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Post Harvest Technology :: Agriculture :: 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.

 

COMPOSITION

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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.

NUTRITIVE VALUE OF PULSES

 

Energy
Kcals

Moisture
g

Protein
g

Fat
g

Mineral
g

Carbohydrates  g

Fibre
g

Calcium
mg

Phosphorus
mg

Iron
mg

Bengal gram, whole

360

10

17

5

3

4

4

202

312

5

Bengal gram, dhal

372

10

21

6

3

1

1

56

331

5

Bengal gram, roasted

369

11

22

5

2

1

1

58

340

9

Black gram, dhal

347

11

24

1

3

1

1

154

385

4

Cow pea

323

13

24

1

3

3

4

77

414

     9

Field bean, dry

347

10

25

1

3

1

1

60

433

3

Green gram, whole

334

10

24

1

3

4

4

124

326

4

Green gram dhal

348

10

24

1

3

1

1

75

405

4

Horse gram, whole

321

12

22

0

3

5

5

287

311

7

 Kherasi dhal

345

10

28

1

2

57

2

90

317

6

Lentil

343

12

25

1

2

59

1

69

293

7

Moth beans

330

11

24

1

3

56

4

202

230

9

Peas green

93

73

7

0

1

16

4

20

139

1

Peas dry

315

16

20

1

2

56

4

75

298

7

Peas roasted

340

10

23

1

2

59

4

81

345

6

Rajmah

346

12

23

1

3

61

      5    

260

410

5

Redgram, dhal

335

13

22

2

3

58

1

73

304

3

Redgram tender

116

65

10

1

1

17

6

57

164

1

Soyabean

432

8

43

19

4

21

4

240

690

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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

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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.

Source

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