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 machine-

In 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 con-

ditioner for subsequent splitting.