Dal Milling Process

By

Er. om

Pulses are basically grain legumes. They occupy an important place in human

nutrition due to their high protein content than cereal grains. In Indian dietary

regime it occupies an important place. Since majority of Indians are vegetarians,

they depend largely on grain legumes (pulses) for their dietary protein. Legumes

contribute a major portion of lysine in the vegetarian diet. They are also a fairly

good source of vitamins like thiamine, machine, riboflavin and much needed iron.

Therefore its quality availability to the common man is a major challenge. Since

recovery was poor in traditional technologies, adoption of modern technology will

go a long way in meeting the need of the common man. Jas enterprise offer mini

dal mill machine. This mini dal mill is simple in construction and easy to operate,

maintain and available at very low cost. Mini dal mill machinery consist of

horizontal tapered roller is covered with emery coating, surrounded by a screen

through which the husk is discharged. The shelled pulses are passes through an

aspirating fan on the oscillating sieve unit, where appropriate grading of pulses is

done. It is run on only 2 HP automatic arrangement are made for collection of de

husked and split pulses, un husked and split pulses, un de husked pulses, broken,

husk in separate containers and bags. This machine offers dust free operation, does

not cause pollution, retains proteins, natural shine et cetera.

Advantage and Special Feature of Mini dal mill Suitable for processing of all types

of pulses Easy to operate Operates by 2 HP electric motor / oil engine Recovery of

head pulses - 78% to 80% Recovery of broken - 1 to 3% De husks and splits

almost all pulses retains proteins and natural shine Special provision for bangle

gram (Chana, chickpeas) by stone chakki for de husking and splitting. Automatic

arrangement of collecting of the following in separate containers and bag de

husked and split pulses Un husked pulses Broken Husk Pollution free operation

Compact size (length 1650 mm x width 900 mm x height 1350 mm )

Process of Dal Milling ( Pulse Milling ) Basic processes in dal milling are

cleaning, grading, conditioning, de husking, splitting, and separation, polishing and

bagging. Major variation is involved with de husking process only. Sometimes

linseed oil is also used during dry milling operation to impart shine or better appeal

to the milled dal. The removal of the outer layer of husk and splitting the grain into

two equal halves is known as milling of pulses. To facilitate de husking and

splitting of pulses alternate wetting and drying method is used.

Pulses

↓

Cleaning, chaffs, dirt, etc.

↓

Pitting

↓

Pretreatment with Linseed Oil

↓

Conditioning

↓

De husking and splitting mixture of husk, small broken and powder

↓

Grading

↓

Polishing

↓

Grade I Pulses

Description of various Mini dal mill's Unit of Operations: Cleaning and grading:

This unit consists of a vibratory inclined sieve, hopper, grain collector, waste

collector and motor. The vibratory sieves are providing with different size holes to

match the requirements of the type of dal being process. The vibrations are

inducing by a cam-operated link, which is mounts on a motor driven shaft

Pitting of dalls: An emery roller machine is used for cracking the husk layer and

for scratching of clean pulses passing through it. This is done for loosening the

husk from sticking to the cotyledons in order to facilitate subsequent oil

penetration in the following unit operations. 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 de husked and split which are separated by sieving.

Pretreatment with oil and water: A screw conveyor allows passing the scratched or

pitted material through it and mixing of some edible oil like linseed oil is complete.

The linseed oil is use at the rate of 1.5 to 2.5 kg/tones of pulses (please refer our

guideline for lintel). These are keeping on floors as required to diffuse the oil.

De-husking and splitting of dal: For de husking of conditioned pulses emery stone

coated emery rollers are used. In one pass about 50-75% of pulses are de husked.

De husked pulses are split into two parts. De husked split pulses are separated by

sieving and the husk is aspirated off. Anti shoplifting pulses and tail pulses are

again de husked and milled in a similar way. For complete de husking and splitting

the whole process is repeated two to three times.

Pre milling of dals: To obtain best results during de-husking and splitting of pulses

in the pulse de husking machine in Jas brand mini dal mills, latest model improved

version, pre milling including grading of pulses size wise is very essential. The raw

pulses are first clean of dust, chaff, stones and other extraneous materials. Sieves

grade cleaned pulses or pulses graded and soaked in water in cement's tanks having

6" depth. Height of pulses soaked is 5" and the water level should be 1" above the

pulses. Soak pulses are taken out of the water after specific time and put on the

sieve. Swollen pulses which do not pass through sieve (from which un-swollen

pulses earlier passed through the same) are ready for heaping in shade whereas the

pulses which pass through the same sieves. Then these pulses are ready for heaping

in shade. Degree of happing of swollen pulses in shade should be 30 to 40. This

process should be continuing until the pulses are swill to the desired extent. There

after the pulses are in thin layers in the open sun drying. Duration of sun drying of

pulses vary according to weather condition prevailing at the time of processing.

