A review of millet types, their nutritional composition and the effects of different processing methods on nutritional qualities

Uma revisão dos tipos de milheto, sua composição nutricional e os efeitos de diferentes métodos de processamento nas qualidades nutricionais

Abstract

Millets are small, seeded crops classified as major (pearl, finger, sorghum) or minor (proso, barnyard, foxtail, kodo, little) millet. Stating them as nutri-cereals, they deliver significant amounts of diverse nutrients that promote health as well as bioactive substances like antioxidants, dietary fiber, macro and micronutrients, and so on, when compared to other grains that are commonly consumed viz. rice, maize and wheat. These nutrients have an important role in the nutritious safekeeping of mankind. They are associated with numerous advantages, including drought tolerance, high yielding in limited water conditions, and they have high nutritious value. A comparative review was conducted among all the millets found in India. Here, the study on the application of processing techniques gives an insight into how nutritional values alter when the millets are being processed, and there is limited research on the loss of nutrients while processing as well as how the shelf-life or storage ability is affected. The easy cultivation process, resilient nature and high nutritional contents of these millets show a new area of research in current scenario of harsher climatic conditions. The purpose of this work is to review different types of millets, their nutritional composition and the effects of different processing methods on nutritional qualities.

Keywords:
millets; processing; nutrition; health; nutrients

Resumo

O milheto é uma pequena cultura com sementes, classificada como milheto principal (pérola, dedo, sorgo) ou secundário (proso, terreiro, rabo-de-raposa, kodo, pequeno). Denominados nutricereais, eles fornecem quantidades significativas de diversos nutrientes que promovem a saúde, bem como substâncias bioativas, como antioxidantes, fibra alimentar, macro e micronutrientes, entre outros, quando comparados a outros grãos comumente consumidos, como arroz, milho e trigo. Esses nutrientes desempenham um papel importante na proteção nutricional da humanidade. Estão associados a inúmeras vantagens, incluindo tolerância à seca, alto rendimento em condições de água limitada e alto valor nutritivo. Uma revisão comparativa foi realizada entre todos os milhetos encontrados na Índia. Aqui, o estudo sobre a aplicação de técnicas de processamento dá uma ideia de como os valores nutricionais se alteram quando os milhetos são processados. Note-se que há pesquisas limitadas sobre a perda de nutrientes durante o processamento, bem como se o prazo de validade ou a capacidade de armazenamento são afetados. O fácil processo de cultivo, a natureza resiliente e o alto teor nutricional desses milhetos mostram uma nova área de pesquisa no cenário atual de condições climáticas mais adversas. O objetivo deste trabalho é analisar diferentes tipos de milheto, sua composição nutricional e os efeitos de diferentes métodos de processamento nas qualidades nutricionais.

Palavras-chave:
milheto; processamento; nutrição; saúde; nutrientes

1. Introduction

Millets are coarse grains cultivated mostly by smallholders and tribal farmers under rain-fed circumstances. Millets can be found throughout Asia, Africa, and Europe. These are among India's earliest cultivated crops. Millets are labeled as “yesterday's coarse grains and today's nutri-cereals” (Rao et al., 2017). A small quantity of millet comprises up to 1000 grains. The term “millet” is of French origin “mille,” means thousand (Chandrasekara and Shahidi, 2010). The seed grain is classified to family Poaceae. The exact origin of millet is unknown, but they were farmed in various areas of the world for thousands of years. The world’s one third population has millet intake in their diet (Tripathi and Vyas, 2023). The most common and significant ones are sorghum millet (Sorghum bicolor L.), pearl millet (Pennisetum glaucum), finger millet (Eleusine carocana), little millet (Panicum sumatrense), kodo millet (Paspalum scrobiculatum), proso millet (Panicum miliaceum), foxtail millet (Setaria italica), barnyard millet (Echinochloa species) (Rao et al., 2017). India, Greece, China, Egypt and Africa produce much of the worldwide marketable millet crop. Millets have excellent nutritional value. Millets play a vital role in many traditional cuisines. People across the Indian states employ different types of millets in their culinary habits. Researchers have observed and found that staple cereals like rice, wheat and maize have lesser nutrition than majority of millets by three to five times and gluten free therefore millets are commonly stated as “superfoods” because of their significant nourishing value (Ashoka et al., 2020). Millets are recognized as prospective crops due to higher nutritive content, which is equivalent to rice, wheat and maize, with documented health benefits. Millets also have higher concentration of non-nutritive components, particularly phenolic compounds and tannins, and phytate, all of which act as “antioxidants”. There is evidence suggesting that phenolic compounds possess antioxidant properties preventing oxidative stress and potentially plummeting the threat of non-communicable ailments within the human body (Kumari et al., 2016). Millets are also known as nutraceuticals and extraordinary foods. Nutraceuticals comprise biologically active components that are valuable for human bodily health and can assist combat chronic diseases. They are gaining popularity as a staple diet in economically developing countries like China, India and several African countries and can be used as animal food providing all the nutrients. Mostly agricultural byproducts were used for animal nutrition. India dominates the global market of millet production in recent years. But millets are underutilized or not at par like other cereals, in terms of their usage due to inadequate knowledge about processing methods creating issues like low quality, flavour, and bioavailability. These concerns can be resolved, making them desirable as food for underprivileged households who suffer from malnutrition (Sarita and Singh, 2016). The Figure 1 one shows the usage of millets.

