Detailed Milling Processes for Specific Pulses

When you cook arhar dal at home, the clean golden-yellow splits you see bear little resemblance to the hard, husk-covered whole grain that enters a mill. The transformation happens through a carefully sequenced milling process – and that process looks quite different depending on which pulse you are processing. Arhar (pigeon pea) and Bengal gram (chickpea) are two of India’s most widely consumed pulses, yet their milling journeys under the dry method diverge significantly, driven by one key factor: how tightly the husk grips the cotyledon inside.

Why milling is not one-size-fits-all 🔗

Pulse milling – commonly called dal milling – is accomplished in three fundamental stages: loosening the husk, dehusking, and splitting. The challenge is that the husk is not equally stubborn across all pulses. The presence of gummy substances between the seed coat and cotyledons is the primary reason some pulses are harder to mill than others, and the quantity and quality of these gums determines whether a pulse is classified as “easy” or “difficult” to process. Pigeon pea, black gram, and green gram fall in the difficult category, while Bengal gram, lentil, and field peas are considered comparatively easy to dehusk.

This difference is directly reflected in dal recovery figures. For hard-to-dehusk pulses like arhar, recovery typically ranges between 70-75%, whereas easy-to-dehusk pulses like Bengal gram yield between 78-85%. Understanding why these numbers differ requires a step-by-step look at how each pulse is actually milled.

Dry milling of arhar (pigeon pea) 🔗

The dry method is the most widely used commercial approach for arhar milling in India. It is more popular than wet milling because, as research has established, the quality of splits obtained from wet milling of pigeon pea tends to be inferior. The dry process typically takes three to four days to complete and involves the following sequence:

Step 1: Cleaning and grading 🔗

Raw pulses received at the mill carry dust, chaff, broken grains, stones, and other foreign matter. Cleaning uses reciprocating air-screen cleaners or reel screen cleaners, which separate out lighter materials through air flow and size-graded screens. Grading ensures uniformity in grain size, which matters because uneven grains respond differently to the abrasion in later stages.

Step 2: Pitting 🔗

Cleaned grains are fed into an emery roller machine, where the narrow, progressively closing gap between the emery roller and its housing applies friction to the grain surface. The husk is cracked and scratched during this step – a process called pitting – to facilitate oil penetration during the next stage. Some grains dehusk and split during this pass; these are separated by sieving.

Step 3: Oil smearing 🔗

This is the step that sets arhar milling apart from simpler pulses. After pitting, the grains are smeared with oil – generally 100 to 500 grams per quintal – using an auger mixer, and then stored for 1 to 5 days. The oil seeps into the cracks made during pitting and gradually weakens the bond between the husk and cotyledon. Generally 150 to 250 grams of oil per 100 kg of pulse is applied, and the oil diffuses between husk and cotyledon, facilitating loosening. Mustard oil and linseed oil are commonly used. Research using response surface methodology has shown that an optimal dehulling efficiency of around 83% is achievable at approximately 10% moisture content and 0.3% mustard oil treatment for pigeon pea.

Step 4: Water treatment and tempering 🔗

After oil conditioning, water is applied at the rate of 2.5 to 3.5 kg per 100 kg of pulse and stored overnight to further loosen the husk. Following sun drying for a period, 3-5 percent moisture is added and the grain is tempered for about eight hours, then dried again. This cycle of alternate wetting and drying continues for two to four days until the pulses are sufficiently conditioned.

Step 5: Sun drying 🔗

Grains are spread in a thin layer on a pucca floor and stirred frequently with a rake or by foot for even drying. This operation makes the dal milling process quite lengthy, often requiring 2-3 days of sun drying. The final moisture content is brought down to about 10-12% before the grains are sent to the dehusking machine.

Step 6: Dehusking and splitting – multiple passes required 🔗

This is where arhar’s difficult nature becomes most evident. Each pass through the emery roller removes only 10-25% of the husk. Dals like arhar require repeated operations by dehusking rollers; rewetting and drying is done to loosen portions of husk that continue to stick after each pass. About 50% of pulses are dehusked in a single pass; the tail pulses and unsplit dehusked grains are re-conditioned and milled again, with the whole process repeated two to three times. Each pass produces 1.5 to 2% fines, which reduces overall dal recovery.

After each milling pass, air-screen cleaners separate husk, brokens, splits (dal), dehusked whole grains (gota), and unhusked whole grains. Only the dal is removed; the rest cycles back. For splitting the gota, water is applied at 1-5 kg per quintal and stored for 2-12 hours, then sun-dried for 4-8 hours before the grain is run through splitting machines such as the under-run disc sheller or impact machine (locally called phatphatia).

