1 Introduction

Advancements in nutritional understanding challenge the food industry to develop novel products featuring unique qualities that can boost public health. Nowadays, after the Covid-19 pandemic, people are getting more concerned with their diet to preserve a healthy lifestyle. We need to replace grains with fortified foods that can help us fight nutritional security and provide a disease-free environment [1,2,3]. The most crucial factor in preserving human health is the nutritional value of food. To maintain total human health and fitness and to address malnutrition, attention should be paid to the dietary quality of food. Consumption of baked food products is common all across the world. All of these cooking recipes use refined wheat flour as their primary ingredients. Which delivers protein, carbohydrates, and some minerals like magnesium, phosphorus, and iron, but it is deteriorating consumers on two fronts: first, it lacks fibre and has a high glycaemic index (GI); second, it contains gluten, which increases the risk of gluten-intolerance and causes allergic reactions such as inflammation of the small intestine in some people, results in deficiency of several vital nutrients in the body due to improper absorption and damages the intestinal mucosa this condition, known as coeliac disease [4]. The only way to begin clinical recovery from this disease is by strictly following a gluten-free diet for the entirety of the patient's life.

Millets are small-seeded grasses in the Poaceae family and are classified as nutritionally dense crops cultivated in dry and semi-arid areas. They hold significant ecological and economic importance as a crucial food and fodder resource for resource-limited farmers. These millets are "coarse cereals" or "cereals of the poor. However, Saleem et al. [5] reported that millet accounts for only 2% of cereal production globally, and developing countries account for 97% of millet production and consumption worldwide. The total quantity of millet produced worldwide in 2019, 2020, and 2021 was 28,273,641.59, 30,825,051.54, and 2021 30,089,625 tonnes, respectively. Africa and Asia emerged as the leading producers of world millet production, contributing around 29,102,808.6 tonnes. It was noticed that the United States of America produced 358,503.81 tonnes, the European Union produced around 591,691.55 tonnes, and the nations of the Australian subcontinent added an estimated 358,503.81 tonnes to the world's millet production [6]. Global millet production reached 31,449,000 tonnes by 2023. India was the world's top producer of millet, accounting for 42% of the total output (13,160,000 tonnes). India's top five states for millet production were Madhya Pradesh (10%), Uttar Pradesh (17%), Haryana (8%) and Rajasthan (39%). Millets are a promising option for achieving food and nutritional security for the poorest regions of our society and their production is depicted in Fig. 1.

Fig. 1
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(Adapted from Sabuz et al. [6]; Yousaf et al. [7], www.mapchart.net)

Production of millets worldwide (million tonnes)

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Moreover, millets are more nutritious than rice and wheat (Table 1) as they are rich in vitamins, protein, and minerals [9]. Fiber-rich foods possess a low glycaemic index and can reduce the risk of oxidative stress, metabolic illnesses, and lifestyle disorders, including obesity, diabetes, heart disease, and digestive system-related problems [10]. Land and weed management practices have numerous applications in crop sustainability [11], land degradation and land management [12], vegetation dynamics in semi-arid environments [13], summer roadside vegetation effect of environment challenges on Sorghum halepense in peninsular Italy [14]. These practices help to sustain millet diversification based on their growing pattern and sustainability in terms of climate change. Fermentation improves starch digestibility, increases mineral bioavailability in millet grains, and reduces antinutrients such as phytic acid. Both pure culture and natural fermentation using microbes such as Saccharomyces cerevisiae, Saccharomyces diastaticus, Lactobacillus fermentum, and Lactobacillus brevis have been observed to raise moisture, fat, and soluble sugars while slightly decreasing protein content. Pure culture fermentation resulted in a decrease in fat content, whereas natural fermentation improved it. Overall, fermentation reduced the pH levels, enhanced oil and water absorption ability, and positively affected the nutritional value, digestibility, and storage quality of millet flour [15, 16].

Table 1 Nutritive values of Indian millets
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Additionally, fermented millet contained more catechol, p-coumaric, ascorbic, and gallic acid than unfermented millet. Balli et al. [16] utilized yeast and Lactobacilli for fermentation to investigate the association between the millet fermentation period. They discovered that the total phenolic content (TPC) detected after 72 h and also found that fermentation significantly raises the concentrations of vitexin 2′′-O-rhamnoside and vitexin. By partially reducing the overexpression of protein tyrosine phosphatase-1 B, these substances may impact type 2 diabetes by acting as negative regulators of insulin receptors. Millets are among the oldest cultivated crops and have been a staple in many traditional diets worldwide. There are several different types of millet, each with its unique composition and characteristics (Tables 2 and 3).

Table 2 Composition of various millets (per 100 g) [21, 22, 24]
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Table 3 Characteristics of various millets [6, 22, 36, 37]
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For thousands of years, fermented foods have been a part of many civilizations all over the world. They impart distinct aromas, textures, and nutritional profiles while also acting as a form of preservation. In many different cultures, fermented foods have been important for food preservation, improving nutritional value, adding flavors and textures to food, supporting the human gut and immune system, and having ceremonial and spiritual importance. Many fermented foods are still essential to global culinary and cultural traditions, having been passed down through years of traditional practices. Many fermented foods have been used historically and traditionally in many cultures: Antiquity of Chinese fermented foods and beverages [25], microbial fermentation of Japanese Miso and Shoyu [26], in Korean fermented foods [27], Garum, a fish sauce of ancient Rome [28], Sauerkraut and Sourdough [29], Injera, a traditional Ethiopian fermented flatbread [30], fermented foods in Nigeria [31], Pozol, a traditional fermented corn drink from Mexico" [32, 33]. Traditional fermented foods and beverages has an important place in human diets. The oldest evidence of fermented foods and beverages comes from Asia from around 8000 B.C [34]. Cuamatzin-García et al. [35] described the role of traditional fermented food and beverages around the world. They have human health promoting effect as they have prevented human body from inflammatory, immune, chronic and gastrointestinal diseases. A step forward in this direction through this review, an attempt has been made to update the current plethora of knowledge gained in terms of different varieties of millets having food applications, bioactive compounds in millets and their health benefits, fermentation to increase the nutrient availability of millets along with various fermented foods, and examples of improvement in millet properties due to fermentation, etc. are explained in the following sections. Therefore, this review provides updated information to fill the gaps in the studies on millet as food and medicine for the healthy lifestyle of the human population.

