Grading and Separation of Brokens in Rice Milling

Once paddy has been husked, whitened, and polished, the job is far from over. The milled rice coming out of the polishing stage is a mix – whole grains, large broken pieces, small fragments, and fine chips all tumbled together. Before this rice can be packaged and sold, it must be carefully sorted into distinct categories based on kernel length and size. This sorting process is called grading and separation of brokens, and it is one of the most commercially significant steps in the entire rice milling chain. It directly determines how much premium head rice a mill can market and how the remaining broken fractions are utilized.

Why grading brokens matters 🔗

Rice is not sold as a uniform product. According to the Food and Agriculture Organization (FAO), when rough rice is milled, kernel breakages naturally occur, resulting in different kernel lengths. These differences are grouped into defined categories – head rice, big brokens, medium brokens, small brokens, and brewer’s rice – each with specific length limits and distinct market values. Broken grain carries roughly half the market value of whole head rice, which is why separating them efficiently is a direct economic priority for any rice mill.

According to the IRRI Rice Knowledge Bank, a well-run rice mill should produce 50-60% head rice, 5-10% large brokens, and 10-15% small broken kernels. Depending on national standards, rice grades in the market may contain anywhere from 5 to 25% broken kernels. Precise grading ensures a mill maximizes the proportion of top-grade head rice while assigning each broken fraction to its most appropriate downstream use.

How broken rice is classified 🔗

Before exploring the equipment, it helps to understand the classification system. The FAO defines these fractions as follows:

Head rice refers to a kernel or piece of kernel whose length is equal to or greater than 8/10 of the average length of an unbroken kernel. Big brokens are pieces smaller than 8/10 but not less than 5/10 of the average unbroken kernel length. Medium brokens fall between 5/10 and 3/8 of the full kernel length. Small brokens are shorter than 3/8 of the whole kernel but will not pass through a 1.4 mm round-perforated sieve. Finally, brewer’s rice consists of the finest fragments – particles that do pass through the 1.4 mm sieve and are typically directed to brewing, distilling, or rice flour production.

This classification system is not merely academic. It forms the basis on which grading machinery is designed and calibrated, and it determines how different fractions are priced and sold across markets.

Equipment used for grading and separation 🔗

Modern rice mills use three main categories of equipment to achieve clean, accurate grading: sifters, plansifters, and trieur (indented) cylinders. Each operates on a different physical principle – size, shape, and length – and they are often used in combination to achieve the best results.

Sifters: the first line of separation 🔗

The simplest form of grading equipment is the oscillating screen sifter. After the whitening and polishing stages, milled rice passes over a series of vibrating screens fitted with perforations of progressively smaller sizes. Whole grains are too large to fall through the upper screens and are retained, while smaller fragments drop through successively finer meshes. The finest particles – chips and dust – pass through the smallest screens and are collected separately.

The IRRI notes that after polishing, white rice is separated into head rice, large and small broken rice, and brewer’s rice using an oscillating screen sifter. Installing a screen sifter to remove small brokens and chips from polished rice is a key recommendation in optimizing mill output, since rice with a high proportion of small brokens or brewer’s rice commands a significantly lower price. The small brokens collected this way can be directed towards rice flour production, adding value rather than waste.

Plansifters: multi-level precision grading 🔗

For larger-volume mills requiring more refined separation, the plansifter is the preferred solution. A plansifter is an advanced multi-deck sifting machine that handles multiple grain size categories simultaneously in a single pass. According to Bühler Group, a leading manufacturer of grain processing equipment, the plansifter’s multiple sieves are arranged in stacks within a single housing, and the sieves can be adapted for different sifting tasks, including the separation of large volumes of brokens of many different sizes.

The grain is fed onto the uppermost sieve and moves downward through successively finer screens as the machine oscillates in a circular, horizontal motion. Each level retains a distinct fraction, so the machine simultaneously outputs top-grade rice, large brokens, medium brokens, and fine chips in separate streams. Bühler also notes that the plansifter’s aspiration connections allow airflow to cool the product and reduce moisture, helping to condition rice for further processing or storage. The circular motion also reduces grain breakage compared to simple reciprocating sifters, making it well-suited to fragile, highly polished rice.

Some advanced plansifter designs can classify milled rice into four grades in a single pass: top grade, ordinary, big broken, and broken – a level of output precision that substantially simplifies downstream packaging and blending operations.

