基于酚类物质的氧化聚集和酚类-蛋白质相互作用研究荔枝汁混浊稳定性散失的机制

The cloud juice was not only nutritious, but also has the characteristics of the original flavor of the fruit. It was great popular for its reality. The cloud litchi juice layered and precipitated during the storage, and this problem was not settled. The mechanism of destabilization was poorly understood. Based on the results of previous experiment that the precipitate of cloud litchi juice contained a large number of bond phenols and proteins, we attempted to do the research following the ideas: the main factors - simulation verification - Inter and intra interaction. In this project, we confirmed the main factors which affect the stability of cloud litchi juice, and verified the influence of phenols and proteins on the stability of cloud litchi juice. The phenol autoxidation polymerized and phenol-protein interaction was further studied. Technology such as spectroscopy, rheology and electron microscope was used in the study. Meanwhile, the changes of molecular structure such as molecular conformation, molecular weight, degree of polymerization and the forces of the interaction between molecule such as covalent bond, and non-covalent bond like hydrogen bond and hydrophobic interaction were mainly studied in the simulation of litchi juice. Through this research, the relationship between litchi juice stability and molecular structure, interactions was expounded, and the mechanism of destabilization of cloud litchi juice was finally clarified. Therefore, this research will provide a theoretical basis for the development of stable cloud litchi juice.

混浊型果汁不仅营养丰富，还具有水果原有的风味特征，更增添了果汁的真实感，受到消费者的广泛欢迎。由于混浊型荔枝汁在贮藏期间易出现分层、沉淀现象，迄今尚缺乏明确的解决方案，对于混浊稳定性散失的机理尚不明确。基于前期荔枝浊汁沉淀物含有大量的结合型酚类物质和蛋白质，本项目按照物质基础-模拟验证-分子内与分子间的相互作用的研究思路，拟采用光谱学、流变学结合电镜等技术，确证影响混浊型荔枝汁不稳定的主要因素，验证荔枝果肉酚类和蛋白质对荔枝汁混浊稳定性的影响，在此基础上进一步研究酚类的自氧化聚集，酚类与蛋白质的相互作用。重点研究分子构象、分子量、聚合度等分子结构和共价键，氢键、疏水相互作用等非共价键的分子相互作用力在模拟荔枝汁体系中的变化规律，阐明荔枝汁稳定性与分子结构、相互作用力之间的关系，进而揭示荔枝汁混浊稳定性散失的机制，为开发稳定的混浊型荔枝汁提供理论依据。

与清汁相比，浑浊型果汁具有更逼真的口感和良好的风味，深受消费者喜爱，市场情景广阔。由于其复杂性，混浊型果汁体系在贮藏过程中极不稳定，容易褐变、分层和沉淀。尽管这些现象对果汁的营养影响不大，但会影响产品的可接受度。本课题基于酚类化合物和蛋白质的氧化与变性，研究荔枝汁在贮藏过程中悬浮不稳定的机理。项目分析了沉淀量、相对浑浊度和ζ电位的变化规律，分离了荔枝浊汁上清液和沉淀物，并研究了蛋白质和酚类物质的含量及结构。结果表明，在贮藏过程中，沉淀量有所增加，上清液中总蛋白和总酚含量降低，而沉淀中谷蛋白和总酚含量显著增加，上清液中表儿茶素、原花青素B2和槲皮素-3-O-芸香糖-7-O-鼠李糖苷的含量明显下降。然而，这两种物质在沉淀中无法检测到。荔枝浊汁的上清总蛋白与沉淀中的球蛋白和醇溶蛋白的亚基分子量主要集中在30-95 kD，远低于谷蛋白（>270 kD），表明分子量大的谷蛋白可能在贮藏过程中直接沉降；上清液蛋白、谷蛋白和醇溶蛋白的变性温度均低于95℃，在果汁灭菌过程中，可导致部分蛋白质变性，从而在贮藏过程中沉降。上清总蛋白、球蛋白、醇溶蛋白的游离巯基含量均超过总巯基的50%，二硫键的含量均比较低，在贮藏过程中更容易去折叠，为后期蛋白质活性位点的暴露、以及絮凝和沉淀提供基础。荔枝汁沉积物的形成主要是由蛋白质的缓慢变性以及表儿茶素、原花青素B2和槲皮素-3-O-芸香糖-7-O-鼠李糖苷的氧化所致。为了提高荔枝浊汁的稳定性，采用单因素和相关实验分析相结合的方法得出最佳的均质条件：压力20 Mpa，均质3次。最优稳定剂配比为：果胶、CMC、黄原胶分别为1.39、3.50、0.50 g/L，此时荔枝浊汁的稳定性最好；最优抗氧化剂配比为抗坏血酸、L-半胱氨酸、亚硫酸钠分别为6.00、1.14、0.07 g/L，此时褐变指数最低。该项目初步阐明了混浊型荔枝汁悬浮不稳定的机理，并探讨了稳定性改善的方法。