Rice Milling SystemRice Milling System

NitatNitat TangpinijkulTangpinijkulPostPost--harvest Engineering Research Groupharvest Engineering Research GroupAgricultural Engineering Research InstituteAgricultural Engineering Research Institute

Department of AgricultureDepartment of Agriculture

Rice MillingRice Milling

The processes of converting paddy into rice• Removing the husk from the paddy• Removing all or part of the bran layer

The basic objective of a rice milling system is to remove the husk and the bran layers, and produce an edible, white rice kernel that is sufficiently milled and free of impurities. Depending on the requirements of the customer, the rice should have a minimum of broken kernels.

Factors Affecting Milling Recovery• Paddy quality• Type of machinery, efficiency, system• Operator• Others

• milling degree• ambient condition

Milling Recovery = Weight produced milled rice x 100%Weight processed paddy

Course : Grain QualityModule 4 : Rice MillingLesson 1 : Introduction to the milling process and its outputs

Good Equipment

Skilled Miller

Good Paddy Rice

the use of good milling equipment and good quality paddy does not guarantee a high quality product

the miller is not skilled,

the mill will always produce poor quality milled rice, despite the skill or the miller or maintenance of the mill.

you use poor quality paddy,

the mill will produce high quality head rice.

you have good quality paddy in a well-maintained mill that is operated by a skilled miller,

ThenIf

Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy Rice Lesson 1 : The quality characteristics

Quality characteristics of paddyQuality characteristics of paddy

These characteristics are determined by the environmental weather conditions during production, crop production practices, soil conditions, harvesting, and post harvest practices

Moisture content has a significant influence on all aspects of paddy quality. To obtain high yields, it is essential that paddy be milled at the proper moisture content.

Paddy is at its optimum milling potential when its moisture content is 14%

Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics

QUALITY CHARACTERISTIC: MOISTURE CONTENT

Immature rice kernels are very slender and chalky and result in the production of excessive bran, broken grains and brewer’s rice

Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics

QUALITY CHARACTERISTIC: IMMATURE

A mixture of varieties in a sample of paddy causes difficulties in milling and usually results in reduced milling capacity, excessive breakage, lower milled rice recovery, and reduced head rice yields.

Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics

QUALITY CHARACTERISTIC: VARIETAL PURITY

Dockage includes chaff, stones, weed seeds, soil, rice straw, stalks and other foreign matter. These impurities generally come from the field or from the drying floor.

Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics

QUALITY CHARACTERISTIC: DOCKAGE

Water, insects and heat exposure can cause paddy to deteriorate through biochemical changes in the grain which may result in the development of off-doors and changes in physical appearance

Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics

QUALITY CHARACTERISTIC: DISCOLORED

Overexposure of mature paddy to fluctuating temperature and moisture conditions leads to the development of crack in individual kernels.

Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics

QUALITY CHARACTERISTIC: CRACKED

• Weight per volume or density, gram/litreWeight per 100 or 1000 grains Thai rice variety: 100 grains weight 2.25 - 3.67 g

• Color of Husk Affecting color of parboil rice

• Color of Brown riceAffecting color of parboil riceMilling recovery and quality of milled rice grade

OTHERS QUALITY CHARACTERISTIC:

•• Grain DimensionGrain DimensionGrain size and shape (length-width ratio) is a varietal

property. Long slender grains normally have greater breakage than short, bold grains and consequently have a lower milled rice recovery

Type of Paddy : Length of brown riceExtra long ≥ 7.5Long ≥ 6.5 < 7.5Medium ≥ 5.5 < 6.5Short < 5.5

Sub-type of Paddy : Length/Width RatioSlender ≥ 3.0Bold ≥ 2.0 < 3.0Round < 2.0

Thai rice variety: Length 7.1 - 7.6 mmSlender 3.1 – 3.4

•• ChalkinessChalkiness• grain with opaque areas in the endosperm, caused by

the loose packing of the starch and protein particles, breaks more readily during milling than clear grain, greatly reducing its market value

• controlled by genetic and environment

white center white belly white back

Good Quality Paddy• uniformly mature kernels • uniform size and shape (vareital purity)• free of fissures and cracks• free of empty or half filled grains • less chalky, red kernel • free of contaminants such as stones and weed seeds • moisture content ~ 14%• high milling yield variety

