Effect of surface roughness and temperature on stainless steel - whey protein interfacial interactions under pasteurisation conditions

The effects of the surface characteristics of 316L stainless steel (SS316L), including chemical composition before and after foulant deposition, surface roughness, and wall temperature, on both the liquid-solid and the solid-solid interfacial interactions have been investigated using contact angle measurements and atomic force microscopy respectively. Wettability of the metal surface was favoured by increased surface roughness (in the range-limited for food contact applications) and wall temperature (within the temperature range used for pasteurisation). A fine surface finish (i.e. mirror) could be an effective intervention to reduce liquid adhesion and the subsequent foulant deposition, especially under thermal treatment. The surface free energy (SFE) of SS316L and its polar and disperse components remained constant from ambient to pasteurisation temperatures (< 80°C). However, as fouling develops, the surface free energy evolved: upon foulant deposition, SFE decreased. An increased polarity (3.4% from 25 to 80°C) of the fouled surface could be related to the exposure the hydrophobic core of reversibly adsorbed β-Lactoglobulin toward the foulant-air interface. Both surface adhesion and Young’s modulus at sub-micron spatial resolution confirmed that the packing within the foulant and molecular orientation on the foulant surface were affected by the temperature of the underlying substrate. Temperature also affected the wetting behaviour of cleaning solutions on surface foulant; as the surface temperature increased from 25°C to 75°C, the contact angle on WPC increased, suggesting an enhanced surface hydrophobicity. Overall, this work highlights the importance of surface parameters on governing the interfacial interactions that are competing for the control of the complex fouling phenomena.