Cleaning grading of grains

cleaning grading equipment, sieving, effectiveness of screen, fair average quality of grains

1. Cleaning and Grading of Grains Devinder Dhingra

2. Cleaning and Grading - Grains Cleaning -To remove foreign and undesirable materials from the threshed grains. Grading - To separate the grains into various fractions on the basis of physical characteristics like size, shape, moisture content, color , texture, foreign matter etc. chemical characteristics like composition, odour , flavor, free fatty acid content (rancidity) etc. Biological factors like germination, insect infestation, insect damage etc. Threshed grains contain foreign matter, undesirable material (dust, soil, chaff, twigs, stones, broken grains, damaged grains, insect infested grains etc.); Harvested grains are not alike. They vary on the basis of size, specific gravity, color etc. due to variety / crop growing conditions and many other factors.

3. Grading is the classification of materials on the basis of commercial value, end usage ( product quality), and official standards. For, example, grading is necessary to avoid the further processing of blemished, spoiled, or products not meeting the quality requirements. Grading is done mostly by hand (e.g. inspection of fruits after washing), but when physical characteristics are also indicative of product quality, grading can be done through machinery . In rice, the white kernels are .separated optically from the off colour grains or from foreign matter, and the lighter unripe tomatoes can be separated from the ripe according to their specific weight.

4. Hand Grading The effectiveness of hand grading depends on the following factors I) Quality of the product 2) Quantity per inspector 3) Experience and physical condition of the inspector 4) Kind of inspection 5 ) The ergonomics during work 6) The speed at which the product move in front of the inspector The inspector can assess a number of quality factor simultaneously, and separate physically the product into certain quality categories, using sets of comparison standards, e.g. colour cards or plastic models. Fruits and vegetables are graded on the basis of state, federal , and international standards. Quality classifications such as "free from damage" or "free from serious damage" are used.'

5. Grading of grains, is based on testing; of smaller quantities. The lots are taken out randomly, and are subsequently evaluated in the laboratory, using proper instrumentation. (FAQ – Fair Average quality of grain procured by FCI (wheat, rice, pulses etc.) is well described) Oilseeds are priced based on oil content.

6. Specifications of Wheat - FCI Wheat shall: a) be the dried mature grains of Triticum vulgare , T compactum , T sphaerococcum , durum, T aestivum and T dicoccum. T b) have natural size, shape, colour and lustre. c) be sweet, clean, wholesome and free from obnoxious smell, discolouration, admixture of deleterious substances including toxic weed seeds and all other impurities except to the extent indicated in the schedule below. d) be in sound merchantable condition. e) not have any admixture of Argemone mexicana and Lathyrus sativus ( khesari ) in any form, colouring matter and any obnoxious, deleterious and toxic material. a) Conform to Food Safety and Standards Act! Rules (formerly PFA).

7. Maximum permissible limits of different refractions in Fair Average Quality of Wheat.

8. FAQ Wheat contd Moisture in excess of 12% and up to 14% will be discounted at full value. Wheat containing moisture in excess of 14% is to be rejected. 2. Within the overall limit specified for foreign matter, the poisonous weed seeds shall not exceed 0.4% of which Dhatura and Akra ( Vicia species) shall not be more than 0.025% and 0.2% by weight respectively. 3. Kernels with glumes will not be treated as unsound grains during physical analysis, the glumes will be removed and treated as organic foreign matter. 4. Within the overall limit specified for damaged grains, ergot affected grains shall not exceed 0.05 %. 5. In case of stocks having living infestation, a cut at the rate of Rupee two per quintal may be charged as fumigation charges. 6. For weevilled grains determined by count, stocks containing weevilled grains in excess,of 1% will be rejected. The rate of cut for weevilled grains will be @ Rs.2 per quintal.

9. Grading contd ….. Besides skilled personnel, special machines are used increasingly for grading. The trend is to develop quality control methods that enable a continuous and quick estimation of the products. Nondestructive optical and physical methods such as colour measurements use X-rays, lasers, infrared rays, and microwaves are promising. Most of the machines which are used in grading can also be used in sorting. X-rays are also employed to detect foreign matter, such as glass splits and stones in packed materials. Machine grading of the product is based on a representative index of quality, e.g. colour, firmness, and pH, or a reasonable combination of these properties.

10. Sorting Sorting, like grading, facilitates subsequent processing operations, such as peeling, pitting, blanching, slicing, and filling of containers. It is beneficial for heat and mass transfer operations, where processing time is a function of the size of the product (e.g. Heat conduction, mass diffusion). Criteria for sorting of materials Physical characteristics: Size, weight and shape Technological characteristics: Processing suitability or compatibility to existing equipment Organoleptic characteristics: Texture, colour, aroma, taste, ripeness or freshness Commercial characteristics: Attractiveness, tradition, variety, utility, price Most of the mechanical sorters are based on the size ofthe materials, but some equipment utilizes differences in shape, density, and surface properties of the products to be sorted. Screens (flat or drum type) are used extensively in sorting various grains, seeds, crystals, and other products of relatively small size. Inclined screens, one on top of the other with horizontal and Vertical oscillations, are effective in grain and seed sorting .

