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Post-Harvest Processing of Millets: Maintaining Quality and Nutritional Value
Post-Harvest Processing of Millets:
Maintaining Quality and Nutritional Value
Goodness Farm
Millets, categorized into two major groups based on the presence of husk on the grains.
Naked millet grains include sorghum (jowar), pearl millet (bajra), and finger millet (ragi), while husked millet grains include foxtail millet, little millet, barnyard millet, proso millet, kodo millet, and browntop millet.
Husked millet grains require dehulling for consumption, which can be achieved using two forces – impact or shear.
However, the processing of millets often compromises on their nutritional value as the bran layer is completely removed due to pest infestation problems. As a result, processors often resort to chemical methods of cleaning or polishing their products.
Nevertheless, the inherent variations in the harvested grains’ characteristics hold significant nutritional profiles.
To maintain the quality of millet grains, proper post-harvest processing is crucial. The process typically involves using different machines such as a grader-cum-aspirator, destoner, and dehuller-cum-aspirator.
The primary cleaning is done using a grader-cum-aspirator, which separates the millet grains from sand particles, stones, sticks, mud balls, and straws. Proper selection of the sieve is important to obtain good quality millet grains. Before processing the grains, their moisture level should be around 11-12%, which can be achieved by drying them in sunlight for 2-3 days. Small millet processing generally requires a triple deck grader with three sieves.
The material coming from the grader is then sent to the destoner to remove small stones and mud balls of the same size as the grains. The destoner works on the principle of gravity, where the graded material falls on the destoner bed, and the lighter material moves towards the front while the heavier material moves towards the rear end. The air adjustment slot needs to be carefully adjusted depending on the material.
To reduce the presence of antinutrients, the millets are thoroughly cleaned and soaked for 3-4 hours, then rinsed and sun-dried or dehydrated to achieve a specific moisture level. The next step involves dehusking the millets using a centrifugal dehuller, followed by another round of soaking and drying. The millets are then cleaned using an aspirator machine and soaked for a final time for 2 hours before being rinsed and dried. The duration of the soaking time varies depending on the type of millet chosen. These additional steps have been backed by research and are exclusively and only done at Goodness Farm.
The raw materials, after proper cleaning, are sent for husk removal using a dehuller-cum-aspirator. Dehullers can be classified into two types: centrifugal dehullers and abrasive dehullers. The quality of the dehuller is measured based on the retention of the bran layer on the millet rice after dehulling.
Centrifugal dehullers have an impeller responsible for husk removal, while abrasive dehullers are classified into emery and rubber roller types. In the emery type dehuller, two grinding stones are used for husk removal, where one stone is stationary and the other rotates at a constant speed (the one used at Goodness Farm). In the rubber roller type, rubber rollers are used instead of stones.
As the dehulling speed and friction differs for every millet variety we calculate the de-husking efficiency, head grain yield, broken percentage, and milling efficiency, the following equation was formulated and used:
De-husking efficiency% = A/B 100
Head grain yield%=C/A100
Broken %=D/A100
Milling efficiency%=EF/100
Overall, proper post-harvest processing of millets is essential to maintain their quality and nutritional value.
Goodness Farm
MJ
