Rice grading and sizing: how length graders work
The grader usually sits at the end of a rice milling line, after whitening and polishing, and it is the machine buyers think about least when specifying a mill.
That is a mistake, because grading determines what the mill is able to sell. Two mills processing identical paddy with identical breakage can sell into completely different markets depending on whether one of them separates its output and the other does not.
Here is the point that matters most, and the one most often misunderstood: a grader cannot lower your broken percentage. By the time rice arrives at the grading stage, the broken grains already exist. Breakage was determined by paddy moisture, husker roll settings and whitening pressure, all of which happened earlier in the line.
What the grader does is separate. That sounds like a smaller job. Commercially it is not.
Why separation is worth money
Take a mill producing output that tests at 12% broken.
Sold as one undifferentiated stream, that rice meets a 15% broken specification. It is priced accordingly, which is well below the premium tier.
Run the same output through a grader, and it divides into a head rice stream testing under 5% broken, plus a separated broken stream that sells in its own right as A1 Super or an equivalent broken grade. The premium fraction now commands premium pricing. The broken fraction is not waste, it is a product with its own established market in brewing, starch, pet food, and household consumption across West Africa and parts of Asia.
The mill did not change its milling performance. It changed what it could sell, and that is usually worth more than a marginal improvement in head rice yield would have been.
In markets where broken rice is genuinely preferred, this logic strengthens further. Senegalese cooking traditionally uses 25% and 50% broken grades by choice, so a mill supplying that market needs calibrated broken output as its primary product rather than a by-product. See the Senegal rice industry analysis.
What grading actually separates
Rice can be sorted by three different physical properties, and the terminology gets used loosely.
Length. The basis of head rice and broken separation, and what the term "rice grader" normally refers to. Head rice is defined as kernels at least three-quarters of the average whole kernel length. Anything shorter counts as broken.
Width and thickness. Used for separating immature and shrivelled kernels, and in some configurations for varietal sizing. Handled by screens rather than by indents.
Colour and appearance. A different machine entirely. Discoloured, chalky and damaged grains are removed by a colour sorter using optical sensors, not by mechanical grading.
A complete grading stage may use more than one of these. The core of it is length separation.
The indented cylinder length grader
The indented cylinder separator, also called a trieur, is the standard machine for separating head rice from broken.
How it works
The machine is a rotating steel cylinder whose inner surface is covered in thousands of small indented pockets. Rice is fed in at one end.
As the cylinder rotates, short grains drop into the pockets and get carried up the rising side of the cylinder. Long grains do not fit properly into the indents, so they stay in the bed of rice at the bottom and travel along the cylinder toward the discharge.
At a certain height on the rising side, gravity overcomes the lifting effect and the short grains fall out of their pockets into a collection trough running down the centre of the cylinder. A screw conveyor in that trough carries them out as the broken fraction.
Long grains reach the far end of the cylinder and discharge as head rice.
The separation is mechanical and continuous, with no screens to blind and no moving parts inside the grain stream beyond the trough conveyor.
The two adjustments that matter
Trough edge position. The collection trough can be rotated, which raises or lowers the edge at which falling grains are caught. Raise the edge and the machine catches only the shortest grains, giving a cleaner head rice fraction but leaving more brokens in it. Lower it and more marginal grains fall into the broken stream, giving purer head rice but a larger broken fraction and lower head rice recovery.
This single adjustment is how an operator moves between target grades, and it is where most grading problems originate.
Cylinder speed. Rotation speed changes the point at which centrifugal force releases grains from the pockets. Too slow and grains fall early, before reaching the trough. Too fast and they are held in the pockets past the trough edge and carried back down. Both reduce separation efficiency.
Indent size
Indent diameter is matched to the grain being processed. A cylinder set up for long grain Indica will not separate short grain Japonica correctly, because the pockets sized to lift a broken long grain will lift a whole short grain.
Mills processing multiple varieties either accept reduced efficiency on one of them, or change cylinders. This is worth raising at the specification stage rather than discovering later, particularly for operations handling both aromatic long grain and local short or medium grain.
Well set up, an indented cylinder grader reaches separation efficiency above 95%.
The plansifter
A plansifter, or rotary sifter, is the other common grading machine. Instead of indents it uses stacked screens with different perforation sizes, driven in a gyratory motion.
Rice travelling across the screens stratifies. Fine material passes through the smallest perforations, intermediate fractions through the next size up, and the largest grains travel across the top deck to their own outlet.
Plansifters are efficient for separating small brokens and brewers rice from the main stream, and for removing rice meal and fine material after polishing. They separate by size rather than strictly by length, which makes them well suited to taking out the very small fractions and less precise than an indented cylinder for the head rice and broken split.
Many commercial lines run both: a plansifter to remove small brokens and fines, then an indented cylinder to make the head rice separation cleanly.
Where grading sits in the line
Grading comes after whitening and polishing, and before colour sorting and bagging.
The order is not arbitrary. Whitening and polishing both create additional breakage, so grading before them would separate a stream that is about to change composition. Colour sorting comes after grading because the sorter works more accurately on a stream of uniform grain size, and because sorting the broken fraction is often unnecessary.
