PULSE MILLING

Milling of Pulses Pulses are rich in proteins and are mainly consumed in the form of dehusked split pulses. Pulses are the main source of protein in vegetarian diet. There are about 4000 pulse mills (Dhal mills) in India. The average processing capacities of pulses milk in India vary from 10 to 20 tonnes/day. Milling of pulses means removal of the outer husk and splitting the grain into two equal halves. Generally, the husk is much more tightly held by the kernel of some pulses than most cereals Therefore, dehusking of some pulses poses a problem. The method of alternate wetting and drying is used to facilitate dehusking and splitting of pulses. In India the dehusked split pulses are produced by traditional methods of milling. In traditional pulse milling methods, the loosening of husk by conditioning is insufficient. Therefore, a large amount of abrasive force is applied fur the complete dehusking of the grains which results in high losses in the form of brokens and powder. Consequently, the yield of split pulses in traditional mills is only 65 to 70 per cent in comparison to 82 to 35 per cent potential yield. It is, therefore, necessary to improve the traditional methods of pulses milling to increase the total yield of dehusked and split pulses and reduce the losses. Varieties, Composition and structure Green gram, red gram, bengal gram, horse gram, cluster bean, field pea, arhar are some of the common types of pulses. The botanical name of Arhar is Cajanas cajon. Its chemical composition and structure are: Moisture

10.35 %

Protein (NX 6.25)

24.19 %

Ether extract

1.89 %

Ash

3.55 %

Crude fibre

1.01 %

Carbohydrate

59.21 %

The average percentage of husk and endosperm in arhar is 15 per cent and 85 per cent respectively Milling of Pulses

In India, there are two conventional pulses milling methods ; wet milling method and dry milling method. The latter is more popular and used in commercial mills, Pulses

Cleaning Chaffs, dirts etc Soaking

Mixing with red earth

Conditioning Dehusking and splitting (Mixture of husk, small broken and powder) Grading Brokens Dehusked split pulses (Grade I pulses)

Flow diagram of wet milling Traditional dry milling method ('DHAL' MILLING) There is no common processing method for all types of pulses. However, some general operations of dry milling method such as cleaning and grading, rolling or pitting, oiling, rnoistening, drying and milling have been described in subsequent paragraphs. Cleaning and grading

Pulses are cleaned from dust, chaff, grits, etc., and graded according to size by a reel type or rotating sieve type cleaner, Pitting The clean pulses are passed through an emery roller machineIn this unit, husk is cracked and scratched. This is to facilitate the subsequent oil penetration process for the loosening of husk. The clearance between the emery roller and cage (housing) gradually narrows from inlet to outlet. As the material is passed through the narrowing clearance mainly cracking and scratching of husk takes place by friction between pulses and emery. Some of the pulses are dehusked and split during this operation which are then separated by sieving. Pretreatments with oil The scratched or pitted pulses are passed through a screw conveyor and mixed with some edible oil like linseed oil (1.5 to 2.5 kg/tonne of pulses). Then they are kept on the floor for about 12 hours for diffusion of the oil. Conditioning of pulses Conditioning of pulses is done by alternate

wetting

and drying. After sun drying

for a certain period, 3-5 per cent moisture is added to the pulse and tempered for about eight flours and again dried in the sun. Addition of moisture to the pulses can be accomplished by allowing water to drop from an overhead tank on the pulses being passed through a screw conveyor. The whole process of alternate wetting and drying is continued for two to four days until all pulses are sufficiently conditioned. Pulses are finally dried to about 10 to 12 per cent moisture content Dehusking and Splitting Emery rollers, known as Gota machine are used for the dehusking of conditioned pulses About 50 per cent pulses are dehusked in a single operation (in one pass). Dehusked pulses are split into two parts also, the husk is aspirated off and dehusked, split pulses are separated by sieving. The tail pulses and unsplit dehusked pulses are again conditioned and milled as above The whole process is repeated two to three times until the remaining- pulses are dehusked and split. Polishing Polish is given to the dehusked and split pulses by treating them with a small quantity of oil and / or water.

