Distribution of pearl millet grain minerals

According to Minnis-Ndimba et al. [23], proton-induced X-ray emission (PIXE) spectrometry has shown to be an effective method for examining the distribution of key mineral elements in millets (Table 1). The endosperm and germ are the two primary sections of pearl millet. The primary site of mineral accumulation, according to the PIXE, is the tissue of the germ (scutellum and embryo). While Ca, Mn, and Zn are concentrated in the pearl millet embryo tissues, Fe, P, and K are substantially concentrated in the scutellum. Fe is comparatively high in the scutellum and other peripheral areas, while Zn is more prominent in the embryo. In actuality, the scutellum has 15% more Fe than the embryo.

On the other hand, the endosperm has comparatively low Zn, P, and Fe levels. It was also observed that the outer grain layers included comparatively significant concentrations of Ca and Fe. This demonstrates that the distribution patterns of these two minerals within the pearl millet grain are different [23]. Additionally, microscopic analysis showed that Zn was mostly dispersed in the pericarp, sub-aleurone, and germ regions, whereas Fe was found in the aleurone layer, scutellum region, and the tip of the germ. This demonstrates once again that the distribution patterns of Zn and Fe are different [37]. Earlier investigations reported that Zn in millet is found in the interior sections of the grain where the germ is lodged in the endosperm [38]. The Zn was absent or present in very small amounts in the peripheral tissues. In addition, Zn is equally distributed in the grain and around 8% is present in the bran of pearl millet, according to Hama et al. [38, 39]. This contrasts with Fe, which is not.

The phases of the INFOGEST protocol

The oral phase

To standardize experimental conditions and enhance result comparability, an international network of excellence on the fate of food in the gastrointestinal tract (the INFOGEST network) has developed a consensus protocol for static and semi-dynamic simulated digestion, which is widely adopted in the scientific community. These protocols replicate the physiological conditions of oral, gastric, and small intestinal digestion phases, with the static method being fully controlled by the analyst, while the semi-dynamic method incorporates gradual pH variations through automated processes. It is recommended to incorporate physiologically relevant electrolytes or salts such as simulated salivary fluid (SSF), simulated gastric fluid (SGF), and simulated intestinal fluid (SIF) during the digestion phases to determine how they impact enzyme activities and digestion outcomes (Figure 2). These solutions mimic the ionic composition of digestive secretions in the various sections to support enzyme activities [40, 41]. Table 2 highlights the different types of electrolytes and their role in the phases of the digestion process. The oral phase encompasses a dilution of food with SSF, combining potential salivary amylase use and simulated mastication for solids or semisolids. This phase is essential for all simulated digestion processes, ensuring uniform dilution across food states [40]. Usually, an approximate pH of 7 at 37°C for 2 min runs if salivary amylase is used in a standard INFOGEST model [41]. The 2 min period mimics the brief period food stays in the oral cavity. In the oral phase, there is less effect on mineral solubility as most minerals are still bound to phytate and complexed with the cell wall [42]. However, other studies observed that fermented and germinated millet floursʼ soluble Fe and Zn increase after α-amylase digestion. Furthermore, oral digestion facilitates the release of minerals in the gastric phase through homogenization and exposure of mineral-binding sites, improvement [43]. It is now a mandatory phase for ensuring consistency in all food matrices before entering the gastric phase [40].

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Figure 2. Schematic route of INFOGEST in vitro digestion models. The figure was created using FigureLabs (https://figurelabs.com).

Table 2. The role of electrolytes in the INFOGEST digestion model [41].

Electrolyte	Phase (s) used	Role
NaCl	Saliva, gastric, and intestinal	Preserves ionic strength, contributes to pH buffering and osmotic balance, and replicates the natural sodium levels in digestive juices.
KCl	All phases	Provides physiological potassium concentration; thus, important for maintaining osmotic pressure and enzyme activity.
KH2PO4	Salivary, intestinal	Acts as a phosphate buffer, stabilizing pH and providing phosphate ions for buffering capacity.
NaHCO3	Saliva, intestinal	Primary buffering agent that neutralizes gastric acid in the intestinal phase and maintains physiological pH (7).
MgCl2·6H2O	All phases	Supplies magnesium ions, essential cofactors for many digestive enzymes (e.g., amylases, proteases, lipases).
(NH4)2CO3	Salivary	Mimics ammonia content in saliva, contributing to pH buffering and enzymatic activation.
CaCl2·2H2O	All phases	Supplies calcium ions, which promote micelle formation in lipid digestion and stabilize enzyme structures (especially amylase and lipase).
ZnSO4	Intestinal	As a cofactor for certain proteolytic enzymes, thus enhancing metalloprotease activity.
Bile salts	Intestinal	For emulsification of fats, solubilization of lipophilic compounds for absorption, and micelle formation.

