Different Methods of Milling Pulses: Wet and Dry Techniques

Every time you cook a bowl of dal, the grains you see have gone through a carefully controlled milling process before reaching your kitchen. Raw pulses – whether arhar (pigeon pea), urad (black gram), or moong (green gram) – come with a tough outer husk tightly bound to the cotyledon inside. Removing that husk cleanly, without breaking the grain, is what pulse milling is all about. According to food processing experts, the entire dal milling operation revolves around three core steps: loosening the husk, dehusking the grain, and splitting it into two equal halves. The two primary techniques used to accomplish this are the wet method and the dry method – and the choice between them has a direct impact on dal quality, recovery rate, and processing time.

What is pulse milling? 🔗

Pulse milling, or dal milling, is defined as the removal of the outer seed coat (husk) and the splitting of the grain into two equal cotyledon halves. This is not purely cosmetic. Removing the husk improves digestibility, reduces cooking time, and makes the grain far more palatable. The challenge lies in the fact that the husk in many pulses is tightly bonded to the cotyledon – particularly in varieties like pigeonpea, black gram, and green gram. Because of this tight bonding, the method of alternate wetting and drying is widely used to facilitate dehusking and splitting. Without proper conditioning, excessive abrasive force is needed to strip the husk, which results in high losses in the form of brokens and powder.

In traditional mills in India, the yield of split and dehusked pulses is only 65 to 75% due to losses, compared to an 88 to 89% potential yield achievable with better conditioning methods. This gap underscores why the selection and execution of the milling method matters so much commercially.

The wet method of pulse milling 🔗

The wet method relies on water and clay-based treatment to loosen the husk before mechanical dehusking. It is particularly well-suited to pulses that are relatively easier to dehusk, as well as to varieties like arhar, urad, and moong that benefit from moisture-based conditioning.

Step 1: Cleaning 🔗

The first step in any milling method is thorough cleaning. Wet method processing begins with cleaning to remove dust, dirt, chaff, stone pieces, immature grains, and other seeds from the pulse lot. This is done using reel-type or reciprocating air-screen cleaners that use different sieve sizes and air currents to separate contaminants. Clean, uniform grain is essential for consistent milling results downstream.

Step 2: Soaking in water 🔗

Easier-to-dehusk pulses are soaked in water for 2 to 8 hours, while more difficult-to-dehusk types such as pigeonpea, black gram, and green gram are treated with red earth. During soaking, water penetrates the seed coat and begins to weaken the bond between the husk and the cotyledon. The grain swells as it absorbs moisture, creating internal pressure that further loosens the husk and prepares it for removal.

Step 3: Red earth treatment 🔗

Red earth treatment is a distinctive feature of the wet method. Cleaned and graded pulses are soaked in water for 4 to 12 hours, then mixed with red earth for 12 to 16 hours, and sun-dried to bring moisture content down to about 10 to 12%. Red earth is not merely a filler material – it imparts a good yellow colour to the end product and helps remove small patches of adhering husk due to its mild abrasive quality. This dual role – colour enhancement and gentle abrasion – makes it a preferred conditioning agent in wet milling, especially for toor dal.

After mixing with 3 to 5% red earth and heaping for 12 hours, the pulses are sun-dried and tempered for 2 to 4 days, and red earth is subsequently separated by sieving. This extended conditioning results in 95 to 98% dehusking efficiency when the pulses are finally run through the milling machine.

Step 4: Drying 🔗

After red earth treatment, controlled drying is essential. Pulses are finally dried to about 10 to 12% moisture content prior to dehusking and splitting. This moisture level ensures the husk is brittle enough to crack cleanly while the cotyledon retains enough toughness to avoid crumbling under the abrasive rollers.

Step 5: Dehusking and splitting 🔗

Carborundum-coated emery rollers are used for dehusking, and in a single pass about 50% of pulses are dehusked. Dehusked pulses are then split into two parts, separated by sieving, and the husk is aspirated off. Unsplit and unhusked pulses are run through the process again. This recycling of partially processed grain is standard practice and ensures high overall yield.

A key advantage of the wet method is that soaking and drying result in less grain breakage and facilitate clean splitting of cotyledons. However, the method is weather-dependent and labour-intensive, and dal produced this way tends to take slightly longer to cook.

The dry method of pulse milling 🔗

The dry method avoids prolonged soaking and instead uses mechanical pitting, edible oil, and a controlled water application to loosen the husk. Dry milling is more popular and widely used in commercial mills, primarily because it is less dependent on weather and offers more consistent processing conditions across seasons.

Step 1: Cleaning and grading 🔗

As with the wet method, the process starts with cleaning. Cleaning of pulses removes dust, chaff, dirt, and grits, while grading segregates the grain legumes by desired shape and size. Uniform grain size is especially important in dry milling because the roller gap in subsequent steps needs to be calibrated accurately.