Ø Good variety, good crop managementØ Harvest at optimum periodØ Drying immediately after harvest to safe level m.c.Ø Proper drying to prevent crackingØ Good aeration during storage to prevent grain deteriorationØ Prevent birds & rodents during storage

Purity of paddyMilling Recovery = Weight produced milled rice x 100%

Weight processed paddy

Purity % Impurity % Milled rice %100 0 68.0099 1 67.3298 2 66.5497 3 65.9696 4 65.2895 5 64.60

Embryo

Bran Ricegrain

BranEmbryo

+ =

Rice flourSplinters

Total bran produced when whitening cracked grains

Cracks

75.065.5 63.568.0Recovery

0.51.5 1.51.0Brewer Rice

75.567.0 65.069.0Milled Rice

7.510.0 10.08.0Bran

83.077.0 75.077.0Brown Rice

17.023.0 25.023.0Husk

100.0100.0 100.0100.0Paddy

JaponicaCrack ImmaturedGood

Paddy Quality

Size of Rice MillSize of Rice Mill

•• Small 12 ton/day• earning from bran, broken or money as milling wage• no grading of milled rice

• Medium 13-59 ton/day• commercial mill• for domestic market

• Large > 60 ton/day• for domestic market and export

43,2752,7313,2328,77928,5331997

44,4364,3865,11410,04924,8871996

44,4724,3905,12910,06524,8881995

44,5604,3955,15610,10424,9051994

45,0194,4005,22310,18325,2131993

KingdomPlainEastern

WholeSouthernCentralNorthernNorth-Year

Source : Factory Control Division, Ministry of Industry

Number of rice mill by region, 1993 - 1997

Number of village 962Number of Rice mill 194

< 5 ton/day 90 (46.4%)> 5 < 20 ton/day 42 (21.6%)> 20 ton/day 62 (32.0%)

Number of rice mill in Suphanburi province (2004)

Capacity (Paddy) Price (F.O.B)Ton/Hour Ton/Day (USD)

2-3 50-80 348,0004-5 100-120 624,0008-10 200-240 1,050,00018-20 420-500 1,824,000

PRICE LIST OF MODERN RICE MILLRICE ENGINEERING SUPPLY CO., LTD.

(BANGKOK, THAILAND)For Month January-April 2008

All price exclude installation cost, electric motor, cable, control etc.

The Aim of Rice Milling

• To attain the highest yield of white rice

• With the best quality– Remove least amount

of hull and bran– Minimum brokens– Little foreign matter

Rice milling system• 0ne step process • Two step process • Multi stage process

In a one step milling process, husk and bran removal are done in one pass and milled or white rice is produced directly out of paddy.

In a two step process, removing husk and removing bran are done separately, and brown rice is produced as an intermediate product.

In multistage milling, rice will undergo a number of different processing steps.

Rice Milling Process

Cleaning

Husking

Paddy Separation

Whitening & Polishing

Grading and Blending

Impurity

Husk

Bran

Head rice

Broken

Paddy

Paddy

Paddy, Brown rice

Brown rice

Milled rice

Paddy into rice mill

Paddy Intake• Quality input = quality output• Garbage in = garbage out!

Paddy intake chute

The quality of milled white rice will be dependant on the quality of the paddy or rough rice coming into the mill as well as the milling process. Good paddy into the mill means quality milled rice output.

Paddy Cleaning• After harvest, foreign matter can be as

high as 5-10%

Rice pre-cleaner

While most paddy has been cleaned after harvesting, some foreign material is still present. This can be as high as 5-10%.

• Remove foreign material such as sand, stones, straw, seeds etc. from paddy

• Prevent damage to the milling machinery• Prevent clogging which cause the reduction

in capacity and efficiency of rice mill• Remove materials that cause a reduction in

the grade of milled rice

Pre-cleaning of Paddy

Paddy Precleaner• Sieve: separate large/ small impurities• Aspirator: separate light impurities• De-stoner: separate same size impurities but

heavier • Trieur (Length Grader) : separate impurities

having same width but different in length• Magnet : separate irons

Open double-sieve precleaner

Examples of perforated sheets

and wire mesh

Closed-type single-action aspirator

precleaner

Closed-type double-action aspirator

precleaner

Double-drum type of Precleaner

(West Germany)

