Crop Process Engineering

Lesson 6. Methods of Milling of Pulses

6.2 Wet milling method of pigeon pea

The grains are soaked in water for 3-12 hours in this method of milling. The soaked pulses are mixed with red earth at about 5 % thoroughly. The mixture is kept in heaps overnight. The whole mixture is then dried in the sun for 2-4 days until the husk of the grain are shriveled and loosened. The pulses are tempered overnight in between these days. By sieving, the red earth is separated from the pulses. The dried grains are dehusked and split in disc sheller. The dhal and other fractions are separated. In a single milling operation, about 95 % of the pulses are dehusked and split. The rest material again pretreated and milled to convert in to dhal. The red earth may facilitate in increasing the rate of drying and in loosening the husk. This method requires about 5 to 7 days for processing of a batch of pulses.

 6.3 CFTRI method of Pigeon Pea milling

There are some other methods like CFTRI method, which eliminates mixing of oil and water for loosening the husk. Clean and graded grains are conditioned by dry heat treatment by two passes through LSU drier with hot air. After each passes through the dryer the grains are tempered for    6 hours in tempering bin. The preconditioned pulses are conveyed to the pearler or dehusker where almost all pulses are dehusked in single operation. The gota (dehusked whole grain) are separated from split pulses and mixture of husk, brokens, etc. Water is added at controlled level to Gota and then collected and allowed to remain as such for about 1 hour. Some of the moistened Gota form lumps of varying sizes. These lumps are fed to the lump breaker to separate them. These Gota are conveyed to LSU dryer where it is exposed to hot air for few hours. The Gota are dried to proper moisture level for splitting. The hot, conditioned and dried dehusked whole pulses are split in the emery roller. The mixture is graded in to Grade I pulses, dehusked whole pulses and small brokens. The unsplit dehusked pulses are again fed to the conditioner for subsequent splitting. Average yield by this method is 80 %, in lesser time and lesser processing cost compared to other methods.

6.4 Pantnagar process of pigeon pea milling

In this process the pigeon pea after cleaning and grading are passed through an emery roller mill for scratching.  The pitted grains are 10% sodium bicarbonate solution and tempered for 4 hours in shade. Then the grains are dried under sun to 9.5% moisture content followed by milling in a concentric cylinder roller mill. The milled grains are cleaned and graded with the help of blower, cyclone separator and reel grader. Dhal recovery is 80 % in this method.

 6.5 CIAE method pigeon pea milling

In this method use of edible oil in pretreatment process is eliminated. First the pigeon pea, green gram and black gram is cleaned and fed to the roller mill developed at CIAE for scratching. After cleaning the scratched grains are soaked in tap water at ambient temperature for 30 minutes in case of pigeon pea and hour for black gram and green. The water is the drained off and the grains are dried to 9-10% moisture content. Such conditioned grains are again fed to the roller mill to produce dehusked split cotyledons.

 6.6 Method of Black gram milling

After cleaning the black grams are subjected to pricking in a rough roller mill for some scratching as well as partial removal of the waxy coating on the black grams. The scratched grains are then coated with 1 to 2 percent oil in the grains. The scratched and oil coated pulses are sprayed in drying yards for sun drying for 4 to 6 hours. The partially dried grains are moistened with a spray of 4 to 5 percent water and kept over night for moisture equilibration. The wetted pulses are then dried in the sun for 3 to 4 days and tempered over nights. Thoroughly dried pulses are de-husked in a roller machine. About, 40 to 50 percent pulses are de-husked and split in first milling operation. The husk and powder are then aspirated off. Then, the split ‘dhal’ is separated from the de-husked whole dhal and un-husked pulses by sieving. Both husked and un-husked whole grains are again dried in the sun and milled as above and the same process is repeated until the desired milling of pulses is achieved. The average yield of dhal is 70-71 percent. Sometimes, the last part of the unsplit grains and partially husked grains are allowed to pass through sheller and polisher machines for splitting and removal of the husk and polisher machines for splitting and removal of the husk, which result in a large amount of losses due to formation of powder and brokens. In some cases, policing is done in a buffing machine. In order to give a white finish and to protected from insect attack a coating of soapstone powder is generally given to these ‘dhals’

 6.7 Method of Bengal gram milling

It is comparatively easy to de-husk and split Bengal gram, Lentil and Peas as their husks are loosely attached to the cotyledons. It requires shorter period of preconditioning prior the milling these pulses. After cleaning, the pulses are pitted in a roller machine. The pitted grains are then wetted with water   (5 to 10 percent) in a worm mixer and then these are kept in helps for a few hours for diffusion of water into the grains. These grains are dried in the sun for a day or two, with overnight tempering. About, 60 to 70 percent dried pulses are then de-husked and split in the first pass by a roller machine. The husk and powder are aspirated off. The split pulses are separated from the un-husked and husked whole grains by sieving. The alternate wetting with 5 percent water and sun drying and subsequent milling operations are repeated till the most of the pulses is converter to ‘dhal’.

 6.8 Method of Lentil and peas milling

The preconditioning and milling of Lentils and Peas are comparable with Bengal gram. The same initial pitting, wetting, conditioning, sun drying and subsequent milling by de-husking and splitting in a roller and aspiration of husk with a blower and separation of split dhal from the mixture of un-husked and husked whole grains with a sieve are being followed. The whole process of preconditioning and milling are repeated till most of the pulses are converted into dhal. However, the conversion of these pulses into dhal is easy as compared to tur. It takes about 3 to 5 days for complete processing of batch of pulses.

 6.9 Method of Green gram milling

In dry milling of green gram, both oil and water treatments are given to the pulses. The wetted pulses are dried in the sun. Then the dried pulses are simultaneously de-husked and split using a de-husking machine. After removal of husk, split dhal is separated from the mixture as usual. The yield of dhal is poor which varies from 62 to 65 percent only.

 References

•  Anon. (2004). Course manual for short course on Post harvest processing and agro processing centres for value addition in rural areas. Post Harvest Technology Scheme, University of Agricultural Sciences, Bangalore.

•  Anon. (2011). Report on Bench Marking of Food Processing Technologies. Indian Institute of Crop Processing Technology, Thanjavur.

•  Chakraverthy, A. (1995). Post harvest technology of cereals, pulses and oilseeds. Oxford & IBH publishing Pvt. Ltd., New Delhi.

•  Chavan, U. D. (2012). Post Harvest Management and Processing Technology: cereals, pulses, oilseeds, fruits and vegetables. Daya Hpuse Pub. , Rome.

•  Pandey, P. H. (1998). Principles and Practices of Post Harvest Technology. Kalyani publishing Pvt. Ltd., Ludhiana.

•  Sahay, K. M. and Singh, K. K. (1994). Unit operations of Agricultural Processing. Vikas Publishing House Pvt. Ltd., New Delhi.