温度和焓值增加。