果蔬超声波-真空冻结组合预处理冷冻干燥特性与机理研究

Long drying time, high energy consumption and high production costs are the major problems restricting the extensive application of freeze-drying technology on fruit and vegetable. Special pre-treatments of materials before freeze-drying are regarded as new ways and methods to improve freeze-drying efficiency and reduce costs.In this study, experiments on freeze-drying technology with ultrasonic and vacuum-freezing combination pretreatments are to be performed, taking some typical fruits and vegetables as the study objects. After ultrasonic or/and vacuum-freezing pre-treating, micro-characteristics of materials including cells structure, network structure and bond energy of water molecules, modality and distribution of water, ice crystals form will be investigated by applying instrumental analysis techniques such as nuclear magnetic resonance, Raman spectroscopy, infrared spectroscopy, low-temperature microscopy imaging etc. Based on computational analysis on the heat and mass transfer process for liquid-gas-solid phase changing of water and temperature variation of materials, dynamic monitoring and analysis of the process parameters, temperature-mass coupled thermodynamic model on fruits and vegetables vacuum-freezing will be established. Subsequently, pre-drying and freezing characteristics of the process and the influence mechanism of ultrasonic treatment are to be analyzed and revealed.Analyzing variation rules on materials moisture content and temperature during freeze-drying after ultrasonic and vacuum-freezing pretreatments, dynamic models of the sublimation drying and desorption drying are to be established. And qualities of freeze-dried products will also be analyzed. Finally, the influence mechanism of ultrasonic and vacuum-freezing combination pretreatments on freeze-drying characteristics and products qualities is to be explained. The research findings can provide a theoretical basis for the optimization of the innovative freeze-drying process and its production applications.

干燥时间长、能耗高、生产成本大是制约果蔬冷冻干燥技术广泛应用的主要问题，物料冻干前特殊预处理是提高冻干效率、降低成本的新途径和新方法。本研究以典型果蔬为对象，开展超声波-真空冻结组合预处理冷冻干燥实验研究，应用核磁共振、拉曼光谱、红外光谱、低温显微成像等仪器分析技术，研究超声波、真空冻结预处理果蔬细胞组织结构、水分网络结构与结合键能、水分形态与分布、冰晶体形态等微观特性；基于水分液-气-固相变及物料降温传热传质理论计算、过程参数动态监测等分析方法，建立果蔬真空冻结物料温度和质量耦合变化热力学模型，分析预干燥和冷冻特性，揭示超声波处理对真空冻结预干燥和冷冻效果的影响机理；研究组合预处理果蔬冷冻干燥物料含水率和温度变化规律，建立升华干燥和解析干燥动力学模型，分析冻干制品品质，揭示组合预处理对果蔬冷冻干燥特性和冻干品质的影响机理。研究结论为该新型冷冻干燥工艺过程优化和生产应用提供理论依据。

本项目针对果蔬传统冻干技术存在的工艺复杂、干燥时间长、生产成本大等难题，以前期提出的真空冻结冷冻干燥一体化冻干新工艺技术为基础，以苹果、马铃薯等果蔬物料为对象，重点探索超声波-真空冻结处理对果蔬一体化冻干特性和品质影响及作用机理。研究发现：超声波处理可增加苹果等物料的真空冻结脱水和降温效果，但影响并不显著（p<0.05）；真空冻结过程中的质量、温度变化规律符合三阶段温度-质量热力学模型；超声处理对苹果真空冻结冷冻干燥一体化冻干工艺的冻干时间、复水比、硬度、表观形态的影响并不显著（p<0.05）；真空冻结冻干工艺、超声-真空冻结冻干工艺的冻干样品表观形态和微观形态保持度均远不及常规冷冻干燥样品；真空冻结冷冻干燥一体化工艺比传统冻干工艺更有益于保存柠檬中挥发性风味化合物，减少冷冻干燥过程中的芳香损失。拓展研究表明热烫预处理可显著强化苹果等真空冻结脱水效果，明显改善一体化冻干样品的原有形态保持。并在此基础上探索研究真空快速冻结、真空冻结冷冻干燥新装置及配套工艺技术，获得原创性发明专利成果。低场核磁共振LF-NMR水分网络结构测定表明，超声波、热烫等预处理对物料中水分自由度状态的改变不足以影响真空冻结过程中的水分蒸发强度和失水程度；细胞组织结构SEM扫描电镜/TEM透射电镜、LM光镜测定等微观特性分析表明，物料真空冻结失水程度与细胞膜透性、细胞组织破坏程度密切相关。研究证实项目前期提出的真空冻结冷冻干燥一体化冻干工艺技术的成功关键在于真空冻结期间脱水效果和冻结程度，真空冻结过程失水越多、冻结越充分，越有利于冻干期间细胞组织多孔结构的形成和产品原有形态的保持。相关研究成果已发表论文8篇，其中EI收录论文3篇，另有在投稿SCI、EI收录论文各1篇，已授权国家发明专利1项、实用新型专利3项。本项目研究结论可为真空冻结冷冻干燥一体化新型冻干工艺过程优化和生产应用提供理论依据，项目取得的原创性发明成果及配套技术具有良好的应用价值。