Cluster article
Wheat Impurities: The Six Classes and How Each Is Removed
A field guide to wheat contamination — coarse, size-graded, density, surface, shape and ferrous impurities — with the machine that removes each, typical levels by origin, and acceptable limits.
Key takeaways
- Six impurity classes, each requiring a different physical separation principle.
- No single machine handles more than one or two classes properly.
- Field stone — same size as wheat, twice the density — is the class that most often escapes undersized cleaning lines.
Class 1 — Coarse impurities
Examples: straw, rope, cob, string, plastic, wood, stones larger than 8 mm.
Removed by: drum sieve (SCQY series).
Typical level: 0.1–0.8% depending on harvest method. Combine-harvested wheat carries more straw than hand-harvested.
Why it matters: beyond the obvious, coarse material blocks screens and wraps shafts. This is a machine-protection stage, not a quality stage.
Class 2 — Size-graded impurities
Examples: sand, small stones, broken grain, weed seeds, chaff fragments.
Removed by: vibrating screen (TQLZ, HZXZ) or rotary screen.
Typical level: 0.5–2.0%.
Why it matters: sand that survives into the mill abrades roll flutes. Weed seeds add off-flavours and, in the case of ergot, toxins.
Class 3 — Density impurities
Examples: field stones, glass, mud balls, coal, brick fragments.
Removed by: gravity destoner (TQSF).
Typical level: 0.05–0.5%.
Why it matters most: field stone has almost exactly the size of a wheat kernel and roughly twice the density. Screens cannot separate it. Only combined airflow and vibration can. An undersized destoner removes almost none and the failure is invisible from outside.
Class 4 — Surface impurities
Examples: dirt, dust, wheat beard, smut spores, bacteria, mould spores, pesticide residue.
Removed by: scourer (FDMW), brush machine (TNSW), aspirator (TXFY).
Typical level: variable; measured as ash contribution.
Why it matters: this is the class that determines flour ash. A single heavy-beat scourer pass removes 0.03–0.06 points of ash from the whole meal.
Class 5 — Shape and length impurities
Examples: broken kernels, shrivelled grain, ergot bodies, sprouted kernels, insect-damaged kernels.
Removed by: indent cylinder (MDJY).
Typical level: 2–8%.
Why it matters: broken kernels mill differently from whole ones and raise ash. Ergot is a food-safety issue with legal limits in most markets.
Class 6 — Ferrous impurities
Examples: wire, bolts, staples, screen fragments, wear particles from machinery.
Removed by: plate magnets (TCXP) and drum magnets (TCXT).
Typical level: trace.
Why it matters: this is the only impurity class that can be introduced by the plant. Every screen and every roll is a potential source of ferrous fragments.
Typical impurity profile by origin
| Origin | Total impurities | Stones | Notes |
|---|---|---|---|
| Well-cleaned commercial wheat | 0.8–1.5% | 0.05–0.15% | Standard European or North American grade |
| Average farm wheat | 1.5–3.0% | 0.15–0.4% | Requires full cleaning line |
| Rain-damaged / late harvest | 3.0–6.0% | 0.2–0.6% | Expect sprout damage too |
| Imported blending wheat | 1.0–2.0% | 0.1–0.3% | Varies widely by origin |
What each class contributes to ash
Approximate ash contribution per 1% of impurity in the incoming wheat:
| Class | Ash contribution |
|---|---|
| Coarse organic | 0.00 (removed before it matters) |
| Sand and small stones | 0.00–0.02 (removed by screens and destoner) |
| Surface dirt | 0.05–0.12 (the dominant contributor) |
| Broken kernels | 0.02–0.04 |
| Bran fines | 0.04–0.08 |
Note that surface dirt dominates. Which is why scourers and aspirators — not screens — are the machines that determine flour ash.
Testing your own wheat
A simple sieve-and-float test tells you most of what you need:
- Weigh 1 kg of representative sample
- Sieve through 2 mm and 1 mm screens; weigh each fraction
- Float the 2–4 mm fraction in salt solution (1.2 specific gravity) — stones sink, wheat floats
- Hand-pick and weigh coarse material
Ten minutes of work gives you an impurity profile accurate enough to specify a cleaning line. Most buyers skip this and accept whatever the supplier recommends.