Figure 1
Uses of millet.

Millets are extremely nutritious crops, abundant in diverse range of dietary supplements (shown in Table 1). Millets are rich in calories, resistant starch, dietary fiber and slowly digesting starch, which promotes continuous release of glucose and satiates. Millets contain more protein and sulfur-containing amino acids like cysteine and methionine than staple grains along with an excellent fatty acid profile (Anitha et al., 2020). Conversely millets have a partial amount of lysine and tryptophan, depending on the cultivar. Millets are an abundant source of vitamin B and E, as well as minerals i.e. phosphorus, magnesium, manganese, potassium, calcium and iron. The Food Safety and Standards Authority of India (FSSAI) reports them as a good source of dietary fiber. Millets are also wonderful grains for treating malnutrition and micronutrient deficits because of their high nutrient density, which includes vitamins, minerals, phytochemicals, and dietary fiber (Birania et al., 2020). There are different types of millets shown below in Figure 2 and Figure 3 shows the classification millets. Buckwheat and Amaranth were termed as Pseudo millets in Figure 3 due to same nutritional profile.

Table 1
Nutritional composition of nutri-cereals v/s staple grains.

Grains	Moisture (g)	Energy (Kcal)	Protein (g)	Carbohydrate (CHO) (g)	Fat (g)	Dietary/Crude Fiber(g)	Calcium (Ca) (mg)	Potassium(K) (mg)	Phosphorus (P) (mg)	Ash (g)	Reference
Sorghum Millet	09.01 ± 0.77	334	09.97 ± 0.43	67.68 ± 1.03	1.73 ± 0.31	10.22± 0.49	27.60 ± 3.71	–	–	1.39 ± 0.34	Rao et al. (2017)
	–	–	10.4	72.6	1.9	1.6	25	–	222		Narwal et al. (2022)
	–	–	10.4	72.6	1.9	1.6	25	–	–	–	Poshadri et al. (2023)
	–	329	10.4	70.7	3.1	2.0	13	363	289	1.6	Nani and Krishnaswamy (2021)
Pearl millet	8.97±.60	348	10.96±.26	61.78±.8.5	5.43±.64	11.49±.62	27.35 ± 2.16	-		1.37±.17	Rao et al. (2017)
	–	–	11.6 5	67.5	5.0	1.2	42	-	296	-	Narwal et al. (2022)
	–	–	11.6	67.5	5.0	1.2	42	–	–	–	Poshadri et al. (2023)
	12.4	361-363	11.6-11.8	67-67.5	4.8-5.0	11.3	42	307	296	–	Ramashia et al. (2019)
Finger millet	10.89 ± 0.61	320	07.16 ± 0.63	66.82 ± 0.73	1.92 ± 0.14	11.18 ± 1.14	364 ± 58	–		2.04 ± 0.34	Rao et al. (2017)
	–	–	7.3	72.0	1.3	3.6	344	-	283		Narwal et al. (2022)
	–	–	7.3	72.0	1.3	3.6	344	–	–	–	Poshadri et al. (2023)
	7.15-13.1	–	7.7	75.0-83.3	1.8	15-22.0	398.0	430-490	130-250.0		Ramashia et al. (2019)
	–	336	7.7	72.6	1.5	3.6	344	408	283	2.6	Nani and Krishnaswamy (2021)
Foxtail millet	–	331	12.30	60.09	4.30	–	–	–	–	–	Rao et al. (2017)
	–	–	12.3	60.9	4.3	8	31	–	290	–	Narwal et al. (2022)
	–	–	12.3	60.9	4.3	8.0	31	–	–	–	Poshadri et al. (2023)
	11.2	331	11.50-12.3	60.9-75.2	2.38-4.3	2.5-8.5	31	250	290		Ramashia et al. (2019)
Barnyard millet	-	307	6.20	65.55	2.20	-	-	-		-	Rao et al. (2017)
	–	–	11.6	74.3	5.8	14.7	14	–	121	–	Narwal et al. (2022)
	-	–	12.2	55.8	3.85	10.1	24	–	–	–	Poshadri et al. (2023)
	11.9	307	6.2	65.5	2.2	1.98	20-22	–	280	–	Ramashia et al. (2019)
