Annex 3.8

TABLES ON MILLING - DEHUSKING PADDY AND POLISHING RICE

Table 3.8.1. Rice mill components for husk and bran removal.

Name	Description	Process	Additional information	Comments	Sourcesa
Beam hammer/pounder	Consists of a wooden mortar sunk on the ground and a pestle fitted at one end of a wooden beam (about 3.7 meters long) which is supported at the fulcrum about 0.8 m from the other end; the operator steps on the short side of the beam thereby raising the pestle end, and then releases it on the mortar	Dehusking and polishing by repeated pounding of the paddy inside the mortar until milled rice is produced.	Usually operated by female family members. One operates the pounder while the helper turns over the grain in the mortar and then cleans the milled rice by the winnowing; in some places, one woman operates the husker.	High friction at the oblonged hole fulcrum, laborious, high percentage brokens and grain loss, by-product (bran and brokens) are lost with husk and during winnowing.	35, 91
Mortar and pestle	Consists of stone or wooden mortar and long heavy wooden pestle which is repeatedly pounded on the paddy inside the mortar	Dehusking and polishing to produce milled rice	Mortar is not sunk on the ground. pounding rice can be done by 4 persons operating synchronously	Laborious, high percentage brokens and grain loss, by-product (bran and brokens) lost with husk and and winnowing.	35

a Numbers in references.Table 3.8.1. Rice mill components for husk and bran removal (continued).

Name	Description	Process	Additional information	Comments	Sources
Wooden manual husking machines	Consists of two hollowed-out hardwood cones, an upper half and an inverted cone lower half.	Husking. Paddy is fed on top and flows between the cones as the upper half is rotated back and forth.	Husk is winnowed; unhusked grain is separated and returned to the machine	High brown rice recovery, no polishing is done, brown rice is nutritious	35
Bamboo-clay husker	Upper and lower cylinders with the outer shell made of woven bamboo splits into which an adhesive mixture of clay, lime and salt is pressed. The surfaces of the upper rotary and lower stationary cylinders are lined with bamboo strips. A long handle is placed on the upper cylinder to serve as crank.	Paddy is fed into the hole at the middle of the upper rotary cylinder and is hulled as it passes along the surface of the cylinders.	Husker be operated by one or two persons	High brown rice recovered, which is nutritious . .	91
Manually operated centrifugal husker	Consists of a rotating disc, a rubber-coated ring, a gear system, two handles and a base.	Separates the husk from the paddy. Two operators alternately push and pull a handle forward and backward rotating the disc through a gearing system. Paddy grain fed into the center of the disc is thrown out radially; the impact on the inside of the rubber-lined ring splits the husk.	Performance is dependent on grain variety, moisture content, and cleanliness as well as the operator. Machine is commercially available.	Laborious, needs separation of the husk and polishing of the grain.	35

Table 3.8.1. Rice mill components for husk and bran removal (continued).

Name	Description	Process	Additional information	Comments	Sources
Centrifugal husker	Consists of a rotating disc, a rubber-coated ring, and a pulley and belt drive from an engine power source.	Operates as the centrifugal husker above except for the engine drive.	Same as the centrifugal husker above; part of the disc in contact with the abrasive paddy wears out very fast.	Has more broken brown rice than the rubber roll husker at low grain moisture content (14% and below).	4
Rubber roll husker	Machine consists of two rubber rolls rotating in opposite directions with adjustable clearance, housing, hopper, feed and discharge openings, and aspirator blower for husk separation.	Husk is removed through shearing action as the grain is passed between the rubber rolls rotating in opposite directions. Resilient rubber rolls do not normally break dried grain but without cracks.	Rubber rolls are sold in different sizes up to about 25 cm diameter; normally, are commercially available.		4, 30, 35,37,45, 70, 78
Disc sheller	Consists of two horizontal emery-faced discs, the upper disc, stationary and the lower rotating on a vertical axis. Huller has husk aspirator.	Paddy fed at the center opening of top disc, flows down and horizontally out by centrifugal force created by the rotating bottom disc. Husk is aspirated as it comes out from the husker.	Friction between the two rotating discs remove the husk. Disc clearance is critical to husking and breaking of the grains.	Results in more broken brown rice than the rubber roller husker.	4, 30, 45, 70, 78

Table 3.8.1. Rice mill components for husk and bran removal (continued).

