Processing of Millets

Abstract

The necessity for countries in sub-Saharan Africa (SSA) to be self-sustaining in the fight against food and nutrition insecurity is of crucial importance to maintain their autonomy. Promoting indigenous, drought-tolerant crops is a potential way of mitigating the impacts of climate change and supplementing maize, whose productivity has declined due to dependency on erratic rain-fed agriculture. Millets are known for their high amount of macro- and micronutrients (such as B vitamins, potassium, phosphorus, magnesium, iron, zinc, copper and manganese). However, millets also contain significant amounts of anti-nutritional factors (polyphenols, enzyme inhibitors and phytates), resulting in low bioavailability of the minerals and proteins. This has led to employing a number of processing techniques during millet meal production to reduce these effects. Hence, this chapter focuses on evaluating millet processing techniques applied (e.g., soaking, dehulling, steaming, controlled germination and roasting) and their influence on the anti-nutritional factors, nutritional composition and functional properties of millet meals based on the available literature reports. This review demonstrated the importance of millet processing technologies in removing anti-nutritional factors that could reduce the bioavailability or bioaccessibility of essential nutrients.

1. Introduction

Billions of people globally have been shown to be experiencing malnutrition as well as food insecurity [1]. The global target set by the United Nations to end hunger by 2030 is far from being attained [2]. Rapid population growth and climate change, compounded with economic meltdown over the past few decades, have greatly impacted food security [3]. It is thus imperative to transform the current food system to achieve food and nutrition security by providing an affordable, healthy and nutritious diet to all. Millets (sorghum, pearl millet and finger millet) are climate-resilient crops that can be grown with minimum inputs even under unfavourable agricultural conditions and are tolerant to heat, drought and floods [4, 5]. This makes them the crops of choice in light of climate change and limited natural resources because good productivity can be obtained even in marginal areas with water scarcity.

Millets have the potential to play a significant role in the fight against malnutrition and food insecurity in children, adolescents and the community at large, as well as promote health and immunity [6]. They contain an abundant source of essential macro and micronutrients such as carbohydrates, proteins, dietary fibre, lipids and vitamins [7]. Phytochemicals and antioxidant properties in millets give them the ability to prevent the onset of cardiovascular diseases, cancer and diabetes, among others. Additionally, millets are key for people with celiac disease [7].

The production of millet as a source of human food to meet dietary requirements has increased over the past few decades [8]. Despite their benefits, millets are still predominantly consumed by the poor population because of a lack of awareness of processing technologies and appropriate preparation methods. The nutrients, bioactive compounds and functions of millet grains can be influenced by preparation techniques such as decortication/dehulling, soaking, germination/malting, milling, parboiling and fermentation [3, 8]. According to Amadou et al. [9], due to the small grain size, dehulling is not favourable to millets as the process causes nutrient loss. However, dehulling of millets can also improve the bioavailability of nutrients and consumer acceptability of millet-based products. Milling removes the bran and germ layers, causing significant loss of fibre and phytochemicals [9]. Hence, there is a need to select an appropriate method for each millet type during the production process.

Processing millets into various value-added by-products, such as flour, flakes, nutritional bars, cookies, cakes and extruded products, can increase their consumption and utilisation to enhance health, nutrition and food security. The use of millet combined with other food crops to develop novel value-added food alternatives is an emerging and promising area of food technology [6]. Nutrition-rich fermented foods and beverages from millet also form a major part of the diet in most African countries. Fermentation by lactic acid bacteria increases the protein content and improves digestibility [10, 11]. Millet processing technologies afford the opportunity to prepare a wide range of products, hence giving consumers a variety of choices of acceptable, savoury and nutritious food products [12].