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Bioaccessibility at a glance

From a nutritional point of view, bioaccessibility is the amount or degree to which micronutrients are released from the food matrix and made available for absorption after digestion in the gastrointestinal tract [45, 46]. Bioaccessibility relies on digestion, and the release of nutrients from the food matrix can be simulated using in vitro methods. It allows investigation of how factors like food processing, food matrix composition, and luminal conditions, such as pH, affect nutrient availability. Additionally, bioaccessibility is influenced by the interactions between components and physicochemical properties, including pH, temperature, and texture [47, 48]. Rodrigues et al. [49] revealed that diverse in vitro digestion (IVD) models exhibit varying levels of complexity and automation.

Bioavailability at a glance

The percentage of a nutrient that is broken down, released from food, absorbed by intestinal cells, and made available for bodily physiological processes is known as bioavailability [47]. Following digestion, the nutrients released are taken up and absorbed throughout the intestinal tract, where bioavailable water-soluble and micellarized fat-soluble nutrients permeate the mucin layer and reach the apical membrane of enterocytes. They are then absorbed within these cells [49]. Bioavailability (in vivo) is affected by several host characteristics that in vitro techniques cannot precisely reproduce [47]. Utilizing human subjects, animal models, Caco-2 cell models, and techniques such as mineral dialysis, the bioavailability of micronutrients in crops has been evaluated [45].

Phytic acid content

According to Gibson et al. [51], phytic acid is defined as myo-inositol hexakisphosphate (IP6), which is composed of an inositol ring with six phosphate ester groups, and the salts Mg, Ca, or K phytate that relate to it. Phytates, which are mixed mineral salts of phytic acid, are a common form of phytic acid found in grains [50]. Food grains of the cereal class, oil seeds, nuts, and legumes are rich sources of phytate [52]. According to Savita et al. [52], it makes up between 3% and 5% of the dry weight of plant seeds. Phytate is mainly stored as a complex salt of minerals (Ca, Mg, K, etc.) and protein in globoids and aleurone particles of cereals and legumes. Krishnan and Meera [37] reported that the aleurone, germ, and sub-aleurone layers of pearl millet contain higher concentrations of phytic acid. Phytate is soluble in water and only slightly soluble in ethanol [53]. However, the formation of insoluble complexes decreases their solubility and hinders the absorption of bound elements and IP6 in the small intestines, thereby lowering their bioavailability [54]. While ferric phytate is insoluble at pH levels between 1.0 and 3.5, phytates become more soluble at pH values around 5.5–6.0 with Ca²⁺, 7.2–8.0 with Mg²⁺, and 4.3–4.5 with Zn²⁺ as the counter ion [55]. The buildup of phytic acid rises as external calcium concentration rises. According to reports, pearl millet has a phytate level of 485 mg/100 g [56]. Furthermore, phytic acid P concentration ranged between 0.198 ± 0.034 g/100 g and 0.410 ± 0.036 g/100 g, according to a thorough analysis of a panel of 145 inbred lines of pearl millet [Pennisetum glaucum (L.) R. Br.] of seed flour that came from landraces in India, West Africa, and Central America and encompassed a wide genetic variety [18]. According to a recent study, pearl millet has an extremely high phytate concentration of 878 mg/100 g [57].