A typical commercial sequence runs: pre-cleaner, destoner, husker, paddy separator, whitener, polisher, grader, colour sorter, bagging. The rice milling process guide covers the full sequence, and what machines are needed in a rice mill plant covers each machine's role.
Sizing a grader for your line
Three things decide the specification.
Throughput. Grader capacity is quoted in tons per hour and needs to match the line ahead of it with margin. A grader running at its limit separates poorly, because separation efficiency depends on grains having room to move and settle into the indents. Undersizing the grader is a common way to lose the benefit of the rest of the line.
Number of passes. A single cylinder makes one separation. Multi-pass configurations run the head rice fraction through a second or third cylinder to lift out remaining brokens, or run the broken fraction again to recover head rice that was carried over. Export-grade operations generally need multi-pass. A cooperative mill supplying a local market usually does not.
Grain type. Indent size must match the variety. Mills processing a single variety can specify once. Mills handling several need either a compromise setting or interchangeable cylinders.
For capacity planning across the whole line: rice mill capacity guide.
Common grading problems
Head rice fraction contains too many brokens. The trough edge is set too high, or throughput exceeds the machine's capacity. Lower the edge first, then check the feed rate.
Too much head rice in the broken stream. The trough edge is too low, or the indent size is too large for the variety. This one costs money directly, because head rice is being sold at broken prices.
Separation efficiency drops over time. Indent pockets wear and lose their defined edge, which makes them less able to grip a grain cleanly. Cylinders are a wear part with a service life, and a grader that separated well two years ago will not separate well indefinitely.
Separation varies through the shift. Almost always an inconsistent feed rate rather than a fault in the grader. Surging feed produces surging separation.
For wider diagnosis: common rice milling problems and how to measure and improve milling recovery.
Do you need a grader?
Not every mill does, and it is worth being clear about that.
A mill custom-milling paddy for local farmers who collect their own rice does not need grading. The customer wants their rice back, not a graded product.
A mill selling into a local market that does not differentiate on broken percentage will not recover the investment quickly either.
A grader earns its cost when the mill sells to buyers who specify a grade. That means supermarket and retail supply, institutional and government contracts, export buyers, and any market where broken rice has its own price. In practice that covers most commercial operations above roughly 10 TPD, and effectively all export-oriented mills.
If you are supplying a market where buyers state a percentage in the contract, you need grading. Without it you cannot reliably produce to specification, and pricing will default to the lowest grade your output can guarantee.
Contact Starlight for rice grading equipment
Starlight supplies rice graders in single and multi-pass configurations, alongside complete milling lines, with indent sizing matched to the varieties you process.
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For terminology used across rice processing: Rice Milling Industry Glossary.
Frequently asked questions
What does a rice grader do? A rice grader separates milled rice by grain length, dividing output into head rice and broken fractions. This allows a mill to sell to a defined grade specification such as 5% or 25% broken, and to sell the separated broken rice as its own product, instead of selling one mixed stream at a lower average price.
How does an indented cylinder rice grader work? A rotating steel cylinder with indented pockets on its inner surface carries short grains upward in the pockets while long grains remain in the bed at the bottom. At a set height the short grains fall into a central collection trough and are conveyed out as the broken fraction, while long grains travel to the end of the cylinder and discharge as head rice.
Can a rice grader reduce broken rice percentage? No. Breakage is created earlier in the process by paddy moisture, husker roll settings, whitening pressure and handling. A grader separates brokens that already exist. To reduce breakage, address husker roll condition and setting, whitening configuration, and paddy moisture at milling.
What is the difference between a length grader and a plansifter? A length grader, or indented cylinder, separates by grain length and is the precise tool for splitting head rice from broken. A plansifter uses stacked screens in gyratory motion to separate by size, and is better suited to removing small brokens, brewers rice and fine material. Many commercial lines use both.
How accurate is an indented cylinder grader? A correctly specified and adjusted indented cylinder achieves separation efficiency above 95%. Accuracy depends on indent size matching the grain, appropriate cylinder speed, correct trough edge position, steady feed rate, and throughput within the machine's rated capacity.
Where does the grader go in a rice milling line? After whitening and polishing, and before colour sorting and bagging. Whitening and polishing generate additional breakage, so grading is placed after them. Colour sorting follows grading because sorters work more accurately on a uniform grain size.
Do indented cylinders wear out? Yes. The indent pockets lose their defined edge over time, which reduces their ability to grip and lift grains cleanly. Separation efficiency declines gradually as a result. Cylinders should be treated as a wear component with a service life rather than a permanent fixture.
Does grading work the same for long grain and short grain rice? No. Indent diameter must be matched to the grain being processed. A cylinder sized for long grain Indica will misclassify short grain Japonica, because pockets sized to lift a broken long grain will also lift a whole short grain. Mills handling multiple grain types need either interchangeable cylinders or a compromise setting with reduced efficiency.
Starlight Machinery is a rice processing machinery manufacturer based in Danyang, Jiangsu, China, supplying combined rice mills, production lines, individual processing machines and spare parts to commercial operators, cooperatives and distributors worldwide.