Rice Milling System
Short Description
Lecture Presentation on Rice Milling...
Description
Rice Milling System
Nitat Tangpinijkul Post-harvest Engineering Research Group Agricultural Engineering Research Institute Department of Agriculture
Rice 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 Module 4 Lesson 1
: : :
Grain Quality Rice Milling Introduction to the milling process and its outputs
Good Equipment
Good Paddy Rice
If
Then
you have good quality paddy in a well-maintained mill that is operated by a skilled miller,
the mill will produce high quality head rice.
you use poor quality paddy,
the mill will always produce poor quality milled rice, despite the skill or the miller or maintenance of the mill.
the miller is not skilled,
the use of good milling equipment and good quality paddy does not guarantee a high quality product
Skilled Miller
Course Module 3 Lesson 1
: : :
Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics
Quality characteristics of paddy
These characteristics are determined by the environmental weather conditions during production, crop production practices, soil conditions, harvesting, and post harvest practices
Course Module 3 Lesson 1
: : :
Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics
QUALITY CHARACTERISTIC: MOISTURE CONTENT
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 Module 3 Lesson 1
: : :
Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics
QUALITY CHARACTERISTIC: IMMATURE
Immature rice kernels are very slender and chalky and result in the production of excessive bran, broken grains and brewer’s rice
Course Module 3 Lesson 1
: : :
Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics
QUALITY CHARACTERISTIC: VARIETAL PURITY
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 Module 3 Lesson 1
: : :
Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics
QUALITY CHARACTERISTIC: DOCKAGE
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 Module 3 Lesson 1
: : :
Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics
QUALITY CHARACTERISTIC: DISCOLORED
Water, insects and heat exposure can cause paddy to deteriorate through biochemical changes in the grain which may result in the development of offdoors and changes in physical appearance
Course Module 3 Lesson 1
: : :
Grain Quality Determining the Physical Characteristics of Paddy Rice The quality characteristics
QUALITY CHARACTERISTIC: CRACKED
Overexposure of mature paddy to fluctuating temperature and moisture conditions leads to the development of crack in individual kernels.
OTHERS QUALITY CHARACTERISTIC: • 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
• Grain Dimension Grain 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 : Extra long Long Medium Short Sub-type of Paddy Slender Bold Round
Length of brown rice ≥ 7.5 ≥ 6.5 < 7.5 ≥ 5.5 < 6.5 < 5.5 : Length/Width Ratio ≥ 3.0 ≥ 2.0 < 3.0 < 2.0
Thai rice variety:
Length 7.1 - 7.6 mm Slender 3.1 – 3.4
• Chalkiness • 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 Ø Ø Ø Ø Ø
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
Purity of paddy Milling Recovery = Weight produced milled rice x 100% Weight processed paddy Purity % 100 99 98 97 96 95
Impurity % 0 1 2 3 4 5
Milled rice % 68.00 67.32 66.54 65.96 65.28 64.60
Cracks Bran
Rice grain
Embryo
Bran Embryo
Rice flour Splinters
+
Total bran produced = when whitening cracked grains
Paddy Quality Good
Crack
Immatured
Japonica
Paddy
100.0
100.0
100.0
100.0
Husk
23.0
23.0
25.0
17.0
Brown Rice
77.0
77.0
75.0
83.0
Bran
8.0
10.0
10.0
7.5
Milled Rice
69.0
67.0
65.0
75.5
Brewer Rice
1.0
1.5
1.5
0.5
Recovery
68.0
65.5
63.5
75.0
