Rice Milling System

8/10/2019 Rice Milling System

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Rice Milling SystemRice Milling System

NitatNitat TangpinijkulTangpinijkul

PostPost--harvest Engineering Research Groupharvest Engineering Research Group

Agricultural Engineering Research InstituteAgricultural Engineering Research Institute

Department of AgricultureDepartment of Agriculture


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Rice MillingRice Milling

The processes of converting paddy into rice

Removing the husk from the paddy

Removing all or part of the bran layer


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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.


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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


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Course : Grain Quality

Module 4 : Rice Mil ling

Lesson 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 notguarantee a highquality product

the miller is not

skilled,

the mill will always

produce poor quality

milled rice, despite

the skill or the miller

or maintenance ofthe 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


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Module 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, cropproduction practices, soil

conditions, harvesting,

and post harvest

practices


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Moisture content has a significant

influence on all aspects of paddyquality. 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%




QUALITY CHARACTERISTIC: MOISTURE CONTENT


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Immature rice kernels are

very slender and chalky and

result in the production of

excessive bran, broken

grains and brewers rice




QUALITY CHARACTERISTIC: IMMATURE


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A mixture of varieties in a sampleof paddy causes difficulties in

milling and usually results in

reduced milling capacity,

excessive breakage, lower milled

rice recovery, and reduced head

rice yields.




QUALITY CHARACTERISTIC: VARIETAL PURITY


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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.




QUALITY CHARACTERISTIC: DOCKAGE


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Water, insects and heatexposure 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




QUALITY CHARACTERISTIC: DISCOLORED


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Overexposure of mature paddy

to fluctuating temperature and

moisture conditions leads to

the development of crack in

individual kernels.




QUALITY CHARACTERISTIC: CRACKED


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Weight per volume or density, gram/litre

Weight 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 rice

Affecting color of parboil rice

Milling recovery and quality of milled rice grade

OTHERS QUALITY CHARACTERISTIC:


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Grain DimensionGrain Dimension

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

property. Long slender grains normally have greaterbreakage than short, bold grains and consequently have a

lower milled rice recovery

Type of Paddy : Length of brown riceExtra long 7.5

Long 6.5 < 7.5Medium 5.5 < 6.5

Short < 5.5

Sub-type of Paddy : Length/Width Ratio

Slender 3.0

Bold 2.0 < 3.0

Round < 2.0

Thai rice variety: Length 7.1 - 7.6 mm

Slender 3.1 3.4


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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


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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

Good variety, good crop management

Harvest at optimum period Drying immediately after harvest to safe level m.c. Proper drying to prevent cracking

Good aeration during storage to prevent grain deterioration Prevent birds & rodents during storage


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Purity of paddy

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

Purity % Impurity % Milled rice %

100 0 68.00

99 1 67.3298 2 66.54

97 3 65.96

96 4 65.28

95 5 64.60


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Embryo

Bran Ricegrain

BranEmbryo

+ =

Rice flourSplinters

Total bran producedwhen whiteningcracked grains

Cracks


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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


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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


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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


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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)


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The Aim of Rice Milling

To attain the highest

yield of white rice

With the best quality

Remove least amountof hull and bran

Minimum brokens

Little foreign matter


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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 isproduced 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 ofdifferent processing steps.


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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


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Paddy Intake

Quality input = quality output

Garbage in = garbage out!

Paddy intake chute

The quality of milled white

rice will be dependant onthe 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.


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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%.


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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


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Open double-sieve precleaner

Examples of

perforated sheets

and wire mesh

Closed type Cl d t


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Closed-type

single-action aspirator

precleaner

Closed-type

double-action aspirator

precleaner

Double drum type of Single drum type of


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Double-drum type of

Precleaner

(West Germany)

Single-drum type of

Precleaner

(Japan)

Magnetic separator


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Magnetic separator

Magnet

Brass

A, B: Simple permanent magnet

C: Rotating brass cylinder and

permanent magnet


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De-stoner


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Paddy inPaddy in

PaddyPaddy

StoneStoneAir streamAir stream


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Stone

Wind direction

rice

Vibration direction


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De h sking


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De-husking

Underrun disc huller

Rubber roll huller

Centrifugal disc huller



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PaddyPaddy

Brown riceBrown riceHuskHusk

Particulars of the under


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

Composition:

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.

