Commercial Scale Milling Processes for Pulses

India processes an estimated 75% of its domestically produced pulses into dal, making pulse milling the third largest food processing industry in the country after rice and flour milling. Behind every packet of clean, split lentils or pigeon pea dal on a grocery shelf lies a carefully orchestrated series of mechanical operations – cleaning, drying, pitting, husk loosening, dehusking, splitting, and polishing. Commercial-scale milling performs all of these steps using purpose-built machines, in a tightly controlled sequence designed to maximize dal yield and quality. Understanding how this process works helps explain why commercially milled dal looks, cooks, and tastes the way it does.

What commercial-scale milling actually means 🔗

Commercial-scale milling involves processing large quantities of pulses in plants with higher capacities compared to household or cottage-level operations. While home-scale processing relies on a mortar-pestle and sun-drying, and yields only 50-60% dal due to breakage, commercial mills are engineered for consistency and efficiency. The yield of dal from commercial mills typically ranges from 68 to 75%, with controlled conditions allowing higher throughput and more uniform product quality across large batches.

The basic unit operations in any commercial dal mill – cleaning, drying, loosening the husk, dehusking, splitting, and polishing – remain consistent. Although the sequence of pre-milling treatment, conditioning, dehusking, and splitting is broadly common, the specific steps and machines vary from mill to mill and region to region. What distinguishes commercial milling is its use of motorized, high-throughput machinery for each of these steps rather than manual labour.

Step 1: Cleaning and grading 🔗

Raw pulses arriving at a mill carry a range of impurities – dust, chaff, weed seeds, broken grains, and foreign particles. In general, raw pulses may contain 2 to 5 per cent impurities such as foreign materials and some insect-infested grains. Removing these before processing is essential, since impurities affect machine performance and end-product quality.

Cleaning and grading of pulses from dust, chaff, dirt, and grits is performed using a reel-type or rotary screen-type cleaner. Two machines are commonly employed at this stage:

Reciprocating air-screen cleaners 🔗

These machines combine two mechanisms – screening and air aspiration – to clean grain efficiently. Air screen cleaners separate grains from weed seeds, chaff, and foreign materials using screens with different sized openings along with controlled airflow. The reciprocating (back-and-forth) motion of the screen deck keeps grain moving continuously across multiple sieve layers, with a top screen removing oversized material and a bottom screen separating smaller particles. An air blast simultaneously blows away lighter impurities like dust and husk fragments. By removing large and small impurities and dust at this stage, processors prolong storage life and protect downstream equipment from wear.

Reel screen cleaners 🔗

Also known as rotary screen cleaners, these use a rotating cylindrical drum fitted with perforated screens. Grain fed into the drum tumbles as the cylinder rotates; particles smaller than the screen openings fall through, while oversized material is discharged from the end. Reel screen cleaners are particularly effective for scalping – the removal of large impurities in an initial rough cleaning pass – and are often used in combination with reciprocating cleaners for thorough pre-processing cleaning.

Step 2: Drying 🔗

Once cleaned, pulses are dried to bring their moisture content to an optimal level for husk loosening and dehusking. Commercial facilities use controlled drying systems to reduce moisture to 8-10%, creating ideal conditions for husk removal. Too much moisture makes husks stick tenaciously, while too little causes excessive breakage. In traditional milling, sun-drying is used for this purpose, which makes the process weather-dependent and slow. Commercial mills use mechanical driers that maintain consistent temperature and airflow regardless of season, enabling year-round production at scale.

Step 3: Pitting – preparing the husk for loosening 🔗

Before any chemical or moisture treatment is applied, the hard outer seed coat of pulses must be physically abraded. This step is called pitting. Cleaned and graded pulses are passed through a roller dehusker in which scratches, cracks, and dents are created on the hard seed coat – a process known as pitting of pulses.

An emery roller machine is used for this purpose. The clearance between the emery roller and its cage gradually narrows from the inlet to the outlet, and cracking and scratching of the husk takes place mainly by friction as the material passes through this narrowing gap. The physical damage to the seed coat is essential – it creates entry points for oil and water in the next step, which would otherwise struggle to penetrate a smooth, intact husk.

Step 4: Husk loosening – oil smearing and water treatment 🔗

Loosening the husk from the cotyledon is the most critical pre-dehusking step, and commercial mills use a two-phase approach: oil application followed by water treatment.

Oil smearing 🔗

Pitted pulse grains are passed through a sieve cleaner to separate splits, husk, and powder, then smeared with oil – generally 100 to 500 grams per quintal of pulses – either manually or using an auger mixer, and stored for 1 to 5 days. During this tempering period, the oil diffuses between the husk and cotyledon, weakening the bond and facilitating loosening of the adhering husk. A worm mixer (auger mixer) is the machine commonly used in commercial mills for uniform oil application across large quantities. Commercial pulse mills use oil smearing or thermal treatment processes to loosen the seed coat – it is one of the defining techniques that distinguishes commercial-scale processing from home methods.

Water treatment 🔗

After oil tempering, a measured amount of water is applied to further loosen the husk. Water treatment involves approximately 2.5 to 3.5 kg of water per 100 kg of pulses, applied and stored overnight, which helps in further loosening of the husk. The pulses are then subjected to drying and cooling before dehusking. The brief moisture exposure causes the seed coat to swell slightly, breaking the remaining adhesion between husk and cotyledon without waterlogging the grain. This controlled combination of pitting, oil, and water treatment is what makes commercial milling more effective than cottage-level processing, where husk loosening is often incomplete.