Fiber

Dietary fiber includes non-starch polysaccharides, lignin, resistant starch, non-digestible oligosaccharides, cellulose, hemicellulose, and pectin according to Krishnan and Meera [37], and pearl millet has 20.8% dietary fiber [70]. Another study has found that the total dietary fiber (TDF) content among pearl millet genotypes ranged from 7.68–16.18 g/100 g [71]. Dietary fiber is classified as either water-soluble, highly fermentable, or non-water-soluble, less fermentable based on its solubility in water and fermentation capacity. Water-soluble highly fermentable fibers include pectin, gums, inulin, galactomanans, soluble hemicelluloses (arabinoxylans or pentosans), and mucilages; non-water-soluble less fermentable fibers include cellulose, insoluble hemicelluloses, lignin, and resistant starch [72]. TDF is the sum of the soluble dietary fibers (SDF) and insoluble dietary fibers (IDF) [73]. Water-soluble non-starch polysaccharides in pearl millet account for approximately 0.66% of grain weight and water-insoluble non-starch polysaccharides for 3.88% [70]. Most of the dietary fiber present in pearl millet grains is insoluble fiber. It is composed of cellulose, hemicellulose, and lignin, whereas SDF is made up of arabinoxylan and β-glucan. From the outer to the inner layers of the grain, these five elements are dispersed in different ratios [74]. Compared with the insoluble dietary fraction, the soluble dietary fraction is often concentrated in the inner layers [74]. IDF ranging between 54 and 72 g/100 g was present in the bran-rich fraction [75]. Jha et al. [75] also reported that most of the dietary fiber in pearl millet was insoluble fiber. The endosperm fraction had an IDF content of 5.47 g/100 g, whereas the bran-rich fraction had a high insoluble fiber level of 63.32 g/100 g. Moreover, they observed that pearl millet’s coarse fraction, containing pericarp and germ, had higher IDF content, while its soluble fiber content is larger than the endosperm-rich fraction [75]. Another study conducted by Goudar et al. [57] revealed that of a TDF content of 11.76 g/100 g in pearl millet, soluble dietary and insoluble dietary fractions were 1.91 and 9.85 g/100 g, respectively. Thus, without any ambiguity, the bran of pearl millet has more IDF than the endosperm.

Oxalate

Oxalate is a salt that is made from oxalic acid. Oxalic acid usually forms strong bonds with different minerals, like Ca, Mg, Na, and K, to form oxalate salts. The insoluble Ca oxalate tends to precipitate (or solidify) in the kidneys or in the urinary tract, resulting in the formation of kidney stones [56, 73, 79]. According to Goyal [80], oxalate is engaged in several processes, including the regulation of Ca, the management of metal toxicity, the suppression of oxidative bursts in response to pathogenic invasion, and programmed cell death. Goudar et al. [57] revealed in a recent paper that the oxalate concentration of pearl millet grain was 26.50 mg/100 g. Prior research by Suma and Urooj [32] on two types of pearl millet obtained from the Mysore, India market revealed an oxalate level of 31.6–58.4 mg/100 g for the MRB variety and 36–64.8 mg/100 g for the K variety. In addition, research by Goyal [80] stated that the oxalate level of pearl millet varied from 0.63–1.98% oxalate content. Plants contain both soluble and insoluble forms of oxalate. Soluble forms are created when oxalate combines with K, Na, and Mg (though Mg oxalate is less soluble than the K and Na salts). Insoluble forms are created when oxalate combines with Ca and Fe. Ca oxalate is not soluble at neutral or alkaline pH levels, but it readily dissolves in acid.

Phenolic compounds

An aromatic ring with one or more hydroxyl groups and a range of substitutions is what defines phenolic compounds, which are a broad class of substances [50, 70]. These are secondary metabolites found in plants. Based on their carbon structure, phenolic compounds are classified into many classes, such as stilbenes, lignans, phenolic acids, and flavonoids. According to Raes et al. [50], they can exist as monomers, polymers, or bound to conjugates. Total phenolics localization studies support the notion that polyphenols are primarily found in the pericarp, aleurone layer, and to a lesser degree in the endosperm, that is, the area directly adjacent to the epicarp of the pearl millet grain [37]. The results of the studies conducted by Krishnan and Meera [37] revealed that the content of galloyls in the pearl millet fine fraction (endosperm) and coarse fraction (bran and germ) was 0.929 and 3.317 mg/g, respectively; additionally, they reported that the content of catechol in the pearl millet fine fraction (endosperm) and coarse fraction (bran and germ) was 1.360 and 3.415 mg/g, respectively. These findings validated the notion that phenolics such as galloyls and catechol are more abundant in the bran and germ and less in the endosperm.

Decortication: mechanical food processing

Millet and certain other coarse grains are typically dehulled and subjected to various treatments before consumption to improve their sensory and edible quality [85]. The pericarp and bran layers are mechanically removed from the pearl millet grain, reducing anti-nutritional factors such as phytate, tannins, and polyphenols, as well as some minerals and vitamins [8]. It was previously not possible to cook decorticated millet as edible, discrete grains like rice and get a soft texture [83]. Yousaf et al. [86] presented two distinct ways for decortication: a modern dehuller and a stone dehuller. Two conventional techniques previously mentioned by Saleh et al. [83] are hand-pounding and mechanical abrasive decortication.

