Direct measurements of interfacial interactions between pectin and κ-casein and implications for the stabilisation of calcium-free casein micelle mimics

Effect of the pectin degree of methylesterification (A: 78%, B: 28%, C: 90%) on the interfacial interaction between pectin (blue) and κ-casein (green) and the behaviour κ-casein coated latex particle in pectin solution at pH 4.6. Using Surface Plasmon Resonance (SPR) it has been shown that the fine structure of the anionic polysaccharide pectin strongly influences its interfacial interaction with a κ-casein layer coated onto a gold surface (via a dextran linker) in the pH range 3.5–6.8, with the highest SPR signal being observed for pectin with the lowest charge density tested (a degree of methylesterification (DM) around 90%). Furthermore, the Brownian motions of κ-casein coated polystyrene beads (used to provide calcium-free ‘model casein micelles’) were studied in pectin solutions using Diffusing Wave Spectroscopy (DWS) and microscopy, and were compared with measurements made on naked beads. At every pH value studied (with the exception of 3.5), bridging of the protein-covered probe particles was observed for pectins of both DM 28 and DM 78. However, no aggregated complexes were found in these model casein micelle systems when pectin of an unusually high DM was used (90%). It was hypothesised that having a limited number of binding regions of spatially limited extent maximises the number of chains binding to the protein layer (as found with the SPR measurement), encourages the formation of loops and trains, and additionally limits the potential for destabilisation via bridging.