Apart from the processing of millet into value-added production for human consumption, it can be processed as a source of livestock feed. Studies have investigated the use of millets in ruminant and non-ruminant diets [13]. For instance, Cisse et al. [14] reported that inclusion of pearl millet of up to 50% in broiler grower and finisher diets had no negative effect on their performance with comparable results to corn on metabolisable energy and digestible amino acids. An enhancement in growth and feed efficiency in broilers was reported after replacing corn with pearl millet [15]. Pearl millet was also reported to be able to fully replace maize in high-supplement diets for confined cattle [13]. Hassan et al. [16] observed that for grazing beef cattle during the dry season, millet processing increases the digestibility of dry matter and dietary nutrients.

Millet’s production and processing have drawn the attention of various food security stakeholders worldwide to enhance its better utilisation and reduce hidden hunger in the world. This chapter discusses the processing techniques of millets, the effect of processing on nutrients, anti-nutritional factors, bioavailability and bioaccessibility of nutrients, and the value-added products from millets.

2. Processing techniques

Processing of millet grains can be defined as the conversion of the millet grains into edible forms with an enhanced quality. Various processing techniques are therefore available for the conversion of millet grains into value-added edible products. These processing techniques can be classified into two basic types: hot processes, in which heat is either applied or created during the treatment process and cold processes, in which the temperature of the grain is not increased significantly [17]. The various processing methods are discussed below.

2.1 Soaking

Soaking involves mixing millets in either cold or hot water for a period of 6 to 24 h at various temperatures ranging between 30 and 70°C [18]. To reduce contamination from harmful microorganisms, hot (above 40°C) or cold water, <7°C, can be used. This softens and swells the grain and reduces the cooking time, while also enhancing digestibility [19], palatability and feed value [20, 21]. Portable water is periodically (e.g., 3 h intervals) changed during soaking to a total soaking time not exceeding 24 h. Kate and Singh [22] also reported increased mineral bioavailability and decreased anti-nutritional compounds such as phytic acid. After soaking, the product can then be crushed through crimped rollers to crush the grain. Unfortunately, soaking does exhibit some disadvantages, including the risk of souring during storage. In addition, the increased seed size, or swelling, after soaking can lead to storage capacity limitations [23]. Furthermore, micronutrients such as iron, phosphorus and calcium present in pearl millet have been found to be reduced following soaking in an acidic medium [22].

2.2 Dehulling or dehusking

Dehulling is the process of removing the outer coat of grain, also known as decortication. The hulls (outer covering of the grain) are high in fibre and low in digestibility in both humans and monogastric animals. When these are removed, feed value improves due to increased voluntary feed intake and digestibility [24]. Furthermore, bioavailability of existing nutrients and a decrease in anti-nutrients have been reported [22]. Other nutritive ingredients, such as iron, zinc and calcium, are compromised. Ikwebe et al. [25] reported decreases in nutrient content ranging from 2 to 7% after dehulling of the following: iron (Fe) from 40.3 to 39.3 mg/kg; zinc (Zn) from 38.53 to 35.97 mg/kg and magnesium (Mg) from 311.06 to 306.45 mg/kg. The decrease in these micronutrients suggests that these nutrients are concentrated in the outer coverings of millet grains [26].

2.3 Steaming

The grain is first steam-cooked for 15–30 min to increase the moisture content to 18–20%. The moistened grain is then passed through rollers, yielding a thin, flat flake that is then dried. Steaming has been shown to increase millet polyphenols, which improves antioxidant capacity [24]. This will have a net positive effect on human nutrition [27]. Steaming has been shown to significantly improve the whiteness of millets [28]. In livestock, flaked grain is considered more acceptable and has a slightly higher digestibility value than unprocessed grains. For example, sorghum starch digestibility increased from 42–91% after steaming and flaking [17].