2 Bioactive compounds in millets

Due to the growing population, malnutrition and food security are significant challenges in the current global context. These issues must be resolved urgently to maintain equitable food distribution with the continuous growth of the population. The plant is the primary source of food. They contribute to plant-based foods' flavor, color, aroma, and nutritional value, including grains like millet. Apart from this, plants possess the property of producing secondary metabolites, compounds that are synthesized alongside primary metabolites. Secondary metabolites serve specific purposes, unlike essential primary metabolites like amino acids, proteins, carbohydrates, and lipids that support growth [38, 39]. Millet grains can be stored for long period without spoilage. This makes the valuable application of mallets for combating malnutrition and ensuring food availability. But millets are sometimes underutilized because of their coarse property, limited use of beneficial foods, and lack of novel techniques and research for diverse product development despite their medicinal benefits, economic potential, and nutritional value [40].

These beneficial substances are often found in food in trace quantities, particularly in fruits, grains, and various types of vegetables, and provide nutritional worth and health benefits [7]. Bioactive compounds offer both therapeutic promise and the ability to influence energy intake. This reduces pro-inflammatory conditions, oxidative stress, and metabolic disorders [40,41,42]. Epidemiological study indicates the consumption of diverse food types abundant in a wide range of bioactive molecules and other antioxidants, including various phytochemicals and vitamins and high levels of phenolic compounds like flavonoids and carotenoids [43,44,45]. Their consumption benefits health, potentially diminishing the risk of conditions like heart-related issues, tumors, Alzheimer's disease, cataracts, stroke, elevated sugar levels, and age-related problems [46]. Little millet (Panicum sumatrense) is a small-seeded variety often used in Indian cuisine for making porridge, upma, and other dishes. It is an agricultural product and a rich source of dietary fiber and phytochemicals. Humans generally ignore it for consumption because of an insufficient awareness of the benefits to their health. Little millet is nutritionally better than other cereals for human health [47]. Little millet complements the diet because of its high sustaining power, lower glycaemic index, and high satiety scores; it also does not create acids [48]. Little millet produces various bioactive phytochemicals like phenols, tannins, and phytates, as well as macro and micronutrients. Little millet also effectively lowers the risk of chronic, anti-inflammatory, and antirheumatic diseases, as it possesses various value-added bioactive compounds such as kaempferol, luteolin, and apigenin, respectively [49, 50].

The bioactive compounds found in millets include ferulic acid, vanillic acid, cinnamic acid, benzoic acid, catechol, ascorbic acid, gallic acid, kaempferol, salicylic acid, syringic acid, protocatechuic acid, chlorogenic, sinapic acids, etc. [51,52,53]. The research conducted by Hithamani and Srinivasan [52] on Eleusine coracana (finger millet) extracts revealed the presence of various compounds, viz., caffeic, syringic, gallic, p-hydroxybenzoic, gentisic, p-coumaric, sinapic, salicylic, and transcinnamic acid. Similarly, Nani et al. [54] had reported extracts analysis from pearl millet indicated the presence of ferulic, gallic, p-coumaric, and syringic acid; amounts were noticed as 199, 15.3, 1350, and 7.4 μg/g. Similarly, Salar et al. [55] reported that ascorbic, gallic, and p-coumaric values were 320, 120, and 160 g/g in pearl millet variety cv. PUSA-415 extracts after HPLC examination. Likewise, the HPLC analysis of extracts from foxtail millet and little millet revealed that it consists specific compounds, including gallic acid (6.91 μg/g and 5.82 μg/g), p-hydroxybenzoic acid (1.07 μg/g and 4.05 μg/g), vanillic acid (2.37 μg/g and 35.69 μg/g), caffeic acid (85.22 μg/g and 78.83 μg/g), chlorogenic acid (19.91 μg/g and 29.53 μg/g), ferulic acid (93.4 μg/g and 129.31 μg/g), sinapic acid (85.36 μg/g and 63.24 μg/g), and p-coumaric acid (3.66 μg/g and 2.71 μg/g) [53]. However, all these bioactive compounds can be divided into the following categories: phenolic acids and flavonoids, which showed various health benefits (Table 4). The various bioactive components in millets showing health benefits are discussed in Fig. 2.

Table 4 Health benefits of bioactive compounds present in various millet varieties
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Fig. 2
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Bioactive compounds present in millets

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2.1 Phenolic acids

All cereals contain phenolic acids (PAs), usually derivatives of cinnamic and benzoic acids. Cereals like sorghum and millet etc., have the most diverse collection of these compounds [61,62,63,64]. PA has been categorized into two classes: hydroxybenzoic acids, originating from benzoic acid, and hydroxycinnamic acids, derivative of cinnamic acid. Hydroxybenzoic acids comprise the predominant free phenolic compounds within finger millet, accounting for approximately 70–71% of the grain phenolic acid content. This group encompasses p-hydroxybenzoic acid, protocatechuic acid, vanillic acid, gentisic acid, gallic acid, and syringic acid. Notably, protocatechuic acid is recognized as the primary free PA present, with a concentration of 45 mg per 100 g of the grain [65,66,67,68]. Moreover, in the context of the free phenolic fraction, small quantities of hydroxycinnamic acids (ferulic, p-coumaric, and caffeic acids) are present. Conversely, the bound PAs in finger millet are predominantly composed of hydroxycinnamic acids, encompassing, coumaric (p-coumaric acid), caffeic, chlorogenic, sinapic, cinnamic (trans-cinnamic acid), and trans-ferulic acid (ferulic). Bound phenolic acid comprises around 30% of the total grain's phenolic acids. Likewise, ferulic acid, coumaric and caffeic acids are the bound phenolic acids in found in millets. The millet flour roughly contains 18.6 mg/100 g ferulic acid, 1.64 mg/100 g caffeic acid, and 1.20–1.25 mg/100 g coumaric acid, respectively. The bound fraction also contains trace quantities of syringic, protocatechuic, and hydroxybenzoic acids [69, 70].

