Whey protein adsorption and aggregation on modified stainless steel surfaces in relation to fouling (2004)

This thesis focuses on the possibilities to reduce fouling by 1.Modifying the stainless steel surface. For this purpose, surfaces modified with state-of-the-art material science techniques, such as SiF3+ and MoS22+ ion implantation, Diamond-like Carbon (DLC) sputtering, DLC, DLC-Si-O and SiOx Plasma Enhanced Chemical Vapour Deposition (PECVD), autocatalytic Ni-P-PTFE and silica coating, have been investigated.2.Adjusting the process conditions in terms of temperature, protein solution residence time and flow conditions. The adsorption of whey proteins, where b-lactoglobulin (b-Lg) is the major constituent, on unmodified and modified stainless steel surfaces was studied by ellipsometry.The modified surfaces were characterised according to their chemical composition, roughness and topography by atomic force microscopy (AFM), and wettability by the Wilhelmy plate method. The size distribution of the whey protein aggregates formed in solution in the flow cell was determined by dynamic light scattering (DLS).The chemical composition of the steel was found not to affect the adsorption behaviour whereas in general a higher surface roughness increased the amount adsorbed. The different modification techniques did not alter the roughness of the surfaces with a 2R finish (cold rolled and annealed in a protective atmosphere), the exception being the Ni-P-PTFE surface, whereas it increased the roughness of the surfaces with a 2B finish (cold rolled, heat treated, pickled and skinpassed). For this surface finish the DLC sputtering and Ni-P-PTFE coating produced surfaces with the greatest roughness. All modified surfaces revealed a similar surface topography with the exception of the Ni-P-PTFE coating, which masked the underlying steel topography. The surface energy was also affected by the different modification techniques where the SiOx-plasmaCVD and Ni-P-PTFE-coating methods produced the most hydrophilic and hydrophobic surfaces, respectively. Adsorption beyond monolayer coverage was found to proceed via protein aggregation on the surface rather than deposition of aggregates on the surface. The amount of protein adsorbed onto the surface was reduced when a larger proportion of whey protein aggregates was present in the solution. An increase in the pH of the solution resulted in a lower initial adsorption rate on unmodified and ion-implanted stainless steel surfaces, while a higher initial adsorption rate was found on the DLC-sputtered and DLC-PlasmaCVD surfaces. This was attributed to a balance between electrostatic and hydrophobic interactions. The DLC-sputtered surface was found to give the lowest amount of protein adsorbed, which was explained by protein conformational changes upon adsorption induced by the surface modification. After the formation of the initial protein monolayer the influence of the surface modification on the rate of the subsequent adsorption / surface aggregation process decreased. However, the final amount of protein adsorbed onto the surface was dependent on the extent of monolayer formation. This showed that the surface modification affected the adsorption process by altering the formation of the first layer of proteins on the surface. Both an increase in the protein solution residence time and in the Reynolds number in the turbulent region led to smaller amounts of protein adsorbed on the surface. The largest reduction was observed when increasing the residence time, which was attributed to a decrease in the concentration of native protein available in the solution. The model developed for protein monolayer and multilayer formation suggested that, increasing the Reynolds number below the unfolding temperature of b–Lg (72°C) resulted in a higher collision frequency between the protein and the surface, and above the unfolding temperature (85°C) the dominant effect was the increase in the extent of protein removal from the surface.