11. Classification of graders and sorters on the basis of their grading principle SI. No. Basis of grading Graders and Sorters 1 Size and Shape Trammel, spiral separator, disc and indented cylinder separator 2 Specific gravity Specific gravity separator, destoner 3 Surface roughness Inclined drapper , velvet roll separator 4 Aerodynamic property Pneumatic/aspirator separator, fluidized bed separator, cyclone separator 5 Magnetic property Magnetic separator 6 Optical property Colour sorter

12. CLEANING & SORTING GRAINS,NUTS AND SEEDS Procedures to clean and sort grains and seeds are similar. Cleaning, sorting and partial or perhaps final grading is based on following factors Size Shape Specific gravity Color Surface characteristics The first 4 factors are important and equipment utilising these factors is available. Surface characteristics as differentiated from shape affect the drag coefficient where an air blast is used for separation. Although it is known to be an effective factor, its importance thus far has not been demonstrated .

13. Pre Cleaners and grader Used for efficient cleaning /grading of all types of cereal grains, pulses and pre-cleaning of all types of seeds of all crops such as wheat, paddy, barley, maize, millets, sunflower, soybean, oil seeds, vegetable seeds, pulses, coffee seeds etc . Based on combination of aspiration and shaking/reciprocating flat screens

14. Flat screens Flat screen separaties according to size alone. The mixture of grain and foreign matter is dropped on screening surface which is vibrated either manually or mechanically. A single screen can make the separation into two fractions; The screening unit may be composed of two or more screens as per the cleaning requirement . Screen/Screens can be either a slotted screen or a round-hole screen or a triangular hole, according to the separation to be made and the flexibility of the machine . The size of slot or diameter of hole depends on the grain to be cleaned and graded.

15. Principle of operation of flat screen grader Products to be sized are deposited from the feed hopper on the top of the upper-most screen . This screen can be either a slotted screen or a round-hole screen, according to the first separation to be made and the flexibility of the machine. In operation, the screens move forward and backward, all screens in a single block moving simultaneously in the same direction. This motion causes the products to move down the inclined screens toward the discharge spouts. Product larger than the hole-openings in any particular screen remain on top of the screen and are discharged at the end of the screen. Product smaller than the openings, because of gravitational force, drop through and are deposited on the top of the next lower screen in the machine. This screening action continues until the products are appropriately sized and discharged according to screen types and sizes in the machine . Rate of feed and rate of vibration are two adjustments common to separators of this type . Shaking and reciprocating screens are inclined slightly and vibrate at 30-1000 strokes/min, separating particles in the size range of 0.2-25 mm.

16. Fine Cleaner For cleaning and grading of high value crop seeds like vegetable seeds, flower seeds, grass seeds, forestry seeds, small lots of hybrid seeds, oil seeds etc. The machine is also very suitable for cleaning and grading of all type of spices, for research stations, breeder and foundation seed multiplication farms, seed testing laboratories etc.

17. ROTARY DRUM CLEANER Suitable for pre-cleaning as well as fine cleaning of all types of cereal grains, seeds, oil seeds, pulses, malt, barley etc.

18. Gravity Separator The machine is specifically meant for removing impurities and achieving very high grading quality in any free flowing granular material, grains, all types of seeds, pulses, spices etc. It removes the impurities and upgrades the material on the principle of specific weight.

19. Gravity Separator – another model

20. Indented cylinder Grader The equipment is suitable for separating short kernels from long, broken or cut kernels from whole kernels & also round seeds from oblong ones.

21. Indented Cylinder Separator The cylindrical separator consist of a horizontal rotating cylinder which has indents on the inside surface. The indents are closely spaced and hemispherical in shape . When the mixture of grain is fed into one end of the cylinder, short grains are picked up by combined effect of fitting into the indents and centrifugal force. These grains are dropped into an adjustable trough inside the cylinder near the top of rotation. A screw conveyor is provided in the bottom of the trough which conveys the material . Generally , the cylinder is kept at slight inclination to facilitate gravity flow of grains in the cylinder. The cylinders with indents of different sizes are available, but the size of all indents in a particular cylinder is same. For different separation needs, indented cylinder has to be changed .

22. Destoner

23. Color Sorter

24. EFFECTIVENESS OF SCREEN The screen effectiveness may be defined as the ability of a screen in closely separating the feed into overflow and underflow according to its size. If the screen functions properly , all material '0' would be in the overflow, while all the material 'U' would be in the underflow. The material balance in a screening operation can be derived as follows. F = mass flow rate of feed, kg/h O = mass flow rate of oversize, kg/h U = mass flow rate of undersize, kg/h (F,O and U pertain to a particular screen having mesh size / opening say “S” ) The screen analysis of the feed (F), Overflow (O) and underflow (U) material is conducted to obtain m f = cummulative mass fraction of oversize material in feed (retained on screen size “S”) m o = cummulative mass fraction of oversize material in overflow (retained on screen size “S”) m u = cummulative mass fraction of oversize material in underflow (retained on screen Size ‘S’)

25. The total quantity of the feed is the sum of overflow and underflow (Screen Size “S”) F = O + U Substituting 0 = F-U and U = F-O Fm f = Om o +Um u and A common measure of screen effectiveness is the ratio of actual amount of oversize material in the overflow to the amount of oversize material entering with the feed. Thus, (effectiveness for oversize material) and ; (effectiveness for undersize material) (1-m u ) represents mass fraction of undersize material in the underflow which passes screen “S” and (1-m f ) is mass fraction of undersize material in the feed which passes through screen “S”   EFFECTIVENESS OF SCREEN

26. Screen Effectiveness Contd Overall effectiveness E= E o E u = Substituting the value O/F and U/F E=  

27. Effectiveness of screen The effectiveness of screening or cleaning efficiency for an air screen grain cleaner as suggested by the Bureau of Indian Standards (BIS) is given below: Cleaning efficiency = Where E= fraction of clean seed at clean seed outlet F=fraction of clean seed in feed G=fraction of clean seed at foreign matter outlet  

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