Soaking: traditional and modern food processing

Using this method, grains are completely submerged in liquid for a predetermined amount of time, activating endogenous enzymes such as phytases that are naturally present in grains. This can remove a significant amount of phytic acid content in grains by activating these enzymes through soaking [87]. The soaking liquid medium can be acidic, neutral, or alkaline; the pH of the soaking medium determines how much an antinutrient is broken down [75]. Five types of pearl millet varieties that were studied had a significant decrease in their phytate content, and their percentage decrease ranged from 23.6–32.4% [88]. The reason for this reduction in phytate content could be that soaking solubilizes phytic acid salts, which are stored in relatively water-soluble form (like K and Na phytate). Additionally, because phytate is present in the aleurone layer and pericarp, it could leach out into the water used for soaking [88].

Fermentation: bioprocess food technology

In addition to increasing the availability and digestibility of nutrients (minerals), fermentation frequently improves the food’s texture, flavor, aroma, and acceptability while also decreasing or degrading certain naturally occurring toxins in the food crop [89]. Fermentation is a metabolic process that uses microorganisms to break down complex materials into simpler forms [90]. In Africa, where modern food preservation techniques are still uncommon, fermentation is commonly used due to its usefulness in food preservation [83]. There are three types of cereal fermentation: controlled, backslopping, and natural (spontaneous). Microorganisms compete during natural fermentation, with the most suited strain becoming victorious. Using a little percentage of a successful batch as starter cultures is known as backslopping. For shorter lag times, controlled fermentation employs strains. Although less efficient, natural fermentation (spontaneous) is widespread in underdeveloped nations. Additionally, solid-state or submerged/liquid fermentation is possible [89].

Milling: mechanical food processing

The main processes involved in milling are grinding, sifting, separation, and regrinding. These are repeated to extract specific parts of the grain, such as endosperm, and to obtain bran as a by-product [91]. The process involves separating endosperm, bran, and germ. To facilitate the production of fine flour, endosperm is ground into small and fine particles; hammer mills and roller mills can be used to reduce the particle size. Additionally, millet grains are typically ground by a nonmotorized grain mill that is operated by hand or another nonelectric method, particularly in rural areas for household purposes. Nevertheless, a manual grain mill that is connected to a gas or electric motor via a pulley system can also be used in this manner.

Blanching: ambient and thermal temperature

Blanching (briefly immersing food in boiling water or steam) is a thermal treatment means of processing food relative to production, harvesting, and transportation [92, 93]. One of the best pre-milling techniques for extending the shelf life of the finished product is blanching. Without significantly altering its nutritional makeup, it might slow down the enzymatic activity [8]. Concisely, blanching, which involves heating steam or hot water, helps reduce or destroy microbiological activity, removes moisture to extend shelf life, maintains food quality to promote consumer acceptability of the final product, and inactivates enzymes that could cause quality degradation.

Heat treatment: ambient and thermal temperature

Prior to the advent of convenience foods, heat treatment was one of the most significant pre-milling processes for pearl millet because it has a relatively short shelf life and serves as a major catalyst for increasing daily consumption [8]. Cooking is regarded as a household method of making basic, traditional staple foods [86].

Germination: bioprocess food technique

It is common to use the phrases “germination”, “sprouting”, and “malting” interchangeably [86, 94]. Steeping in water, imbibition, endogenous metabolic activation, and radical emergence are all steps in the process. Hydrolytic enzymes are activated and synthesized from scratch during sprouting. Improved mineral bioaccessibility and the release of trapped minerals result from the hydrolysis of phytate by hydrolytic enzymes such as phytase activity [86]. Additionally, pearl millet’s germination lowers its phytate content. Furthermore, the decline was greater at a grain-to-water ratio of 1:5 than it was at 1:2. Thus, the phytate level was significantly decreased by a longer duration and a larger amount of soaking water [88].

