Choosing Stainless for Parts That Touch Food

September 21, 2026

Food equipment is one of the few places where the finish on a machined part matters as much as its dimensions. A part that is out of tolerance gets rejected at inspection. A part that cannot be cleaned gets rejected by an auditor, usually after it is installed.

Start with the environment, not the grade

The instinct is to write 304 because it is the food grade everybody names. That is right often enough to be dangerous.

What decides the grade is what the surface actually meets:

• Dry product, no wash down. 304 is comfortable.
• Wet product, water rinse. 304 usually holds.
• Salt, brine, cured meat, cheese. Chloride. 316L.
• Citrus, tomato, vinegar, fermented product. Acid plus chloride. 316L.
• Chlorinated sanitiser or caustic clean in place. 316L, and check the concentration and temperature the supplier recommends.
• Continuous brine or sea water processing. Past 316L, into 2205 duplex.

The chemistry is the same story as everywhere else in this family. The molybdenum in 316L lifts the PREN from about 19 to about 24, and that is the step that survives chloride. There is more detail in 303, 304 and 316L compared.

Write the environment on the drawing or in the RFQ. A sentence such as daily clean in place with 2 percent caustic at 70 C tells a shop more than the word food does.

What is not allowed anywhere near food

303 and any free-machining grade. The sulphide inclusions are pit initiation sites and they open up into small cavities when the surface is polished. Those cavities hold product and hold bacteria. The cost saving is real and it is not worth the argument with an auditor.

Martensitic grades in wet service. 410, 416 and 420 rust in a wash down bay. 440C blades exist in food processing but they are cleaned and dried deliberately.

Anything unpassivated. Machining smears free iron from the tooling across the surface. That iron rusts and the rust contaminates product. Passivation is not optional on a food contact part.

Finish is a requirement, not a preference

Cleanability is about the microscopic profile, not about how shiny a part looks. Crevices, tool marks, folded metal and open inclusions all hold residue that survives cleaning.

3-A Sanitary Standards, the reference most food equipment in North America is built to, call for product contact surfaces at 32 microinch Ra or better, which is roughly 0.8 micrometre. That is achievable by machining with a good finishing pass, and it is much easier to hold with a polishing or electropolishing step after.

Two points buyers get wrong:

Ra is an average, not a guarantee of no defects. A surface can average 0.6 micrometre and still carry a deep tool mark or a torn inclusion. If cleanability genuinely matters, specify electropolishing, which removes the peaks and opens nothing up. The passivation and electropolishing article explains the difference, and the surface finish guide covers what each route delivers.

Internal surfaces are the hard part. A bore, a cross hole or a blind pocket is where cleaning fails. Design them out where you can. Where you cannot, say on the drawing that the internal surface is product contact, because it changes the process and the price.

Design features that fail audits

• Sharp internal corners that a brush cannot reach. Ask for a radius.
• Blind threads in product contact areas. They trap and they cannot be verified.
• Crevices at joints, including where a machined part meets a weld. See machining stainless weldments.
• Horizontal ledges inside a vessel that hold liquid after draining.
• Unfinished drilled holes where the drill left a rough bottom.

None of these are machining defects. They are design decisions that a shop will build exactly as drawn.

Standards, and what they mean for a machined part

3-A Sanitary Standards cover dairy and food processing equipment design. They specify materials, surface finish and cleanable geometry. A machined component inside a 3-A system usually has to meet the material and finish requirements even when the component itself is not separately certified.

NSF/ANSI 51 covers food equipment materials. It is a materials and construction standard, applied to the finished equipment.

FDA 21 CFR 177 covers food contact substances. It matters mostly for the plastics and elastomers around your stainless part rather than for the stainless itself.

For a machined component the practical requirement is almost always: correct grade, documented, at the specified finish, passivated, with certificates. Certification of the assembly is the equipment builder’s job, and we do not certify anything. We match your RFQ with shops that already work to these requirements, and you confirm their documentation directly with them.

What to write on the drawing

Material: 316L stainless, ASTM A276, mill certificate with heat number required. Product contact surfaces: Ra 0.8 micrometre maximum, marked on the drawing. All product contact surfaces passivated per ASTM A967, citric, Method 1, certificate required. Internal corners R3 minimum in product contact areas.

That specification is quotable. Food grade stainless, polished is not, and the three quotes you get back will not be comparable. What to put in a stainless RFQ has the full list.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• When to Machine 17-4 PH and When to Age It
• Duplex 2205 and the Parts That Actually Need It