Step 7: Polishing 🔗

To finish, 2 to 3 kg of castor oil per quintal of arhar dal is mixed in to give the product a glossy appearance. This polishing step is done in a buffing or cone polisher machine and is as much about marketability as it is about processing.

Dry milling of Bengal gram (chickpea) 🔗

Bengal gram has a comparatively loosely attached husk and is classified as an easy-to-dehusk pulse. It is comparatively easy to dehusk and split Bengal gram, lentils, and peas, as their husks are loosely attached to the cotyledons. This translates to a simpler, faster process with less reliance on intensive oil treatment.

Step 1: Cleaning and grading 🔗

The process begins identically to arhar – thorough cleaning to remove impurities, followed by grading for size uniformity. This step is critical in all pulse milling to ensure efficient and consistent downstream processing.

Step 2: Pitting 🔗

Bengal gram is similarly passed through an emery roller machine to create scratches and cracks on the seed coat. However, because its husk is not as tightly bonded to the cotyledon, the pitting step alone begins the loosening process more effectively than in arhar.

Step 3: Water treatment – the primary conditioning agent 🔗

Unlike arhar, Bengal gram does not require the intensive multi-day oil treatment. For the dry method, 2.5 to 3.5 kg of water per 100 kg of pulses is applied and stored overnight to loosen the husk. Water is the primary conditioning agent here. The CFTRI method, commonly used for Bengal gram, uses dry heat preconditioning with two passes through an LSU dryer followed by six-hour tempering at each pass, after which almost all grains are dehusked in a single operation.

Step 4: Sun drying 🔗

After water treatment, grains are sun-dried to bring moisture down to the appropriate milling level. The drying duration is shorter compared to arhar because fewer conditioning cycles are needed. Mechanical dryers, where available, significantly reduce drying time.

Step 5: Dehusking and splitting – fewer passes needed 🔗

Because Bengal gram’s husk is more loosely attached, the preconditioning and milling of lentils and peas – which follow a similar protocol to Bengal gram – can be completed in about 3 to 5 days total, and conversion into dal is significantly easier than tur. The grain does not require the same number of repeated dehusking passes as arhar. Depending on the adherence of the husk, pulse grains are passed through the mill between two and eight times, but Bengal gram typically requires passes toward the lower end of this range.

After dehusking, the gota is conditioned with water and then split using an under-run disc sheller or roller mill. Husk is aspirated off using blowers, and dal splits are separated from whole and partially husked grains by sieving.

Step 6: Polishing 🔗

Dal is polished to improve its glazing appearance and market value, using materials like water, oil, soapstone powder, or selkhari powder applied to the dal surface. For Bengal gram, this step is relatively straightforward as the splits are uniform and require less corrective surface treatment.

Side-by-side comparison: arhar vs Bengal gram in dry milling 🔗

The table below summarises how the two processes differ:

What determines milling losses 🔗

Even in well-run mills, not all of the pulse becomes usable dal. Commercial dal mills achieve yields of 68-75%, while the potential yield of common dals can be as high as 85-89%. Milling losses come primarily from small brokens and fine powders generated during scoring and simultaneous dehusking and splitting operations. Each additional milling pass for arhar adds to these losses. This is why optimising the pre-milling treatment – specifically the level of oil, moisture, and conditioning time – directly improves profitability. Studies on pigeon pea varieties have shown that mustard oil treatment yields maximum hulling efficiency, with one variety recording up to 79.4% hulling efficiency under optimal oil treatment conditions.

For Bengal gram, the lower number of passes and better recovery rates mean less powder generation and fewer brokens – which is partly why chana dal commands consistent quality grades in the market with lower processing overhead.

The role of machinery 🔗

Both pulses rely on carborundum emery rollers as the primary dehusking tool. Linseed oil is commonly applied to impart shine or better market appeal to the milled dal. Splitting is carried out using under-run disc shellers (URD), roller mills, or impact machines. For Bengal gram specifically, a hand-operated pulse dehusking machine with an emery-coated inverted metal cone has been developed for small-scale rural processors, capable of processing 50-60 kg of splits per hour and yielding 75-80% dal from pigeon pea and Bengal gram. At larger scales, tapered and cylindrical roller dehuskers with adjustable clearances handle both varieties, with the gap settings calibrated based on the hardness and size of the specific pulse being milled.

What do you think? Given that arhar requires far more intensive treatment and yields less dal than Bengal gram, should millers be compensated differently for processing harder pulses – and how might that affect the price of tur dal in the market? Also, as mechanical dryers can reduce the 2-3 day sun-drying period for arhar to just a few hours, do you think the widespread adoption of dryers in small dal mills would meaningfully change dal recovery rates and reduce post-harvest losses at scale?

References

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