Physical and Mechanical Properties of Pulses

Every time a handful of dal is tipped into a pot, it might seem like a straightforward act – but behind that simple step lies a set of precise physical and mechanical characteristics that determine whether the grain mills cleanly, splits evenly, cooks in time, and stores without spoilage. For food technologists and grain engineers, understanding the physical and mechanical properties of pulses like gram, urad, arhar, and pea is not academic exercise – it is the foundation for designing efficient processing systems, reducing post-harvest losses, and delivering consistent quality dal to the consumer.

Why physical and mechanical properties matter in pulse processing 🔗

Pulses such as gram (chickpea), urad (black gram), arhar (pigeon pea), and pea are among the most important protein-rich food crops in India, together accounting for a major share of national pulse production. Yet processing them into clean, split dal is far from uniform across varieties. The key reason is that each pulse comes with its own set of dimensional attributes, shape characteristics, and composition ratios – all of which interact with milling machinery in distinct ways.

According to researchers studying pulse milling, dal processing of legumes is primarily influenced by grain size, husk content, the adherence of husk to the seed, and cotyledon texture. When any one of these properties varies – as it does significantly across gram, urad, arhar, and pea – the equipment settings, pre-treatment duration, and expected recovery rates all need to be adjusted accordingly. This is why a precise understanding of these properties is essential before any pulse enters a commercial dal mill.

Key physical properties: dimensions and shape 🔗

Linear dimensions 🔗

The three principal dimensions of a pulse grain – length (L), width (W), and thickness (T) – are the starting point for characterizing any grain. These are measured in millimeters and vary considerably between pulse species and even between varieties of the same species. For instance, pea seed has an average length of about 7.80 mm, width of 6.41 mm, and thickness of 5.55 mm at a moisture content of around 10% dry basis. Arhar (pigeon pea) grains tend to be smaller and more elongated, while gram grains are larger and rounder in comparison to urad.

These dimensions are directly relevant to the design of sieves, screen separators, and size-grading equipment in dal mills. Grains that are longer or thicker require wider sieve apertures and different roller gap settings. Getting these dimensions wrong during machinery setup can cause excessive breakage or incomplete dehusking – both costly outcomes in commercial milling.

Geometric mean diameter and sphericity 🔗

While raw dimensions describe size, sphericity describes how closely a grain resembles a perfect sphere. It is calculated from the three principal dimensions and expressed as a ratio between 0 and 1 (or as a percentage). A sphericity of 1.0 indicates a perfect sphere. Pulses with higher sphericity roll more freely, flow through machinery more uniformly, and are generally easier to grade and clean.

Gram, being one of the rounder pulses, exhibits a sphericity range of approximately 0.7678 to 0.8122 – a relatively high value that reflects its compact, near-spherical form. Urad grains, being slightly flatter and more irregular in shape, tend to have lower sphericity values. Research on black gram reports an average sphericity of around 66%, while masoor dal achieves values close to 98.69% – illustrating how dramatically shape can differ between pulse species. For pea, sphericity values range from 0.836 to 0.851 across varying moisture levels, making pea among the more spherical common pulses.

Sphericity also influences how grains respond to abrasive milling rollers. More spherical grains tend to rotate freely on the roller surface, resulting in more even abrasion around the circumference and more uniform husk removal. Less spherical grains may press against rollers unevenly, leading to patchy dehusking or breakage along flat edges.

Composition properties: cotyledon and husk content 🔗

Cotyledon content 🔗

The cotyledon is the edible inner portion of the pulse grain – the protein- and starch-rich fraction that ultimately becomes dal. Cotyledon content is expressed as a percentage of total grain weight. Higher cotyledon content means more edible yield from the same weight of raw pulse – a key economic indicator for millers.

Gram stands out with a cotyledon content of approximately 87.21% to 88.71%, which is notably high among commonly processed pulses. This means that for every 100 kg of raw gram, nearly 87-89 kg of usable cotyledon is potentially available – though actual dal recovery will be lower once milling losses are accounted for. This high cotyledon-to-total-grain ratio partially explains why kabuli chickpea varieties deliver better dal recovery (70-71%) compared to desi types (66-66.38%) due to differences in seed coat thickness.

In contrast, arhar (pigeon pea) has a higher proportion of husk relative to its total grain weight, which directly reduces milling recovery. The average husk content in arhar is around 15%, leaving an endosperm (cotyledon) content of approximately 85%. Gram’s husk content by comparison is significantly lower, at around 10-12%, which is one reason it is among the easier pulses to process.

Husk content and its implications 🔗

Husk (seed coat) content directly governs how difficult a pulse is to dehusk and how much of the milling effort is spent on coat removal versus useful grain processing. A thicker, more tightly adhering husk demands more abrasive passes, more pre-treatment time, and yields more husk waste.

For hard-to-dehusk pulses such as arhar, moong, and urad, dal recovery during milling is typically 70-75%, while for easier-to-dehusk pulses like bengal gram, lentil, and peas, recovery ranges from 78-85%. This difference is directly traceable to the physical husk properties – its thickness, toughness, and the strength of the adhesive gum sheet that bonds it to the cotyledon surface.

Research confirms that the seed coat-to-cotyledon adherence is tight in desi chickpea but loose in kabuli types – and this structural difference explains why desi varieties require more intensive milling to achieve the same dal quality. Understanding husk content as a physical property is therefore inseparable from making milling decisions: it informs choices about the number of roller passes, oil application rates, and soaking duration.

Mechanical properties and their processing significance 🔗

Bulk density and true density 🔗

Bulk density refers to the mass of grains per unit volume of bulk material, including the air spaces between grains. It is a critical parameter for designing storage bins, hoppers, and pneumatic conveying systems. True density measures the density of the grain material itself, excluding inter-grain voids. Together, these two values determine porosity – the percentage of empty space within a grain bulk.