	–	300	11.0	55	3.9	13.6	12		280	4.5	Nani and Krishnaswamy (2021)
Proso millet	–	341	12.50	70.04	1.10	–	–	–	–	–	Rao et al. (2017)
	–	–	12.5	70.4	1.1	2.2	14	–	206		Narwal et al. (2022)
	–	–	12.5	73.0	4.2	2.2	14	–	–	–	Poshadri et al. (2023)
Little millet	14.23 ± 0.45	346	08.92 ± 1.09	65.55 ± 1.29	2.55± 0.13	06.39 ± 0.60	16.06 ± 154	–	–	1.72 ± 0.27	Rao et al. (2017)
	–	–	7.7	-	–	7.6	27	–	–		Poshadri et al. (2023)
	–	329	9.7	60.9	5.2	7.6	17	126	220	5.4	Nani and Krishnaswamy (2021)
Kodo millet	14.23 ± 0.45	331	08.92 ± 1.09	66.19 ± 1.19	2.55 ± 0.13	06.39 ± 0.60	–	–	–	1.72 ± 0.27	Rao et al. (2017)
	–	–	11.0	66.6	3.6	10.0	27	–	–	–	Poshadri et al. (2023)
	12.8	309	9.8	65.9-66.6	1.3	2.47	27	144	188	–	Ramashia et al. (2019)
Wheat (Whole)	10.58 ± 1.11	321	10.59 ± 0.60	64.72 ± 1.74	1.47 ± 0.05	11.23 ± 0.77	39.36 ± 5.65	–	–	1.42 ± 0.19	Rao et al. (2017)
	–	—	11.8	71.2	1.5	1.2	41	–	306	–	Narwal et al. (2022)
	–	–	11.8	78.2	0.5	0.2	10	–	–	–	Poshadri et al. (2023)
	–	348	14.4	71.3	2.3	2.9	41	363	306	1.8	Nani and Krishnaswamy (2021)
Rice (Raw Brown)	09.33 ± 0.39	353	09.16 ± 0.75	74.80 ± 0.85	1.24 ± 0.08	04.43 ± 0.54	10.93 ± 1.79	–	–	1.04 ± 0.18	Rao et al. (2017)
	–	–	6.8	78.2	0.5	0.2	10	–	160		Narwal et al. (2022)
	–	–	6.8	78.2	0.5	0.2	10	–	–		Poshadri et al. (2023)
	–	362	7.5	78.2	2.4	10.2	10	130	160	4.7	Nani and Krishnaswamy (2021)
Miaze (Dry)	09.26 ± 0.55	334	08.80 ± 0.49	64.77 ± 1.58	3.77 ± 0.48	12.24 ± 0.93	8.94 ± 0.61	–	–	1.17 ± 0.16	Rao et al. (2017)
	–	–	11.5	66.2	3.6	2.7	20	–	348	–	Narwal et al. (2022)
	–	–	11.1	66.2	3.6	2.7	10	–	–	–	Poshadri et al. (2023)
	–	358	12.1	66.2	4.6	2.3	10	270	89	1.8	Nani and Krishnaswamy (2021)

Figure 2
Types of millets: (a) Sorghum millet; (b) Finger millet; (c) Pearl millet; (d) Little millet; (e) Proso millet; (f) Foxtail millet; (g) Barnyard millet; (h) Kudo millet.

Figure 3
Classification of millets based on their grain size and two pseudo millets due to their same nutritional profile.

2. Types of Millet

Different types of millet have been described in the following with their local name, family and environmental conditions to grow.

2.1. Sorghum Millet (Sorghum bicolor L)

It is commonly referred to as Jowar. This crop is cultivated majorly in India and Africa and among the oldest cereal grains. Sorghum is a safe substitute for people with celiac disease or gluten sensitivity (Ambati and Sucharitha, 2019).

2.2. Pearl Millet (Pennisetum glaucum)

Asian and African continent cultivars have grown this multipurpose crop typically for feed, food and forages. Its common names in different languages of India are bajra, kambu, sajje, bajri, kamban, sajjalu, etc. (Satankar et al., 2020). This grain is sturdy cereal as it can grow in severe environments as in nutrient-deficient soils. It is a high-energy food that has more proteins, fiber, and minerals like iron and zinc. The phytochemicals present in the grain are of antioxidant properties (Srivastava et al., 2023).