Germination: bioprocess food technique

Hydrolytic enzymes like phytase are activated and undergo de novo synthesis when sprouting occurs. Additionally, as germination time increases, pearl millet’s phytase activity gradually rises. After 72 h of germination, the entire pearl millet’s phytase activity rose from 0.190 μmol/g to 1.890 μmol/g in the coarse fraction and 0.461 μmol/g in the fine fraction. According to Krishnan et al. [76], the fractionated native grains’ phytase activity showed that the enzyme was primarily concentrated in the coarse fraction as opposed to the fine fraction. Enhanced mineral bioaccessibility and the release of trapped minerals resulted from increased phytase activity, which also causes phytate hydrolysis [94]. When compared to raw pearl millet flour, the Mg, Ca, Na, and Zn levels of the three-day-germinated millet flour increased [34]. Moreover, germination raised the Fe content of millet somewhat, according to Sow et al. [95]. Thorat et al. [88] found that, however, the Fe and Zn contents of germinated and malted pearl millet grains decreased when compared to the raw millet samples. In this study, the grain: water ratio during soaking resulted in the loss of minerals, with a large volume of water to grains leading to a significant loss in Zn, and vice versa in the case of Fe. They observed 8.8% to 17.5% loss upon germination, and they hinted that the low percentage of Fe could be due to the period of germination, which was short, 48 h, in contrast to other researchers who examined the effects of germination for longer periods of time, up to 96 h. After 4 days (96 h) of germination, Zanabria Eyzaguirre et al. [96] found that the pearl millet’s total Fe concentration had decreased by 50%.

Fermentation: bioprocess food technology

In some fermented foods that are popular in India, fermentation raised the concentrations of Mg, Fe, Ca, and Zn; this rise was correlated with a decrease in phytates [97]. On the other hand, since bacteria break down protein and carbohydrates during fermentation, dry matter loss may also contribute to the rise in mineral content [98]. Food grain fermentation increases the minerals’ bioavailability [63]. Fe levels in the millet were very slightly reduced during the fermentation process, going from 7.56 mg/100 g to 7.55 mg/100 g. The short soaking time after fermentation was the cause of the minor reduction [95]. Research indicates that Hausa “koko” with bread and “akara”, a fermented porridge made from pearl millet in Ghana, has a high Fe content (15 mg/100 g) and a high Mg content (1.2 mg/100 g), but low levels of Na and Cu. The reason for the relatively high Fe content in the Hausa “koko” with bread and “akara” could be the processing method (soaking millet, milling, fermentation, and cooking), which also likely involves the processing of “akara”. The Mg content was a result of using whole grains, which are the richest sources of Mg. The low Ca content is caused by phytate’s inhibitory effect, as the phytate:Ca molar ratio (8:2) was very high above the critical level of 0.24, but it was advantageous for Zn and Fe bioavailability in the diet.

Soaking: conventional and contemporary techniques of processing food

Soaking is a pretreatment that facilitates grain processing and is the most important procedure in cereal germination and fermentation processing, according to the review by Feizollahi et al. [60]. They also discussed how endogenous phytase activity and pH-dependent leaching of soluble phytate salts into the soaking medium affect the phytate level after soaking, and how discarding soak water prior to these processing steps lowers phytate, since phytate is soluble in water. Leaching of phytate is influenced by the characteristics of the phytate, which is a type of salt containing various minerals, including K, Ca, or Mg. It has been found that soaking causes the overall Fe and Zn contents to drop. It was discovered that soaking millet grains in water decreased their Fe and Zn concentrations; this could be because the minerals in the soaking water leached [88, 99]. According to other research, the Fe content of pearl millet flour decreased dramatically when the soaking duration increased from 0 to 12 h [100]. Additionally, a 24 h soaking resulted in greater Zn (18%) and Fe (48%) losses in dehulled millet grains relative to Fe loss in intact millet grains. Once more, the losses were attributed to the minerals leaching into the soaking medium because of a concentration gradient, and the variations in the percentage losses of the minerals were attributed to the locations of Fe and Zn [101]. Zn is more concentrated in the embryo, but Fe is more prevalent in the pericarp [23]. Reducing anti-nutrients through soaking has been shown to improve mineral bioavailability [102]. Jha et al. [75] investigated how soaking circumstances affected the bioaccessibility of Fe and Zn in Indian pearl millet. Apart from endosperm soaked in water, they discovered that soaking grain fractions reduced the amount of Fe. All bran fractions, except the bran soaked in water, showed a decrease in Zn content. Endosperm and bran-rich fractions showed an increase in Fe bioaccessibility because of the solubilization of insoluble fiber and a decrease in phytate content. Zn bioaccessibility, however, declined in the endosperm-rich fraction, suggesting that different inhibitory factors had different effects on mineral bioaccessibility within the grain. The effects of pre-treatments that are typically applied in homes on Fe bioavailability are helpfully understood by means of this kind of research [103]. It has been reported that soaking causes Zn and total Fe loss. Zn and Fe levels drop by roughly 20–40%, with losses increasing as soaking times are extended from 12 to 24 h and, in the case of Fe, when more water is used during soaking (grain to water ratio of 1:5) [83]. Zinc losses in the first case and Fe losses in the second case were largely caused by the type of millet and the huge volume of soaking water medium [83].

