Millets are considered to be crops that would be among the important ones in the future because they can resist pests and diseases as well as could tolerate and grow under harsher environmental conditions in arid and semi-arid regions of Asia and Africa. Not just are they sustainable when environmental changes take place, but they are also quite nutritious and are called Nutri-cereals. Most of the millet is more nutritious than rice, wheat, and corn in terms of vitamins, dietary fiber, proteins, and minerals including calcium and iron.
Millets are normally processed prior to consumption for several reasons. Firstly, the processing is done to remove inedible portions as well as to improve the shelf life. Processing also helps in improving nutritional properties as well as the millets become more edible with improvement in sensory properties. Not all processing steps are done in all millets and for all applications. Depending on the needs some may be carried out. Processing may include dehulling, soaking, germination, roasting, drying, polishing, and milling or grinding. These will improve the acceptability of millets. Many other processes may be referred to as secondary processes and include fermentation, parboiling, cooking, puffing, popping, malting, baking, flaking, extrusion, etc., which are aimed at improving digestibility and nutrient bioavailability. However, some nutrients may be lost in some of these.
Antinutrients are compounds produced by plants for their natural defense against pests. When consumed uncooked or without any treatment, they may be detrimental to humans. They reduce nutrient absorption causing deficiency and may cause bloating. Millets e.g. ragi may contain, tannins, phytates, oxalates, trypsin inhibitors, etc. Anti-nutritional factors may be one of the disadvantages of millets over wheat and rice.
Phytic acid has a strong chelating ability forming complexes with potassium, calcium, iron, zinc, magnesium, etc. reducing their bioavailability. Pearl millet or bajra contains goitrogenic compounds namely, derivatives of phenolic flavonoids like C-glycosyl flavones. Not only they may be responsible for health problems, but their metabolites can also cause off-odors in flours during storage.
Some of the pre-treatments or processing techniques such as bran removal, soaking, germination, fermentation, and autoclaving will reduce the anti-nutrients. In some cases, there will also be nutrient losses but in others there is gain.
Soaking before cooking helps reduce anti-nutrients. That improves mineral bioavailability. Some losses occur in Zn and Fe as they are lost in soaking water. Soaking in hot water decreases phytic acid and increases bioavailability. Germination and fermentation also affect minerals in millets. Germination causes catabolism of saponins and polyphenols that inhibit mineral bioavailability. Germination also activates phytase-specific phosphatase enzymes. This releases minerals bound to phytates increasing its availability. Fermentation improves the availability of many minerals including Ca, Fe, and Zn. Fermentation and germination improve the contents of most vitamins.
Decortication not only reduces antinutrients but also removes mineral content like Ca, Fe, and Zn because they are present in bran. Polishing and bran removal result in losses of vitamins such as riboflavin, thiamine, niacin, and folic acid, as these are present in bran and germ.
Boncompagni et al. 2018, Antinutritional Factors in Pearl Millet Grains: Phytate & Goitrogens Content Variability & Molecular Characterisation of Genes Involved in Their Pathways PLoS ONE 13 (6): e0198394