Step 2: Pitting 🔗

This is the step that distinguishes the dry method from all others. Clean 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. The emery roller machine has a housing (cage) with a gradually narrowing clearance from inlet to outlet. Cracking and scratching of the husk takes place mainly by friction between pulses and the emery surface as the material passes through the narrowing clearance. Pitting opens up the seed coat surface so that oil applied in the next step can penetrate effectively.

Step 3: Oil smearing 🔗

Once pitted, the grains are coated with edible oil. The pitted pulse grains are smeared with 100 to 500 grams of oil per quintal of pulses, either manually or with an auger mixer, and stored for 1 to 5 days. Linseed oil is commonly used. During this tempering period, the oil diffuses between the husk and cotyledons, weakens the bond, and facilitates loosening of the adhering husk. The oil essentially acts as a chemical wedge, working its way into the micro-cracks created during pitting.

Step 4: Water treatment 🔗

After the oil has had time to work, water is applied to the grain. Water treatment of 2.5 to 3.5 kg water per 100 kg of pulses, applied overnight, helps in further loosening of the husk. At the end of the storage period, water is applied to the grains and stored for a further 12 to 14 hours, then sun-dried for 1 to 3 days before milling. This combination of oil and water conditioning is what gives the dry method its effectiveness without relying on prolonged soaking.

Step 5: Drying and cooling 🔗

After water conditioning, pulses are conditioned by alternate soaking, drying, and tempering, with moisture content brought to 10 to 12% before dehusking and splitting. Sun drying remains the most economical option, though heated air dryers operating at 60 to 120°C are also used in larger commercial mills. The complete dry milling cycle – from cleaning to dehusked splits – typically takes 3 to 4 days.

Step 6: Dehusking, splitting, and polishing 🔗

The conditioned and dried grains are fed into emery roller dehuskers. The dry milling process results in approximately 75% dal recovery. After splitting, a final polishing step is carried out. Polishing is completed by treating dehusked and split pulses with a small quantity of oil and/or water to impart a glazed surface appearance that improves consumer acceptance and market value.

Wet vs. dry method: how to choose 🔗

Both methods share the same three-stage goal – husk loosening, dehusking, and splitting – but differ significantly in their approach, time requirements, and output characteristics. Here is a direct comparison of key factors:

Suitability by pulse type: The wet method works best for arhar, urad, and moong. The dry method is more versatile and is used across a wider range of pulses in commercial settings, including chana and masoor.

Processing time: The wet method with red earth treatment requires about 5 to 7 days for processing a batch, while the dry method takes 3 to 4 days.

Weather dependency: The wet method has the disadvantage of being weather-dependent and labour-intensive, making it unsuitable for humid climates or monsoon seasons. The dry method offers more consistent year-round operation.

Dal quality and cooking: Dal produced by the wet method cooks better but takes longer to cook. The colour and appearance are also enhanced by the red earth treatment.

Commercial preference: For large-scale commercial mills, the dry method with oil and water treatment is more commonly adopted due to its throughput consistency and lower weather risk. For household milling, water treatment alone is more popular.

The role of polishing in final dal quality 🔗

Regardless of the milling method used, polishing is a standard finishing step that affects market value. Linseed oil is used to impart shine or better appeal to the milled dal. In some mills, soapstone powder or selkhari powder is applied to improve surface glaze. While polishing is primarily cosmetic, it does influence consumer perception and therefore pricing in retail markets.

Common losses in pulse milling and how to minimize them 🔗

Milling losses are a persistent challenge in the industry. Each pass through the dehusker produces around 1.5 to 2% fines, reducing overall recovery. These losses stem from excessive abrasion during dehusking – often a result of insufficient husk conditioning. Proper pre-treatment (whether wet or dry), accurate moisture management, and correct roller gap settings are the primary controls a miller can use to reduce breakage and maximise split dal recovery.

Another factor is the number of passes required. For complete dehusking and splitting, the whole process may need to be repeated two to three times. Each additional pass adds to processing time but also to cumulative grain losses, making the initial conditioning quality especially critical.

What do you think? Given that the wet method produces better cooking quality but takes longer and depends on weather, while the dry method is faster and more commercially scalable – which factors do you think should matter most when a small-scale pulse processor is choosing between these two methods? And with increasing demand for minimally processed, additive-free foods, do you think the use of red earth in the wet method could become a point of consumer concern in the future?

References

1. https://www.aesmofoodmachine.com/pulse-process/
2. https://www.aesmofoodmachine.com/pulse-processing-plant/
3. https://course.cutm.ac.in/wp-content/uploads/2020/06/LEC-56-pulse-milling.pdf
4. https://kcpelearning.com/showcourse/pulse/intro-to-pulse-milling
5. https://www.studocu.com/in/document/parul-university/food-technology-ii/pulse-milling/43718356
6. https://indiaagronet.com/indiaagronet/post_harvest/pulses.htm
7. https://course.cutm.ac.in/wp-content/uploads/2020/06/Session-4.4-%E2%80%93-Pulse-Milling.pdf