2.1.1 Hydroxybenzoic and hydroxycinnamic acids

The availability of a benzene ring accompanied by one or more OH groups is a distinguishing structural feature of phenolic substances. These structures cover a wide range of complexity, from simple phenolic entities like benzoic acids to complex phenolic polymers. Hydroxybenzoic acids (HBAs) share a common C6-C1 skeletal framework. Their composition involves incorporating methoxyl and hydroxyl groups into an aromatic ring. Examples of HBAs include p-hydroxybenzoic, syringic, 4-hydroxybenzoic, gallic, vanillic, and protocatechuic acids [71]. Millets contain various HBAs: gentisic, vanillic, gallic, syringic, salicylic, and protocatechuic p-hydroxybenzoic acid [72]. Yuan et al. [73] also reported the presence of various HBAs derivatives in proso-millet, including protocatechuic aldehyde, vanillic acid, p-hydroxybenzaldehyde, and p-hydroxybenzoic acid at 280 nm wavelength.

Hydroxycinnamic acids (HCAs) exhibit a fundamental nine-carbon (C6-C3) framework accompanied by a double bond in their side chain, either in cis or trans configuration. Notable among HCAs are caffeic, o-coumaric, p-coumaric, m-coumaric, ferulic, and cinnamic acids, which are widely recognized. Previous findings indicated that hydroxycinnamic acids identified within proso-millet included chlorogenic acid, ferulic acid, caffeic acid, and p-coumaric acid. The principal dehydroxybenzoic acid reported was primarily syringic acid [74]. In another study by Yuan et al. [73] HCAs (including chlorogenic acid, caffeic acid, p-coumaric acid, and ferulic acid) were quantified using a wavelength of 320 nm. Concerning millets, within the array of identified phenolic acids, it was observed that Foxtail millets (FM) displayed a more noteworthy alteration following germination than Proso millets (PM).

Regarding the free HCAs, caffeic, p-coumaric, sinapic, and ferulic acids showed a more pronounced increase in Foxtail millets, while vanillic acid exhibited a more significant increase in Proso millets. Likewise, regarding bound HCAs, FM revealed a more pronounced increment than PM for compounds, namely p-coumaric, vanillic, ferulic, caffeic, and sinapic acids. A study by Pradeep and Sreerama [75] showed the elevated concentrations of total hydroxybenzoic acids (gallic, p-hydroxybenzoic, and vanillic acids) and hydroxycinnamic acids (chlorogenic, p-coumaric, sinapic, caffeic, and ferulic acids) in various variety of millets namely foxtail, proso, and barnyard millets (48-h germination and raw). This elevation in hydroxybenzoic and hydroxycinnamic acids is likely attributed to activating or biosynthesizing enzymes responsible for the increased synthesis of these PAs [76, 77]. During germination, a transformation from bound phenolics to free phenolics takes place. Phenolic acids are also produced through the shikimate pathway, controlled by shikimic enzymes, involving synthesizing various PAs or amino acids. The augmentation in the concentration of free phenolics was observed, while the germination of legumes, pseudocereals, and cereals was linked to the hydrolysis of cell walls and enzymatic degrading components. This process releases bound phenolics, as indicated in studies by Alvarez-Jubete et al. [77]; Kumari et al. [78].

However, sinapic acid and its derivatives have been found to possess antioxidant, anti-inflammatory, and anticarcinogenic properties, effectively inhibiting lipid peroxidation. Notably, Khare et al. [79] reported the identification of sinapic acid of value 7.9 ppm in kodo- millets, revealing its potential anti-obesity effects. Ferulic acid presents many health advantages, including anticarcinogenic, antioxidant, anti-allergic, antimicrobial, antithrombotic, anti-inflammatory, antiviral, hepatoprotective, and vasodilatory properties. Additionally, it has also been linked to improved sperm viability. In Kodo millet, p-coumaric acid is an antioxidant, displays anti-inflammatory effects, and demonstrates potential as an anticancer agent. It also diminishes lipid and cholesterol oxidation peroxidation and impacts low-density lipoprotein (LDL) [80, 81].

2.2 Flavonoids

Flavonoids constitute a prominent group among naturally occurring phenolic compounds. They encompass various types, such as flavonols, isoflavones, and anthocyanins, each distinguished by distinct chemical components. These compounds possess a three-ring structure with multiple substitutions and low molecular weight. Rutin, the pioneering flavonoid, was initially extracted from oranges in 1930. However, numerous flavonoid compounds, including their derivatives from diverse plant sources, have been identified and extensively studied for their biochemical and biological properties [82,83,84,85]. Cereals, particularly millet, stand as a notable reservoir for flavonoid composites. A diverse range of these composites has been detected in grains and millet leaves; nevertheless, limited efforts have been directed toward quantifying their concentrations. In finger millet, the soluble form of flavonoids is predominant and has been documented at an elevated level of 2100 µg/g, surpassing the content in other millet varieties [86,87,88,89,90].

Moreover, finger millets contain distinct esterified forms of flavonoids, distinguishing them from other varieties of millets, including Kodo-millet [91]. Furthermore, a substantial presence of condensed tannins or proanthocyanidins, mainly in oligomeric or polymeric forms of flavonoids, has been observed. Procyanidin (including B1 and B2) is identified as the primary dimer [92]. These phenolic products are acknowledged for their diverse beneficial properties, including antiproliferative, anti-inflammatory, anti-allergic, and antioxidative effects.