2.3. Finger Millet (Eleusine coracana)

Finger millet ranks as the sixth most cultivated crop after staples like rice, wheat, maize, sorghum and pearl in India. Normally it is called Ragi and in Assamese, it is called Marua (Muthamilarasan et al., 2016). Finger millet, recognized for its excellent nutritional content, has a lot of calcium, which is important for bone health. A significant amount of dietary fiber, phenolic compounds, and important amino acids were found in it (Srivastava et al., 2023).

2.4. Little Millet (Panicum sumatrense)

It is one of the course cereals consumed in the form of rice (Nandini et al., 2019). It belongs to the poaceae family and panicoideae subfamily. Little millet is a significant produce cultivated for human eating and animal fodder, mainly in the tribal zones of Chhattisgarh, Andhra Pradesh and Madhya Pradesh.

2.5. Foxtail Millet (Panicum italicum L.)

Foxtail millet is recognized as kakun, kangni, tenai and navane, is an unirrigated produce primarily cultivated in China, India, and Bangladesh. Rich in dietary fiber and minerals. Foxtail millet also contains phytosterols and flavonoids associated with benefits of cholesterol reduction and anti-inflammatory effects (Srivastava et al., 2023).

2.6. Proso millet (Panicum miliaceum)

Proso millet is an extensively cultivated harvest in South-east Asian nations like India, Sri Lanka, Pakistan and Nepal. More than half a million hectares of proso millet are grown in India, primarily in the states of Tamil Nadu, Karnataka, Andhra Pradesh, Uttarakhand and Andhra Pradesh. It is a short-duration crop that is highly tolerant of heat and drought and matures within 60 to 90 days of plantation. Proso millet is an energy-dense seed that is rich in protein, fiber, and vital minerals like magnesium, manganese, and phosphorus. It has a low glycemic index (Srivastava et al., 2023).

2.7. Barnyard millet (Echinochloa species)

India leads worlds as the principal cultivator of barnyard millet, with respect to production (0.147 mt), and area (0.146 m ha) with an average productivity of 1034 kg/ha for three years (Renganathan et al., 2020). It is cultivated for both food and fodder making it a multipurpose crop. It serves as a source of protein and dietary fiber providing a good equilibrium of insoluble and soluble components (Sharma et al., 2020).

2.8. Kodo millet (Paspalum scrobiculatum)

Kodo millet, which is extensively cultivated in arid and semi-arid regions of Africa and India, where the soils are of poor quality. The grain’s color ranges from light red to dark gray and is bound in a hard, complicated husk (Bunkar et al., 2021).

Cluster analysis of nutritional composition of nutri cereals v/s staple grains were prepared with the help of heat map as shown in Figure 4. Data indicates similarity in energy (high energy) content in proso millets and barnyard millets while rice, kodo and little millet were exhibiting dissimilarity in terms of potassium and phosphorus content among them. Similar observation of dissimilarity in energy content of finger millet can be seen. Again, in terms of energy content the sorghum millet and wheat are quite similar but dissimilar in terms of potassium and phosphorus content. However, in the case of foxtail millet’s energy content the similarity with pearl millet and dissimilarity with maize can be seen. The ash, fat, fiber, moisture and protein content seem quite similar in all of them. But in case of potassium and phosphorus dissimilarity among all is seen. The carbohydrate content of barnyard, kodo, little, sorghum, wheat, maize and foxtail millet are quite similar with each other. But in terms of calcium (high) content finger millet is very dissimilar with others.

Figure 4
Heat map of nutritional composition of nutri-cereals v/s staple grains.

Millets were primary crop in India since ages, accounting for over one-third of the total food supply. Although millets are more nutrient-dense than other cereals, only impoverished and traditional people consume them in their diet. Due to its richness in calcium (0.38%), dietary fiber level (18%), and presence of phenolic components (0.3–3%), millets are well known for their nutritional value (Devi et al., 2014). When compared to rice, millet provides a greater source of protein (8%), calories, dietary fiber, fat (4%), and they contain approximately 65–75% carbohydrates. It contains huge amounts of non-starchy polysaccharides, minerals and calcium. The dietary composition of diet plays a vital role for human wellbeing and maintenance of the body’s metabolism. Millets provide 320 to 370 kcal of energy per 100 g. Millets with more roughage offer numerous health advantages, which include better gastrointestinal health, blood lipid profiles, and blood glucose levels. They are gluten free having a low glycemic index, advantageous for individuals with celiac disease and diabetes. Millets also contain a variety of phytochemicals which contribute to health benefits, such as polyphenols, phytosterols, lignins, phycocyanins, and phytoestrogens. These phytochemicals protect against age-related degenerative diseases such as type 2 diabetes, cardiovascular disease, and cancer by functioning as antioxidants, immunomodulators, and detoxifying agents. One study revealed that millets contain approximately 50 unique phenolic groups and their derivatives, including ferulic acid, flavones, flavanols, and flavanonols, all of which have strong antioxidant capabilities (Azeez et al., 2022). Figure 5 shows the health benefits associated with the millets.