2.4 Controlled germination

This is a natural processing technique that is an extension of the soaking method. The germination period ranges from 48 to 72 h at an ambient temperature of 25 to 30°C. More than 48 h of germination (sprouting) has been shown to result in a loss of grain dry matter with negligible nutritional improvement [22]. Controlled germination enables biological activation of grains, which improves their nutritional and functional properties [23]. Saleh et al. [21] reported an increase in protein digestibility after germination, which was attributed to a reduction in anti-nutritional factors such as phytic acid, tannins and polyphenols, which are known to interact with proteins, forming insoluble complexes that reduce their bioavailability for the body. Controlled germination reduces the component of hydrolysable tannins in sorghum while increasing phosphorus content, improving the grain’s feeding value for both humans and animals [29]. Humidity, oxygen and temperature are all critical environmental factors in controlled germination.

2.5 Roasting

Roasting is a form of heat treatment in which the grain is oven-roasted between 120 and 180°C for 3–15 min. It can also be done traditionally with a gas fireplace and an iron pan used as a hotplate that is heated by a flame [30]. It improves the flavour, palatability, voluntary feed intake and feed efficiency of millets [24]. It also destroys heat-sensitive anti-nutritional factors like phytates and tannins [31]. This increases the nutritional value of the final product for both humans and monogastric animals [32]. Roasting at high temperatures around 120°C has been shown to reduce phytates by 34.89% [30]. Wang et al. [24] found that roasting time and temperature affect total polyphenol levels in millets. For ruminant animals, it increases the flow of undegradable digestible dietary flow to post-ruminal sites [17].

2.6 Grinding

This is the process of reducing grain size to a form suitable for the required recipe [22]. A hammer mill can be used for this process. The final grain particle size is determined by the size of the sieve used. The method is simple and inexpensive. It generally improves the digestibility of small hard grains such as sorghum, increasing their feed value. On the other hand, grinding has been shown to significantly reduce the shelf life of millet grains by increasing the free fatty acid (FFA) content, which can cause rancidity in millet flour [22].

Ruminants prefer coarse to fine grinding. Grinding is also crucial in piggery as they are poor chewers of feed. Grinding is essential for maximum performance in poultry kept under intensive conditions as they do not have access to grit, which is used in the gizzard to aid grain breakdown [17]. Excessive grinding should be avoided because it does not improve digestibility or performance and can cause problems with pig health. Fine grinding can produce pasty, unpalatable, dusty material that can be inhaled and irritate the eyes [17]. It may also induce vomiting; as a result, it affects palatability and, thus, voluntary feed intake. However, it is important to note that fine grinding is essential in human food production.

2.7 Cooking

Cooking of millet is typically completed under steam pressure. The end product can be rolled to produce a product similar to steam-flaked grain. Kate and Singh [22] reported complex changes in physicochemical and functional properties of millets as a result of cooking, including gelatinisation, protein denaturation and the release of bound phenolic and antioxidant compounds. Cooking cereals improves palatability and digestibility slightly compared to unprocessed grains. The duration and temperature of cooking are critical factors to consider when preserving nutritional constituents, particularly minerals and phenolic contents that are heat-labile [22]. McDonald et al. [17] reported improved sorghum grain utilisation through pressure cooking.

2.8 Pelleting

In animal feed production, the feed is ground and then forced through a thick dye (moulder) to form pellets. The method is typically used to reduce the dustiness of feeds. Pelleting results in thorough mixing of the diet as the food is forced through the pelleting machine. In comparison to powdered feeds, pelleting allows for easier mechanical feeding. Pellets can be fed on the ground or in windy conditions since dustiness is reduced. In comparison to meals/mash, pellets do not separate feed ingredients in transit. The advantages of pelleting include that it increases the flow of nitrogen and amino acids into the small intestines [32]. In addition to this, pelleting improves voluntary feed intake, digestibility and feed value. The method is commonly used in rabbit and poultry feeds, among others.