2.2.1 Flavonols

Flavonols, a subgroup of flavonoids containing ketone groups, play a pivotal role in proanthocyanin biosynthesis. Among the extensively researched flavonol compounds are myricetin, kaempferol, fisetin, and quercetin. These compounds are notably present in lettuce, tomatoes, onions, apples, grapes, berries, tea, millet, and red wine. Incorporating flavonols into a diet has various health advantages, including antioxidant properties and a lowered risk of vascular diseases [93]. Commonly studied types like kaempferol, quercetin, myricetin, procyanidin B1/B2, rutin, and catechin have been identified in various millet varieties, contributing to their antioxidant properties and potential advantages similar to other flavonol-rich foods.

In previous research, millet bioactive, mainly flavonoids, have been noted for their potential in countering cancer therapies. However, this involves inducing programmed cell death, inflammation and proliferation inhibition, cell cycle arrest, curtailing processes like cancer and angiogenesis, impacting epigenetic modification, regulating proteasomal activity, and validating gene targets [94]. A significant catechin compound, Epicatechin-3-gallate, has shown promising effects in earlier studies related to stroke issues, obesity, diabetes, Alzheimer's, and Parkinson's disease [95]. In a study by Khare et al. [79], catechin (1.10 ppm) was quantified in Paspalum scrobiculatum, revealing its anti-obesity potential. Among these flavonols, troxerutin (also known as trihydroxyethyl rutin) has been identified, contributing to the flavonoid composition of millets. Similarly, quercetin derivatives, such as avicularin, spiraeoside, and quercetin 7-O-rutinoside, are present within millets. Another flavonol, dihydroquercetin (also known as taxifolin), has been detected in millets [96]. Likewise, various studies showed different features regulated by nutritional genes of targeted crops (Table 5).

Table 5 Nutritional genes/transporters of different target crops [97,98,99,100]
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2.2.1.1 Flavones

Flavones represent another significant subclass within the realm of flavonoids. These compounds are notably prevalent as glucosides in various sources, such as Ginkgo biloba, celery, parsley, mint, red peppers, and citrus fruits. Nevertheless, the precise categorization of flavones within millets can fluctuate depending on the distinct millet type and its unique composition. Notably, recognized flavones such as luteolin, apigenin, and tangeritin are recurrently identified in various dietary sources [101]. The distribution and prevalence of these flavones within millet varieties contribute to a potential array of favorable effects, notably encompassing antioxidant and anti-inflammatory attributes. These flavones are acknowledged for their ability to impact an array of physiological pathways and mechanisms that could augment overall health. However, a comprehensive understanding of the diverse spectrum of flavones within various millet types might require further systematic exploration to offer an all-encompassing grasp of their categories and quantities [102]. Flavones identified in millets encompass compounds like di-C, C-hexosyl-apigenin, butin, and syringetin 7-O-hexoside. Furthermore, tricin derivatives have been recognized within millets, including tricin 4'-O-β-guaiacylglycerol, tricin 5-O-hexosyl-O-hexoside, and tricin 7-O-hexosyl-O-hexoside. These compounds fall under the flavonoid category and contribute to the overall bioactive diversity within millets [57].

2.2.1.2 Flavanones

Various flavanones can be found in different millet varieties, contributing to these grain-rich bioactive compounds. Flavanones constitute a diverse group of molecules with distinct structures and potential health benefits. The specific array and quantities of flavanones within millets can differ based on the unique millet type and composition. Flavanones offer numerous health benefits due to their capacity to scavenge free radicals (acting as antioxidants) and their anti-inflammatory potential to lower cholesterol levels [103, 104].

Flavanones, a group of bioactive compounds in various millet varieties, offer potential health benefits. Among them, hesperetin 7-rutinoside, also known as hesperidin, is abundant in citrus fruit millets and is associated with antioxidant and anti-inflammatory effects. Another flavanone, homoeriodictyol, contributes to millet's bioactive content and is recognized for its antioxidant and potential anti-inflammatory properties. Naringenin, a well-studied flavanone in millets, provides antioxidant capabilities and could have protective effects against chronic diseases. Similarly, naringenin 7-O-glucoside, or prunin, is part of millets' flavonoid profile and contributes to their antioxidant potential. Naringenin chalcone, a member of the flavanone group, shares potential health-promoting attributes with other flavanones. Furthermore, liquiritigenin, classified as a flavanone, adds antioxidants and anti-inflammatory properties to millets. The presence of these flavanones in millets enriches their nutritional value and may contribute to various health advantages. It's important to note that further research is needed to comprehensively explore these flavanones' quantities and potential effects in millets [57].

3 Health benefits of millet

Patients with celiac disease can utilize millet as they are gluten-free. Consuming millet reduces blood glucose responses and glycosylated hemoglobin, lowering the risk of developing diabetes. Oxidative stress is reduced by removing free radicals from the body using the phenolic compound present in millet. Millets can reduce hypertension by oxidizing low-density lipoproteins, anticancer properties, and inhibiting DNA damage [105]. The various health benefits of millets are presented in Fig. 3.

Fig. 3
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Health benefits and various applications of millets

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3.1 Diabetes

Millets demonstrated the benefits of lowering pancreatic amylase and glucosidase, which decreased postprandial glucose levels. Lowering the enzyme-mediated breakdown of complex carbohydrates can reduce hyperglycemia. Aldose reductase is an enzyme that catalyzes the reduction of glucose to sorbitol, which is part of the polyol pathway. Excessive activity of aldose reductase, particularly in conditions of high blood sugar (hyperglycemia), can lead to the accumulation of sorbitol. Varieties of millet contain compounds that have been found to inhibit aldose reductase. Specifically, some studies have shown that certain polyphenols and flavonoids present in millets have aldose reductase inhibitory activity. These bioactive compounds can help to reduce the conversion of glucose to sorbitol, thereby potentially lowering the risk of diabetic complications, including cataracts [106, 107]. Although millets contain a significant amount of magnesium, it reduces the risk of Type II Diabetes. Magnesium is an essential component that aids in raising the effectiveness of insulin and glucose receptors by creating many enzymes that break and digest carbohydrates and regulate insulin action [108]. Moreover, consuming millet products reduces glucose fasting by 32% and eliminates insulin resistance by 43% [109].

