Last reviewed 1 Oct 2026 · 8 min read
Wheat milling
Structure of the wheat kernel
| Part | Share | Notes |
|---|---|---|
| Endosperm | about 81–83 % | Starch and gluten proteins; the source of white flour |
| Bran (pericarp, seed coat, aleurone) | about 14–16 % | Fibre, minerals, B vitamins |
| Germ (embryo) | about 2–3 % | Oil, protein, vitamin E; removed in white flour (it shortens the shelf life) |
The crease (a deep groove running along the kernel) makes it difficult to peel the bran completely — hence the roller system of progressive grinding rather than a single pass.
Cleaning and conditioning (tempering)
- Cleaning — magnetic separators, sieves, aspirators, destoners, scourers (to remove surface dirt), and disc separators remove foreign material; cleaned wheat is passed to the tempering bins.
- Tempering (conditioning) — water is added to raise the moisture to about 15–17 % and the wheat is rested for 8–24 hours (hard wheat longer). Purposes: toughen the bran so that it comes off in large flakes rather than powdering (which would darken the flour); mellow the endosperm so that it grinds easily into flour; improve the separation of bran from endosperm.
Roller flour milling
The milling process is a gradual reduction: the grain is broken, the endosperm particles (semolina, middlings) are separated and purified, and reduced to flour by successive roller passages, each followed by sifting.
- Break system (usually 4–5 breaks) — corrugated (fluted) rolls with differential speed (2.5 : 1); the grain is opened and the endosperm scraped from the bran in chunks (semolina and coarse middlings) with as little bran as possible broken.
- Plansifter — a stack of oscillating sieves that separate the stock into coarse (bran), semolina and middlings, and flour, passing each fraction to the next stage.
- Purifiers — air and sieve machines separate bran specks from endosperm particles in the semolina by size, density and air velocity.
- Sizing (grading) rolls — reduce the semolina to uniform size.
- Reduction system (8–12 passages) — smooth rolls, differential speed (1.25 : 1); the purified middlings are ground to flour and sifted in turn.
- Bran finishers (bran brushers/dusters, scourers) — recover adhering endosperm.
- Flour blending, bleaching and fortification, packing.
Extraction rate and products
Extraction rate .
| Product | Extraction | Description |
|---|---|---|
| Maida (refined/white flour) | about 70–75 % (straight-run white flour 72–78 %) | Fine, white; from the endosperm only; low in ash (about 0.5 %); for bread, biscuits, cakes |
| Suji (rawa, semolina) | by-product of milling (about 10–15 % of wheat in a flour mill) | Coarse endosperm particles |
| Atta (whole-wheat flour) | 95–100 % (or 90–95 %) | Contains bran and germ; made in chakki mills or roller mills; for chapatis |
| Wheat bran | about 14–25 % | Feed, fibre supplements |
| Wheat germ | about 1 % (separate stream) | Oil and nutritional supplements |
- Ash content (mineral) of flour rises with extraction because bran has more ash; it is a grade indicator (refined flour: about 0.5 %; whole-meal: about 1.5–2 %).
- Wheat types: hard wheat (high protein, vitreous; bread) vs soft wheat (low protein; biscuits); durum wheat (pasta, semolina); the Triticum aestivum (bread wheat) is the commonest.
Chakki (stone) milling of atta
The stone mill (chakki) has two circular stones (millstones) — one fixed, one rotating at about 400–900 rpm. The wheat is ground in one pass between the stones, giving whole-wheat atta with the bran and germ. The product is coarser, the taste preferred for chapatis; heat from friction can raise the temperature and reduce keeping quality. Modern atta chakkis use steel or composite plates with sieving.
Quality tests for flour
Moisture, ash, gluten content (wet gluten 25–35 % for bread flour), falling number, water absorption (farinograph), granulation, colour (Pekar test), and freedom from insects and foreign matter.
A mill processes 100 t of wheat per day and obtains 74 t of maida, 12 t of suji and 14 t of bran (including germ).
Maida extraction ; total flour and semolina ; the bran/offal (a mass balance of 100 t, assuming no loss).
Maize (corn) milling
Kernel structure
Maize kernel: endosperm about 82 % (hard, vitreous in flint corn; floury in dent corn), germ about 11–12 % (large, rich in oil), pericarp (bran) about 5–6 %, tip cap 1 %. The large germ makes the grain oil-rich, which shortens the shelf life of meal.
Dry milling
Used for maize meal, grits, flour and flakes; degermination is the key step.
- Cleaning and tempering (moisture to about 20–22 %).
- Degerming (degermination) — in a degerminator (an impact/attrition machine) that removes the germ and bran from the endosperm.
- Roller milling — the endosperm is reduced to grits (coarse), meal, flour, sifted and purified; the germ is separated by a gravity table.