Roasting: thermal food processing technique

By browning the surface of the food, roasting is a processing method that uses dry heat from an oven, open flame, or other heat source to improve flavor. It has been observed that adding roasted pearl millet to beef sausages boosted the sausages’ mineral contents overall, especially those of Mg, Fe, and Ca [104]. The roasted pearl millet improved the mineral content of the sausages by adding minerals. The amount of Fe, Mg, and Ca in the sausages was considerably higher when roasted pearl millet flour was utilized.

Decortication: mechanical food processing technique

The effects of 2, 4, and 6 min decortication on Fe-binding phenolics (galloyls and catechols) were investigated by Krishnan and Meera [37]. They found that decorticating pearl millet for 4 min enhanced the galloyls and catechols in the bran fraction while decreasing the grain fraction. However, the galloyl and catechol contents of the bran and endosperm fractions, respectively, decreased after an additional 6 min of decortication. The galloyls and catechols are more restricted to the outer regions of the grain, below the epicarp region, which explains the results that have been found [37].

Starter cultures

On solid media, the degradation of phytate by yeast phytases has been investigated. According to Greppi et al. [113], Pichia kudriavzevii, which was isolated from “Gowe” and “Mawe”, affected the profuse production of phytase, which in turn caused the breakdown of phytic acid. Additionally, on both phosphate-free and high-phosphate media, the Pichia kudriavzevii TY13 strain isolated from “togwa” produces higher levels of phytase to break down [114]. The ability of the isolated yeast to break down food-grade phytin complexes in the food matrix was not tested in either study, nor were the amounts of various minerals released during the breakdown of phytate quantified. Raghavendra [115] did, however, screen for phytate-degrading lactic acid bacteria (LAB) isolated from several Indian foods in a thesis work. The investigation found that 20 isolates with the ability to break down phytate on de Man-Rogosa-Sharpe (MRS) agar containing phytate and MRS agar containing Na phytate in addition to CaCl₂ were obtained from the intestines of chickens, marine fish, raw milk, and red rice. Accordingly, such cultures can break down food-grade phytin complexes, which enhances the bioaccessibility of Ca, Mg, and Zn in malted finger millet seed coat and soymilk. The levels of free Ca, Mg, and Zn increased noticeably, improving their bioaccessibility. This is because the phytin complex was destroyed by the action of phytic acid LAB, which led to the release of the bound Mg, Ca, and Zn.

Optimization

The food industry has come to terms with the fact that one major indispensable tool for addressing challenges is optimization. In optimization, favorable conditions are figured out and improved for optimal outcomes. The production systems geared towards the identification of the best conditions are done systematically [116]. The old approach of using one-factor-at-a-time has been metamorphosed to response surface methodology, which is characterized by experimental modelling and variable interactions. Rheem [117] further buttressed this view by indicating that response surface methodology focuses on using statistical and practical means for experimental designs, response modelling, and factor level optimization.