3.2 Cancer

Millets contain antinutrients such as phenolic, tannins, and phytates that can lower the chance of developing and spreading cancer. Linolic acid, which is present in millet, aids in preventing tumors. Due to tannins and polyphenols, sorghum has antimutagenic and anticarcinogenic characteristics. According to Pawar et al. [110] millet grains' phenolic acids, phytate, and tannins serve as “antinutrients”. According to a recent study, millet phenolics may prevent the start and progression of in vitro cancer [111]. Likewise, millets contain lignans that, when digested, can be converted into animal lignans by the gut microbiota. According to Peterson et al. [112], these animal lignans have shown preventive properties against several chronic diseases, including cancer. A recent finger millet seed purified extract study revealed significant anti-proliferative activity on K562 chronic myeloid leukemia. This happens because finger millet seeds consist of a bifunctional complex of -amylase-trypsin inhibitor, also known as RBI (ragi bifunctional inhibitor), which simultaneously inhibits trypsin and -amylase [113].

3.3 Celiac disease

Gluten consumption causes the genetically sensitive state known as celiac disease. Millets improve digestion as they are gluten-free. So, it decreases the irritability produced by common cereal grains that contain gluten. By managing their digestive system, one can retain more nutrients and reduce their risk of getting more serious gastrointestinal conditions like gastric ulcers or colon cancer. The high fiber content of millet helps treat and prevent various ailments, such as constipation, excessive gas, bloating, and cramps [114].

3.4 Cardiovascular disease

The three amino acids, thionine, threonine, and lecithin, found in finger millets, contribute to removing extra liver fat, lower cholesterol levels, and forming unnecessary fat. However, finger millet has a low serum triglyceride concentration. By decreasing plasma triglycerides, ingesting finger millet reduces the risk of cardiovascular disease [115]. Millet, a rich source of potassium and magnesium, acts as a vasodilator, decreasing blood pressure and reducing the chances of heart attacks and strokes, particularly in people with atherosclerosis [113].

3.5 Bones development and repair

Phosphorus, abundant in Pennisetum glaucum, supports the development of bones and repair procedures. Phosphorus is the key component for growth and bone development, as well as for the production of adenosine triphosphate, a primary energy source of the human body [22, 116].

3.6 Detoxification

Several millet components can remove toxins from the body. The millet contains antioxidants and nutraceuticals, which help in the body’s removal of toxins and foreign substances. Millet-based prebiotic and probiotic beverages purify the body by removing harmful substances from the body [117].

3.7 Antimicrobial

Millets’ secondary metabolites have antibacterial and antifungal properties. Bisht et al. [118] noted that the bacterial pathogens Escherichia coli, Bacillus cereus, Listeria monocytogens, Proteus mirabilis, Salmonella typhi, Pseudomonas aeruginosa, and Yerisina nteroclitica that millets suppress. Moreover, millet is claimed to have antifungal properties [96]. The efficacy of seed protein extracts from various millet species was tested in vitro to stop the growth of Rhizoctonia solani, Macrophomina phaseolina, and Fusarium oxysporum. Protein extracts from pearl millet can effectively stop all three tested phytopathogenic fungus growth [119].

4 Fermentation to increase the nutrient availability of millet

Fermentation is used to improve these phytochemicals and nutritional qualities of the millet. However, fermentation is a biological phenomenon wherein microorganisms like bacteria, yeast, and fungi break down complex organic substances into simpler forms, usually without oxygen. This process produces energy and various end-products, such as alcohol, organic acids, gases, or other metabolites. Humans have used fermentation for thousands of years to make multiple food and beverages, including beer, wine, cheese, bread, yogurt, sauerkraut, kimchi, and many more. It is also utilized in a wide range of industrial procedures, like for producing biofuels, pharmaceuticals, and enzymes [120,121,122]. Fermented food has numerous applications in beverages for human health and industry [123]. It continues to be an essential process in human civilization, contributing to producing a wide range of products we use daily. Solid substrate fermentation (SSF) offers advantages such as low cost of production, simplicity, and potential for utilizing agricultural residues as substrates. It also provides a unique micro-environment for microbial growth, resulting in distinctive flavors, textures, and nutritional profiles in fermented products. Overall, fermentation improves the comparative nutritional value, food shelf-life, protein content, digestibility, mineral availability, and functional qualities, lowers phytic acid concentration, and gives the food product superior sensory properties [124]. Pearl millet flour's nutritional and phytochemical components are enhanced through fermentation with baker's yeast, which the food industry can employ to produce nutritious, functional food products [47].

Nutrient content increased significantly when Bacillus natto was used to ferment millet bran. Soluble dietary fiber (SDF) increased by 92.0%, β-glucan increased by 164.4%, polypeptide increased by 111.4%, polyphenol increased by 32.5%, flavone increased by 16.4%, and total amino acid content increased by 95.4%, respectively. Additionally, compared to the unfermented millet bran extract (0.99 mmol g−1, 32.1%, and 35.1%, respectively), the fermented millet bran extract (FMBE) showed improved glucose adsorption capacity (2.1 mmol g−1), glucose dialysis retardation index (75.3%), and α-glucosidase inhibitory activity (71.4%, mixed reversible inhibition) [125]. Fermenting finger millet with Lactobacillus salivaricus increased tryptophan by 17.8% and lysine by 7.1%, respectively. In addition, improvements were noted for thiamine, riboflavin, niacin, BV, and NPU [126]. According to Zhang et al. [127], millet grains prove an excellent substrate for fermentation using the fungus Monascus ruber, which yields Monacolin K. The compound may be used to formulate drugs that lower hypercholesterolemia. Furthermore, Dias-Martins et al. [128] have identified Fura, Kimere, and Lohoh as examples of fermented pearl millet products. A naturally fermented probiotic beverage is koko sour water. Ogi or Akamu, Agidi or Eko, Fura, Ndaleyi, Kunun-zaki, Burukutu, and Otika are examples of traditional fermented millet dishes and drinks in Africa [129]. In Africa, Uji and Togwa are pleasant fermented beverages, whereas Jandh is a popular traditional fermented alcoholic beverage in Nepal. In Burkina Faso, ben-saalga is a fermented millet gruel. The probiotic potential of Lactobacillus and Pediococcus isolated from Omegisool, a Korean millet-fermented alcoholic beverage, was examined. Seawater fish, cooked millet, and salt are the main ingredients of the fermented fish rice meals known as nerezushi and sikhae, which are of Japanese and Korean descent [5, 130]. Another study noted that pearl millet starch was significantly affected by the Rhizopus azygosporus and Aspergillus sojae fermentation process, which produced pores on the granule surface and reduced particle size. Aspergillus sojae fermentation resulted in a greater number of pores than Rhizopus azygosporus. Amylose content, swelling capacity, and solubility were all altered, with an increase in amylose content (15.96%) and solubility (20.41%) and a decrease in swelling ability (13.15 g/g). These findings demonstrate the potential of solid-state fermentation as an affordable method of modifying starch in pearl millet [131].