Single-drum type of Precleaner

(Japan)

Magnetic separator

Magnet

Brass

A, B: Simple permanent magnetC: Rotating brass cylinder and

permanent magnet

De-stoner

Paddy inPaddy in

PaddyPaddy

StoneStoneAir streamAir stream

Stone

Wind direction

rice

Vibration direction

De-husking

• Underrun disc huller• Rubber roll huller• Centrifugal disc huller

Underrun disc huller

PaddyPaddy

Brown riceBrown riceHuskHusk

Ratio of W/D ~ 1/6 or 1/7Composition:

• Emery grit 14, 16• Silicium carbide grit 16

Peripheral speed ~ 14 m/s

Particulars of the under-runner disc huller: V = peripheral speed (recommended 14 m/s); W = width of coating; D = stone diameter; and W/D = 1/6 or 1/7.

Under-runner disc huller peripheral speed-curve for V = 14 m/s.

D = stone diameter = 700 mm (0.7 m); and V peripheral speed = 14 m/s.

V = ¶ x D x n (m/s)60

n = 60 x V = 60 x 14¶ x D 3.14 x 0.7

= 380 rpm.

Diameter Capacity Motor(mm) (kg of paddy/hr) (hp)750 450-600 31000 700-1000 3.51250 1000-1400 41400 1600-2100 5.5

Capacity and power requirement of underrun disc huller

• Advantage§ Operational simplicity, low running cost§ The abrasive covering can be remade at the

site

• Disadvantage§ Given grain breakage § The abrasions to outer bran layers

Underrun disc huller

Rubber Roll Huller

Rice Huller

Rollers

Dehusking principle of rubber roll huller:• The rubber roll huller consists of two rubber rolls.• Both rolls have the same diameter.• One has a fixed position, the other is adjustable to obtain

the desired clearance between the two rolls. • The rolls rotate in opposite directions.• The adjustable roll speed running about 25% slower than

the fixed one.

74010001025010250

90012003768.5220

90013202.5646150

(rpm)(rpm)(Inch)(mm)(Inch)(mm)

Lowspeed

HighspeedWidthDiameter

Size and speed of the rubber rollWhen the rolls are new, their peripheral speed is about 14 m/sso that a smaller roll runs faster than a larger one.

The relation between paddy variety and hulling capacity• The wear for long grain variety is higher than short grain.• The wear of adjustable roll is lower than the fixed one.

Durability of Rubber Rolls

15302.52035340754601106

140240817030010

Long grain(ton)

Short grain(ton)

Type (Inch)

Diameter

6.03.82.225425410

4.01.91.22201506

2.51.250.92201004

(hp)Short grain

Long grain(mm)(mm)(Inch)

MotorCapacity (ton/hr)WidthSize

Capacity and power requirement for different rubber roll hullers

Rubber Roll Huller

• Advantage§ Hulling efficiency is higher.§ Reduce grain breakage and loss of small

broken.§ The silver skin of brown rice is undamaged.§ Sieving the husked products is unnecessary.

• Disadvantage§ The cost of replacing rubber rolls.

Dehusker

Sieve

Paddy

Brown rice, Broken, Paddy, Bran, Husk

Coarsebran

Husk

Brewerrice

Husk Aspirator

Brown rice, Broken, Paddy

Paddy Separator

Husk Separation

Husk Aspirator

Plansifter &Husk Aspirator

Closed Circuit Husk Aspirator

1 เกลียวลําเลียงแกลบ2 เกลียวลําเลียงเมล็ดออน3 เกลียวลําเลียงขาวกลอง/

ขาวเปลือก4 แผนปรบัลม5 แผนปรบัลม

Rubber roll huller with husk aspirator

Huller with Husk Aspirator

(BUHLER: DRHC/DRSC)

Paddy Separation

• Removal of paddy from brown rice

After husk separation, a mixture of 85-90% brown rice and 10-15% paddy is fed into the paddy separation stage. The paddy must be separated before the brown rice goes to the bran removal stage. The separated paddy is returned to the husker for dehusking.

• Weight per volume of paddy is less than that of brown rice,• Specific gravity of paddy is lower than that of brown rice,• Dimensions of paddy are longer, wider and thicker,• Coefficient of friction is different.

