Selecting the most appropriate materials and fabricating them into hygienic food-processing equipment is a process that should start by considering the detailed definitions of both the product and the process in terms of their chemistry, stresses, times and temperatures, including those which may result from, for instance, vibration or 'water-hammer' or thermal shock, all of which are common in food-processing plant. Then the detergents and sanitizing chemicals required to remove process soils and bring the plant back to an acceptable microbiological standard may introduce additional physical or chemical demands. (reference)
Naturally, in terms of both engineering and hygiene, some of the early stages of food preparation, such as in the orchard or the abattoir, are less demanding and involve fewer hazards than the later stages of cooking and packaging, and so low grade materials, such as painted mild steel, may be adequate for them, but after these initial stages, maintainable hygiene quickly becomes critical. This places special constraints on the design process.
All surfaces, including their joints, must be smooth and must have neither ridges nor crevices which could harbor organic materials. Projections, edges and recesses should be reduced to a minimum. Components should preferably be assembled by welding or continuous bonding. The inside surfaces of pipes and vessels must have curves of a radius sufficient to allow thorough cleaning. There should be no 'back-waters' where soils can accumulate and become a problem to remove and no drain traps which can retain process or cleaning fluids. Cleanability is critical to hygiene.
Essential to cleanability is a superior surface finish. Generally, the rougher a surface is, the more readily soils will adhere to it and the more difficult it will be to clean, particularly by cleaning in place which may not be able to apply the mechanical action which can help to remove process soils. The surface roughness of sheet or tube, usually expressed by Ra values, is relatively easy to measure where the surface is accessible, but is frequently more important inside pipes and vessels, which are less easy to inspect.
Surfaces subjected to wear or abrasion will, as they become less smooth, become more difficult to clean. In the processing of dry foods, the friction as they pass along pipes can cause significant abrasion of the bore. Over time this can lead not only to roughness and the resultant cleaning problems but also to very localized rises in temperature which can encourage microbial growth. If the food passing along the pipe is not dry but a fast-moving fluid containing abrasive particles, the internal surface can suffer erosion, particularly where there are any projections or features such as weld-runs to increase turbulence. Turbulence-induced pressure waves can also cause cavitation by creating bubbles and then causing them to collapse violently, damaging the surfaces of, for instance, impellers or valve-seats.
Stainless steel's inherent hardness maintains the smoothness which enables it to resist the adhesion of soils and biofilms and renders it very easy to clean and sanitize. Indeed, stainless steel has been proved in clinical tests to be significantly more hygienic than other food-contact surfaces. Claims have, of course, been made that other (non-ferritic) materials offer strong antibacterial properties by effectively poisoning virulent pathogens, so having to depend less upon the smoothness and abrasion resistance of their surfaces, but stainless steel remains hygienic without imparting any of its constituents either to pathogens or to the food being prepared.
|Stainless steel's inherently smooth surface has an additional, but less obvious, advantage. When used for vertical surfaces in kitchen areas stainless steel forms a most effective barrier up which vermin cannot climb, significantly restricting their freedom to spread throughout a building. For this reason, stainless steel is increasingly being used to face the vertical surfaces of vehicle loading-bays in factories as these are favorite entry-points for mice. (photo courtesy)||