U d di h ll


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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.

Capacity and power requirement of


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Diameter Capacity Motor

(mm) (kg of paddy/hr) (hp)

750 450-600 3

1000 700-1000 3.5

1250 1000-1400 4

1400 1600-2100 5.5

Capacity and power requirement of

underrun disc huller


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AdvantageOperational simplicity, low running cost

The abrasive covering can be remade at the

site

Disadvantage

Given grain breakage

The abrasions to outer bran layers


Rubber Roll Huller


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Rubber Roll Huller

Rice Huller

Rollers


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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.

Size and speed of the rubber roll


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74010001025010250

90012003768.5220

90013202.5646150(rpm)(rpm)(Inch)(mm)(Inch)(mm)

Lowspeed

HighspeedWidthDiameter

Size and speed of the rubber roll

When the rolls are new, their peripheral speed is about 14 m/s

so that a smaller roll runs faster than a larger one.


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Durability of Rubber Rolls


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Durability of Rubber Rolls

15302.5

20353

40754

601106

1402408

17030010

Long grain(ton)

Short grain(ton)

Type(Inch)

Capacity and power requirement for


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Diameter

6.03.82.225425410

4.01.91.22201506

2.51.250.92201004

(hp)Shortgrain

Longgrain(mm)(mm)(Inch)

MotorCapacity (ton/hr)WidthSize

different rubber roll hullers

Rubber Roll Huller


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Rubber Roll Huller

AdvantageHulling 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.

Husk Separation


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Dehusker

Sieve

Paddy

Brown rice, Broken, Paddy, Bran, Husk

Coarsebran

Husk

Brewerrice

Husk Aspirator

Brown rice, Broken, Paddy

Paddy Separator

Husk Aspirator


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Closed Circuit Husk Aspirator


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1 !"#$%"&'"#$()!"*

2 !"#$%"&'"#$(+",-./.0

3 !"#$%"&'"#$(12'%!"2.(/

12'%3"4.!

4 )5/0367*"+

5 )5/0367*"+


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Huller with

Husk Aspirator


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Husk Aspirator

(BUHLER: DRHC/DRSC)

Paddy Separation


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y p

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.


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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


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Compartment-type Separator


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The 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,

high

Paddy, brown ricePaddy, brown rice


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PaddyPaddy

low

high

Brown riceBrown rice

Paddy, brown ricePaddy, brown rice


PaddyPaddy

P ddP ddHi hHi h HUSKERHUSKER


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PaddyPaddy

aa

HighHigh

LowerLower

AdjustmentAdjustment

SpeedSpeed 9090 -- 120 strokes/min120 strokes/min

SlopeSlope

StrokeStroke

HUSKERHUSKER

WHITENERWHITENER

Tray type separator


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First rubber roll huller


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Second rubber roll huller

Whitening Machine


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Removes bran layer to produce white rice.

Amount of bran removed (usually 8-12%)

Whitening


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Whitening Actions

Abrasive type

High speed / Lowpressure

Griding action

Impact action

Friction type

Low speed / Highpressure

Tearing action

Cutting action


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Milled rice & bran


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Friction type

Smooth

surface

Large

Abrasive type

Rough

surface

Fine

Whitening Machine


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Vertical abrasive whitening cone

Horizontal abrasive whitener

Horizontal friction-type whitener

Bottom-Up Vertical friction-type whitener

Vertical Abrasive

Whitening ConeAbrasive

cone

Air

Feeding


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Whitening Cone

Level

adjust

Wiremesh

Rubberbrake

Bransweeper

Bran

Rice

Number of Brakes = Cone Diameter - 2100


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100

Peripheral Speed 13 m/s

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


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


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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

( g )