Optimization of soaking duration and water volume

Soaking and fermentation are wet processing steps adopted during the preparation of fermented foods. Optimization of these processing steps is critical for maximizing Fe, Ca, and Zn bioaccessibility, reducing antinutrients such as phytate, and retaining desirable organoleptic properties [118]. Soaking directly affects enzyme activation, phytate solubilization, and physical leaching of water-soluble nutrients and is the first major hydration step in many fermented food productions. Prolonged soaking (> 24 h) coupled with high volume of water has a tendency to increase the loss of total minerals through diffusion into the soak water, while short-to-moderate soaking (6–12 h) frequently reduces phytate and limits mineral leaching [119, 120]. Thus, the grain: water ratio is considered a significant parameter for mineral bioaccessibility outcomes. Larger water volumes, such as 1:5 (w/v) millet-water, are often characterized by high dilution, thus increasing the concentration gradient that drives soluble mineral and phytate complexed mineral to diffuse out of the grain, collectively leading to absolute mineral loss and phytate decline [121, 122]. Conversely, low water ratios such as 1:2–1:3 (w/v) limit leaching; however, reduce the effectiveness of endogenous phytase or exogenous microbial activity because of limited mobility. Thus, the optimization of soaking conditions is an essential practical means not only to balance sufficient water to permit grain hydration but also to promote enzymatic hydrolysis for antinutrient degradation while minimizing unnecessary dilution and discarding of the soak liquor. Recent studies suggest that 12 h soaking with a moderate grain:water ratio of 1:2–1:3 achieved significant phytate decline with acceptable mineral retention in millets and related cereals [119, 120]. Below the moderate grain:water ratio of 1:2, a decrease in mineral content has been reported [123] due to a decrease in mobility of endogenous and exogenous phytases from microbial activity [119, 120]. Stronger evidence from research earlier supported the preference of moderate, millet to water ratios, demonstrating a balance between a grain:water ratio of 1:3. However, it is noteworthy that the nutritional value of a processed food depends on what remains and its bioaccessibility (Figure 6). Therefore, though losses of minerals during soaking are inevitable, it does not always equate to the nutritional value of the final product [119, 120]. Feitosa et al. [124] and Luo and Xie [125] arguably reported that discarding soak water alters Fe and Zn bioaccessibility in legumes. Furthermore, it has been noted that while soaking reduces phytate levels and increases prospective mineral bioaccessibility, the simultaneous leaching of minerals into soak water may counteract these benefits, leaving the bioaccessible mineral fraction with little to no net improvement [63, 126].

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Figure 6. Techniques for improving pearl millet’s mineral bioaccessibility. The figure was created using FigureLabs (https://figurelabs.com).

Tuwo or tuo

A traditional treat from Northern Ghana and Nigeria, “tuwo” is prepared from millet, maize, or sorghum, depending on the producer’s preference. It is a solid meal that's formed into balls or swallows. It can be made using whole grains or dehulled grains. Fine flour is created by milling the grains. If the flour is made from whole grains, the hulls are removed by appropriately sieving the flour. When making “tuwo or tuo-zaafi”, water is heated to boil in a saucepan, some flour is sifted into cold water to create a slurry, and the cold slurry is then added to the boiling water to ensure proper boiling. After that, the food is heated for a further 10 to 15 min, stirred, and served with any preferred soup [84]. The millet flour is sieved and then gradually added to the pot along with the boiled water and slurry. Stir until the desired thickness is reached.

Ben-saalga/Hausa koko

“Ben-saalga”, also known as Hausa “koko”, is a fermented gruel made mostly from millet and produced in many traditional production units throughout Ghana and Burkina Faso. After washing and soaking the grain, pearl millet is ground, kneaded, sifted, settled, and cooked. In small amounts before grinding, aromatic components, including ginger, pepper, black pepper, and mint, are typically added, depending on the “ben-saalga” or “koko” producer’s custom. The supernatant from the settling process is collected and heated to almost boiling for 40 min during the final cooking stage. For consistency, the paste is then added to the supernatant and cooked for 7 min while stirring. Depending on the sourness degree of the supernatant, most processors boil either just the supernatant or a combination of fresh water and the supernatant for the manufacture of Hausa “koko”. To get the right consistency for Hausa “koko”, the heated supernatant or diluted supernatant is then gradually added to the sediment in an aluminum basin or pot while being constantly stirred.

Bushera

The most popular traditional drink made in Uganda’s western highlands, where sorghum and millet are significant commercial and staple crops, is “bushera”. Both adults and small toddlers use the product. “Bushera” is prepared in a variety of ways. To manufacture flour, germinated millet or sorghum grain is mixed with boiling water and allowed to cool to room temperature. Next, incorporate the germinated millet or sorghum flour and allow the mixture to ferment at room temperature for one to six days [15].