Figure 5
Benefits of millets to human being (AO-Antioxidant, FR- Free radical, NC-Nerve cell).

3. Antinutrients Factors of Millets

Antinutrient substances like tannins, dietary fiber, trypsin inhibitory factors, phenols and phytates, which hinders the enzyme activity causing metal chelation (Sruthi and Rao, 2021). It has been deduced from the studies that antioxidant activity can be owed to phenols, tannins, and phytates presence playing a noteworthy role in wellbeing of human affecting ailments. Antinutrients are phytochemical substances made naturally by the plants for defense mechanisms. These antinutritional agents interfere with food absorption, resulting in lower nutrient bioavailability and utilization (Bora et al., 2019). Plant-based diets include antinutrients namely phytates, tannins, oxalates and inhibitors of chymotrypsin. A disadvantage of millets is that they contain more antinutritional elements than wheat and rice. Antinutrients are removed from food grains using pretreatment or processing processes including dehulling, germination, soaking, fermentation, autoclaving etc. These methods improve the minerals’ bioavailability such as Zn, Fe, and Ca and absorption of protein (Birania et al., 2020).

4. Processing Technologies for Millet

Agricultural processing remains the vital aspect of food and nutrition security in today’s era. There are certain significant technologies utilized in manufacturing food products that are related to sensory enhancement, nutritional characteristics, and convenient use. This makes processing a significant chore since it boosts bioaccessibility of nutrients and palatibility although decreasing antinutrients (Rao et al., 2016). Dehusking/decortication, soaking, fermentation, milling, germination, heating, autoclaving, malting, and roasting are some of the procedures used in processing. These procedures change the nutritive content, physical and functional behavior of food by causing changes in physical-chemical characteristics (Gowda et al., 2022). Further the commonly used processes have been described. Figure 6 illustrates the classification of general processing methods used for millets.

Figure 6
Classification of processing methods.

4.1. Dehulling and Decortication

Before being consumed, millet and few other abrasive grains are typically dehulled where hard outer coating is being removed and given various treatments to enhance their edible and sensory qualities. Dehulling, also known as decortication, is one of the primary processing techniques used in cereals and millets which removes the outer layer of the grains (Rao et al., 2017; Sruthi and Rao, 2021). This was traditionally accomplished by manual pounding with stones or wooden mortars, which frequently led to severe nutrient loss, endosperm and bran loss along with the husk, and major grain breakage. Additionally, the procedure took a lot of time and effort (Jaybhaye et al., 2014). Improved abrasive dehullers were created to get around these restrictions. These machines include a hopper for feeding grain and abrasive plates that remove the husk. Centrifugal force propels the grains onto the plates, and a cyclone separator effectively separates the husk (Kate and Singh, 2021).

4.2. Soaking

Grain soaking is a widespread process during processing of food. It reduces antinutritional substances namely phytic acid and phytase activity to ameliorate mineral absorption (Bhuvaneshwari et al., 2020). Soaking time affects the antinutrient components of the seed.

4.3. Germination

Germination is commonly a popular household processing procedure that could greatly enhance the dietary worth of millet-based diet items by unlocking the activity of enzymes (Chandraprabha and Sharon, 2021). During germination, the grains were typically soaked for 2-24 h before being spread on a moist fabric for roughly 24-48 h or kept in incubation at 30 ̊C for 48 h (Ramashia et al., 2019). Germination, when carried out under controlled conditions, stimulates biological activities in the grain, improving its functional qualities and nutritional value (Hassan et al., 2020).

4.4. Malting

Malting is a commonly used processing technique in millets that involves three sequential stages—steeping, germination, and kilning (Kate and Singh, 2021). This method involves soaking grains in water, letting them germinate to encourage the growth of sprouts and enzyme activity, and then drying them to a moisture content of 3–5% to stop the enzymatic activity. Large-scale operations usually use hot air at 80–220°C, which is circulated through the grain bed until the appropriate moisture content is obtained, even if drying can be accomplished naturally under the sun (Habschied et al., 2023). Handa et al. (2017) has reported that malting loss occurs due to conversion of carbohydrates into simple sugars and further during growth of embryo their utilization is done. The set-up conditions were ambient for the hydration of grains during malting, where the grain constituent gets changed by the endogenous enzyme (Nadeem et al., 2010).