2.9 Malting

Another processing technique commonly used in millets is malting. This is a three-step process carried out in sequence [22]:

1. Steeping (e.g., soaking of grains in water),

2. Germination (sprouts development and enhancement of enzymatic activity) and

3. Kilning (e.g., grain drying to stop enzymatic activity). This process dries the grain down to 3–5% moisture and arrests germination. It can be done using natural sun heat. However, where huge volumes of grain are involved, large volumes of hot air, 80–220°C, are blown through the grain bed [33], driving away moisture until desired moisture levels are reached.

Malting enhances the sensory properties, nutritional quality and digestibility of grains while lowering anti-nutrient levels. The kilning process yields malts of varying colours and flavours [33]. Polycyclic aromatic hydrocarbons (PAHs) are known to appear at higher temperatures and are generally considered undesirable in food and beverages because they pose a serious health risk to humans [33]. They are caused by pyrolytic processes that involve the incomplete combustion of wood, organic matter, coal or oil. PAH molecules have varying degrees of carcinogenicity, but all contribute to the overall carcinogenicity of foods exposed to smoking or other sources of PAHs.

2.10 Extrusion

Extrusion is a process of gelatinising and cooking a product until it is fully cooked, resulting in the production of various types of food. This process involves the use of heat or steam and pressure. Grains will move under pressure through an extruder, and by the time it exits (extrusion), the anti-nutritional factors will have been deactivated due to high temperatures in the extruder [16]. This affects protein content because it reacts and changes its levels and structure during the process [34]. Extrusion is a processing technique that reduces the protein content, solubility and water retention of millet varieties. The protein content may decrease due to denaturation of proteins, which has been observed to occur at temperatures above 50°C [17, 29]. Akharume, Santra and Adedeji [34] found that the average denaturation temperature of proteins in Proso millet was 82.1 ± 3.5°C. Hydrophobic properties decrease in response to extrusive processes, but emulsion stability remains unaffected. Emulsion stability influences the colour, texture and succulence of millets [35]. The stability is desirable because it prevents the separation of the oil and water components, resulting in a blending effect that is desirable for maintaining the quality, freshness and integrity of cooked products [36]. Extrusion improves feed value by inactivating anti-nutritional factors, and it may also promote undegraded digestible dietary protein for ruminant animals due to the high-temperature heating involved [24].

3. Effects of processing on nutrients

Millets are usually consumed after being subjected to a variety of processing techniques, such as heating, soaking, dehulling, steaming, roasting, germination, fermentation and other processing methods, which may alter nutritive content. Millets are bestowed with a wealth of nutrients, fibre and antioxidant properties that support and boost metabolism, augment heart health and control blood pressure. Among the nutrients is magnesium, which plays a role in maintaining normal blood pressure. Its adequate intake is associated with a lower risk of hypertension and cardiovascular complications. Millets contain amino acids, lecithin and methionine, which help in bringing down cholesterol levels by eliminating excess fat. The amino acid, tryptophan, in millets lowers appetite and thus assists in weight control [37]. Reduction in weight and cholesterol levels is essential in controlling blood pressure and keeping the heart healthy. These crops help balance cholesterol in the human body [21]. Millets contain essential fats, just the right amount to give our body adequate fat and help prevent excess weight. Millets are low in calories and have a low glycaemic index, making them a good choice for those on a weight-loss diet. The amino acid, threonine, in millets hinders fat formation in the liver. The soluble fibre in millets results in highly viscous intestinal contents that possess gelling properties which delay the absorption of carbohydrates by the intestines and thus reduce weight gain [38]. Their dietary fibre keeps you feeling full after eating, and proteins and other nutrients make them a better option to choose for the weight loss journey.

3.1 Effects of processing on protein

There are some changes in millet’s nutritional properties with respect to processing methods. Processed millet has, on average, increased protein content of approximately 13.75 g/100 g compared to raw seeds (10.60 g/100 g) [24]. This is due to the concentration of the protein content through the removal of hulls and other processes. Heat-moisture treatment of millet usually has a minimal effect on the protein content. The hydrophobicity of the protein is not altered much by heat-moisture treatments; therefore, there are not many changes to the protein in millet from such processing techniques [39].