Pulses Cleaning Chaff, dirts, etc. Pitting Mixture of husk and brokens (feed) Pretreatment with oil Conditioning Dehusking and splitting Mixture of husk + Brokens + powder (feed) Grading Polishing Grade I Pulse Flow diagram of dry milling of pulses Commercial milling of pulses by traditional methods It is discussed earlier that the traditional milling of pulses are divided into two heads, namely, dry milling and wet milling. But both the processes involved two basic steps : (i) Preconditioning of pulses by alternate wetting and sun drying for loosening husk and (ii) subsequent milling by dehusking and splitting of the grains into two cotyledons followed by aspiration and size separation using suitable machines.

100 per cent-

dehusking and splitting of pulses are seldom achieved particularly in cases of certain pulses like tur, black gram and green gram. Of them tur is the moat difficult pulses to dehusk and split. Only about 40 to 5O per cent tur grains are dehusked and split in the first pass of preconditioning and milling. As sundrying is practiced the traditional method is not only weather dependent but also it requires a large

drying yard to match with the milling capacity. As a result it takes 3 to 7 days for complete processing of a batch of 20 to 30 tonnes of pulses into dhals. Moreover milling losses are also quite high in the traditional method of milling of pulses. In general, simple reciprocating or rotary sieve cleaners are used for cleaning while bucket elevators are used for elevating pulses. Pitting or scratching of pulses is done in a roller machine. A worm mixer is used for oiling as well as watering of the pitted pulses. The machines used for dehusking are either power driven disc type sheller 'chakki’ or emery-coated roller machine, which is commonly known as 'gota1 machine. The emery roller is encaged in a perforated cylinder. The whole assembly is normally fixed at a horizontal position. The Engelberg type rice hullers are also used for dehusking of return unhusked black gram and green gram pulses in some parts of South India, where coarse stone powder at 0.5 to 0.75 per cent level is mixed with the grains as a abrasive material. Sometimes either a cone type polisher or a buffing machine is employed for removal of the remaining last patches of husk and for giving a fine polisher to the finished dha. The cone polisher is similar to the polishing machine used for polishing of rice (i.e. for removal of bran from brown rice). The buffing machine is equipped with a rotating paddle having leather straps which can remove the last patch of husk and can give a fine polish to the dehusked pulses. Blowers are used for aspiration of husk and powder from the products of the disc sheller or roller machine. Split dhals are separated from the unhusked and husked whole pulses with the help of sieve type separators. Sieves are also employed for grading of dhals. In general, the raw pulses may contain 2 to 5 per cent impurities (foreign materials), some insect infested grains and extra moisture. Though the clean pulses contain about 10-15 percent and 2-5 per cent germs, the yield of dhals commercial dhal mills varies from 68-75

per cent.

It may be noted that the average potential yields of common

dhals vary from 85 to 89 per cent. These milling losses in the commercial pulses mills can be attributed lo small brokens and fine powders found during scoring and simultaneous dehusking and splitting operations. Some of the commercial milling methods commonly followed for different pulses are briefly described in the subsequent paragraphs. Dry milling of tur

The dry milling of tur is generally practiced in M.P and U,P, as it can give, higher turnover in terms of capacity of mill having the same area of drying facilities. In this method the pulses are subjected to pitting in a roller and then they were subjected to oil treatment by applying 0.5 to 2 per cent linseed or any other edible oil in worm mixers. The pulses are then spread in the drying yard for sundrying for 2 to 4 days. The pulses are tempered by heaping and covering during the nights in between these days. After thorough sundrying the pulses are once again moistened uniformly with about 5 per cent water and kept as such on heaps overnight for uniform moisture equilibration. Then these grains are passed through the rollers for dehusking and splitting. About 50 per cent grains are dehusked and split in first operation. After removal of the husk by aspiration the split dhal is separated from a mixture of husked and unhusked whole pulses.

The mixture is once again moistened by

spraying water and dried in the sun and then dehusked and split as before either in roller or in an under runner disc sheller, where around 30 per cent of the grains are dehusked and split. The above process of alternate wetting and drying is repeated until almost all the remaining pulses are converted into split dhal. The average yield of dhal ranges from 68 to 75 per cent.