4.5. Fermentation

Fermentation is a traditional processing method used to improve the nutritional content and digestibility of foods made from millet. It depends on microbes to break down complex carbohydrates, but enzymatic hydrolysis uses enzymes directly to accomplish the same goal. Yeast or a sourdough starter made from previous batches of fermented millet can be added to start the fermentation process. Enzymes like α-amylase and β-glucanase improve digestibility and nutritional quality by breaking down complex carbs into simpler sugars by enzymatic hydrolysis. This method is also used to produce malt for brewing (Dixit and Ravichandran, 2024). Fermentation utilizes microorganisms to modify the nutritional properties of meals and brews. Fermentation can happen naturally or stimulate deliberately to alter the perceivable features like aroma, flavor and texture of the edibles. Fermentation helps to augment the nutritional and beneficial features of food, thereby offering consumers improved health benefits (Rezac et al., 2018). Here protein is enhanced by improving digestibility and boosting the lysine content. Helps in inhibiting pathogenic microorganisms by producing antimicrobial ingredients (Xiang et al., 2019).

4.6. Popping

Popping or puffing is the high-temperature processing technique involves subjecting millets to heat for a brief period of time, usually between 180 and 200°C (Dixit and Ravichandran, 2024). Arkhipov et al. (2005) described the process of popping where expansion of moisture causes it to pop out through the outer shell of the kernel during heating. While expansion of pre-gelatinized kernels occurs due to sudden release of water in the process of puffing (Hoke et al., 2007). Popping or puffing is an ancient food processing technology used to make ready-to-eat items from expanded cereals and grain legumes. Popped or puffed items are used as snacks and base of various supplementary foods as they are pre-cooked (Mishra et al., 2014).

The processing methods described above cause an effect on the nutritional content of the grain. The effect has been elaborately described below for dietary fiber, protein, vitamins, fats, flavonoid and carbohydrate components.

5. Effect of Processing Methods

5.1. Effect on the shelf life/storage ability of millets

The processing methods like thermal (microwave processing, infrared treatment, radio frequency) and non-thermal (cold- plasma, ultrasound processing, pulsed light, high-pressure, gamma radiation) methods were explored for the storage stability of millets. It is well known that millets have lower shelf life compared to other staple cereals owing to the lipase activity and free fatty acids (FFA). They cause rancidity and bitterness. It has been reported that thermal processing does enhance the shelf life but causes protein denaturation, which is a matter of concern. Though it can be reduced by reducing the exposure time of millets to heat. The acceptability of millets by consumers reduces due to physical and practical changes (Kaur et al., 2023). The treatment processes must be standardized for each millet to ensure nutritional security by maximizing the food quality and nutrient content. The thermal processes generate more carbon footprint as they are energy intensive (Latha Ravi and Rana, 2024). Non thermal processing can reduce emissions and energy usage. Carbon footprints can be lowered if renewable sources of energy are applied (Periakaruppan et al., 2023).

5.2. Effect on nutritional properties of millets

5.2.1. Effect of processing on protein

Millets are exceptional storehouses of protein. It is believed that as a plant their protein content is at par, with significantly reduced levels of saturated fats compared to animal proteins. Dehulling, milling, soaking, and heating are simple ways for reducing antinutrient levels and increasing in vitro protein digestibility (Sharma and Niranjan, 2018). Germination (at 25°C for 40 h), fermentation, alkaline cooking and popping improved the quality of protein of foxtail millet. Due to proteolytic enzymes synthesis by microflora during fermentation causes the degradation of antinutritional phytate and conversion of insoluble protein to soluble protein (Rani et al., 2019). A study showed an increase in the quantity of protein in proso millet by 9.5% post pan-frying. Sprouting for 96 h of proso millet causes an increase in protein concentration (Morah and Etukudo, 2017). The concentration of protein increased upto 8.12% from 7.92% in the kodo millet by the process of puffing or popping (Jaybhaye et al., 2014). While another found that proso millet had higher protein after sprouting for 96 h (Morah and Etukudo, 2017). Studies also reported the protein concentration in pearl millet after germination amplifies from 14% to 26%, Malting process in pearl millet significantly boosted the protein content once soaked for 24 h and for 18 h germination (Morah and Etukudo, 2017).

Decortication lowers the substantial damage of proteins and amino acids such as arginine, lysine, and histidine, by eliminating roughly 12% to 30% of the exterior bran, husk, and germ components of seeds. Gowda et al. (2022); Sharma et al.,(2015) explored the outcome of germination on the nutritive content of foxtail millet. It was also revealed that total protein content rose following the germination phase. A study conducted by Abioye et al. (2018) reported surge in protein content of the finger millet due to germination.