Size 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
Number of rice mill by region, 1993 - 1997 Year
North-
Northern
Eastern
Central
Southern
Plain
Whole Kingdom
1993
25,213
10,183
5,223
4,400
45,019
1994
24,905
10,104
5,156
4,395
44,560
1995
24,888
10,065
5,129
4,390
44,472
1996
24,887
10,049
5,114
4,386
44,436
1997
28,533
8,779
3,232
2,731
43,275
Source : Factory Control Division, Ministry of Industry
Number of rice mill in Suphanburi province (2004) Number of village Number of Rice mill < 5 ton/day > 5 < 20 ton/day > 20 ton/day
962 194 90 (46.4%) 42 (21.6%) 62 (32.0%)
PRICE LIST OF MODERN RICE MILL RICE ENGINEERING SUPPLY CO., LTD. (BANGKOK, THAILAND) For Month January-April 2008
Capacity (Paddy) Ton/Hour Ton/Day 2-3 4-5 8-10 18-20
50-80 100-120 200-240 420-500
Price (F.O.B) (USD) 348,000 624,000 1,050,000 1,824,000
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 Paddy into rice mill
Cleaning
Impurity
Paddy
Husking
Husk
Paddy, Brown rice Paddy
Paddy Separation Brown rice
Whitening & Polishing
Bran
Milled rice Head rice
Grading and Blending
Broken
Paddy Intake • Quality input = quality output • Garbage in = garbage out! 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 intake chute
Paddy Cleaning • After harvest, foreign matter can be as high as 5-10% While most paddy has been cleaned after harvesting, some foreign material is still present. This can be as high as 5-10%.
Rice pre-cleaner
Pre-cleaning of Paddy • 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
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
Brass
Magnet
A, B: Simple permanent magnet C: Rotating brass cylinder and permanent magnet
De-stoner
Paddy in
Paddy
Air stream
Stone
Vibration direction Stone rice
Wind direction
De-husking • Underrun disc huller • Rubber roll huller • Centrifugal disc huller
Underrun disc huller Paddy
Brown rice Husk
Particulars of the underrunner disc huller: V = peripheral speed (recommended 14 m/s); W = width of coating; D = stone diameter; and W/D = 1/6 or 1/7.
Ratio of W/D ~ 1/6 or 1/7 Composition: • Emery grit 14, 16 • Silicium carbide grit 16 Peripheral speed ~ 14 m/s
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 ¶xD 3.14 x 0.7 = 380 rpm.
Capacity and power requirement of underrun disc huller
Diameter (mm) 750 1000 1250 1400
Capacity (kg of paddy/hr) 450-600 700-1000 1000-1400 1600-2100
Motor (hp) 3 3.5 4 5.5
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
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.
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.
Diameter
Width
High speed
Low speed
(mm)
(Inch)
(mm)
(Inch)
(rpm)
(rpm)
150
6
64
2.5
1320
900
220
8.5
76
3
1200
900
250
10
250
10
1000
740
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 Type (Inch)
Short grain (ton)
Long grain (ton)
10
300
170
8
240
140
6
110
60
4
75
40
3
35
20
2.5
30
15
Capacity and power requirement for different rubber roll hullers Size
Diameter
Width
Capacity (ton/hr)
Motor
(Inch)
(mm)
(mm)
Long grain
Short grain
(hp)
4
100
220
0.9
1.25
2.5
6
150
220
1.2
1.9
4.0
10
254
254
2.2
3.8
6.0
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.
Husk Separation Paddy
Dehusker Brown rice, Broken, Paddy, Bran, Husk Brewer rice
Sieve
Coarse bran
Husk Aspirator
Husk
Brown rice, Broken,
Paddy
Paddy Separator
Husk Aspirator
Plansifter & Husk Aspirator
Closed Circuit Husk Aspirator
1 เกลียวลําเลียงแกลบ 2 เกลียวลําเลียงเมล็ดออน 3 เกลียวลําเลียงขาวกลอง/ ขาวเปลือก 4 แผนปรับลม 5 แผนปรับลม
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.
Paddy and brown rice have different characteristics • • • •
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.
