乳清蛋白表面结构变化对超滤浓缩过程的影响机制

Whey is a multi-protein system, the membrane fouling mechanism is very complex during concentration by ultrafiltration. In our previous study, it was found that the protein surface morphology has an important impact on its filtration behavior. However, the regularity and mechanism need to be further studied. The study is to reveal the effect of micro-environmental changes including ion concentration, pH, transmembrane pressure on whey protein surface structure during ultrafiltration process. This project is to study the protein surface structure changes including protein aggregate shape, size, surface roughness and surface groups, etc. in retention, membrane pore and membrane surface by monitoring the micro-environmental changes including ion concentration, pH and transmembrane pressure, etc. with the feed across the membrane into the permeation during the ultrafiltration process. The aim is to explore the effect of time and environment on the surface structure of whey protein during ultrafiltration. The formation of membrane fouling was mainly from the membrane pore blockage and membrane surface protein adhesion. Based on membrane fouling model, XDLVO theory and collapse effect, the effects of protein surface structure changes on the interaction between proteins and the membrane during membrane concentration were analyzed to reveal the mechanism of membrane pore blockage and membrane surface protein adhesion caused by the structural changes of whey protein during ultrafiltration. This project aims to reveal the membrane fouling mechanism of whey protein ultrafiltration from the perspective of the whey protein surface structure changes. Then provide a basic theory for high efficiently control membrane fouling during whey ultrafiltration process.

乳清作为多蛋白体系，在超滤浓缩过程中形成膜污染的机制十分复杂。课题组前期研究发现蛋白表面形态对其动态过滤行为有着重要的影响，但具体规律和机制有待深入研究。本项目通过监测乳清蛋白在超滤浓缩过程中，随透过液的渗出，蛋白周围微环境(包括离子浓度、pH和跨膜压力等)的变化对截留液中、膜孔中和膜表面粘附的蛋白表面结构(包括蛋白团聚体形状、尺寸、表面粗糙度和表面基团等)的影响，探索超滤场中时空变化对乳清蛋白表面结构的影响及其变化规律。膜污染的形成主要来源于膜孔堵塞和膜表面蛋白粘附。基于膜污染模型、XDLVO理论和塌缩效应等分析膜浓缩过程中蛋白表面结构变化对蛋白与蛋白之间，蛋白与膜的相互作用的影响，揭示乳清蛋白在超滤场中表面结构的变化规律对其形成膜孔堵塞和膜表面蛋白粘附的机制。旨在从乳清蛋白表面结构随时空变化的角度揭示乳清蛋白超滤过程中形成膜污染的机制，为高效控制乳清蛋白超滤过程中的膜污染提供理论依据。

我国的乳清蛋白目前还依赖于进口，昂贵的膜材料和依赖于进口清洗剂使得乳清蛋白的膜浓缩成本提高，让乳品企业望而生却。本项目考察乳清蛋白超滤过程中随着超滤时间的增加，离子环境变化对蛋白结构的影响，探究乳清蛋白膜浓缩过程中表面结构的变化规律。建立蛋白表面结构变化与膜相互作用关系，揭示乳清蛋白超滤过程中的膜污染机制。考察了Na离子调控对蛋白表面结构的影响，为进一步减轻膜污染提供理论基础。研究发现在超滤第7 min时，α-螺旋和β-折叠含量降低，氢键被破坏，疏水性增强，Ca离子含量上升，与相邻蛋白质通过离子桥相连形成络合物，发生膜孔堵塞，增强膜污染，通量下降。7 min后饼层阻力增大，α-螺旋含量的增加有利于膜污染的降低，膜表面蛋白7 min主要存在C-H键的伸缩振动，分子间和分子内部氢键作用力增加，色氨酸残基被掩埋。15 min AFM图像显示粗糙度最高。Ca离子和Mg离子含量升高蛋白在膜表面堆积形成饼层。蛋白表面张力结果显示柔性结构和较高疏水表面积的蛋白质可以强烈的吸附在界面上，与AFM观察结果相对应。随着过滤时间的延长，接触角逐渐增大，疏水作用增强，膜通量降低，离子强度升高，溶液中存在的阳离子会中和物质表面的负电荷，屏蔽物质表面的含氧基团，降低蛋白和水分子之间形成氢键的机会，疏水性增强，膜污染增加。粘聚自由能的正负值反应亲疏水性指标，7 min时亲水性减弱，膜污染加剧。蛋白质-膜与蛋白质-蛋白质之间的各作用力大小顺序均依次为极性作用力ΔGAB＞范德华力ΔGLW＞双电层作用力ΔGEL。基于以上研究，在不同浓度和不同超滤时间，采用Na离子调控，研究发现，在超滤0 min时，加入10 mmol/L NaCl调控，超滤24 min后，膜总阻力最低，通量最高。截流液蛋白α-螺旋含量上升，膜表面蛋白C-H键伸缩振动强度降低，在3500-3300 cm-1 O-H吸收峰增强，羟基的伸缩振动加强，膜表面蛋白分子间氢键和分子内氢键作用力增加。在7 min前或临近7 min加入Na离子调控有助于减轻孔隙堵塞的形成，提高过滤效率。