During the extrusion process, protein solubility and structure are decreased and disrupted when applied under high pressure and temperature [40]. Germination has been reported to improve the protein content with better water solubility and oil absorption capacity.

3.2 Carbohydrates

In general, processing can destroy starch granules in millet to varying degrees and reduce them as well. With the decrease in particle size of ultra-fine powder, the solubility increases but the gelatinisation temperature decreases; the freeze-thaw stability, enzymolysis and sedimentation properties are also significantly improved [41]. In addition, fermentation with the bacterial strain, Saccharomyces cerevisiae reduces starch content in millet varieties with subsequent increase in carbon dioxide and ethanol production throughout the fermentation period. Moreover, pH is significantly reduced, which activates the phytase enzymes, which hydrolysis phytate, thereby reducing the phytate content [41]. This reduction is important as it enhances the bioaccessibility of major nutrients.

During processing, the starch molecules in millet are heated in an aqueous or moist environment, causing them to swell, rupture and burst. The starch gets gelatinised, permitting greater enzymatic digestion by amylases. Cooking increases the digestibility of carbohydrates [42]. Millets are low-glycaemic index (GI) foods and can help keep blood sugar from spiking after meals. They contain indigestible carbohydrates that help control blood sugar, fibre and non-starchy polysaccharides; hence, millets are a good whole grain, especially if you have type 2 diabetes [43]. Type 2 diabetes is when the body makes less insulin than is needed, but in type 1, the body does not make any insulin.

Hydrothermal processing in millet fractionates the starch into amylose and amylopectin fractions, causing a decrease in cold, hot water soluble and hemicellulose B fractions and an increase in pectic polysaccharides, hemicellulose A and cellulosic fractions [44]. Natural fermentation of millets can improve their lower cooking quality, taste, low bioavailability and palatability. Fermentation helps break down nutrients in food, making them easier to digest than their unfermented counterparts. As a result, fermented millets provide many health benefits such as antioxidant, antimicrobial, anti-fungal, anti-inflammatory, anti-diabetic and anti-atherosclerotic activity [45].

Fermentation and germination also affect the amylose content and the structure of amylopectin in millets. These treatments yield more pores in the granule surface and change the crystallinity of the flour. Changes in these structural properties will affect the gelatinisation temperature and application of the treated flour. Igbetar [46] states that soluble sugar is low in millet flour (2.61%), and it increases twofold and sixfold after cooking and fermentation for 72 h respectively. This suggests that fermentation and cooking increase the availability of soluble sugars. There is reduction of total levels of sugar during roasting which is attributed to caramelisation and other reactions [47].

3.3 Fats

Millets are highly nutritive, but their fat content is generally low, which has a positive impact on human health. Processing of the millets results in alterations in the fat content of the millets [48]. For example, there is a reduction in the fat content of millet milk (0.74–0.6%) when compared to that of unprocessed millet milk, which is around (9.1%). The total fat content remains unchanged due to parboiling, which is a heat treatment process. However, some portion of the endosperm fat migrates towards the periphery of the grain, and the oil globules in the aleurone layer are disrupted. During processing, the specified ratios of compositing lead to an increase in the concentration of fat [49]. Pearl millet has omega-3 fatty acids, alpha-linolenic acid, eicosapentaenoic and docosahexaenoic as their fat nutrient [50].

Steeping and fermentation of millets leads to an increase in fat fraction in the treated flour coupled with better water absorption capacity and hygroscopicity. Dey et al. [51] postulate that the effect of processing on some millet varieties is to remove toxins, reduce fat and anti-inflammatory factors due to the presence of millets containing adverse compounds (e.g., hexadecenoic acid, methyl ester, stigmasterol, C-sitosterol and pregnenolone) which exhibit a range of activities (e.g., antimicrobial, nematocidal and anticancer). Other processing techniques such as soaking, grinding or milling have less impact on the fat content present in the millet varieties. The impact varies from one technique to the other and also depends on the nature of the nutrient being tampered with. Finger millet is a good source of essential fatty acids and antioxidants [16]. The fat component in millet will determine the presence of fat-soluble vitamins, and any processing technique that impacts negatively on the fat fraction will inevitably impact on the vitamins also, particularly the fat-soluble ones.