Types of paddy separators • Compartment (table) type • Tray type • Screen type
(BUHLER: DNTB)
Compartment-type Separator The main part of this paddy separator is the oscillating compartmentassembly 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,
Paddy, brown rice
high low
Paddy
Brown rice
Paddy, brown rice Paddy
Brown rice
High
Paddy
a
HUSKER
Adjustment • Speed 90 - 120 strokes/min • Slope • Stroke
Lower
Brown rice
WHITENER
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 Whitening Actions • Abrasive type High speed / Low pressure • Griding action • Impact action • Friction type Low speed / High pressure • Tearing action • Cutting action
Milled rice & bran Friction type
Abrasive type
Smooth surface
Rough surface
Large
Fine
Whitening Machine • • • •
Vertical abrasive whitening cone Horizontal abrasive whitener Horizontal friction-type whitener Bottom-Up Vertical friction-type whitener
Feeding Air
Vertical Abrasive Whitening Cone
Abrasive cone Wire mesh
Bran sweeper
Bran Rice Level adjust
Rubber brake
Number of Brakes = Cone Diameter - 2 100 Eg. Cone Diameter Number of Brakes
= 600 mm = 600/100 – 2 = 4
Peripheral Speed 13 m/s
Capacity of Cone-type Whitener (kg of brown rice / hr) Diameter
Power
1 Cone
2 Cones
3 Cones
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
Composition Whitening Cone No. 1 and No. 2 Emery grit 16 Silicium carbide grit 16 Silicium carbide grit 18
25% 50% 25%
Whitening Cone No. 3 Emery grit 18 Silicium carbide grit 18
25% 75%
Horizontal abrasive whitener
Horizontal abrasive whitener
Abrasive roll Screw
Counter Weight
Bran Aspirator
Resistance pieces
Abrasive grit #30, 36
Screen
Long grain
Short grain
900 450
% Bran
00
0
30 45 60
90
Characteristics of Abrasive Action Milling (Speed 12 - 13 m/s)
Shaping Action Factors Condition
Flat Shape Slim shape Round Shape
Peripheral Speed
Low
Medium
High
Roll Grit
Small
Medium
Large
Pressure
High
Medium
Low
Horizontal friction-type whitener
Perforated sheet Milling roll
Brown rice
P control
Milled rice
Bran
Aspirator
Screw Perforated sheet
Characteristics of Low-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
Polisher
(BUHLER: DRPE)
Vertical Polishing Cone – Brush type
Vertical Polishing Cone - Leather type
Horizontal Polisher
Whitening System System
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
Vent Friction
Hu Friction
Short grain (Japonica) 1
Abrasive Vent Friction Vent Friction
Milling System for Indica-type variety
ABRAS
ABRAS
ABRAS
Abrasive
ABRAS ABRAS
ABRAS
FRICT
Friction
FRICT
Combined System vs Low Speed System
Combined System (High + Low speed)
1st Vertical whitening roll 2nd Vertical whitening roll 3rd Hor Humidifying friction polisher
Grading Size of brokens according to Thai Rice Standard (1997)
General standard
Japanese standard
A from B : By thickness A from C : By thickness / length B from C : By length
A from B :
By length
Bold grain A from B/C : By thickness / length B from C : By length Slender grain A from B/C : By length B 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 Small brokens
Large brokens
Whole kernels
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.
scales
Bag filler and weighing
Color Sorter
Conveying System Scaling and Packaging System Duct Collecting System
Milling Quality Assessment
Assessment the milling quality of paddy • Sampling / dividing • Measure moisture content • Cleaning sample • Milling test • Grading
Sample divider
Scale
Sieve
Aspirator
Moisture Meter Resistance type Capacitance type
Oven for m.c. determination 130oC 1 hr 103oC 72 hr 105oC 5 hr
A, B : Weight per Volume Tester C : Volume Measuring Cylinder D : Counting plate
Rubber roll huller
Abrasive-type Whitener
Length Grader, Trieur
Friction-type Whitener
Friction-type Whitener with Length Grader
Procedures for Milling Quality Assessment 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
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 kernels WMR = 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)
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