Kunu or kunun-zaki

Nigerians make “kunu”, a wholesome non-alcoholic beverage, from a variety of cereal grains, including rice, sorghum, millet, and maize. People of all age groups frequently utilize it as a dietary supplement or to relieve their thirst. “Kunu” is inexpensive and a good substitute for fizzy drink products, which offer their customers little to no nutritional value [84, 137]. To make “kunu”, the millet grain must first be cleaned and steeped in twice as much water as it has volume. Next, the grains must be washed and spiced with ginger, red pepper, cloves, and black pepper. Finally, the steeped millet grains and spices must be wet-milled in a grinding machine, sieved to remove the chaff, and the supernatant must be decanted from the slurry. Finally, the slurry should be divided into two equal parts, with one half being added to boiling water and stirred for 5 min. Lastly, the other half should be mixed with the cooled half-slurry. After adding adequate water, the mixture is stirred and given time to settle. The filtrate, which has been sweetened with granulated sugar, is then suitably mixed and sieved through a muslin cloth to produce the freshly processed millet beverage. The product is bottled in plastic bottles [84].

Fura

After being slightly moistened with water, the millet grain is processed in a locally manufactured disc attrition mill. The grain is pulverized in a hammer mill and sieved after the hull is removed, after it has dried in the sun. To make a smooth dough, mix pearl millet flour, powdered ginger, powdered black pepper, and water (95°C) in a mortar and pestle. The dough is formed by hand into balls, put in a pan of boiling water, and cooked for 30 min. While the balls are still hot, they are kneaded once more until a smooth, slightly elastic mass is achieved. After that, the dough is formed into “fura” balls [15] and floured [84].

Ogi

This fermented gruel made from cereal is made and consumed in West Africa, primarily in Nigeria. According to Amadou et al. [15], “ogi” is a porridge made from fermented cake or paste derived from millet, sorghum, or maize. It is frequently used as a baby’s weaning diet, an adult’s breakfast, and a sick person’s diet [136]. The processing includes washing and steeping the grains for 12 to 72 h, decanting the fermenting water, wet-milling the grains to create a paste, wet-sieving the paste to create a filtrate, and allowing the filtrate to settle and ferment for one to three days. Warm water typically reduces the amount of time cereal needs to soak. Additionally, the producer’s preferred pH and taste determine how long “ogi” takes to ferment [136]. “Ogi” is the origin of numerous regional cuisines that go by different names. Boiling the slurry produces porridges. Other types of “ogi” include “eko” (stiff gel) and “akpan” (fluid or semi-solid cooked “ogi”) [15, 136, 138].

Togwa

This is a non-alcoholic drink made by fermenting mixtures of millet, sorghum, cassava, and maize. In Tanzania, “togwa” is an indigenous fermented dish that is frequently eaten by adults and small children. Sorghum and millet, or sorghum and maize, are combined in a 1:1 (w/v) ratio to produce “togwa”. After that, water is added and combined to create a slurry in a 1:9 (w/v) ratio, which is boiled to create gruel. After cooling the gruel to around 30°C, sorghum malt and aged “togwa” are added. The mixture is allowed to ferment for one to three days. The preparation of “togwa” is akin to that of Mahewu in South Africa. The kind of inoculum utilized differs primarily in that the Mahewu recipe uses wheat flour as an inoculum [136].

Oshikundu

“Oshikundu” is a traditional non-alcoholic fermented cereal beverage that is drunk in Namibia’s Ohangwena, Omusati, Oshana, and Oshikoto areas. When yeast is utilized for fermentation, the beverage can turn into an alcoholic one. Malted sorghum meal and pearl millet (“mahangu” meal) are used to make it. This beverage is made by women in Northern Namibia for household use, but it is also offered for sale in public markets. The beverage is consumed in a single day due to its incredibly short shelf life of less than 6 h. It has the potential to be commercialized due to its processing method, which is why Namibia’s school meal program has included it. To prepare the “mahangu” meal, boiling water is added, the mixture is allowed to cool to room temperature, and it is periodically stirred. Malted sorghum meal is then added to the mixture. In some regions, the “mahlangu” meal and malt sorghum meal are blended before the boiling water is added. The mixture is then blended and allowed to cool [132, 135]. At this stage, adding bran is optional, but regardless of the producer's strategy, it is added. Previously fermented “oshikundu” is also added. Water is added to the mixture to dilute it. The fermented “oshikundu” is ready to consume after the mixture has been allowed to ferment for several hours at room temperature.

Mahewu

“Mahewu” is a nonalcoholic beverage made from cereal that is produced and consumed in Zimbabwe. It is refreshing and can be used as an additional food source for young children. To make it, prepared millet or sorghum malt is combined with cooked and cooled maize porridge. The mixture is then allowed to ferment in a plastic bucket or clay pot at room temperature for 16–48 h [134].