Wooster Tim J, Augustin Mary Ann
Food Science Australia, Werribee, Australia.
J Colloid Interface Sci. 2007 Sep 15;313(2):665-75. doi: 10.1016/j.jcis.2007.04.054. Epub 2007 Apr 29.
The effect of the steric layer thickness on the flocculation stability of beta-lactoglobulin-carbohydrate diblock copolymers was assessed. The diblock copolymers were created by conjugating beta-lactoglobulin to maltose or a series of different M(n) maltodextrins using the Maillard reaction. The thickness and spatial arrangement of the interfacial layers were assessed via latex adsorption and selective enzymatic digestion studies. An increase in the molecular weight of the maltodextrin (900, 1900 and 3800 Da) increased the interfacial thickness (1.1, 2.5 and 7.3 nm, respectively). No detectable change to interfacial thickness was observed upon the attachment of maltose. The increase in the interfacial layer thickness scaled with the hydrodynamic size of the carbohydrate. The beta-lactoglobulin-maltodextrin conjugates were found to have a diblock architecture, with the protein anchored at the surface and the carbohydrate protruding into the aqueous continuous phase. The stability of oil-in-water emulsions formed using the conjugates was assessed by exposing them to salt (150 mM NaCl or 0-20 mM CaCl(2)), heat alone or heat in the presence of 150 mM NaCl. Conjugation of a 900 Da maltodextrin provided sufficient steric stabilization to prevent flocculation in high salt environments. The effect of the (number) density of the steric layer was also assessed by controlling the average number of maltodextrins attached per beta-lactoglobulin molecule. The steric layer density at which emulsions became unstable was a function of carbohydrate M(n). Emulsions made from the 900 Da maltodextrin conjugate became unstable below a steric layer density of one tail per 7.5 nm(2), whilst emulsions made from the 1900 Da maltodextrin were unstable below a steric layer density of one tail per 9.5 nm(2). This trend was expected and can be explained by the stronger van der Waals attraction that arises from the closer interdroplet separations that are permissible with the shorter maltodextrins. The excellent flocculation stability of Maillard conjugate emulsions is thought to arise from the combined effects of weak electrostatic repulsion from the screened protein surface charge and steric repulsion from the attached carbohydrate layer. This means that attachment of a relatively thin steric layer is enough to stabilize the emulsions against flocculation. These findings have important implications for the development of commercial processes to manufacture protein-carbohydrate Maillard conjugate emulsifiers. Furthermore the work provides a greater empirical understanding of the relationship between interfacial architecture and colloidal stability, and may provide the means for greater theoretical understanding of biopolymer stabilization of interfaces.
评估了空间层厚度对β-乳球蛋白-碳水化合物二嵌段共聚物絮凝稳定性的影响。通过美拉德反应将β-乳球蛋白与麦芽糖或一系列不同平均分子量（M(n)）的麦芽糊精共轭来制备二嵌段共聚物。通过乳胶吸附和选择性酶消化研究评估界面层的厚度和空间排列。麦芽糊精分子量增加（900、1900和3800 Da）会使界面厚度增加（分别为1.1、2.5和7.3 nm）。连接麦芽糖后未观察到界面厚度有可检测到的变化。界面层厚度的增加与碳水化合物的流体力学尺寸成比例。发现β-乳球蛋白-麦芽糊精共轭物具有二嵌段结构，蛋白质锚定在表面，碳水化合物突出到水连续相中。通过将使用共轭物形成的水包油乳液暴露于盐（150 mM NaCl或0 - 20 mM CaCl₂）、单独加热或在150 mM NaCl存在下加热来评估其稳定性。连接900 Da麦芽糊精可提供足够的空间稳定作用，以防止在高盐环境中絮凝。还通过控制每个β-乳球蛋白分子连接的麦芽糊精平均数量来评估空间层（数量）密度的影响。乳液变得不稳定时的空间层密度是碳水化合物M(n)的函数。由900 Da麦芽糊精共轭物制成的乳液在空间层密度低于每7.5 nm²一个尾巴时变得不稳定，而由1900 Da麦芽糊精制成的乳液在空间层密度低于每9.5 nm²一个尾巴时不稳定。这种趋势是预期的，并且可以用较短麦芽糊精允许的更近液滴间距产生的更强范德华吸引力来解释。美拉德共轭物乳液优异的絮凝稳定性被认为源于筛选后的蛋白质表面电荷产生的弱静电排斥和连接的碳水化合物层产生的空间排斥的综合作用。这意味着连接相对较薄的空间层就足以使乳液稳定以防絮凝。这些发现对开发生产蛋白质-碳水化合物美拉德共轭乳化剂的商业工艺具有重要意义。此外，这项工作为界面结构与胶体稳定性之间的关系提供了更丰富的经验理解，并可能为从理论上更深入理解生物聚合物对界面的稳定作用提供方法。