Paddy and brown rice have different characteristics

Types of paddy separators• Compartment (table) type• Tray type• Screen type

(BUHLER: DNTB)

Compartment-type SeparatorThe main part of this paddy separator is the oscillating compartment-assembly where the actual separation of paddy and brown rice takes place. The compartment-assembly consists of a number of compartments in one, two, three, or sometimes four decks.The number of compartments depends on the capacity of the rice mill,

PaddyPaddy

low

high

Brown riceBrown rice

Paddy, brown ricePaddy, brown rice

Paddy, brown ricePaddy, brown rice

Brown riceBrown rice

PaddyPaddy

Brown riceBrown rice

PaddyPaddy

aa

HighHigh

LowerLower

AdjustmentAdjustment•• Speed Speed 90 90 -- 120 strokes/min120 strokes/min•• SlopeSlope•• StrokeStroke

HUSKERHUSKER

WHITENERWHITENER

Tray type separator

First rubber roll huller

Second rubber roll huller

Whitening Machine

• Removes bran layer to produce white rice. • Amount of bran removed (usually 8-12%)

Whitening Actions

• Abrasive typeHigh speed / Low pressure

• Griding action• Impact action

• Friction typeLow speed / High pressure

• Tearing action• Cutting action

Whitening

Milled rice & bran

Friction type

Smoothsurface

Large

Abrasive type

Roughsurface

Fine

• Vertical abrasive whitening cone• Horizontal abrasive whitener• Horizontal friction-type whitener• Bottom-Up Vertical friction-type whitener

Whitening Machine

Vertical Abrasive Whitening Cone

Leveladjust

Wiremesh

Abrasivecone

Rubberbrake

Bransweeper

BranRice

Air

Feeding

Number of Brakes = Cone Diameter - 2100

Peripheral Speed 13 m/s

Eg. Cone Diameter = 600 mmNumber of Brakes = 600/100 – 2 = 4

Diameter Power

mm hp Long grain Short grain Long grain Short grain Long grain Short grain

500 4 350 420 570 680 680 800

600 7.5 550 650 890 1100 1050 1250

800 10 750 900 1230 1450 1460 1725

1000 15 1000 1200 1700 2000 1900 2300

1250 20 1350 1600 2200 2600 2600 3000

1500 25 1700 2000 2700 3200 3200 3800

1 Cone 2 Cones 3 Cones

Capacity of Cone-type Whitener(kg of brown rice / hr)

Composition

Whitening Cone No. 1 and No. 2Emery grit 16 25%Silicium carbide grit 16 50%Silicium carbide grit 18 25%

Whitening Cone No. 3Emery grit 18 25%Silicium carbide grit 18 75%

Horizontal abrasive whitener

Aspirator

CounterWeight

Bran

Abrasive roll

Screw

Horizontal abrasive whitener

Resistance pieces

ScreenAbrasive grit#30, 36

Shortgrain

Longgrain

% Bran

0 30 45 60 90

00

450

900

Characteristics of Abrasive Action Milling

(Speed 12 - 13 m/s)

Shaping Action Factors

LowMediumHighPressure

LargeMediumSmallRoll Grit

HighMediumLowPeripheral Speed

Round ShapeSlim shapeFlat ShapeCondition

Horizontal friction-type whitener

AspiratorMilled riceBran

P control

Milling roll

Perforated sheet

Screw

Perforated sheetBrown rice

Characteristics ofLow-Speed Machine

(Friction-type)

Effect of Ventilation on Low Speed Machine (Friction-type Whitener)

Bottom-Up Vertical friction-type whitener(SATAKE)

Polisher

• Humidifying Rice Milling Machine• Vertical Polishing Cone – Brush type• Vertical Polishing Cone – Leather type• Horizontal Polisher – Leather type

Humidifying Rice Milling Machine

Nozzle

Bran

Water addition rate 0.3 - 0.4 %

Mold Growth on Rice from Non-Humidifying and Humidifying Milling

Humifying rice-milling machine is used in the last step of whitening system

Characteristics of Humidifying Milling

Machine

(BUHLER: DRPE)

Polisher

Vertical PolishingCone

– Brush type

Vertical Polishing Cone - Leather type

HorizontalPolisher

Whitening SystemSystem 1 2 3 4 5

Long grain (Indica)