After sun drying, the pulses are again heap in shade so moisture contents in the

pulses may become uniform as need in them for de-husking. Then the pulse are

graded again fed in to the pulse de husking machine in Jas mini dal mill in graded

lots to achieve he best results with minimum brokenness. For the guidance of the

entrepreneurs, the process of details of pre-milling of pulses after proper cleaning

is show here after in a tabulated chart.

Note: - Period of soaking in water will vary according to weather condition and

quality of pulses. Duration of sun drying will vary according to weather condition.

Pulses should not be warm at the time of De-husking/splitting. Pulses should be

de-husk and split in graded lots only. Pulses shown at one and two must be dry

within 3 days to get better results. Their processing in any season should be

avoiding. The de-husked pulses mixed with the un-husked pulses and the husks are

again de-husked in the Jas brand mini Dal mill as usual. In second pass all, the

pulses are de-husked.

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.

COMPOSITION

Green gram, red gram, bengal gram, horse gram, cluster bean, field bean, cow pea

are some of the common types of pulses.In general, their protein content is high

and is commonly more than twice that of cereal grains, usually constituting about

20 per cent of the dry weight of seeds. The protein content of some legumes like

soyabean is as high as 40 per cent.

Pulse seeds are also sources of other nutritionally important materials, such as

vitamins and minerals.

Carbohydrates: Food pulses contain about 55-60 per cent of total carbohydrates

including starch, soluble sugars, fibre and unavailable carbohydrates.

Minerals: Pulses are importantly sources of calcium, magnesium, zinc, iron,

potassium and phosphorus.

Vitamins: Pulses contain small amounts of carotene, the provitamin A.

TOXIC CONSTITUENTS OF PULSES

The seeds of pulses include both edible and inedible types. Even amongst the

edible legumes toxic principles occur and their elimination is important in order to

exploit them for edible purposes. Two thermoliable factors are implicated in toxic

effects. Inhibitors of the enzymes trypsin, chymotrypsin and amylase

haemagglutinins, which impede the absorption of the products of digestion in the

gut. In addition, legumes also contain a goitrogen, a toxic saponin, cyanogenic

glycosides and alkaloids.

Elimination of Toxic Factors

It has already been indicated that soaking, heating and fermentation can reduce or

eliminate most of the toxic factors of the pulses. Correct application of heat in

cooking pulses can eliminate most toxic factors without impairment of nutritional

value. Cooking also contributes towards pulse digestibility. Heat causes the

denaturation of the proteins responsible for trypsin inhibition, haemagglutination

and the enzyme responsible for the hydrolysis of cyanogenic glycosides. The mode

of application of heat is important. Autoclaving and soaking followed by heating

are effective. Another way of eliminating toxic factors is by fermentation, which

yields products more digestible and of higher nutritive value than the raw pulses.

Processing: Processing of pulses is of primary importance in improving their

nutritive value. The processing methods used are soaking, germination

decortications, cooking and fermentation.

Soaking: Soaking in water is the first step in most methods of preparing pulses for

consumption. As indicated above, soaking reduces the oligosaccharides of the

raffinose family. Soaking also reduces the amount of phytic acid in pulses.

Germination: Germination improves the nutritive value of food pulses. The

ascorbic acid content of pulses increases manifold after 48 hours germination.

Germinated and sprouted pulses have been used to prevent and cure scurvy. The

riboflavin, niacin, choline and biotin contents of all pulses increase during

germination. The germination process reduces and/or eliminates most of the

antinutritional and toxic factors in several pulses.

Decortication: A simple method is to soak the seeds for a short time in water; the

husk takes up more water than the seeds and may be easily separated by rubbing

while still moist. In the alternative, the soaked grains may be dried and the husk

removed by pounding and winnowing. Roasting also renders the husk easier to

separate. Roasted legumes like those of Bengal gram and peas are widely used in

India.

Cooking: Cooking destroys the enzyme inhibitors and thus improve the nutritional

quality of food pulses. Cooking also improves the palatability.

Fermentation: The processing of food pulses by fermentation increases their

digestibility, palatability and nutritive value. Fermentation process improves the

availability of essential amino acids and, thus, the nutritional quality of protein of

the blend. In general, the nutritive value of the legume based fermented foods has

been shown to be higher than their raw counterparts.

Pulse milling

Pulses are usually converted into Dhal 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 dhal milling are cleaning, dehusking, splitting, separation and

bagging. Major variation is involved with dehusking process only. Dhals 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.

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.

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, moistening, 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 operations 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.

Commercial milling of pulses by traditional methods

The traditional milling of pulses is 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 Red gram, black gram and green gram. Of them Red gram is the

most difficult pulses to dehusk and split. Only about 40 to 50 per cent Red gram

grains are dehusked and split in the first pass of preconditioning and milling. As

sun drying 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.

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.