Pulses are the dried, edible seeds of the Leguminosae (also known as Fabaceae) family of plants, commonly known as legumes. With high nutritional value, pulses offer several health benefits associated with their consumption. Particularly rich in proteins in their cotyledons, pulses also are low in fat and high in fiber while containing essential minerals and no cholesterol. Due to how slowly their carbohydrates are digested, they also help control obesity and diabetes. They’re also beneficial for the environment, as they don’t need any additional nitrogen-based fertilizers and actually introduce nitrogen into the soil they grow.
Pulse crops are grown worldwide, and though featuring different nutritional characteristics than other food crops, they can be milled in much the same way as cereal grains. As food, their seeds are either consumed whole or reduced to become split grains, dehulled split grains, and flour. Whether whole or reduced, these seeds can all be milled into flours, similarly to how wheat and other cereal crops are reduced, with dry seeds broken down into dehulled split grains and then ground into flour.
Benefits of Pulse Milling
Pulses are a good source of protein, fiber, and complex carbohydrates. Most pulses have a protein content of 22-24 percent, while cereal crops typically offer only 7-15 percent protein. They contain essential amino acids, leucine, and lysine complement other cereal grains nutritionally. Along with their nutrients, their favorable impact on the environment makes them an important crop for feeding the world’s population now and into the future.
The most common types of pulse grains milled for human consumption include:
- Chickpeas (Garbanzo Beans)
- Common beans
- Dry peas
- Mung beans
- Pigeon peas
- Urd beans
- Yellow split peas
Grinding pulses into flours allows them to be utilized along with cereal-based foods to increase their nutritional value. Pulse flour can even partially replace wheat flour in bread, cakes, cookies, noodles, pasta, and snack foods. There are also other types of pulses that are grown and consumed by both people and animals in various areas globally. Pulses like dry peas, fava beans, and lupins are often used for animal feed.
Why Mill Pulses When Dry?
There are numerous advantages to milling dry pulses rather than wet ones. As per cereals that are ground into flour, dried pulses can be more easily made into whole, dehulled, fiber-rich, protein-rich, and starch-rich flours.
Advantages of dry pulse milling include:
- Consuming similar amounts of energy when compared to other processes.
- Improving quality by removing gums between husk and cotyledons.
- Removing dirt and other debris improves appearance, digestibility, taste, and quality.
- Saving time by drying pulses via machine.
Pulses are the third most widely milled plant after wheat and rice, with the dry milling process largely preferred over wet milling.
Pulse Milling Techniques & Processes
Flour made from pulse milling is used worldwide, primarily in Southeast Asia. The challenge when milling pulses involves producing a consistently uniform flour in particle size. Pulses become viscous during the flour milling process, so the tendency of residue to stick to equipment is a factor that processers need to consider. Additionally, milling with the hull or seed coat intact affects its milling properties, including flow rate, particle size distribution, screening, and yield.
Four key techniques used in reducing particulate size include:
- Attrition milling: Relying on a horizontal vessel that rotates within, a solution utilized to reduce particle size, attrition mills form spherical particles that cause the material to flow freely. This category includes ball mills, which can reduce particles to less than a micron.
- Direct-pressure milling: This technique uses two hardened surfaces to crush or pinch material. It involves either one or two rotating bars, along with a stationary plate that reduces the material to 800-1000 microns. Roll mills and cracking mills fall into this category.
- Impact milling: This involves striking a wide area with a hard object to fracture particles, normally with blunt, hammerlike blades, as is the case with hammer and pin mills. Pulse milling via impact is often used to produce whole flours, employing a horizontal or vertical rotating shaft featuring hammer bars contained in a steel drum. Impact mills reduce particles until they can move through a metal screen.
- Knife milling: Using a sharp blade to apply shear force to large particles, these mills cut particles down to predetermined sizes. They also minimize the amount of fine material in the process, using a rotating assembly featuring sharp blades. Dicing and Guillotine mills are two examples capable of reducing large chunks of material to between 250-1200 microns.
The pulse milling process also involves dehulling, splitting, and reducing pulses to flour. Dehulling, also called decortication, involves removing seed coats and pulse hulls. Removing the hulls is especially important for lentils and peas, as their hulls contain lipids that block the absorption of nutrients, while dehulling beans or chickpeas is more difficult. Splitting loosens the cotyledons, the embryonic seed leaves of a pulse, which contains much of these seeds’ proteins.
Certain characteristics of pulse milling equipment affect the pulse milling process, such as:
- Blade (or hammer) profile: How blades or hammers are positioned determines the degree of reduction. The type, shape, and number of blades or hammers also reduce material, with knife-like blades producing moderate granulation and impact implements providing a more forceful reduction.
- Feed rate: Uniform feeding offers the most effective means for milling, with machines typically using 15-60 rotations per minute (rpm); the higher the feed rate, the more energy the mill utilizes.
- Feed throat: This is where material is introduced into the milling chamber via gravity feed to the mill’s rotating implements.
- Rotor speed: Affecting particle size distribution, the faster the speed of the rotor, the finer it grinds. Flat blades typically reach speeds between 3000-7200 rpm, so are used in finer grinding applications, whereas sharp blades at speeds of 1000-3000 rpm produce coarser flours.