DRY

milling of black gram

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 grain; are then coated with 1 to 2 per cent oil in a worm mixer and then heaped over night for diffusion of the oil m the grains The scratched and oil coated pulses are sprayed in drying yards for sundrying for 4 to 6 hours. The partially dried grains are moistened with a spray of 4 to 5 per cent water and kept over night for moisture equilibration. The wetted pulses are then dried for 3 to 4 days in the sun and tempered over nights in between these drying periods. The thoroughly dried pulses are dehusked in a roller- About 40 to 50 per cent pulses are dehusked and split in first milling operation. The husk and powder are then aspirated off. Then the split dhal is separated from the dehusked whole dhal and unhusked pulses by sieving. Both husked and unhusked 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 per cent. 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, which result in a large amount of losses due to formation of powder and brokens. In some cases polishing is done in a buffing machine. In order to give a white finish and to protect from insect attack a coating of soapstone powder is generally given to these 'dhals’. Dry milling of Bengal gram, Lintel and peas. It is comparatively easy to dehusk and split Bengal gram. Lentil and Peas as their husks are loosely attached to the cotyledons. It requires shorter period of preconditioning prior to milling these pulses. After cleaning, the pulses are pitted in a roller machine. The pitted grains are then wetted with water (5 to 10 per tent) in a worm mixer and then these are kept in heaps 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 in between these days. About 60 to 70 per cent dried pulses are then dehusked and split in the first pass of a roller machine-The husk and powder are aspirated off. The split pulses are separated from the unhusked and husked whole grains by sieving. The alternate wetting with 5 per cent water and sundrying and subsequent milling operations are repeated till the most of the pulses are converted to dhal. The preconditioning and milling of Lentils and Peas are comparable with bengal gram. The same initial pitting, wetting, conditioning, sundrying and subsequent milling by dehusking and splitting in a roller and aspiration of husk with a blower. Separation of

split dhal from the mixture of unhusked 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, conversion of these pulses into dhals are easy compared in tur. It takes about 3 to 5 days for complete precessing of a batch of pulses, Dry milling of Green Gram In dry milling of preen gram, both oil and water treatments aft given to the grains. The wetted grains are dried in the sun. Then the dried pulses are simultaneously dehusked and split using a dehusking 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 per cent only. Wet milling of tur In wet milling of tur the grains are soaked in water for a period of 3 to 12 hours. The soaked pulses are thoroughly mixed with wet red earth at about 5 per cent level The mixture * is kept in heaps over night. The whole mixture is then dried in the sun for 2 to 4 days until the husk of all grains are loosened. The pulses are tempered overnight in between these days. The red earth is then separated from the pulses by sieving. The sundried grains are dehusked and split in a disc sheller (chakki). Dhal and other fractions are separated as usual. It is claimed that about 95 per cent of the grams can be dehusked and split in a single milling operation. The split dhal is separated from the mixture usually. The rest of the unhusked and husked whole grains are preconditioned and milled as above for conversion of these grains into dhal. Though the above wet milling of tur is popular in South India, the purpose of using earth is not well understood. However, it is believed that the red earth facilitates in increasing the rate of drying and in consequence in loosening the husk. So also the earth may act as a milling aid on account of its abrasive nature. It is also considered that dhals produced by following wet method are attractive in colour and good taste. The wet method requires 5 to 7 days for complete processing of a batch of grains. The flow diagram of wet milling of tur is given below

MODERN

CFTRI METHOD of PULSES MILLING

Cleaning Cleaning

is

done

in

rotary

reel

cleaners

to

remove

all impurities

from pulses and separate them according to size. The cleaned pulses are conditioned in two passes in a dryer (LSU type) using hot air at about 120°C for a certain period of time. After each pass, the hot pulses are tempered in the tempering bins for about six hours. The preconditioning of pulses helps in loosening husk significantly. Dehusking The preconditioned pulses are conveyed to the Pearler or dehusker where almost all pulses are dehusked in a single operation. The dehusked whole pulses (gota) are separated from split pulses and mixture of husk, brokens, etc., and are received in a screw conveyor where water is added at a controlled rate. The moistened goto is then collected on the floor and allowed to remain as such for about an hour.

The flow diagram of the modern milling of pulses by CFTRI Method as given below is self-explanatory.

Lump Breaking Some of the moistened gota form into lumps of varying sizes. These lumps are fed to the lump breaker to break them. Conditioning and Splitting After lamp breaking the gota is conveyed to LSU type of dryer where it is exposed to hot air for a few hours. The gota is thus dried to the proper moisture level for splitting. The hot conditioned and dried dehusked whole pulses are split in the emery roller. All of them are not split in one pass. The mixture is graded into Grade 1 pulses, dehusked whole pulses and small brokers. The unsplit dehusked pulses are again fed to the conditioner for subsequent splitting.