Name	Description	Process	Additional information	Comments	Sources
Huller mill (Engleberg, steel roller husker)	Consists of a fluted cylinder on a shaft enclosed in a hollow cylinder with cast iron top and perforated metal bottom, an adjustable blade, hopper, a pulley and metal frame.	Separates the husk and the bran in one operation. The helical ribs at the inlet push paddy to the discharge side. The straight ribs of the cylinder rotate the grain inside while the blade stops the rotation of the grain causing intense pressure and friction separating husk, bran, germ and brokens which drop at the screen perforation. Milled rice is discharged at the outlet.	Some hullers are provided with aspirator to remove husk and powdery materials mixed with milled rice. Grounds husk, bran, brokens, germs and powdery materials are discharged mixed. Milling recovery is usually lower by a minimum of 4% compared with other type of rice mills.	Small capacity milling done in one pass. Generally very poor milling performance due to process limitation. Can be used as polisher or whitener to remove bran.	23, 30, 35, 37, 70
Pearling or whitening cone	Consists of an inverted frustum of a steel cone which is lined with reinforced emery-carborundum and rotates on a vertical shaft. A stationary screen which fits outside the cone and strips of rubber brakes provide the frictional resistance for polishing the brown rice.	Part of the bran is removed as the grain flows from the top through friction between emery stone, screen and rubber brakes. Bran discharges separately.	Rotating cone can be lowered to adjust clearance; usually used in series of up to 3 units for gradual polishing of the grain.	Grain must be paddy-free; polisher does not remove husk in the process, bran removal can be regulated.	30, 70, 78

Table 3.8.1. Rice mill components for husk and bran removal (continued).

Name	Description	Process	Additional information	Comments	Sources
Horizontal abrasive roller whitener	Consists of a cylindrical carborundum formed on a hollow shaft with screw iron roll at inlet and cap cover at the outlet enclosed in a perforated steel cylinder. At the inlet is a hopper shutter, regulating valve and resistance piece. A weight at the outlet of an adjustable resistance piece is installed at the perforated cylinder to control degree of whitening.	Primarily for bran removal. As brown rice is fed, the iron screw roll pushes the grain to the outlet side. The emery roll scours the bran as it passes with the resistance piece regulating the extent of bran removal. Discharge is regulated by the resistance plate through the adjustable weight.	Grain coming out from this machine has a rough surface. It must be pass through a polisher to remove the remaining bran and smoothen the grain surface. Paddy will not be husked by this machine.	This machine has high capacity and is usually used for commercial installations.	4, 30, 45, 70, 78
Iron roll friction whitening machine	Consists of a milling roll, a screw iron roller and a lock nut fixed on a hollow shaft. This is enclosed in a hexagonal perforated screen provided with inlet and outlet controls.	The pressure and friction between the rubbing surfaces of the grain, the screen, and the rotating cylinder smoothen the surface of the grain.	Air is injected into the shaft and enters the milling chamber through perforation of the shaft; milling roll cools the grain and separates bran from the grain.	This is the last section for whitening the grain in big rice mill installations.	4, 30, 45, 70, 78

Note: Machines not directly involved in the removal of the husk and bran during the milling process were not included in the above table. These ancillary equipment are the paddy cleaners, husk and bran aspirators, destoner, paddy separator, automatic weigher, brown rice thickness grader, automatic weighing and bagging grain, elevators, conveyors and others which may be added by the mill operator.

Table 3.8.2. Comparative rice milling tests.

	Actual	Commercial milling1				Laboratory milling
Milling System	Capacity (kg/hr)	Milling recovery	Head rice	Broken rice	Brewer's rice	Milling recovery	Head rice	Broken rice	Brewer's rice
		Percent
Rubber roll single pass (Kyowa)	225	69.75	60.77	38.50	0.95	72.65	82.94	14.97	2.09
Rubber roll multipass (Satake)	4200	69.43	77.71	21.94	0.35	71.33	80.00	17.25	2.75
Rubber roll steel huller combination	300	68.47	59.05	40.50	0.45	71.31	86.07	11.50	2.43
Cone2	630	68.36	74.55	24.61	0.84	70.90	80.22	17.37	2.41
Stone disc steel huller combination	444	65.56	53.68	43.63	2.69	68.80	80.00	17.67	2.33
Steel huller2	380	64.50	29.18	68.86	2.06	72.29	81.19	16.45	2.36

¹ Tests for each milling system were replicated four times (4x).
² Average values from four different mills.
Source:

Table 3.8.3. Rice milling methods practiced in selected West African countries.

Country	Milling method practised, %
	Hand pounding	Village mill1	Commercial mill2
Benin	30	30	40
Gambia	92	6	2
Ghana	62	24	14
Ivory Coast	46	31	23
Nigeria	92	6	2
Senegal	Data not available	Data not available	Data not available

¹ Predominantly U.K.- Engleberg types
² Japanese rubber roll dehusker - friction whitener or Engleberg whitener