Gram, being a denser, compact grain, generally exhibits higher bulk density compared to arhar, which is lighter and more irregular in surface texture. For pea, bulk density decreases from about 712 to 647.5 kg/m³ as moisture content rises from 10% to 35% dry basis, while true density similarly falls from 1160.5 to 1085.0 kg/m³. This moisture-dependence of density is important because pulses received at mills often carry higher ambient moisture, which affects how tightly they pack in storage and how they respond to mechanical forces.

Coefficient of friction 🔗

The coefficient of static friction determines how easily pulse grains slide over different surfaces – which directly influences the design of chutes, inclined conveyors, and roller machines. Higher friction means more resistance to flow; lower friction means grains move too fast and may not receive adequate processing time in contact with abrasive surfaces.

Pea seeds show progressively increasing coefficients of static friction against rubber (0.388-0.413), galvanised iron (0.360-0.409), aluminium (0.292-0.351), and stainless steel (0.270-0.311) as moisture content increases. For pulse processing equipment, this means stainless steel surfaces allow smoother grain flow, while rubber-lined surfaces generate more grip – useful for controlling grain movement in dehusking chambers.

Hardness and rupture force 🔗

Mechanical hardness – the resistance of a grain to compression and deformation – is a property that directly governs the force settings on milling rollers and the energy required for splitting. Harder grains require greater force for dehusking and splitting, but excessive force risks cracking the cotyledon, increasing breakage and reducing the proportion of whole dal in the final product.

Hardness in pulses is strongly moisture-dependent: as moisture content rises, grains become progressively less brittle and more resistant to cracking under impact. Research shows that bulk density, true density, and hardness in pulse grains all decrease with increasing moisture content. This explains why both wet and dry pre-treatment methods – which precisely manipulate grain moisture – are central to achieving the right balance between husk brittleness and cotyledon toughness during milling.

How these properties shape pulse processing decisions 🔗

Impact on dehusking and milling 🔗

All the physical and mechanical properties discussed above converge at the milling stage. Pulse milling is accomplished in three major steps: loosening of the husk, dehusking, and splitting. The success of each step depends on a correct read of the grain’s properties. Conditioned pulse grains are subjected to mild abrasion inside roller machines, which removes 10-25% of husk in a single pass; the number of passes required depends directly on husk adherence and grain hardness.

For difficult-to-process pulses like arhar and urad – which have tighter husk adhesion and lower sphericity – the milling process requires more pre-treatment cycles, often involving oil application (150-250 g per 100 kg) and extended tempering periods of one to three days. Gram, by contrast, with its higher sphericity and thinner husk, generally moves through the milling process with fewer passes and higher recovery rates.

Impact on cooking quality and storage 🔗

Physical properties also influence cooking behavior. Various processing techniques including drying, milling, soaking, and cooking are applied to peas and other pulses to improve their functional and physicochemical properties. Cotyledon content affects water absorption during cooking – denser cotyledons take longer to hydrate and soften, which is why dal from high-cotyledon varieties like gram tends to cook more uniformly once the husk is fully removed.

For storage, bulk density and porosity determine air circulation and moisture movement within a grain bulk. The true density of pea grains is significantly higher than water, which is an important property for food grain handling and storage structure design. Higher-porosity grain bulks are more susceptible to moisture uptake and insect infestation during storage, which makes measuring and controlling these properties essential for post-harvest management.

Property variation and its practical implications 🔗

One aspect worth emphasizing is that physical properties are not fixed – they vary with moisture content, variety, and even growing conditions. Sphericity, geometric mean diameter, angle of repose, and coefficient of static friction all increase linearly with rising moisture content, while bulk density and hardness decrease. This means grain properties at harvest may differ significantly from those at the mill – and machinery must be calibrated accordingly.

Varietal differences add another layer of complexity. Dal recovery is positively correlated with 100-seed weight and grain volume, but negatively correlated with true density – meaning that heavier, bulkier grains with lower density tend to mill better. This insight has direct implications for plant breeders and processors: varieties bred purely for agronomic yield, without attention to these physical traits, may perform poorly in the dal mill, resulting in economic losses throughout the supply chain.

Understanding these properties also informs equipment upgrades. A milling line designed for gram cannot simply be repurposed for arhar without adjusting roller gaps, changing abrasive surfaces, and modifying tempering time – because the two grains differ significantly in sphericity, husk adherence, and mechanical hardness. Getting this right is what separates efficient, low-loss milling operations from those burdened with high breakage rates and poor dal recovery.

What do you think? Given that physical properties like sphericity and cotyledon content vary between pulse varieties – and even between different lots of the same variety – how should dal millers account for this variability in real-time processing decisions? And with plant breeders historically focusing on yield over milling traits, do you think physical and mechanical property benchmarks should be made mandatory selection criteria for new pulse variety releases?

References

1. https://vajiramandravi.com/upsc-daily-current-affairs/mains-articles/pulses-production/
2. https://bbrc.in/wp-content/uploads/2018/03/BBRC18_017.pdf
3. https://www.sciencedirect.com/science/article/abs/pii/S0260877406002305
4. https://link.springer.com/article/10.1007/s11694-015-9242-7
5. https://course.cutm.ac.in/wp-content/uploads/2020/06/LEC-56-pulse-milling.pdf
6. https://www.aesmofoodmachine.com/pulse-process/
7. https://environmentandecology.com/wp-content/uploads/2025/03/MS13-Moisture-dependent-Some-Physical-Properties.pdf
8. https://www.mdpi.com/2304-8158/12/13/2527