Fourier transform infrared (FTIR) spectroscopy is a novel technique for evaluating molecular changes within food biopolymers, facilitating the examination of chromatographic patterns of polyphenols and functional groups. FTIR spectra effectively indicate the alterations resulting from fermentation treatments of the fermented millets.

In optimized white finger millet probiotic beverage, peaks in the 1541.12–1558.48 cm−1 range were attributed to C = C stretching, which was assigned to vibrations by aromatic compounds. Likewise, ultrasound treatment showed a promising method for enhancing the bioavailability and nutritional composition of the white finger millet. In this context, FTIR results of the ultrasound-treated white finger millet-based probiotic beverage showed the presence of various hydrophilic and hydrophobic groups [132]. Thus, each functional group of millet plays a significant role in the nutritional properties of the millet and its by-products. Therefore, various processing techniques and treatments affect the millet's nutritional composition (Table 6).

Table 6 Processing technology used for millet improvement [133, 134]
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Bacillus natto fermented millet bran dietary fibre (MBDF) exhibited a porous, honeycomb-like structure indicating increased porosity compared to unfermented-MBDF due to adsorption improved oil-holding capacity, water-holding capacity, and cholesterol binding capacity [135]. Relatively high porosity was observed in the extrudate of germ millet, characterized by large, disaggregated particles and a somewhat smoother and more uniform shape due to the extrusion cooking process, which led to the leaching of starch granules [136].

Similarly, the fermented millet flour samples and prepared biscuit samples were compared to observe the morphological features. As indicated by the micrographs of the acquired flour, the structure of pearl millet flour (NF) exhibited a composition of more minor, irregular, and tightly packed granules, in contrast to fermented millet flour (FF) and malted pearl millet flour (MF). In FF and MF, a consistent and smoother structural arrangement was evident, implying that the processes of malting and fermentation had induced alterations that contributed to the establishment of uniform configurations in their respective samples. Due to their increased openness, these alterations likely contributed to the flour samples' enhanced water absorption capacity (WAC), oil absorption capacity (OAC), and swelling capacities. The granules across the three flour samples exhibited predominantly rounded or polygonal shapes featuring fewer pores. Notably, the scanning electron microscope (SEM) images of the FF and MF samples also displayed comparatively less compactness, more significant orientation variance, and heightened porosity, attributed to the influence of osmotic stress. The discernible indentations within the MF samples can be ascribed to the hydrolysis of starch, leading to sugar formation and protein degradation into amino acids. This aligns with the observed increase in water absorption capacity (WAC) in the MF and NF samples, resulting from starch and protein breakdown into monomers. Analyzing the microstructure of the biscuit samples, a distinct arrangement of granules became evident. The baking process distorted the granular structure, resulting in larger granule sizes with a configuration reminiscent of a "honeycomb-like" structure. This is akin to the findings concerning finger millet processed at high temperatures. This transformation can be attributed to the gelatinization of starch and the denaturation of proteins, leading to alterations in the initial flour sample's structure [137].

In the fermented sorghum sourdough, the concentrations of p-coumaric acid, ferulic acid, and specifically caffeic acid were noted to be higher (P < 0.001) compared to that in unfermented control samples [138]. An increase in free p-coumaric and bound sinapic acids was noted for fermented white sorghum; however, only free p-coumaric acid content increased in fermented red sorghum [139]. In this regard, Salar et al. [55] performed the qualitative and quantitative bioactive compound analysis of unfermented pearl millet grains (UFMG) and fermented pearl millet grains (AFMG) by HPLC. Phenolic acids represent secondary bioactive metabolites that are widely distributed in natural resources. Among these resources, pearl millet is a plentiful reservoir of bioactive components, including phenolic compounds and condensed tannins. To assess the presence of bioactive compounds within aqueous fraction of millet flour (AFMF) and ultrasonic fraction of millet flour (UFMF), six standard compounds were selected for screening: gallic acid, ascorbic acid, benzoic acid, p-coumaric acid, cinnamic acid, and catechol. The phenolic acid composition of both AFMF and UFMF was analysed.

Qualitative and quantitative analyses were performed through HPLC (High-Performance Liquid Chromatography) techniques. Visual representations of the HPLC chromatograms notably, both UFMF and AFMF exhibited the presence of three predominant compounds: ascorbic acid, gallic acid, and p-Coumaric acid. However, AFMF displayed a higher content of bioactive compounds than UFMF. Chandrasekara and Shahidi [140] used diethyl ether and ethyl acetate to extract the phenolic compounds of the millets. The HPLC analysis noted the soluble and bound fractions of raw millet grain. This might result from the chemicals in the raw grain changing during the heat treatment, followed by the in vitro simulation of enzymatic digestion and microbial fermentation. However, stable molecules found in the extracts have in-vitro antioxidant capabilities further revealing that altered phenolics might act as active antioxidative agents in vivo and shield the gastrointestinal tract from oxidative damage. Gabriele et al. [141] used spectrophotometric, fluorometric, and HPLC–DAD analysis to explore the effects of fermentation on phenolic components, antioxidant activity, and inflammatory potential in millet (Table 7).

