Last reviewed 1 Oct 2026 · 9 min read
Structure of paddy
Paddy (rough rice) consists of:
| Part | Share (by mass) | Notes |
|---|---|---|
| Husk (hull) | about 20–22 % | Outer protective cover (lemma and palea); high in silica; a fuel and by-product |
| Bran (pericarp, seed coat, aleurone and germ) | about 8–10 % | Rich in oil, protein, vitamins (B), fibre |
| Endosperm (white rice) | about 68–70 % | Mostly starch; the edible white rice |
Brown rice is paddy with the husk removed (about 78–80 % of paddy); white (milled) rice has the bran and germ removed (about 68–70 %). Head rice is whole or large pieces (at least about three-quarters of the grain); broken rice is smaller pieces.
Objectives of milling
- Remove the husk (de-husking, shelling or hulling).
- Remove the bran and germ (whitening or polishing) to give a white, palatable, longer-keeping rice.
- Obtain the highest possible yield of head (whole) rice with minimum breakage.
- Recover by-products — husk, bran, brokens.
Steps in a modern rice mill
- Pre-cleaning — remove stones, straw, dust and other impurities (sieves, aspirator, destoner, magnet).
- De-husking — rubber-roll huller (or under-runner disc huller) removes the husk from paddy.
- Husk aspiration — air separates the husk from the brown rice and paddy.
- Paddy separation — separates un-hulled paddy from brown rice (paddy table, compartment separator) and returns the paddy to the huller.
- Whitening (de-branning) — removes the bran layers in one or two passes (abrasive and friction whiteners).
- Polishing (optional) — smoothens the surface and gives a gloss (brush or friction polishers; water mist).
- Grading — separates head rice, brokens, and brewers' rice (indented-cylinder or sifter).
- Colour sorting — removes discoloured or damaged grains.
- Packing and storage.
Types of rice mills
| Type | Description | Features |
|---|---|---|
| Traditional (hand-pounding, chakki, dhenki) | Pounding or stones; very low capacity | Poor yield and quality |
| Huller mill (Engelberg huller) | A single-pass machine: a ribbed cast-iron cylinder rotating in a perforated screen; husk and bran removed together | Cheap and simple; low head-rice yield (about 55–60 %), high breakage; common in small rural mills |
| Sheller-cum-polisher mill | Shelling (rubber-roll or disc shellers) followed by a cone polisher | Medium quality; better yield than the huller |
| Modern (multistage) rice mill | Separate steps — cleaning, shelling, paddy separation, whitening, polishing, grading | High milling recovery (about 65–70 %) and head rice of 60–65 % of paddy; capacities 1–10 t/h; the standard in commercial mills |
| Mini rice mill | A compact version (0.5–1 t/h) | Used in villages |
De-husking (shelling) machines
Rubber-roll husker (sheller)
- Two rubber-covered rolls rotate in opposite directions at different speeds (about 25 % speed difference; peripheral speed about 15–20 m/s) with an adjustable gap.
- Paddy passes between them; the differential speed creates a shearing force that splits the husk while the rubber cushions the kernel (low breakage).
- Husking efficiency 80–90 % in one pass; roll life depends on the paddy handled (a pair of rolls can process about 100–200 t of paddy depending on conditions).
- Best for low breakage; the commonest commercial husker.
Under-runner disc husker (sheller)
- Two cast-iron discs coated with an abrasive (emery) mixture; one rotates, one is stationary (under-runner in some types).
- The paddy is sheared and abraded between the discs; the gap is adjusted for the grain size.
- Cheaper than the rubber-roll type but gives more breakage and discs wear quickly.
Impact huskers and centrifugal shellers
Paddy is thrown at high speed against a rubber or abrasive ring; the impact removes the husk. Lower efficiency; used in some small units.
Whitening (de-branning)
Abrasive whitener
A rotating cone or cylinder coated with an abrasive (emery) inside a perforated screen. The rice is scoured by abrasion; bran is rubbed off and passes through the screen. Higher capacity, lower pressure, but gives more breakage if run too hard; the grains are less glossy.
Friction (jet) whitener
A ribbed (steel) rotor inside a perforated screen presses the grains against each other and against the screen; bran is removed by friction (grain-on-grain). Works at higher pressure, gives a better finish and lower breakage than the abrasive type. Air jets or water mist cool the rice and improve the polish.
Polisher
A brush or a friction cone with a water mist polishes the white rice to a glossy surface. Over-polishing removes the aleurone layer (rich in nutrients).
Degree of milling
The degree of milling is the extent to which bran is removed — under-milled, reasonably well-milled, well-milled, extra well-milled — judged by the colour and bran layers left (or measured by a milling-degree meter, which reads the whiteness). More milling gives whiter rice, but lower yield, more breakage, and loss of nutrients (B vitamins, minerals, fibre).