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How Does a Flour Mill Work? From Wheat Kernel to Flour Bag
A plain-language walkthrough of what happens inside a flour mill — cleaning, tempering, breaking, reducing, sifting, purifying and packing — with the physics behind each step.
Key takeaways
- A wheat kernel is not flour. Milling is the business of separating starchy endosperm from bran and germ without contaminating either.
- Nothing is 'ground to powder' in one step — the kernel is opened gradually across 9 to 15 separate passages.
- Bran is 14–16% of the kernel and 20× richer in minerals than endosperm, which is why ash content measures bran contamination.
The kernel you are trying to take apart
A wheat kernel is roughly:
| Component | Share of kernel | What it contains |
|---|---|---|
| Endosperm | 80–83% | Starch and protein — this is flour |
| Bran | 14–16% | Fibre, minerals, some protein |
| Germ | 2–3% | Oils, vitamins, enzymes |
The whole job of a flour mill is to separate the 80% endosperm from the 16% bran without dragging bran into the flour. That sounds simple. It is not, because the bran is glued directly onto the endosperm with no clean interface.
Step 1 — Cleaning
Wheat arrives with stones, straw, dirt, broken kernels and metal. Every one of these either damages the machines downstream or ends up in your flour.
Six to nine machine stages remove them by size, density, surface abrasion and magnetism. See the cleaning handbook for detail.
Step 2 — Tempering
Water is added and the wheat rests for 12–24 hours. The bran becomes leathery and the endosperm softens.
Without this step the bran shatters into dust during grinding and the flour turns grey.
Step 3 — Breaking
The first roll passage — called 1B for first break — runs fluted rolls at a large differential (2.5:1). The fast roll holds the kernel while the slow roll shears.
The kernel splits. Endosperm comes away in chunks. Nothing is reduced to flour yet, and that is deliberate: grinding everything at once would smear bran through the flour.
Output from 1B goes to a sifter, which separates:
- Fine flour (small fraction)
- Coarse middlings (go to break 2 or sizing)
- Bran with adherent endosperm (goes to break 2)
Step 4 — More breaks
Break 2 runs the bran again with tighter rolls, scraping off more endosperm. Then break 3, break 4. Each passage recovers what the previous one missed.
After 4 breaks, bran still carries 6–8% residual starch. After 5, it drops to 4–6%.
Step 5 — Reduction
Clean endosperm chunks from the sifters go to reduction passages. These run smooth rolls at a much smaller differential (1.25–2.0:1), gradually reducing particle size.
Reduction produces flour. Break passages produce the material that reduction works on.
Step 6 — Sifting
Every passage output goes to a sifter. A plansifter is a stack of frames with different mesh sizes, gyrating at 245 rpm.
It separates each stream into flour, coarse middlings, fine middlings and bran. The fine material goes forward; the coarse goes back to another passage.
Step 7 — Purifying
Middlings of the same size but different purity are separated in a purifier, which uses airflow over a vibrating deck. Pure endosperm is lighter and more angular, so it floats and travels. Bran-contaminated particles settle.
This is the machine that turns “well-sifted flour” into “low-ash patent flour”.
Step 8 — Packing
Flour is check-sifted one last time, magnets remove any ferrous material, and the packer fills bags to ±0.2% accuracy.
What comes out
| Output | Share of wheat | Destination |
|---|---|---|
| Flour | 72–78% | Human food |
| Bran | 20–26% | Animal feed |
| Germ | 2–3% | Oil extraction or feed |
| Losses | 1–3% | Cleaning and dust |
The thing that surprises most people
Flour milling does not grind wheat into powder. It takes wheat apart, one passage at a time, and each passage is designed to separate rather than to pulverise.
That is why a mill with more passages extracts more flour from the same wheat — and why a 4B5M flow sheet beats a 3B4M on economics even though it costs more.