Composition


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Composition

Whitening Cone No. 1 and No. 2

Emery grit 16 25%

Silicium carbide grit 16 50%


Whitening Cone No. 3

Emery grit 18 25%




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Aspirator

Counter

Weight

Bran

Abrasive roll

Screw

Resistance pieces


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ScreenAbrasive grit

#30, 36


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Short

grainLong

grain


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Characteristics of

Abrasive Action Milling(Speed 12 - 13 m/s)


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Shaping Action Factors


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LowMediumHighPressure

LargeMediumSmallRoll Grit

HighMediumLowPeripheral Speed

Round ShapeSlim shapeFlat ShapeCondition

Horizontal friction-type whitener


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Characteristics of

Low-Speed Machine

(Friction-type)

Effect of Ventilation on Low Speed Machine

(Friction-type Whitener)


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Bottom-Up Vertical friction-type whitener(SATAKE)


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Polisher


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Humidifying Rice Milling Machine

Vertical Polishing Cone Brush type

Vertical Polishing Cone Leather type

Horizontal Polisher Leather type

Humidifying Rice Milling Machine


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Nozzle

Bran

Water addition rate 0.3 - 0.4 %


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Mold Growth on Rice from Non-Humidifying

and Humidifying Milling


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Humifying rice-milling machine is used in the

last step of whitening system


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Characteristics of

Humidifying Milling

Machine


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Polisher


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(BUHLER: DRPE)


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Vertical Polishing

Cone


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Brush type

Vertical Polishing Cone - Leather type


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Horizontal

Polisher


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Whitening System

System 1 2 3 4 5


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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


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Abrasive Friction

ABRAS ABRAS ABRAS ABRAS ABRAS ABRAS FRICT FRICT

Combined System vs

Low Speed System


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Combined System

(High + Low speed)


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1st Vertical whitening roll

2nd Vertical whitening roll

3rd Hor Humidifying friction

polisher

Size of brokens according to Thai Rice Standard (1997)

Grading


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g ( )

General standard


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Japanese standard

A from B : By thickness

A from C : By thickness / length

B from C : By length


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A from B : By length

Bold grain

A from B/C : By thickness / length


Slender grain

A from B/C : By length


Grading of White Rice

After polishing,


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white ricecontains: Head rice

Large brokens

Small brokens

The white rice isseparated using Sifter

Length grader

Rotary Seiver

Rotary Sifter


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White Rice Grading

One double layers sifter


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and two trieurs inparallel.

One double layers sifter

and three trieurs in

series.

Rice Mixing

An efficient rice mill will produce:


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50% head rice (whole kernels)

5-15% large broken and

5-15% small broken kernels

Depending on the countrys standards, ricegrades in the market will contain from 5-25%broken kernels.

Rice Mixing


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Whole

kernels

Largebrokens

Smallbrokens

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


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50 kg sacks whichmust be accurately

weighed and labeled.

Bag filler andweighing

scales

Color SorterColor Sorter


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Conveying SystemConveying System


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Duct Collecting SystemDuct Collecting System

Scaling and PackagingScaling and Packaging SystemSystem


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Milling Quality AssessmentMilling Quality Assessment

Sampling / dividing

M i t t t

Assessment the milling quality of paddy


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Measure moisture content Cleaning sample

Milling test

Grading

Sample divider


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Scale Sieve


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Aspirator


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Moisture Meter

Resistance type

Capacitance type


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Capacitance type

Oven for m.c. determination130oC 1 hr

103oC 72 hr

105oC 5 hr

A, B : Weight per Volume Tester

C : Volume Measuring Cylinder


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C : Volume Measuring CylinderD : Counting plate

Rubber roll huller


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Abrasive-type Whitener


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Length Grader, Trieur


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Friction-type Whitener


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Friction-type Whitener with Length Grader


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Using Abrasive-type Whitener

Weigh clean paddy 250 g

Procedures for Milling Quality Assessment


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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

Using Friction-type Whitener (De-husking method)


Procedures for Milling Quality Assessment


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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


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Moisture Content = Wt of Water x 100Wt of Sample

After Drying Weight = (100 Initial MC ) x Initial Weight


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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)


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Thank you


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Rice Milling System

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