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Processing of pulses

Pulses constitute essential components of vegetarian diet. Pulses are major source of protein in Indian vegetarian diet. These are main source of protein providing most of the essential amino acids to a certain degree. Economically, pulses are cheapest source of protein. Pulses are Bengal gram, pigeon pea, black gram, green gram, lentil, etc. Pulses are mainly consumed in the form of dehusked split pulses, as these are rich in proteins. In vegetarian diet pulses are main source of protein. There are about 4000 pulses mills (Dal mills) in India with varying capacity of pulse mills ranging from 10 to 12 tonnes per day.

Pulse milling:
Pulses are usually converted into Dal by decutilating and splitting. Both dry and wet milling processes are employed. By and large carborundum emery rollers are used for dehusking and burr grinders for splitting. Decuticling is seldom complete in single pass requiring multiple passes, each pass producing 1.5 to 2% fines reducing recovery of dal.

Basic processes in dal milling are cleaning, dehusking, splitting, separation and bagging. Major variation is involved with dehusking process only. Dals like Arahar, urad, moong and lentil are difficult to dehusk as a result repeated operations by dehusking rollers are required. Rewetting and drying is done to loosen portions of husk sticking after repeated rolling. Linseed oil is used to impart shine or better appeal to the milled dal.

The removal of the outer husk and splitting the grain into two equal halves is known as milling of pulses. To facilitate dehusking and splitting of pulses alternate wetting and drying method is used. In India trading milling methods produce dehusked split pulses. Loosening of husk by conditioning is insufficient in traditional methods. To obtain complete dehusking of the grains a large number of abrasive force is applied in this case as a result high losses occur in the form of brokens and powder. Yield of split & pulses in traditional mills are only 65 to 75% due to the above losses compared to 82 to 85% potential yield.

Procedure for pulse milling:
There are two types of conventional pulse milling methods in India. Wet milling and dry milling method. Flow diagram of wet milling is given below:




Mixing with red soil


Dehusking and splitting


Dehusked split pulses

Dry milling of Pulses :
For all types of pulses there is no common processing method. However, some general operations of dry milling method such as cleaning and grading, rolling or splitting, soiling and moistening, drying and milling are considered.




Pretreatment with oil


Dehusking and splitting



Grade I pulses

Cleaning and grading:
Cleaning of pulses from dust, chaff, dirt, grits, etc. and grading according to size is performed by a reel type or rotative screen type cleaner.

An emery roller machine is used to obtain cracking and scratching of clean pulses passing through it. For loosening the husk to facilitate the subsequent oil penetration this is required. Gradually the clearance between the emery roller and cage (housing) is narrowed from inlet to outlet. Cracking and scratching of husk takes place mainly by friction between pulses and emery as the material is passed through the narrowing clearance. During the operation some of the pulses are dehusked and split which are separated by sieving.

Pretreatment with Oil:
A screw conveyor allows to pass the scratched or pitted material through it and mixing of some edible oil like linseed oil is completed. The linseed oil is used at the rate of 1.5 to 2.5 kg/tonne of pulses. These are kept on floors for about 12 hours to diffuse the oil.

Pulses are conditioned by alternate wetting and drying. Moisture (3.5%) is added to the pulses after sun drying for a certain period and tempering is done for about eight hours. The grain is dried in the sun again. By allowing water to drop from an overhead tank on the pulses which are passed through the screw conveyor addition of moisture to the pulses are achieved. Until all pulses are sufficiently conditioned the whole process of alternate wetting and drying is continued for two to four days. Pulses are finally dried to about 10 to 12% moisture content.

Dehusking and Splitting:
For dehusking of conditioned pulses emery rollers called as Gota machine are used. In one pass or single operation about 50% of pulses are dehusked. Dehusked pulses are split into two parts. Dehusked split pulses are separated by sieving and the husk is aspirated off. Unsplit dehusked pulses and tail pulses are again dehusked and milled in a similar way. Till the running pulses are dehusked and split of the whole process is repeated tow or three times.

Polishing is completely by treating dehusked and split pulses with a small quantity of oil and/or water.

CFTRI modern method of pulses milling
Steps involved in CFTRI method of processing are
To remove all impurities from pulses and separate them according to size cleaning is done in rotary seed cleaners.

Pre conditioning
LSU type dryer is used to condition the clean pulses into two passes using hot air at about 1000C for a certain period of time. Hot pulses are tempered after each pass in the tempering bins for about six hours. In loosening husk pre conditioning of pulses helps significantly.

Pearler or dehusker are used to dehusk the preconditioned pulses in a single operation where almost all pulses are dehusked. Separation of dehusked whole pulses (gota) from split pulses and mixture of husk, brokens etc. (chunni-Bhushi) is done which are received in a screw conveyor where water is added at a controlled rate. The moistened gota are collected on the floor and allowed to remain as such for about half an hour.

