In dairy pasteurization equipment, fouling is an ongoing problem. Indeed, when heated, milk and its derivatives generate mineral and proteinaceous deposits on stainless steel walls. This heat-induced fouling impairs the process through the addition of an increasing thermal resistance to the system. Deposits are also a threat to food safety as they provide micro-organisms with good settlement opportunities. As a consequence, fouling mitigating strategies are needed.

Biomimetic surfaces in particular, inspired from the surface morphology of lotus leaf could be considered for their self-cleaning abilities. Its dual-scale roughness (i.e. a micro roughness supporpsed by nanoscale roughness) allows for the composite Cassie-Baxter wetting state due to air remaining trapped between the liquid and the solid surface. As a result, those surfaces possess very high contact angles (typically higher than 150o) and very low contact angle hysteresis (typically less than 10°). However, a major limitation of this type of surface is the difficulty to maintain a stable Cassie-Baxter state over time: depending on the experimental conditions (pressure, vibration, evaporation, surface defect) the liquid penetrates sooner or later into the structures degrading their anti-biofouling properties. To overcome this limitation, it has been proposed to impregnate the textured surface by a liquid of low surface tension (usually an inert oil not miscible with water). This led to SLIPS surfaces (Slippery Liquid-Infused Porous Surfaces). Even if these surfaces present low contact angle, their hysteresis is also almost null whatever the experimental conditions leading to antifouling properties [1].

This work aims at designing Cassie-Baxter and SLIPS surfaces and test them in dairy processing conditions to assess their antifouling properties. To this end, 316L stainless steel surfaces were texturized via femtosecond laser irradiation to generate dual-scale (cauliflower-like) structures [2]. Some of the fabricated surfaces underwent further modifications: (i) silanization with perfluorodecyltrichloro-silane or (ii) silanization followed by impregnation with a fluorinated oil to create Slippery Liquid Infused Porous Surfaces (SLIPS) [3]. All surfaces were tested for their fouling properties in a pilot pasteurization equipement (UMET-PIHM, Institut National de la Recherche Agronomique, Villeneuve d'Ascq) [4] allowing to mimick industrial conditions of the pasteurization process.

Thorough characterizations were performed on the surfaces before and after fouling, to (i) establish clearly their surface properties (wettability, surface energy, roughness) and (ii) to investigate the impact of the different surface properties on heat-induced dairy fouling compared to a native stainless steel as reference. A wide range of analytical tools such as Goniometry, cross-section Electron Probe Micro-Analysis X-ray mappings, and Scanning Electron Microscopy were implemented to this end.

Outstanding results were obtained regarding antifouling properties of dual-scaled roughness surfaces in dairy processing conditions, with a reduction of fouling by more than 90% in weight.

References

[1] T.-S. Wong, S. H. Kang, S. K. Y. Tang, E. Smythe, B. D. Hatton, A. Grinthal, and J. Aizenberg, “Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity,” Nature, vol. 477, pp. 443–447, 2011.

[2] A.-M. Kietzig, S. G. Hatzikiriakos, and P. Englezos, “Patterned Superhydrophobic Metallic Surfaces,” Langmuir, vol. 25, no. 8, pp. 4821–4827, 2009.

[3] A. K. Epstein, T.-S. Wong, R. A. Belisle, E. M. Boggs, and J. Aizenberg, “Liquid-infused structured surfaces with exceptional anti-biofouling performance,” PNAS, vol. 109, no. 33, pp. 13182–13187, 2012.

[4] M. Jimenez, G. Delaplace, N. Nuns, S. Bellayer, D. Deresmes, G. Ronse, G. Alogaili, M. Collinet-Fressancourt, and M. Traisnel, “Toward the understanding of the interfacial dairy fouling deposition and growth mechanisms at a stainless steel surface: A multiscale approach,” J. Colloid an interface Sci., vol. 404, pp. 192–200, 2013.