Millet grains are small-seeded tiny grains often termed as nutri-cereals or dryland cereals. It includes sorghum, pearl millet, finger millet, little millet, foxtail millet, proso millet, barnyard millet, Kodo millet and others. These millets are gluten-free and are an important source of dietary proteins, carbohydrates, fibres, vitamins, minerals like magnesium, calcium, manganese, tryptophan, phosphorus and antioxidants that can be consumed by people belonging to all age groups. FAO declared 2023 as the International Year of Millets. India is the largest producer of millets with 41.0% of global market share production¹. Parboiling is one of the latest processing methods that is widely used for paddy, wheat and barley to improve its milling efficiency, nutritional and functional qualities.
Parboiling, being a hydrothermal treatment involves the boil-soak-boil method, in which three basic processes are involved and they are as below:-
1. Soaking of millets at an ambient temperature of about 70°C until saturation within a time period of 10 to 24 hours facilitates the absorption of water. Soaking of whole grains for a longer time period can decrease phytic acid content by 20 to 30 percent². Cooling of these soaked grains to room temperature causes the amylase molecules to re-associate among themselves to form a tightly packed structure, making kernels harder with a glossy appearance. Soaked grain may be pre-dried under direct sun or in dryers before steaming.
2. Steaming of soaked and dried millets can gelatinize the starch in the endosperm of the millet grains. Steaming can be done at elevated temperatures in autoclaves or steamers or containers in form of a bucket with perforations and surmounted by a kettle made of cast aluminium.
3. Drying of steamed millets under open sun or dryers is done to reduce water content by up to 10% before milling/decortication.
Effect on nutritional and functional quality of Millets:
Micronutrients contained in the bran moves to the endosperm of the millet grains during parboiling, thereby making it more nutritious. Parboiling of millets decreased protein digestibility, due to the formation of disulfide cross-linkage and altered secondary structure of protein. Protein digestibility decreased by about 17 to 31% in sorghum millets and about 4.5 % for pearl millets. This treatment leads to reduction in total starch and rapid digestible starch content and increases residual starch content, due to leaching of some starch during parboiling³. Parboiling increased the phenolic content of millets, due to the release of phenolic compounds bound to cell wall and moved to inner endosperm of the grains⁴. It was reported that parboiling improved certain properties such as increased the hardness values, reduced water uptake and swelling index values and cooking time in foxtail and little millets⁵.
Effect on anti-nutritional factors of Millets:
Though, millets are a rich source of nutrients, as compared to cereal crops such as wheat, rice, barley, the presence of anti-nutritional factors like phytates/ phytic acids will reduce mineral bio-availability such as calcium and phosphorous; and trace elements such as Zn, Fe, Cu and Mn. Presence of phytates inhibits starch and protein digestibility. Parboiling treatment has the ability to reduce phytate concentrations in millet grains and also improves the nutrition content⁴ and this was reported in Pearl Millets².
Effect on polishing/decortication of Millets:
Parboiling seals the cracks in the endosperm of grain that undergoes retro-gradation and its structure hardens the endosperm during drying, making it translucent and also increased the rate of head rice recovery. This parboiling process makes the grains tough, enhances millets dehulling efficiency, increases its resistance to breakage during milling (abrasive/ friction forces) operation. It improves dehulling and milling efficiencies, increases cooking characteristics of grains and reduces the nutrient losses during milling and cooking. Parboiling treatment increased the head rice yield anywhere between 18 to 57% for Pearl Millet and about 24% in Proso Millet with varied milling time³.
Effect on shelf-life of Millets:
Parboiling increases grain resistance to insect attacks and improves its nutritional quality. Inactivation of enzymes like lipase will take place during the boiling step in parboiling treatment. Lower fat acidity values were observed during parboiling, due to inactivation of lipoxygenase and peroxidase enzyme, which in turn resulted in improvement of shelf life of such parboiled millets made into flours. Though, parboiling improves the stability of flour, it alters its major physico-chemical properties and the appearance of the food products that may reduce consumer acceptance. To control such negative effects, further investigation is necessary.
Effect on glycaemic values of Millets:
Parboiling treatment decreased the glycaemic index values and increased residual starch content, due to the complex formation between amylose and lipids. This residual starch content remains undigested, passes to the large intestine and is converted into short-chain fatty acids on its fermentation, thereby decreasing postprandial glycaemic and insulinemic responses, lowers plasma cholesterol and triglycerides, increases satiety, improves bowel and colonic health and increases micro-nutrients absorption. Hence, it can be concluded that parboiling can be a suitable pre-treatment method to make millets as pre-biotic ingredients⁴.
Conclusion
The time and temperatures during soaking and steaming steps of parboiling will play a crucial role in affecting different properties and cause irreversible swelling of grains of millets, thereby modifying their structure and shape. The increase in soaking temperature and steaming pressure also increases the milling yield and head rice recovery. Parboiling helps to heal cracks in grain, reduces grain breakage and damaged kernel, easy de-husking and increases milling yield. This process leads to concomitant changes in the physical, chemical and nutritional properties of millets affecting dry-milling, shelf life, resistance to spoilage by insects and mould, cooking, organoleptic and processing qualities of millet grains.
References:
1. https://www.mordorintelligence.com/industry-reports/millets-market
2. Sene, S., Gueye, M. T., Sarr, F., Sow, M. S., Diallo, Y. Gaye, M. L. (2018).Optimization of Parboiling Process Pearl Millet (Pennisetum Glaucum [L.]R.Br.) GB 87-35 Variety. Journal of Food Process Technology, 9(8), 1 – 4.
3. Bora, P., Ragaee, S., & Marcone, M. (2019). Effect of parboiling on decortication yield of millet grains and phenolic acids and in vitro digestibility of selected millet products. Food Chemistry, 274(July 2018), 718–725.
4. Kaushik, N., Yadav, P., Khandal, R. K., & Aggarwal, M. (2021). Review of ways to enhance the nutritional properties of millets for their value‐addition. Journal of Food Processing and Preservation, 15550.
5. Varadharaju, N. & Ganesan S. (2017). Effect of parboiling (thermal treatment) on de-hulling and cooking qualities of little millet (Panicum sumatrense) and foxtail millet (Setaria italica), 17th International Conference on Food & Nutrition May 22-24, Las Vegas, USA.
6. Nantanga, K. K., Seetharaman, K., de Kock, H. L., & Taylor, J. R. (2008). Thermal treatments to partially precook and improve the shelf-life of whole pearl millet flour. Journal of the Science of Food and Agriculture, 88(11), 1892-1899.
About the Author:
N. Sai Prasanna
M.Tech (Food Process Engineering),
IIT Kharagpur, India.
Email ID: sprasanna557@gmail.com