Lump breaking
Lumps of varying sizes are formed by some of the moistened gota. A lump breaker is used where these lumps are fed for further breaking.

Conditioning and Splitting
The gota, after lump breaking is conveyed to LSU type dryer where it is exposed to hot air for a few hours. For splitting the gota is dried to the proper moisture level. In emery roller the hot conditioned and dried dehusked whole pulses are split. In one pass all of them are not split. Grading of mixture is done to get grade I pulses, dehusked whole pulses and small brokens. For subsequent splitting the unsplit dehusked pulses are again fed to the conditioner.

Machinery for pulses processing

Pulses are processed in India for converting the raw material into variety of products. Machinery is available for production of dal dehusked splits, roasted and pulse flour and polished pulses. Selection of machine depends upon the type of pulses crop to be processed and the product desired.

Hand operated machines
Domestic dal mill: Domestic dal mill is one of the oldest existing unit having multipurpose use. It consists of two circular stones mounted on one another in horizontal plane. Upper stone is rotated to get size reduction of the material fed from the central opening on the top stone. This manually operated unit is commonly used for making splits of variety food legumes. The traditional domestic dal mill has been modified (KVIC 1982-83) and a grading and splitting unit (4 quintal/day capacity) developed for Bengal gram.

Pulse dehusking machines: A hand operated pulse dehusking machine has been developed for small scale rural processors. It consists of emery coated inverted metal cone fixed to a vertical shaft rotating inside a chemical wiremesh screen with the arrangement for clearance adjustments. This machine has a capacity of 50 to 60kg splits/hour at 60 to 70rpm and gives 75-80% dal yield from pigeon pea and Bengal gram. This machine is commercially manufactured for use of small processors. This is useful unit for rural or small scale use for supplementing of farm income. Its usefulness is further strengthened owing to increased recovery of splits by about 3 to 5%.

Soybean dehuller: A hand operated soybean dehuller, 35kg/ hour capacity consisting of two concentric cylinders, a dividing mechanism, a blower fan and feed hopper has been developed. There is a gradual decrease in clearance from feed inlet to discharge outlet i.e. from 10 to 7mm. Dal (splits) husk and brokens are separated. Dal recovery of 84.5% is reported with dehulling efficiency of 95.5% with 3-4% brokens. Such a simple and efficient unit is useful for small primary processing of soybean and other food legumes.

Power operated machines:
Burr mill: Small capacity burr mill with 200mm diameter grinding stones (vertical) was developed for grinding purposes. It was tried for dal making and capacity was 72kg per hour and 52kg per hour for soybean and Bengal gram respectively. The burr mill at an optimum capacity of 64kg per hour gave a dal yield of 77.7% at 3.50mm clearance.

Roller machine: Abrasive, carborundum roller cylindrical mill of 100kg per hour capacity, run by 1.5kw electric motor was developed for dehusking and splitting of food legumes viz.pigeon pea gram and black gram. It consisted mainly of a 300mm diameter cylindrical emery coated roller to give 13.5 to 14m/s speed at 850-900rpm and dal (splits) recovery 74.75%. Though the unit is similar to traditional miller machine and gives more or less the same dal recovery. It is useful for small scale production (8 to 10q per day) of dal in villages & towns.

Cylinder concave dehuller: The machine developed at GBPUAT, Pautnager consists of a 380mm long and 220mm diameter cylinder with trunketed surface, and a concave. The kernels are fed to the unit along the full length of the cylinder. The machine developed at IARI, New Delhi has cylinder concave set consisting of 150mm diameter mild steel pipe of 6mm thickness coated with 36 week emery cloth to form a cylinder. The concave has 1x1mm grooves along the length and spaced at 25mm apart. The unit run by one H.P. electric motor when operated at 1080rpm (8.48m/s) give a soybean dal yield of 85% at 8.69% moisture content (d.b) with 94% hulling efficiency. The energy requirement of unit was 2.15 Wh/kg with a milling capacity of 140kg per hour. These units have been proved to be of high utility value due to higher recovery rates of splits and improved dehusking efficiency.

Concentric double cylinder: This machine give acceptable recovery rates for difficult to mill pulses viz. Pigeon pea, and this could be quite useful in manual as well as power operated conditions for variety of food legumes.

Under runner disc (URD) Sheller: The URD sheller (300mm dia, 100mm thick, 900rpm) can be used for milling of bengal gram (variety G.T.62). It give 83% recovery of splits with 6% brokens. The energy consumption increases with increase in feed rate upto 250kg/h and remains constant (2KW) upto 300kg/h and then decreases.

Centrifugal Sheller: The unit with impeller diameter 218mm, 3 vanes, 10mm thickness and 340mm casing diameter is useful for splitting of pigeon pea. The moisture content of grains was raised to a level of 17% d.b. followed by 24 hour equilibrium drying to a 10.5% moisture content d.b.splitting at 2100rpm and with tarpauline padding mounted on the inner side of the casing for 0.92 effectiveness.