The effect of nanometer dimension topographical features on the hygienic status of stainless steel.

Wear of food contact surfaces through abrasion may increase the surface roughness and introduce different topographical features. Both of these properties may enhance retention of soil and microorganisms and affect the surface cleanability. To test this hypothesis, stainless steel surfaces with topographical features and surface roughness (Ra) values simulating those of worn in-use surfaces were prepared. Surfaces were imaged and Ra values determined using atomic force microscopy (AFM). These ranged from 23 to 900 nm. Surfaces were sprayed with standardized cell suspensions of Pseudomonas aeruginosa or Staphylococcus aureus and allowed to air dry and were then cleaned using a nonionic detergent delivered via a manual linear cleaning device. There was a 2-log reduction in numbers attached after cleaning, but there was no significant difference (P > 0.05) between the cleanability of the surfaces in terms of the numbers of cells per unit area remaining after cleaning, although cells appeared to be retained within topographical features. Thus, the simulated effect of wear of a hygienic food contact surface did not affect its cleanability after a one-off microbiological soiling event. AFM provided hitherto unavailable information on the topography of worn stainless steel surfaces. In future work, the surfaces will be repeatedly challenged with an organic soil-microorganism mixture after cleaning events, to provide a more rigorous, realistic test.