Step 5: Dehusking using roller dehuskers 🔗

With the husk properly loosened, the pulses are ready for mechanical dehusking. Roller dehuskers coated with carborundum are used to dehusk pulses. Two types of rollers – cylindrical and tapered – are available for dehusking. The abrasive surface of the carborundum roller peels away the loosened husk through friction as the grain passes through the gap between the roller and its housing.

In a single pass or operation, about 50% of pulses are dehusked. Dehusked pulses are split into two parts, and the dehusked splits are separated by sieving while the husk is aspirated off. Unsplit dehusked pulses and tail pulses are again dehusked and milled. The entire process is repeated two or three times until the running pulses are fully dehusked and split.

Some pulse varieties are particularly stubborn. Red gram is the most difficult pulse to dehusk and split, with only 40 to 50 per cent of grains dehusked in the first pass of preconditioning and milling. For difficult-to-mill pulses like arhar, urad, and moong, rewetting and drying are done to loosen portions of husk that remain stuck after repeated rolling operations.

Step 6: Splitting 🔗

Once the husk is removed, the dehusked whole grain – called gota – needs to be split into two cotyledons. Most pulses naturally split during the dehusking process itself. However, a portion of gota remains unsplit and requires a dedicated splitting step. Splitting of gota is carried out by treating the grain with water in a ratio of 1:2 to 1:3, followed by tempering for 12 hours and splitting in a disk sheller. The brief re-moistening softens the natural seam between the two cotyledons, allowing the disk sheller to cleave them apart cleanly with minimal breakage.

At this stage, blowers are used for aspiration of husk and powder from the products of the disc sheller or roller machine. Split dals are separated from unhusked and husked whole pulses with the help of sieve-type separators. Sieves are also employed for grading the dal by size before it moves to the final polishing step.

Step 7: Polishing 🔗

The final stage gives dal its characteristic smooth, bright appearance. Polishing is completed by treating dehusked and split pulses with a small quantity of oil and/or water. Linseed oil is used to impart shine or better appeal to the milled dal. The oil is applied in controlled quantities, and the dal is tumbled in a polishing drum that gently buffs each grain to a uniform finish.

Polishing is more than cosmetic. A well-polished surface makes the dal easier to clean before cooking, reduces surface dust, and improves shelf appearance – all of which matter for commercial grading and consumer acceptance. The polishing process requires careful balance – insufficient polishing leaves the dal looking dull and unappealing, while excessive polishing removes valuable nutrients and creates too much dust. Modern polishers use adjustable pressure systems and dust collection to optimize this final step.

The CFTRI method: a weather-independent alternative 🔗

A major limitation of the traditional commercial milling method described above is its dependence on sun-drying. As sun-drying is practised, the traditional method is not only weather-dependent but also requires a large drying yard to match milling capacity. As a result, it takes 3 to 7 days for complete processing of a batch of 20 to 30 tonnes of pulses.

To address this, the Central Food Technological Research Institute (CFTRI), Mysore developed a modern milling method that eliminates weather dependency entirely. The CFTRI method eliminates the use of edible oil in the milling process. Cleaned and graded pigeonpea grains are fed into a roller mill for scratching. The whole and split grains are soaked in water at ambient temperature for 25-30 minutes to produce a moisture content of about 35%, then dried to 10% moisture content. The dried grains are then milled in a cylindrical abrasive mill to produce dehusked split dal, which is separated using an air-screen grain cleaner. The average recovery for pigeonpea by this method is approximately 75%, achieved in a far shorter processing time than traditional approaches.

Milling losses and yield efficiency 🔗

Despite the sophistication of commercial milling, losses remain a significant concern. The average potential yields of common dals vary from 85 to 89%, yet yield in commercial mills typically ranges from 68 to 75%. These milling losses can be attributed to small brokens and fine powders generated during scoring and simultaneous dehusking and splitting operations.

Decortication is seldom complete in a single pass, and each pass produces 1.5 to 2% fines, which progressively reduces the recovery of dal. This gap between potential yield and actual yield is one of the key drivers pushing the industry toward improved pre-milling treatments, better roller calibration, and mechanized alternatives like the CFTRI method that can reduce losses while maintaining throughput.

What do you think? Given that commercial milling still falls 10-15% short of the potential dal yield, which aspect of the process – pre-milling treatment, dehusking machinery, or splitting technology – do you think offers the most room for improvement? And with the CFTRI method eliminating oil and reducing processing time, what barriers might prevent smaller dal mills from adopting such modernized techniques?

References

1. https://www.studocu.com/in/document/parul-university/food-technology-ii/pulse-milling/43718356
2. https://www.nextechagrisolutions.com/blog/tag/pulses-milling-process/
3. https://agritech.tnau.ac.in/postharvest/pht_pulses_processing_commercial%20milling.html
4. https://indiaagronet.com/indiaagronet/post_harvest/pulses.htm
5. https://www.flamanagriculture.com/grain-cleaner-c122
6. https://www.westrup.com/pre-cleaners
7. http://ecoursesonline.iasri.res.in/pluginfile.php/154231/mod_resource/content/1/Lesson_15.htm
8. https://pub.isprd.in/index.php/jfl/article/view/2154
9. https://cftri.res.in/kweb/technologies/FM/vdm.pdf