Lecture 7: Introduction to Post harvest handling of Pulse - important unit operation of pulse milling- Cleaning, Conditioning, Pitting, drying, dehusking and splitting, grading.

Lecture 8: Domestic scale and Commercial scale dhal milling Process , Wet milling and Dry milling of pulses, Differentiate between wet milling and dry milling, Advantages and disadvantages, Modern milling methods - CFTRI methods

Lecture 9: Working Principle Dhal Mill - Corborandum roller, Emery roller

Lecture 10: Factors affecting milling quality and quantity of pulse, milling efficiency formula and Problems in dhal milling industry

Milling

• Pulses are mostly consumed in the form of dehusked splits, commonly known as dal.
• The outer layer of the grain (husk) is attached to the protein and starch bearing cotyledons of the pulse grains.
• In some grains like pigeon pea, mung bean and urad bean, this bonding is strong due to the presence of a layer of gums in between the husk and the cotyledons. These are known as difficult-to-mill pulses.
• In other grains like chickpea, pea, lathyrus etc., this bonding is comparatively weaker. Such grains can be milled easily and are categorized as easy-to-mill pulses.
• The process of removal of husk from the cotyledons is called dehusking and the entire process of dehusking and subsequent splitting of cotyledons, its cleaning, polishing and grading is known as milling.
• Dehusking improves product appearance, texture, product quality, palatability and digestibility.
• Essentially milling process involves cleaning, grading, pitting, treatment milling and polishing operations. Usually milling processes are described for the toughest to mill pulse grains i.e., pigeon pea.

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Traditional Milling

Stone quern mill.

Pre-milling treatments

• The treatment is given for loosening of husk from cotyledons, which is attached through a gum layer is called pre-milling treatment.
• Mostly premilling treatments are developed for pigeonpea.
• Water soaking, oil and water application, mixing of sodium bi-carbonate solution and thermal applications are commonly recommended and adopted pre-milling treatments.
• For commercial milling in large capacity dal mills, oil and water treatment is commonly adopted, whereas for household milling, water treatment is popularly used.
• Different methods are employed in different regions depending upon type of grain.
• This also varies from mill to mill.
• Pre-treatments can be broadly classified into i) wet treatment and ii) dry treatment.

Wet treatment

• In this method of treatment, soaking and drying are considered as effective technique to loosen the husk.
• This method has the advantage of facilitating dehusking and splitting the cotyledons, giving less breakage.
• This can be attributed to lower deshusking percentage of grains in water treatment process.
• However, it has the disadvantage of being weather dependent and labour intensive.
• Dal produced by this method cooks better but takes longer time to cook.
• Commonly adopted red earth treatment is considered as wet method.
• In this method, grains are thoroughly mixed with a paste of red earth after soaking in water for about 12 hours and heaping for about 16 hours.
• The grains are spread in thin layer in drying yards for 2–4 days.
• When dried, the red earth is removed by sieving and the grains are then milled on power operated stone or emery coated vertical chakki to yield dal.

Dry treatment

• Dry milling treatment is reported to produce dal that cooks faster, however, losses due to broken and powdering are high.
• In dry method, oil/water application followed by drying are important steps in processing of pulses.
• In this process, after cleaning and grading, grains are pitted and then mixed with about 1% oil (linseed), thoroughly and spread for sun drying in thin layer, for 2–3 days.
• At the end of drying, 2-5% of water is sprayed, mixed thoroughly and tempered for overnight.
• Tempered grains are dehusked in roller machines to give dehusked grains and dal.

Pitting and Tempering

• Pitting
• Use of emery-coated roller is a common practice in commercial dal mills.
• The emery coating is used for abrasive or refractory action.
• Whole pulses are passed through abrasive roller machine for scratching of seed to facilitate the entry of oil/water in the grain during pre-milling treatment.
• Tempering
• Once the pre-milling treatment is given, conditioning is done to have uniformity of treatment throughout the grain mass.
• This process gives time for better penetration of oil/water beneath the seed coat to dissolve gums.

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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.

Cont…

• 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.

CFTRI mini dhal mill

PKV mini dhal mil

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Under runner disc (URD) Sheller

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.

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.

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.

Method of Black gram milling

• 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’

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’.

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.

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.

Dehusking and splitting Dal

• Dehusking and splitting Dal mills by and large use emery rollers for dehusking and splitting.
• In case of pigeonpea, more than 3 passes are required for complete milling.
• Other pulses take one or two passes in emery mill in order to achieve maximum milling.
• The physical, chemical and structural strength of grain coupled with the functional and mechanical characteristics of processing units jointly play an important role.
• Grain properties such as hardness, load deformation behaviour, shape, size density and variety of grain etc. have considerable effect on dal yield.
• The machine parameters such as roller speed, clearance, emery size etc. have vital role to play on dal recovery.
• As a result of milling, unhusked and dehusked whole grains, split cotyledons, broken, husk and powder are obtained.
• Husk and powder produced during milling is generally separated with the help of aspirator and are used as cattle feed.

Polishing

• Polishing is done to increase consumers appeal and is a form of value addition, though not desirable.
• Dal is polished in different ways, such as nylon polish, oil/water polish, leather and makhmal polish.
• Generally polishing is done using soap stone, oil or water.
• Polishing gives uniform look and shine to each grain.

Hulling efficiency