Trieur (indented) cylinders: length-based separation 🔗

Sifters and plansifters work primarily on the size and width of grains. However, broken rice fragments that are similar in width to whole grains – but shorter in length – can pass through conventional screens without being separated. This is where the trieur cylinder, also called an indented cylinder or length grader, becomes essential.

A trieur cylinder is a large rotating drum whose inner surface is lined with precisely shaped indented pockets or cells. As described by Seedburo Equipment Company, broken or short kernels are lifted by the indent pockets that line the inside surface of the cylinder as it rotates. The operator controls the separation by adjusting the position of an internal collection pan. Short fragments – the brokens – are small enough to be picked up and carried to the top by the pockets, where they fall into the internal trough and are discharged separately. Longer whole grains are too large to be retained in the pockets and simply roll along the bottom of the cylinder to exit from the other end.

Manufacturers note that when the length of broken kernels is more than half that of whole grains, thickness and width alone cannot reliably differentiate them – length-based separation by the trieur cylinder becomes the only reliable method. When used together with a conventional sifter, the combination can achieve near-zero broken content in the head rice output stream.

The relationship between grading and head rice recovery 🔗

Efficient grading directly affects a mill’s head rice recovery rate – the single most important commercial metric in rice milling. Head rice is classified as kernels that are 75-80% or more of the length of a whole kernel, and maximizing this fraction requires not only careful husking and whitening but also accurate grading at the end of the process. The FAO emphasizes that grading prior to milling also offers advantages: immature grains are separated, more precise adjustment of the huller becomes possible – minimizing breakages – and independent milling of graded lots is achievable.

For precision grading, IRRI recommends using a length or indent grader to attain a higher degree of grading accuracy beyond what sifters alone can achieve. This two-stage approach – sifter followed by trieur cylinder – is the standard practice in high-output commercial mills.

Marketing different grades of broken rice 🔗

A key advantage of precise grading is that it transforms what might otherwise be considered milling waste into separate, marketable products. Each fraction of broken rice has an established commercial purpose. The FAO’s framework for milled rice grading acknowledges the distinct commercial identity of big brokens, medium brokens, small brokens, and brewer’s rice, each targeted at different buyers and industries.

Large brokens are sold for direct retail consumption in markets where broken rice is preferred – notably in West Africa and parts of South and Southeast Asia, where it is used in traditional dishes like thieboudienne or congee. Medium and small brokens find buyers in the food processing industry for products like rice noodles, puffed rice snacks, and breakfast cereals. The finest fraction – brewer’s rice – is sold to breweries, distilleries, and rice starch or rice flour manufacturers. The IRRI notes that small brokens can be utilized to produce rice flour, making them a useful by-product rather than a loss.

This multi-stream output means that a well-graded mill wastes almost nothing. The economic logic is straightforward: a mill that sells four or five distinct product grades will always outperform one that blends everything together into a lower-quality, lower-value mixed product.

Quality standards and grading consistency 🔗

As the FAO notes, quality has become one of the dominant factors for consideration in the rice industry, and the first step toward achieving quality rice is grading. Setting up modern post-harvest facilities alone cannot solve the quality problem completely – grading must be consistent and systematic. Grading conducted at regular intervals across all stages of post-harvest operations ensures uniform quality monitoring and forms a reliable basis for comparison between grain quality before and after storage.

National and international rice trade standards specify the permissible broken content for each rice grade. Importers and bulk buyers routinely test shipments using standardized laboratory methods – including the use of indented plates, mechanical graders, and sieve sets – to verify that the declared grade matches the actual product. A mill that invests in accurate grading equipment not only produces better rice, it builds the credibility to access premium markets.

What do you think? Given that broken rice fractions each serve distinct industrial and consumer markets, should rice mills be designed from the outset with specific grading configurations to target those markets – or is a standardized grading setup sufficient for most operations? And as consumer preferences for specialty rice products grow globally, how might grading technology need to evolve to meet increasingly precise quality specifications?

References

1. https://www.fao.org/4/x5048e/x5048e02.htm
2. http://www.knowledgebank.irri.org/training/fact-sheets/postharvest-management/item/modern-rice-milling-fact-sheet
3. http://www.knowledgebank.irri.org/step-by-step-production/postharvest/milling
4. https://www.buhlergroup.com/content/buhlergroup/global/en/products/supersift_plansifter.html
5. https://www.chinagrainmachine.com/info/rice-grading-plansifter-103071495.html
6. https://seedburo.com/products/3444
7. https://www.wintone-machinery.com/machines/length-grader.html