5.2.2. Effect of processing on carbohydrate

The culinary methods such as pressure cooking, soaking, sprouting, and autoclaving have the potential to influence the quantity of readily available carbohydrates in dietary grains (Rao et al., 2017). The carbohydrates in pearl millet flour, showed no significant change during the initial hours (24 and 48h) of germination, but drastically reduced after 72 h (Bello et al., 2017). Because carbohydrate levels are dependent on these grain qualities viz. the decrease in crude protein, moisture, fat, and ash, corresponds to an increase in carbohydrate during foxtail millet germination. The studies carried out to observe the impact of sprouting and fermentation on the carbohydrate concentration of pearl millet reported that germination considerably surges the concentration of total soluble sugar as well as both non-reducing and reducing sugars (Rani et al., 2018; Pawase et al., 2016). Due to breakdown of starch during the germination process of proso millet, surge in the whole quantity of sugars was observed (Balasubramanian et al., 2014). The digestible starch content was reduced by the process of parboiling in proso millet and pearl millet, reducing its glycemic index (1.6 to 3.9%). It suggests the role of parboiling in formulating various products for metabolic diseases (Bora et al., 2019). Total amount of carbohydrates increased by 16% in finger millet after the hydrothermal and decortication process (Dharmaraj and Malleshi, 2011).

5.2.3. Effect of processing on dietary fiber

The major source of roughage is contributed by the bran fraction of the millet which is not available as they are characterized as complex polysaccharides. Substantial reduction in fiber components is observed after decortication and dehulling. For maximizing the fiber content in millets, it is imperative to regulate the extent of dehulling as most of the millets are consumed in decorticated form (Yousaf et al., 2021). According to a study (Sharma and Niranjan, 2018) on the effects of milling on foxtail millet's fiber components, the unsolvable dietetic fiber content including cellulose, hemicellulose, and lignin in the powdered part was lesser in comparison to entire millet flour. Conversely the fiber amount of foxtail millet showed significant rise with longer germination times. The fiber level in pearl millet increased from 0.77% to 0.87% after malting it for 24 h (Obadina et al., 2017). Before cooking, millets must be soaked to reduce the antinutrients which enhances the bioavailability of minerals. Millet grains that have been soaked increase the “in vitro solubility” of minerals like iron and Zinc by 2–23%. Germination and fermentation influenced the mineral composition of pearl millet flour (Rani et al., 2018). Sharma et al. (2017) stated an enhancement in mineral content from 232.82 to 251.73 mg/100 g after germination at 38.75°C for 36 h in kodo millet. Fermentation increased the accessibility of Calcium, Iron, Phosphorus, and Zinc by 20%, 27%, and 26% respectively. Fe availability increased in vitro from 2.19 to 3.29 mg/100 g of pearl millet after being bleached for 90 s (Rani et al., 2018). Roasting at various periods and temperature reported to lessen the fiber amount in millet. Puffing and popping thermal processes also reduced fiber content of millets (Chauhan and Sarita, 2018). The fiber contributed by the exterior bran covering reduces constipation and type 2 diabetes. So, millers must be discouraged to polish millets as unpolished contributes to good health.

5.2.4. Effect of processing on vitamins

The accretion of minerals and vitamins naturally occurs in the germ, aleurone, and pericarp of the grain. Vitamins like riboflavin, thiamine, niacin, folic acid (Shahidi and Chandrasekara, 2015) and pyridoxine (Rao et al., 2017) are rich in millets. It has been shown that the vitamin composition of pearl millet can be influenced by both germination and fermentation. Researchers deduced an increase in the levels of certain vitamins such as thiamin (Rani et al., 2018) and niacin (Saleh et al., 2013) after fermentation. Heating treatments like roasting, boiling etc. cause loss of heat-labile vitamins, so upto certain temperature and time the heating must be done. Saleh et al. (2013) conducted one study on the dietary and storage features of West African millet cuisine, where they observed a decrease in vitamin B₂ by 31.4%, 34.3%, and 45.7% when grain was processed into meal, flour, and fura, respectively. In refined millet flours the germ and bran components gets removed during polishing resulting in the loss of vitamins creating a millet of low nutritional profile (Gowda et al., 2022). Because majority of the vitamin content in millets is hoarded in the outer layer. Cooking time of millet gets reduced after soaking, increasing the nourishing content. Germination of millets and developing byproducts from those millets will contribute to reducing the loss of significant vitamins.