Table 7 Detection of polyphenolics in various types of millet
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In the small intestine, the hydroxycinnamic acids, such as ferulic acid found in grains, are mostly absorbed [137]. This has demonstrated that a small amount of ferulic acid was absorbed after it was released from insoluble fiber in the large intestine. Furthermore, at the gut level, these phenolics may work locally to prevent the breakdown of other dietary antioxidants. As a result, they could improve the human body's overall antioxidant state and protect against illnesses linked to oxidative stress. The other various studies also suggested that the phenolic compounds in millet grains may experience structural changes both during digestion and during microbial fermentation in the body [142,143,144,145].

Fermentation has a range of beneficial effects on millets, impacting their texture, flavor, nutritional content, and bioavailability of nutrients. Microorganisms such as bacteria, yeast, and fungi act on millet grains, breaking down complex compounds into simpler forms. One major effect is the reduction of pH levels due to the production of organic acids, which aids in product preservation and flavor enhancement. Fermentation also boosts antioxidant activity by producing bioactive compounds, neutralizing harmful free radicals, and improving overall health. The process enhances the flavor and aroma of millets, making them more palatable, while also improving their nutritional profile by breaking down anti-nutritional components like phytic acid, leading to better absorption of minerals such as iron, zinc, and calcium. Additionally, fermentation extends the shelf life of millet products by preventing the growth of spoilage organisms and improves digestibility by breaking down complex polysaccharides and proteins. This is particularly beneficial for those who struggle with digesting certain millet components [146]. Fermentation also increases the bioavailability of nutrients, reducing compounds that inhibit mineral absorption, thus helping prevent deficiencies. Some fermented millet products may contain probiotics that support gut health, digestion, and the immune system. Prebiotics produced during fermentation can further nourish beneficial gut bacteria [147]. The process also reduces anti-nutrients like tannins and lectins, enhancing nutrient absorption [148, 149]. Textural changes occur as well, with millet grains becoming softer, contributing to a different mouthfeel and eating experience [150]. In certain cases, fermentation can reduce natural toxins or antinutrients, making millet safer to consume [151]. The formation of bioactive compounds during fermentation may have anti-inflammatory effects, potentially helping to reduce inflammation associated with chronic diseases. Fermented millet also has a lower glycemic index compared to non-fermented millet, making it useful for regulating blood sugar levels, particularly for people with diabetes [106]. Furthermore, fermentation enhances the quality of millet proteins by increasing the availability of essential amino acids, which is especially important for those following plant-based diets [48]. The process may also reduce the allergenic properties of millet, making it suitable for individuals with sensitivities or allergies [59]. Lastly, the combination of improved digestion, enhanced nutrient content, and potential modulation of gut microbiota through fermented millet consumption can contribute to greater satiety and aid in weight management.

5 Food development from millet

Sorghum is a staple crop in African countries, including some parts of India. It is consumed as traditional food. The main causes of this reduction include the replacement of sorghum with other fine cereal grains, increasing income patterns, and some government initiatives that favor other cereals. There are numerous nutrient-dense dietary items made from millet.

5.1 Roti

Unleavened Indian flat-bread known as sorghum roti goes by a variety of names in various Indian languages, such as chapati (Hindi), rotla (Gujarati), bhakri (Marathi), and rotte (Telugu) [152].

5.2 Multigrain flour

Several grains, such as multigrain, are used for flour development, also known as composite flours. Blending processed and unprocessed grains and pulses prepares composite flour, fulfilling the major nutrition needs. Processed sorghums are utilized in multigrain flour formation and can elevate roti's taste and nutritional and nutraceutical properties [153]. In-finger milted fortified chapattis increases taste, possesses antidiabetic ability, and is effective for taking diabetic patients. In constipation problems, high fiber content flour, mainly composite flour, is efficiently beneficial [154].

5.3 Fermented foods

In the food processing industry, fermentation is a crucial process that serves many essential functions [155]. In the Asian subcontinent, traditional fermented foods like Idli, Dosa, and Ambli have a long history of using LAB for the fermentation of millet [156]. According to research by Tamene et al. [157], LAB fermentation of millets increases the number of micronutrients, vitamins B and phylloquinone (K), and amino acids like lysine and folate in the fermented food items. The malted and fermented flour cookies included less fat and ash, more essential and non-essential amino acids, and more moisture. However, malted biscuits were the most well-liked due to their higher sensory acceptability through fermentation and malting [158].

Various studies have shown an increase in the nutritional and functional properties of fermented millet foods. The study conducted by Liu et al. [159] revealed the Rhizopus oryzae based fermentation for the nutritional constituents of adlay millet seeds. Lactobacillus pentosus isolated characterized by probiotic properties and L-tryptophan production properties through millet fermentation [160] and, addition of prosco millet-based bran fibre for Gluten-free Proso millet-based dough and cake [161]. Broomcorn Millet Huangjiu for flavor quality [162], fermentation effect on non-alcoholic fermented food and beverages [163], changes in flavour compounds in the fermentation of millets [164], probiotic Pichia kudriavzevii strains for the ability to enhance folate content in traditional cereal-based African fermented food [165], Eleusine coracana (L.) grains for their germination and fermentation [166], lactic acid bacteria for pearl millet-soybean for development of foods for young children [167].

Examples of fermented millet products from various countries viz., fermented millet porridge (Uji, Kenya), fermented millet-based drinks (Bhajani, India), fermented millet flour (Kunu, Nigeria), fermented millet bread (Injera, Ethiopia), and fermented millet snack (Popped millet, various countries). The studies on fermented malted millet-based products from different countries, such as, in Africa [124], the fermentation effect of folate on African cereal-based foods [168], role of millet in the production of Hausa koko, a Ghanaian fermented Cereal Porridge and role of bacteria and yeast [169, 170], and studies on Nigerian fermented food [171], African traditional fermented foods poto-poto and dégué [172], pearl millet (Pennisetum glaucum) for production of ben-saalga fermented gruel from Burkina Faso [173], fermented food from Zimbabwe [174], Ndaleyi, a Nigerian fermented pearl millet food [175].

