Pulses could be further processed to produce flour. The type of the flour and therefore the milling method will be determined by the final product which will be made of the flour. For example, the particle size range and distribution will have a a major effect on the functionality of flour like water absorbtion and viscousity. This, in turn, will influence the properties of the final product like texture and porosity.
Four Milling Techniques
There are four principal techniques used to bring about the size reduction necessary for processing. These are impact milling, attrition milling, knife milling, and direct-pressure milling.
Impact milling involves use of a hard object to strike a wide area of the particle to fracture it. A rotating assembly then uses blunt or hammer-type blades, such as with hammermills, pin mills, cage mills, universal mills, and turbo mills. The impact technique is recommended for pulse milling applications where the side particle size distribution is tolerated.
By contrast, attrition milling relies on a horizontal rotating vessel filled with a size-reduction solution. Treated to grinding media, the materials tend to be turned into free-flowing, spherical particles. This method, which includes the ball mill, can reduce 1,000 micron (20-mesh) particles of friable materials down to less than 1 micron. This process is well suited to produce very fine particle size but will be limited in producing coarse flours like the ones used in bakery and confectionary
With knife milling, a sharp blade applies high, head-on shear force to a large particle, cutting it to a predetermined size, while also minimizing fines. A rotating assembly of sharp knives or blades is used to cut the particles. Examples like knife cutters, dicing mills, and guillotine mills can reduce two-inch or larger chunks or slabs of material, including elastic or heat-sensitive materials, to 250 to 1,200 microns.
Direct-pressure milling occurs when a particle is crushed or pinched between two hardened surfaces. This can involve two rotating bars or one rotating bar and a stationary plate and can typically reduce one-inch or larger chunks of friable materials down to 800 to 1,000 microns. Examples include roll mills, cracking mills, and oscillator mills.
Roller Mills could be configured to crush the particles (with flat rolls) or cut them (in corrugated rolls) thus providing a wider range of different flours which could be manufactured. The particle size could be controlled to be as large as grits or as fine as 100 micron. This range typically covers the different flour types used in bakery, confectionary, extrusion and pasta
The rotor speed, feed rate, screen size, screen type, and moisture content of peas all affect pea milling quality. Rotor speed is the primary factor and can significantly impact the milling process.
The feed throat introduces material into the milling chamber. A gravity feed throat delivers material tangentially to the rotation of the blades.
The type, quantity, and shape of a milling blade helps determine the degree of reduction achieved. The blade pro le offers the flexibility of a knife on one side and an impact tool on the other, with the former being used for gentle granulation and latter for more aggressive reduction.
Milling is most effective if the product is fed uniformly into the feed throat using a variable feed system (15 to 60 rpm). It should be noted that high feed rates increase energy consumption.
The rotor speed affects particle size distribution and as a general rule, and with all other variables remaining constant, the faster the rotor speed, the finer the grind.
Rotor speeds of 3000 to 7200 rpm are used with at blades in ne-grinding applications such as with coarse and ne pea our and other legume ours, while speeds of 1000 to 3000 rpm are used with sharp blades in coarse grinding applications.
The screens can be round or rectangular, with screen thickness and the total open surface area of the screen affecting the comminuting (i.e., pulverizing) operation. The diameter of the screen holes doesn’t necessarily designate the particle size of the finished product as impacted particles follow a tangential trajectory from the blades and approach the screen at a shallow angle. The higher the rotor speed, the smaller the angle under which the particle approaches the screen and the smaller the screen openings appear to the particle.
In the search for new food protein and fiber resources, commercial facilities have begun focusing on extracting protein concentrates from pulses via a process called fractionation that allows researchers to separate out component ingredients to obtain the desired concentrates and isolates.
Dietary fiber in pulses is actually captured as a byproduct of the process by which protein and starch concentrates are acquired from pulse seeds. The results are generally richer in fiber when obtained from the hulls.
The separation of pure legume starch is difficult because of the presence of a highly hydrated fine fiber (cotyledon cell wall material) and the strong adherence of large amounts of insoluble proteins.
Fractionation typically takes the form of a dry or wet method: air classification or wet milling. Dry and wet separation processes have been used for some time to fractionate grain legumes for both experimental purposes and industrial applications.
The traditional wet process is intended for food applications. Using this method, the hulls are removed from the seeds and then milled or ground into flour. The legume our is pulped using a decomposing agent like an alkaline solution to pull out the protein, which is then dried. The solid matter left after the protein has been separated out is screened through a series of sieves to recover the starch. Like fiber, pea starch is usually made available as a byproduct of protein extraction.
Another wet approach includes soaking the whole grain, followed by straining the result, now a slurry, through a cloth. This is common in Thailand, the Philippines, and other Southeast Asian countries as a means for removing the hull to extract the starch. In some West African countries, whole peas, or the remains from stone grinding, are soaked in water and agitated until the hull separates and is then captured by sieving.
For chickpeas and dehulled split yellow peas, starch fractionation involves steeping the seeds in warm water with toluene (i.e., a colorless, water-insoluble, flammable liquid often used as a solvent) to prevent fermentation. This is followed by wet grinding and repeated screening.
Lentils are better served by a similar method that also includes resuspension in a 0.2 percent sodium hydroxide (NaOH) solution, which dissolves most of the protein.
The wet method is particularly useful if the pulse is to be ground into a paste for further processing. On the other hand, if it is to be dried, the wet method can be difficult and time consuming, often involving the loss of nutrients in the soak-water.
The dry method uses a mill and air classification process to break down the dehulled seeds and separate out the starch and protein fractions. Dry processes have been employed more successfully with grain legumes than with other legume varieties because in legumes starch is the principal storage compound rather than oil.
Removal of the loosened hulls from the grain in the dry-milling technique is commonly done in small machines. These usually take the form of under-runner discshellers or grinders with emery or stone contact surfaces. A plate mill is sometimes used to both hull and split the soaked and dried grains.
In India, the grains are oil treated and sun dried before being mixed with two percent to three percent stone powder. They are then hulled in a rice-huller. The usable pieces are removed by sieving, while the hull, powder, and small bits remain in the stone powder.
Wet Versus Dry
In a comparative study of dry and wet milling, dry air classification of pea our containing 22 percent protein and 55 percent starch yielded fractions containing 53 percent protein and 83 percent starch. The protein fraction also contained some broken starch granules in addition to most of the lipid, ash, sugars, flavor, and color compounds in the flour.
The protein isolated from wet milling contained 88 percent protein and refined starch contained less than 1.0 percent protein. The refined fiber was light colored and relatively free of other constituents. The main drawback of the wet milling method is the resulting loss of protein and starch in the whey and washes, as well as the expensive effluent recovery requirements.
Puffing of legumes has been practiced in Asia, Africa, and Latin America for years. It is achieved by first subjecting the pulse to high temperatures, about 176 degrees F (80 degrees C), for a short time. Water is then added and allowed to absorb over-night. The grain is finally roasted, which prompts the cotyledons to expand, thereby splitting the hull so that it can be more easily removed.
Studies show that in addition to moisture conditioning or moisture addition prior to heating, puffing can be improved by certain hardening agents such as calcium phosphate, egg white, gums, calcium, or sodium caseinate