柠檬一体化真空冷冻干燥过程单萜烯挥发性风味化合物动态消长机制研究

Monoterpenes volatile flavor compounds are the main aroma components in lemon fruits . The integrated vacuum freeze-drying technology, in contrast with other methods, has unparalleled performance in the retaining of volatile compounds of dried lemon product, but the action mechanism is not yet clear. In this project, the morphological and distribution characteristics of monoterpene volatiles and other macromolecules in the cellular tissue system of lemon during the vacuum-freezing process will be precisely and quantitatively analyzed by the single molecule detection techniques including immune transmission electron microscopy, quantum dot label fluorescence analysis etc. The ultra-structural changes of lemon cell interstitial and organ tissue during the integrated freeze-drying will be analyzed by computerized tomography and atomic force microscopy etc. We will construct a cell-wall-cross diffusion model and a intercellular-porous diffusion model to explore the physical diffusion mechanism of monoterpene volatiles in the dried layer. The mutual evolution of monoterpenes volatiles and their closely related precursor substances and new generators in the cellular tissue system will be investigated to explore the chemical pathways of hydration, oxidation and degradation for lemon monoterpenes volatiles, and the cellular chemical mechanism of monoterpenes volatiles will be revealed. Finally, dynamic variation mechanism of monoterpenes volatile flavor compounds during the integrated freeze-drying of lemon could be comprehensively revealed on the basis of physical diffusion and histochemistry of cell tissue, which will provide theoretical basis for regulating the volatile flavor compounds during the freeze-drying of lemon.

单萜烯挥发性风味化合物是柠檬果实香气组分构成主体，一体化真空冷冻干燥技术对柠檬挥发性风味化合物保留具有显著优势，但作用机制尚不清楚。本项目通过免疫透射电镜、量子点标记荧光分析等单分子检测技术定位、定量分析柠檬真空冻结过程单萜烯挥发物及大分子物质在细胞组织体系内的存在形态、分布特性；通过计算机辐射断层扫描、原子力显微成像等研究一体化冻干过程细胞间隙及器官组织超微结构变化，构建挥发物在干燥层细胞孔室的跨薄壁扩散模型以及细胞间隙多孔扩散模型，模拟验证单萜烯挥发物迁移过程；定位、定量分析单萜烯挥发物及其密切相关的前体物质、新生物质等在细胞组织体系内相互演变关系，探索单萜烯挥发物水合、氧化、降解途径，揭示单萜烯挥发物动态变化的细胞组织化学作用机制。从细胞组织物理扩散、组织化学方面综合阐明柠檬一体化真空冷冻干燥过程单萜烯挥发性风味物质动态消长机制，为柠檬冷冻干燥加工挥发性风味物质调控提供理论基础。

香气大量损失是果蔬冻干加工的一面短板。本项目在前期对柠檬一体化冻干保香效果的研究基础上，重点探索单萜烯等主要挥发性风味物质在一体化冻干过程中的动态变化规律及作用机制。实验探明了柠檬整果挥发性风味物质一体化冻干后的流失特性，D-柠檬烯等主要挥发性风味物质平均保留率<30%。在新鲜柠檬组织中，外果皮挥发性风味物质种类和含量最多，汁胞和海绵层次之，囊衣最少。外果皮和汁胞一体化冻干过程中，挥发性风味物质种类减少，总含量呈先快后慢的下降趋势，氨基酸、有机酸、可溶性糖、脂肪酸含量及LOX、ADH、AAT酶活性等总体均呈下降趋势。在物理作用方面，分析认为挥发物变化主要由选择性扩散理论和微区包封理论的综合作用结果，随着干燥进行，细胞组织中的纤维素、果胶等浓度增加、固化，形成包封微区，碳水化合物氢键缔合作用，增加了挥发物扩散阻力，直至微区被密封，挥发物迁移停止。创新应用X射线计算机断层扫描显微成像和激光共聚焦显微成像技术，建立了柠檬外果皮中水相、油相分布及冻干过程结构变化成像分析方法，基于匀质扩散理论构建了冻干水分扩散模型，成像比较水相灰度和油相荧光强度变化速度，以水相扩散速度为参照计算出油相聚集速度，进而探明冻干过程中挥发性精油的迁移规律。在化学作用方面，发现很多挥发性风味物质与上述非挥发性风味物质及关键酶活性之间存在大量正相关和负相关（p<0.05），这些氨基酸、有机酸、可溶性糖、脂肪酸等可能作为风味前体物质参与了酶促反应、美拉德反应、加热氧化和热降解等一些反应，影响了挥发性风味物质的轮廓构成及变化。相关研究成果已发表论文16篇，其中SCI收录7篇、EI收录2篇、中文核心期刊7篇，另有4篇SCI论文在外审和返修，申请发明专利3项，2项发明专利技术成果获得许可转让，培养毕业研究生3名，参加国际学术会议做大会汇报1次。本项目研究结论可为冻干柠檬挥发性风味物质调控提供理论基础，项目取得的原创性发明成果及配套技术具有良好的应用前景。