5.4 Papad

Currently, various processed food items are being developed. Some of the most popular uses include chakli, papad, and idle, which are rapidly expanding. Several sorghum and millet cultivars in India have been created with more production and biochemical components. Sorghum and millet may be highly beneficial with increased future demand for food items for local and industrial usage. Papad is a top-rated culinary product in India that has been used for a long time. According to Begum, they add up to 55 to 60% finger millet flour to papad preparation, resulting in a correct texture. Methods for creating papad include boiling millet flour in water to gelatinize it, slicing it thinly, and freezing it. After rolling the dough, dry the pieces (6% moisture content) [176]. However, now papad can be used to cook food with a black colour that transforms into a lighter colour when fried.

5.5 Dosa

Samai Dosa is made using one cup of small millet, half a cup of processed rice, one tablespoon of fenugreek, and salt to taste. Soaked fenugreek, black gram, and rice seeds are utilized in the recipe. All the soaking components are thoroughly ground and fermented overnight. Further, the dosa was prepared by combining all the processed ingredients [177].

5.6 Millet sweets and desserts

Traditional milk sweets like rasmalai, jalebi, gulab jamun, and khoa are famous all over South Asia. Various ingredients are included in composite milk candies, such as dried fruits, various types of flour, milk, milk solids, raw or roasted almonds, and milk. South India now has the nation’s first millet milk ice cream shop entirely vegan. A vegan millet milk ice cream is available in an ice cream shop in Trichy, Tamil Nadu. Foxtail millet flour was used to make burfi instead of 57% of Bengal-gram flour, and adding this flour considerably decreased blood sugar and cholesterol levels [178].

5.7 Noodle- vermicelli

Malted finger millet (0%-50%), wheat semolina (10%-50%), and 2% salt were used to create nutrient-rich vermicelli [179]. They discovered that 30% malted flour had the best sensory results. Factors revealed higher protein, fibre, and mineral levels (calcium, iron, and phosphorus) compared to a vermicelli control sample. Devi et al. [130] used a cold extrusion process to create gluten-free sweet vermicelli. The components were pearl millet (30%-80%), sorghum (15–30%), roasted green gram (20–23.5%), guar gum (1–2%), and sugar (12–20%). Based on the superior nutritional, sensory, and three-month shelf life of pearl millet. It is concluded that gluten-free sweet vermicelli can be produced from it. At the commercial scale, sorghum, green gram, guar gum, and sugar proportions are 48%, 15%, 23.5%, 1.5%, and 12%, respectively.

5.8 Alcoholic beverages

In Arunachal Pradesh, Mauda, one of the famous finger-millet-based beverages. To produce mauda, millets are roasted for 30 min, then cooling and cooking until they soften. Fermentation is done in a perforated basket covered with Ekam leaves for 4–7 days. After fermentation, hot water is added from the top, and liquid is gathered in a jar. This fluid is known as mauda [180]. Similarly, Sur, is also a fermented beverage made from finger millet that is predominantly produced in various districts of Himachal Pradesh [181].

5.9 Non-alcoholic products

Appalu, a traditional sweet dish especially in Andhra Pradesh and Telangana, is made from rice flour but a mixture of pearl millet flour, and Bengal gram flour is also used for Appalu production. Also add some sesame seeds, carom seeds, chili powder, and salt to make dough. Then, divide the dough into tiny balls and flatten it into a spherical form. Then they are cooked and served hot. This millet version not only offers a different flavor but also provides added health benefits like higher fibre, minerals, and nutrients compared to the traditional rice flour version. Likewise, Samai Payasam, is also a traditional South Indian dessert made using samai (little millet). It is a delicious and healthy alternative to the usual rice-based payasam, offering a rich flavor with the added benefits of millet [182, 183].

6 Millets as fodder for animals

In India, animal nutrition generally consists of agriculture by-products such as rice, wheat, oil cakes, maize silages, bran and pellet feeds, premixes, and supplements. Millets can be encouraged in animal husbandry, particularly in the last mile rural region, to make farmers self-sufficient by offering a consistent revenue source despite agriculture uncertainty. States like Uttar Pradesh, Maharashtra, Haryana, Punjab, Rajasthan, etc., are hubs for animal husbandry and dairy-related products. The grass commonly referred to as sorghum (jowar) is cultivated as grazing fodder or is harvested green to generate silage and hay. Modern dairy encourages the use of sorghum silage as a food source. After harvest, the remaining stalk is usually grazed because it maintains its greener color for longer. The simplest and inexpensive way to prepare sorghum grain for cattle is to grind it. Likewise, Corn’s entire replacement with millet (50% of layers diets) had the same result on the egg production rate [184]. Feeding laying hens upon pearl millet is believed to be beneficial because it contains lower Omega-6 and higher Omega-3 fatty acids [185]. Sorghum’s nutrition profile is similar to corn’s and is complementary to the protein source generally used in chicken feeds worldwide. Additionally, near about, a 15% undergrounded pearl millet diet can be offered to laying hens as a maize substitute in soybean-based diets. Pearl millet-based diet for laying hens with flaxseed or soyabean oil supplementation can be provided to ensure egg production, yolk colour, and polyunsaturated fatty acid.

7 Conclusions

Due to growing nutritional knowledge, the food business faces problems developing novel food products with distinctive qualities that can enhance people's health. These dietary components, or polyphenols, found in millet varieties have several health advantages, including antibacterial, antioxidant, anti-diabetic, and hypocholesterolaemia. They also protect against diseases linked to food, particularly in rural areas. So, the new perspective on millets highlights their potential to address various challenges facing our world today, from nutrition and health to sustainability and cultural heritage. Their versatility, adaptability, and potential to support human and environmental well-being make millets a significant player in shaping the future of food and agriculture. It's necessary to highlight that while there is promising research on bioactive compounds and the health benefits of millet fermentation, more studies are needed to comprehend the extent of these effects and their mechanisms fully. Furthermore, individual responses to fermented millet may vary based on genetics, overall diet, and health status.