- Screen: These are typically rectangular or round, with their open surface area affecting how the equipment pulverizes material. Screen hole diameters don’t indicate finished product particle size, as higher rotor speeds and the angle at which particles approach the screen influence particle size more.
Depending on the method, pulse milling results in different particle size ranges. A Canadian study identified pin mills producing the finest and most uniform particle sizes. Other milling methods had difficulty in milling cotyledons and hulls down to the same particle size, which caused differences in function of flours made via each method. Another example saw hammer mills produce flour that gelatinized at lower temperatures initially compared to other pulse milling methods.
Prater G-Series Hammer Mill & Mega Mill for Pulse Milling
Prater Industries makes several different pieces of equipment that can be utilized for milling pulses. These include various iterations of Prater’s G Series Full-Screen Hammer Mill and Mega Mill Hammer Mill. Prater’s G Series descend from Prater’s initial product line of hammer mills. This proven system maximizes the use of the entire screen area to enhance equipment efficiency and improve end-product quality. The Mega Mill design offers quieter and smoother operation with its bearing and shaft configuration providing more uniform grinding with minimal heat build-up.
Prater’s Full-Screen Hammermill
Featuring reversible rotors that support electronically-balanced top feeding operations, Prater’s full-screen hammer mill can be operated around the clock to provide uniform grinding operations while evenly wearing the mill’s hammers.
Other benefits and features of Prater’s G Series Hammer Mill include:
- Easy access to change screens and maintenance
- Hammer tip speeds of 14-21 thousand feet per minute
- Stainless steel construction
- Throughput capacity can be increased without increasing power requirements
- Wider rotors to accommodate various hammer configurations
The G Series full-screen hammer mills come in models with averages between 5-300 horsepower and with screens between 440-2700 square inches (2838.7-17419.32 square cm).
Prater’s Mega Mill
Capable of running efficiently with lower airflow than standard hammer mills, Prater’s Mega Mill reduces downtime and maintenance while also requiring minimal power.
Other benefits and features of Prater’s Mega Hammer Mill include:
- Designed for use with a pneumatic conveyor system to reduce dust
- Easily removable rotor and screens
- Large doors with hinges and unique rotor design make cleaning and maintenance easier
- Maximizes horsepower to screen ratio for uniform grinding and optimal capacity while reducing heat build-up
- Multiple hammer shapes and styles available
- Precision-built bearing assembly and innovative rotor assembly offer smoother performance and longer lifespan
- Robust design allows longer life
- Well-planned positioning of interrupter plates allows easy screen removal and intensifies grinding action
Regulation of capacity and particle sizes can be achieved through changes in the number of holes in and the size of the screen, along with a number of hammers, varying the hammers’ tip speed, hammer thickness, and the clearance between the screens and hammers.
Other Equipment for Pulse Milling
Other equipment that can be utilized in pulse milling operations include the Rotormill from IPEC (International Process Equipment Company) and Prater’s CLM Air Classifying Mill with the MAC Air Classifier. The Rotormill offers a means by which waste products such as hulls, shells, and skins can be processed. The combination of CLM and MAC proves excellent for protein shifting and fractionation in the pulse milling process.
Requiring no specialized foundation, IPEC’s Rotormill presents a long gap design that eliminates the need for screens. It comes in eight models that range from 15-750 horsepower.
Other benefits and features of IPEC’s Rotormill include:
- Easy access to mill’s interior through large doors
- Handles abrasive and crumbly materials better than classifying mills or fine grinders
- Offers continuous fine milling with high throughput
- Robust rotor bearings on both bottom and top
- Saves time and money by allowing several simultaneous operations
- Simple to adjust or replace internal parts
- Well-balanced rotor
Prater’s CLM & MAC
Prater’s CLM mill and MAC air classifier are used in combination to form a formidable production partnership. As a classifying impact and closed-circuit mill, the CLM is a single particle size reduction unit that grinds in two stages to produce a fine powder. The MAC Air Classifier then concentrates the proteins to provide protein enrichment, typically 2.5X the original feedstock protein level for Yellow Split Peas.
Benefits and features of Prater’s CLM Air Classifying Mill includes:
- Choice of stainless steel or welded carbon steel construction
- Designed for easy replacement of blades, jaws, and screens
- Interchangeable jaw and screen configurations offer greater versatility
- Provides interstage air classification that allows precise adjustment to and control of particle size
- Provides narrow and highly uniform particle size distribution
- Reduces material to uniform sizes due to precision tolerances
- Requires minimal maintenance
- Secondary pneumatic airflow boosts cooling capabilities
- Utilizes a closed-circuit, dual-stage grinding to reduce material to ultrafine particles
Used in tandem with the CLM air classifying mill, Prater’s MAC Air Classifier has a precision fit rotor that provides a sharp cut in the particle size distribution. Made from carbon or stainless steel, it offers precise control through variations in rotor speed. Its adjustable secondary airflow system captures near-sized particles, while low system resistance means less power is consumed. Additionally, the MAC comes with optional ceramic, polyurethane, rubber, or tungsten lining for use with abrasive materials.