- Products: corn grits (for breakfast cereals, corn flakes, brewing), corn meal, corn flour, germ (oil), bran and feed.
Wet milling
Used to separate the grain into starch, protein (gluten), fibre and oil.
- Steeping — maize is steeped in water containing 0.1–0.2 % sulphur dioxide at 48–52 °C for 30–48 hours; softens the kernel and loosens the starch–protein bonds.
- Coarse grinding and germ separation — hydrocyclones separate the germ (lighter) from the rest.
- Fine grinding and fibre (bran) separation — sieves (DSM screens) remove the fibre.
- Protein–starch separation — centrifuges or hydrocyclones: gluten (protein) being lighter is separated from starch.
- Starch washing, drying, or conversion to glucose, high-fructose corn syrup, dextrose, sorbitol, ethanol.
- Products: corn starch, corn syrup, corn oil (from the germ), gluten meal (60 % protein), gluten feed, steep liquor.
Maize quality
Moisture 12–14 % for storage; aflatoxin is a risk in damp storage; hardness (flint vs dent) affects milling.
Milling of pulses (dal making)
Pulses (arhar/tur, moong, urad, chana, masoor, peas) are de-husked and split to make dal. Pulse milling is complicated because the husk (seed coat) adheres firmly to the cotyledons through a gummy layer; so a pretreatment is applied.
Traditional (wet and dry) processes
- Cleaning and grading.
- Pitting (scarification) — a roller (emery-coated) scratches the husk to allow oil and water to penetrate.
- Oil application (a thin coating of edible oil, 0.2–0.5 %), mixing, and sun-drying (tempering) for 12–24 hours — loosens the husk; or water-soaking and drying (wet method; sweating).
- Dehusking in a huller (emery-coated roller or disc huller, or a stone chakki) — the husk is removed and the pulse splits.
- Aspiration — separates the husk from the dal.
- Sieving (grading) — separates dal, brokens, powder and un-hulled grains; un-hulled grains go back for a second pass.
- Polishing (optional) — with a little oil or water, for a bright finish; by brush polishers.
Modern pulse mills
- Pitting-conditioning-dehusking are done continuously with improved rollers, aspirators and graders; CFTRI (Mysuru) and CIAE (Bhopal) have developed mills and wet-conditioning methods.
- Recovery: typically 70–75 % dal from the pulse (higher with the improved process: moong 80 %, chana 70–75 %, tur 72–76 %); husk 10–15 %, brokens and powder 8–10 %.
- Losses are reduced by pretreatment, proper roller settings and optimum moisture.
Special cases
- Moong (green gram), urad — whole, de-husked (washed) or split; urad has a mucilaginous husk and is softened by wetting.
- Chana dal — Bengal gram is split and polished; the husk is hard — pitting and mild moisture conditioning are important; besan (flour) is made from the dal by grinding.
- Tur dal — hard husk; oil and water treatment with tempering for 1–2 days improves the yield.
- Pea dal and masoor dal — de-husked and split by emery rollers.
100 kg of tur (arhar) is milled. It gives 74 kg of dal, 14 kg of husk and 12 kg of powder and brokens.
Dal recovery . The loss is the brokens and powder (12 %) — the process target is to reduce this by proper conditioning and roller adjustment; the husk is a by-product (feed).
By-products and uses
- Wheat bran — feed, fibre; maize germ — oil; gluten — feed, textured protein; pulse husk and brokens — cattle feed; chuni (broken dal) — feed or flour.
Frequently tested points
- Wheat: endosperm about 82 %, bran about 14–16 %, germ 2–3 %; tempering moisture 15–17 %.
- Roller milling: breaks (corrugated rolls) → sizing → purifier → reductions (smooth rolls) with plansifters.
- Maida extraction about 70–75 %; atta 95–100 %; ash indicates grade.
- Maize: dry milling (degermination; grits, meal) vs wet milling (steeping with SO₂; starch, gluten, oil).
- Pulse milling: pitting, oil/water conditioning, dehusking, splitting, polishing; dal recovery about 70–75 %.
- Skipping tempering (the bran powders and darkens the flour).
- Treating maida and atta as the same (extraction rate and bran content differ).
- Dehusking pulses without pretreatment (poor recovery, high brokens).
- Wheat is cleaned, tempered and ground by roller systems — break, sizing, purification and reduction — with plansifters.
- Flour grades depend on extraction rate and ash; chakki atta keeps the bran and germ.
- Maize is milled dry (grits, meal) or wet (starch, gluten, oil, syrups).
- Pulse milling loosens the husk (pitting, oil, water), dehusks, splits and polishes.
- By-products — bran, germ, gluten, husk — are used as feed and for oil.