Structure of mixed beta-lactoglobulin/pectin adsorbed layers at air/water interfaces; a spectroscopy study

Based on earlier reported surface rheological behaviour two factors appeared to be important for the functional behaviour of mixed protein/polysaccharide adsorbed layers at air/water interfaces: (1) protein/polysaccharide mixing ratio and (2) formation history of the layers. In this study complexes of beta-lactoglobulin (positively charged at pH 4.5) and low methoxyl pectin (negatively charged) were formed at two mixing ratios, resulting in negatively charged and nearly neutral complexes. Neutron reflection showed that adsorption of negative complexes leads to more diffuse layers at the air/water interface than adsorption of neutral complexes. Besides (simultaneous) adsorption of protein/polysaccharide complexes, a mixed layer can also be formed by adsorption of (protein/)polysaccharide (complexes) to a pre-formed protein layer (sequential adsorption). Despite similar bulk concentrations, adsorbed layer density profiles of simultaneously and sequentially formed layers were persistently different, as illustrated by neutron reflection analysis. Time resolved fluorescence anisotropy showed that the mobility of protein molecules at an air/water interface is hampered by the presence of pectin. This hampered mobility of protein through a complex layer could account for differences observed in density profiles of simultaneously and sequentially formed layers. These insights substantiated the previously proposed organisations of the different adsorbed layers based on surface rheological data.