Hygiene in the food industry – why is only machining not enough in the food industry?
An engineer from the maritime or defense industries would associate machining primarily with dimensional precision and fatigue strength. However, in the food industry (Food Grade), an additional, critical characteristic appears: surface roughness.
Even the most precise CNC milling leaves behind tool marks. Although the detail may hold the dimension with accuracy of up to 0.01mm, under a microscope its surface resembles a jagged mountain landscape. Organic residues accumulate in these microscopic “valleys”, becoming a breeding ground for bacteria. Standard cleaning processes (CIP – Cleaning in Place) are unable to “flush out” microorganisms from such deep crevices. A technical “tidbit”: A bacterium such as Listeria monocytogenes has a size of approximately 0.5 – 2.0μm. If your surface after milling has a roughness of Ra = 3.2μm, you create a “safe shelter” for bacteria, in which they are protected from the flow of disinfectants.
Technical „trivia” – bacterium such as Listeria monocytogenes has the size of approximately 0.5 – 2.0μm. If your surface after milling has a roughness of Ra = 3.2μm, you create a “safe shelter” for bacteria, in which they are protected from the flow of disinfectants.
Ra and Rz Parameters – key indicators that must not be confused
In technical specifications, we most often encounter the Ra parameter (arithmetic mean deviation of the profile). However, experienced engineers know that this is not the end of it.
Ra: Informs us about the general condition of the surface. The EHEDG (European Hygienic Engineering & Design Group) norm requires that for surfaces in contact with food, it should be a maximum of 0.8μm.
Rz: This is the maximum profile height (from the highest peak to the lowest valley). It is extremely important, because it is precisely the deep “craters” (even with a low average Ra) that can be a habitat for biofilm.
Comparison of machining methods and their impact on hygiene
| Machining Method | Typical Ra (μm) | Suitability for the Food Industry | Bacteriological Risk |
| CNC milling (roughing) | 3,2 – 6,3 | Only structural elements (external) | Critical |
| Turning (finishing) | 1,6 – 2,5 | Machine elements without food contact | High |
| CNC grinding | 0,4 – 0,8 | Standard for pipelines and tanks | Low (compliant with norms) |
| Polishing (mirror) | < 0,2 | Dairy industry, pharmaceuticals, biotechnology | Minimal |
Bacteria love “micro-shelters” – the mechanism of biofilm formation
The issue of hygiene in stainless steel is not just a matter of aesthetics. It’s pure science. When the surface is rough, adhesion occurs – bacteria adhere to the metal and begin to produce an extracellular matrix (EPS)
This is how biofilm forms. According to sanitary reports, biofilm is up to 1000 times more resistant to antibiotics and cleaning agents than free-living bacteria. The only effective way to combat it is prevention: removing “anchorages” through professional polishing.
From raw block to hygienic mirror
Many facilities offer only machining, leaving the client with the problem of finishing. At Edba, the process is integrated, which provides a guarantee of microbiological safety.
- Design Stage: We select material allowances for final polishing.
- Precise Machining: We perform turning or milling with parameters aiming to minimize surface stresses.
- Mechanical Grinding: We remove the geometric structure left by the tool edge.
- Specialized Polishing: We close the metal structure until the desired Ra is achieved.
Why is this important for the maritime or defense industries? A smooth surface is not only about hygiene, it also ensures the highest corrosion resistance. In the maritime industry, polished 316L steel is significantly less susceptible to pitting because sea salt has nowhere to “settle”.
Passive layer and grain size
Did you know that aggressive grinding can destroy steel’s resistance? During polishing, we ensure not to overheat the material. Overheating (burning) destroys the chromium oxide layer, which protects the steel from rusting. Our CNC grinding is a thermally controlled process which ensures that the element remains chemically stable.
Real-life example: It’s like with the body of an expensive car. If you use polishing paste that is too coarse and rub in one place, you’ll destroy the paint. With stainless steel, you destroy its natural “immunological resistance”.
FAQ – Knowledge in a nutshell
1. Does stainless steel after turning always require polishing?
In the food industry: almost always. Even precise turning rarely goes below Ra = 1.6μm without additional operations, which is a value too high for direct contact with food.
2. What is better: mechanical polishing or electropolishing?
Mechanical polishing (grinding) perfectly levels macroscopic irregularities. On the other hand, electropolishing “smooths” the surface at the atomic level and passivates it. The best results are achieved by combining both methods, which we often do in the most demanding projects.
3. Does polishing affect part strength in the railway or military industries?
Yes, positively! Removing notches (microscopic scratches) after machining drastically increases the element’s fatigue strength. A smooth surface means fewer places where a crack can initiate.
4. How to verify that the contractor actually achieved Ra = 0.8μm?
Demand the measurement protocol from a profilometer. At Edba, every critical element undergoes quality control and results are documented, which is crucial during HACCP audits.
Modern production does not tolerate compromises. Whether you are building systems for the navy or juice bottling lines, surface quality determines success.ing systems for the navy or juice bottling lines, surface quality determines success.