Processing of Farm Crops: Processing of Pulses, Crop Process Engineering | Crop Process Engineering Notes - Agricultural Engg - Agricultural Engineering PDF Download

Introduction

 5.1 Definition of terms used in pulse processing

Gram: Dry legume seeds with husk

Decortications/dehusking: Removal of outer husk from gram

Splitting of pulses: Breaking a seeds in to two halves

Milling of pulses: Removing the husk and splitting the grains into two halves

Dhal: Split decorticated grains

Gota: Unhusked pulse grains coming out of the roller mill

Dust/chuni: Powder obtained as byproduct from the milling operation of pulses

Brokens: Broken grains coming out of the roller along with dhal

Cleaning: Removing the various inorganic impurities present in the raw grains

Grading: Separating the cleaned grains based on size

Conditioning: loosening of the husk to facilitate its separation from the kernel, thus reducing the milling losses(e.g. Hydrothermal treatment, water treatment, heating, use of salt or chemicals)

Polishing: Mixing edible oil and water with dhal by publishing through a screw conveyer to impart desirable color and shine to milled pulses

Grading of dhal: separation of dhal as per the size and soundness

 5.2 Pulse milling process

Basic processes in dhal milling are cleaning, de-husking, splitting, separation and bagging. Major variation is involved with de-husking process only. Dhals like Arahar, urad, moong and lentil are difficult to de-husk as a result repeated operations by de-husking 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 dhal.

The removal of the outer 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. In India trading milling methods produce de-husked split pulses. Loosening of husk by conditioning is insufficient in traditional methods. To obtain complete de-husking of the grains a large number of abrasive forces is applied in this case as a result high losses occur in the form of broken and powder.

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. The dry milling quality is found to be good and on an average 70-72 % dhal recovery is reported. Wet milled dhal is of better taste than dry milled dhal, but takes longer cooking time.

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 here.

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. Husk is cracked and scratched in this operation. 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/tone of pulses). Then they are kept 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 % 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 % moisture content.

De-husking and Splitting: Emery rollers, known as Gota machine are used for the de-husking of conditioned pulses About 50 % pulses are de-husked in a single operation (in one pass). De-husked pulses are split into two parts also, the husk is aspirated off and de-husked split pulses are separated by sieving. The tail pulses and unsplit de-husked pulses are again conditioned and milled. The whole process is repeated two to three times until the remaining- pulses are de-husked and split.

Polishing: Polish is given to the de-husked and split pulses by treating them with a small quantity of oil and/or water.

 5.3 Equipments required for pulse processing

Vibratory sieve separator: For cleaning of raw grains

Cleaner with aspirator:  For removing lighter impurities such as dust, husk, brokens of insects etc.

Destoner: To separate mud and stones from raw grains

Auger Mixer/Screw conveyor: For mixing water/oil/red earth during conditioning and polishing of grains/dhal

Pulse scourer: An emery scourer (emery roller) machine is used to obtain cracking and scratching of clean pulses passing through it. For loosing the husk to facilitate the subsequent oil penetration this required. Gradually the clearance between the emery roller and cage (housing) is narrowed from inlet to outlet. Cracking and scratching of husk takes mainly by friction between pulses and emery as the material passed through the narrowing clearance. During the operation some of the are de husked and split which are separated by sieving.

Pulse splitter: For splitting and dehusked grains into two halves

LSU Drier: For conditioning of the grains before milling

Emery coated rollers: For dehusking and milling of pulses.

Rotating reel grader: For separating the dhal based on size and soundness, in which separate compartments with different size screens are present. Various sizes of dhal though the openings of these screens and graded dhal are bagged separately.