1 Abrasive Abrasive Abrasive - -

2 Abrasive Abrasive Abrasive Vent Friction -

3 Abrasive Abrasive Abrasive Hu Friction -

4 Abrasive Abrasive Abrasive Vent Friction Hu Friction

Short grain (Japonica)

1 Abrasive Vent Friction Vent Friction Vent Friction Hu Friction

Milling System for Indica-type variety

Abrasive Friction

ABRAS ABRAS ABRAS ABRAS ABRAS ABRAS FRICT FRICT

Combined System(High + Low speed)

Combined System vsLow Speed System

1st Vertical whitening roll2nd Vertical whitening roll3rd Hor Humidifying friction

polisher

Size of brokens according to Thai Rice Standard (1997)

Grading

General standard

Japanese standard

A from B : By thicknessA from C : By thickness / lengthB from C : By length

A from B : By length

Bold grainA from B/C : By thickness / lengthB from C : By length

Slender grainA from B/C : By lengthB from C : By length

Grading of White Rice

• After polishing, white rice contains:– Head rice– Large brokens– Small brokens

• The white rice is separated using – Sifter – Length grader

Rotary Seiver

Rotary Sifter

White Rice Grading

One double layers sifter and two trieurs in parallel.

One double layers sifter and three trieurs in series.

Rice Mixing• An efficient rice mill will produce:

– 50% head rice (whole kernels)– 5-15% large broken and – 5-15% small broken kernels

• Depending on the country’s standards, rice grades in the market will contain from 5-25% broken kernels.

Rice Mixing

Whole kernels

Large brokens

Small brokens

Variable speed motors allow the mill operator to mix the appropriate amount of whole kernels with large and small brokens.

Rice Weighing• Rice is normally sold in

50 kg sacks which must be accurately weighed and labeled.

Bag filler and weighing

scales

Color SorterColor Sorter

Conveying SystemConveying System

Duct Collecting SystemDuct Collecting System

Scaling and PackagingScaling and Packaging SystemSystem

Milling Quality AssessmentMilling Quality Assessment

• Sampling / dividing• Measure moisture content• Cleaning sample• Milling test• Grading

Assessment the milling quality of paddy

Sample divider

Scale Sieve

Aspirator

Moisture MeterResistance typeCapacitance type

Oven for m.c. determination130oC 1 hr103oC 72 hr105oC 5 hr

A, B : Weight per Volume TesterC : Volume Measuring CylinderD : Counting plate

Rubber roll huller

Abrasive-type Whitener

Length Grader, Trieur

Friction-type Whitener

Friction-type Whitener with Length Grader

Using Abrasive-type Whitener•• Weigh clean paddy 250 g• De-husk paddy with the rubber roll husker• Weigh brown rice• Whiten brown rice w the abrasive type whitener for 90 s• Weigh milled rice• Separate broken from milled rice w the trieur for 90 s• Weigh head milled rice

Procedures for Milling Quality Assessment

Using Friction-type Whitener (De-husking method)•• Weigh clean paddy 100 g• De-husk paddy with the rubber roll husker• Weigh brown rice• Whiten brown rice w the friction-type whitener for 25 s• Weigh milled rice• Separate broken from milled rice w the length grader for 90 s• Weigh head milled rice

Procedures for Milling Quality Assessment

Using Friction-type Whitener (Un-husking method)• Weigh clean paddy 100 g• Whiten paddy with the friction-type whitener for 30 s• Weigh milled rice• Separate broken from milled rice with the trieur for 90 s• Weigh head milled rice

Procedures for Milling Quality Assessment

Brown Rice Recovery = Wt of Brown Rice x 100 %Wt of Paddy

Milled Rice Recovery = Wt of Milled Rice x 100 %Wt of Paddy

Head Rice Recovery = Wt of Head Rice x 100 %Wt of Paddy

Milling Degree = (Wt of WBR – Wt of WMR) x 100 %Wt of WBR

WBR = Weight of whole brown rice 1000 kernelsWMR = Weight of whole milled rice 1000 kernels

Moisture Content = Wt of Water x 100 Wt of Sample

After Drying Weight = (100 - Initial MC.) x Initial Weight (100 - Final MC.)

Eg. Paddy 500 t with initial MC 26% Final MC after drying is 14%Then After Drying

Weight = (100 - 26) x 50000 = 430,23 t (100 - 14)

Thank you