5.2.5. Effect of processing on fats

Fat plays a crucial role in absorption and transportation of vitamins A, D, E, and K in our body and in brain development. The amount of fat is influenced by the duration of germination. In comparison to the non-germinated specimen, the uncooked and optimized flour of germinated foxtail millet displayed fat amounts of 4.4% and 3.6%, respectively and it can be attributed to the usage of fat during germination as a source of energy. During germination in foxtail millet, the fatty acid oxidation and lipid hydrolysis process are attributed in reduced fat content (Saleh et al., 2013). The fat content in foxtail millet is reduced by 27.98%, due to high-pressure soaking affecting the grains nutritional profile (Sharma and Niranjan, 2018). Throughout the process of germination, free and soluble nutrients were created by enzymatic activity. While a study on pearl millet researchers have observed that the fermentation increases crude fat concentration from to 3.71% from 1.83% (Rani et al., 2018). The amount of linoleic acid, linolenic acid and unsaturated fatty acid in millet might upsurge after the extrusion processing (Kharat et al., 2019) resulting in damage of vitamins due to reduced fat content (Yang et al., 2022). The amount of crude fat reduced significantly in finger millet by 0.71%, 0.06% and 1.3-0.63 g/100 g in the process of roasting, puffing and popping, respectively (Chauhan and Sarita, 2018).

5.2.6. Effect of processing on antioxidants and flavonoid composition

Bhuvaneshwari et al. (2020) took five millets for the study and observed the antioxidant components of the plant after soaking, sprouting and germination. They reported high saponin content after 24 h of soaking and germination time in pearl millet. And in the same study the little millet showed high phytase activity. A study by Sharma et al. (2015) analyzed germinated foxtail millet for total flavonoids. After germination, the free, bound, and total flavonoids of foxtail millet increased considerably. The same research group in the year (Sharma et al., 2016) analyzed the outcome of germination on barnyard millet flavonoid concentration. After germination, the free, bound, and total flavonoid concentrations of barnyard millet increased significantly from 9.48 to 45.57 mg RU/g, 19.54 to 26.35 mg RU/g, and 29.02 to 71.92 mg RU/g, respectively. Chauhan and Sarita, (2018) reported a decreasing effect on the total antioxidant capacity of millet treated by a quadratic relationship between temperature and roasting time. The antioxidant content of millet increases as the roasting temperature rises. According to Azeez et al. (2022), total flavonoid content (TFC), ferric reducing antioxidant power (FRAP), diphenyl-2-picryl-hydrazil radical scavenging activity (DPPH), total phenolic content (TPC), Azino-bis-3-ethylbenzthiazoline-6-sulfonic acid (ABTS) scavenging activities of uncooked finger millet were increased post germination and fermentation process by 30.86%, 30.33%, 29.24%, 31.97%, and 78.55% respectively. The rise in total phenolics seen in germinated flour is attributed to enzyme activation aiding the synthesis of phenolic components. Significant increase in total phenolic compounds was stated in Browntop millet after fermentation (Saini and Sasmal, 2021).

6. Future Outlooks and Conclusions

To promote these underutilized crops various collaborations, implications of policies locally, nationally and internationally are required. Storage, processing methods and consumer/farmers awareness is crucial for its implication. Supply chain must be maintained to maintain the flow from farmer to consumer. The millets were polished commercially by using paddy machinery which results in low quality by affecting the nutrients, their digestibility etc. So, there is a need for the development of appropriate machinery targeting each millet. Studies targeting various pre-treatment methods resulting in efficient quality in each millet as pre-treatment techniques are vital in maintaining the bioavailability of nutrients. Optimizing the extraction and purification process of bioactive compounds must be catered for further studies. In silico and in vitro studies were carried out to study their role in various lifestyle diseases but more research targeting life-threatening diseases must be conducted for their effective role. Response/effect of millet depends on the individuals’ genetics, health status, and overall diet.

Climate disasters have been vocal enough for various climate change policies and agricultural sustainability. And often demand and supply of grains is insufficient, attributed to several climatic factors that have led to the usage of millets as an alternative food source. Millets advocate the term resilient crops providing numerous advantages to our health, agricultural system and environment. The nutritional content of these alternate food sources is high which has led to call them as “superfoods”. The application of processing technique contributes an acceptable range of support to increase its nutritional values. A new dawn of research has begun in the current scenario of climatic deterioration. Newer processing methodologies are generating hope among the people for better use of the crop. Processing methods must be developed keeping in mind the carbon footprints generated.

Acknowledgements

I would like to express my sincere gratitude to Assam down town University for providing the necessary infrastructure and research facilities essential for carrying out this work. I am also thankful to the university for awarding me the ADTU Junior Research Fellowship (JRF), which greatly supported and enabled me to pursue my research effectively.

Data Availability Statement

The entire data set that supports the results of this study was published in the article itself.

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Edited by

• Editor:
  Takako Matsumura Tundisi

Publication Dates

• Publication in this collection
  09 Jan 2026
• Date of issue
  2025

History

• Received
  02 June 2025
• Accepted
  07 Oct 2025