莲子干燥过程中热量、水分传递与淀粉凝胶化研究

Food drying plays an important role in food preservation, representing a very important unit operation of food processing, which could suppress activities of microorganisms, enzymes or ferments in food and keep nutrition. During heat-sensitive natural products drying, phsycohemical reactions in nutrition composition and changes in organizational structure always take place due to inner high moisture distribution and high temperature distribution. The changes and reactions not only affect the product quality,but also affect the transport processes, which especially happen during drying of the large size lotus seeds.This research focuses on starch gelatinization and transport processes inside lotus seeds during drying in order to innovate drying technology, optimize drying process and obtain desired quality of lotus seeds.Relationship between transport of heat and mass and starch gelatinization will be studied.The moisture distributions inside the lotus seeds will be visualizably, non-invasivly obtained by magnetic resonance technique(imaging and relaxation time),and thermodynamic data obtained by differential scanning calorimetry. The mechanism of starch gelatinization will be clarified based on the experimental data.Quality degradation in lotus seeds will be studied and heat and mass transfer model will be established, which will help produce realistic computer simulations of drying processes. The innovative drying method and optimal drying process will be determined, which can keep lotus seed starch from gelatinizing and hardening, increase rehydration of dried lotus seeds, reduce drying time and save energy costs. Results and information obtained from this study can assis in innovation in drying technology and process of lotus seeds drying as well as reducing drying time and improving pruduct quality, and will also enrich theory of transport phenomena in porous materials.

热敏性天然产物干燥过程中，由于物料局部存在高温、高水分的分布，会使有效（营养）成分发生物理、化学变化和组织结构改变，影响产品质量，这些改变又影响到后续的传热传质过程。莲子颗粒较大，干燥时间长，这种影响尤其突出。本项目研究干燥过程中莲子淀粉形态变化与内部传递过程的相互作用，解决淀粉凝胶化造成莲子结壳、硬化，不利于干燥、复水的问题；采用热分析、核磁共振技术，通过实验测量，获得淀粉凝胶化热分析数据、水分分布与变化规律；在此基础上，探索淀粉颗粒形态变化与内部温度、水分状态和含量的关系，揭示莲子干燥过程中淀粉凝胶化机理；建立质、热传递理论模型；掌握莲子质量降解规律，为莲子新型干燥技术和工艺的确立提供理论基础。本项目的开展可缩短莲子干燥时间，提高干燥产品质量，降低能耗，丰富高淀粉类、多孔物料干燥过程的客观认识，为热敏性物料新型干燥技术的开发提供依据。

莲子是江西等地的特色农产品。莲子颗粒大，淀粉含量高，不适宜的干燥易引起莲子淀粉凝胶化，造成微观结构改变，进而影响到产品质量，如硬度、糊化特性等。本项目主要研究莲子干燥过程中的传递现象以及淀粉凝胶化过程，解决淀粉形态结构变化造成莲子结壳、硬化，不利于干燥、质量退化问题，确定莲子高品质干燥技术和工艺，并为干莲子后续加工提供基础、科学数据。项目采用低场核磁共振技术（成像和弛豫时间）研究了干燥过程中莲子内部水分分布状态和变化规律，利用差示扫描量热技术（DSC）对莲子干燥过程中淀粉热特性与凝胶化过程进行了研究，获取了淀粉凝胶化基础数据；开展了莲子变温干燥、先热风后微波分段干燥等试验，确立了莲子变温干燥、分段干燥工艺。研究发现，半结合水是莲子的特征水分，占新鲜莲子总水分的85.59%，其脱除过程呈现指数特征，且流动性趋向变差。相比恒温干燥，变温干燥有效扩散系数显著增加，且随温度升高而增大，有利于提高干燥效率；Midilli模型可以准确模拟莲子热风干燥过程；高温干燥伴随着淀粉凝胶化，引起晶形、微观结构改变，产品硬度增大；采用近“淀粉凝胶化温度”（峰顶温度70℃左右），即干燥温度范围为60～70℃的变温干燥，莲子外形饱满、产品品质高。基于流体仿真技术，项目开发、优化了中高温热泵变温干燥设备，为莲子高品质工业化干燥提供了技术支持。项目研究成果丰富了项目组“基于避免物料内部局部高温、高湿同时出现造成有效（或营养）成分发生物理化学变化，提出对热敏性物料采用变温、分段干燥”、“近淀粉凝胶化温度干燥”的学术思想，为热敏性天然产物的高质量、低能耗干燥提供了相关理论和研究方法。项目相关成果已在干燥设备生产企业、农产品加工企业中得到推广应用。