The malting process in millets significantly lowers the fat contents of the malted samples. Raw millet would contain significantly higher fat content that has not undergone any processing. Malting generally improves the digestibility of these fats in millet, and as a result, malted grains have extensively been used in weaning and geriatric foods [16].

3.4 Minerals

Mineral-wise, millet is a rich source of iron, zinc, copper, manganese and potassium, and its mature kernels are rich in vitamin A but deficient in vitamins B and C [52, 53]. The grains consist of iron and calcium in high concentration when compared to other cereal grains [52]. Finger millet grains also contain high amounts of magnesium and phosphorus. Absorption and utilisation of these nutrients in the human body contribute to the reduction of chronic diseases such as lowering of high blood pressure, ischaemic strokes, cardiovascular diseases, cancers, obesity and type II diabetes [49, 54].

Recent studies have shown that some processing methods such as malting, fermentation, decortication, soaking and steaming can improve the bioavailability of these nutrients [37, 46, 47]. Fermentation and germination can improve the bioavailability of iron significantly by around three times and contribute to the requirement of women by around two times. The processing methods, which include grinding, shelling, high pressure, ultra-sound and microwaving, affect the eating quality characteristics and physicochemical properties of millets and cause changes in polyphenol content. Flour from whole grain finger millet was found to have a higher calcium content (325 mg/100 g) than those made from decorticated grain (222 mg/100 g) [48].

The presence of zinc in millet grain was found to be useful in assisting men’s health and the immune system. Raw grain was found to contain less zinc than processed grain. Soaking, boiling and germinating pearl millet can reduce phytate and phosphorus content, while also increasing the concentration of calcium, magnesium, iron and zinc [55, 56, 57]. These techniques, in particular, impact differently on the contents of different minerals; for instance, a processing technique can have the effect of increasing the bioavailability of one mineral while decreasing or inhibiting that of the other [26].

3.5 Vitamins

Millets contain fat-soluble vitamins, which play an integral role in many physiological processes such as vision, bone health, immune function and coagulation [58]. Millet is rich in B vitamins, especially niacin, pyridoxine and folic acid [59]. These B vitamins are especially important to pregnant and breastfeeding individuals as they aid in foetal brain development and reduce the risk of birth defects. Niacin is essential for the body to convert carbohydrates, fat and alcohol into energy, while pyridoxine is important for normal brain development and a healthy nervous and immune system [59]. Folic acid is essential in the formation of red blood cells and protein synthesis. Soaking helps to increase the nutritional contents of the grain and reduces the cooking time of these millets.

Grinding millet into flour retains its inherent nutritional benefits, providing a good source of vitamins, fibre and essential nutrients. The slow gentle grinding action minimises heat generation, which can help retain the nutritional integrity of the millet flour. As a result, stone ground millet flour may contain higher levels of essential nutrients, such as fibre, vitamins and other minerals, than flour produced using mechanical milling methods [60].

Heat treatments will destroy the heat-labile vitamins in millet, and these include roasting, boiling, parboiling, etc. Cooking and extrusion can reduce the vitamin content; as such, heat treatments should be done over a short time period and under controlled temperatures so that the best outcome is achieved [26].

4. Effects of processing on anti-nutritional factors

Millets contain readily available nutrients and energy sources; hence, scientists, agriculture industries and food security policies are giving more attention to millet processing for its better utilisation [8]. It gets considerable attention because of its nutritional quality related to its high content of dietary fibre, protein and starch patterns, as well as high mineral levels [61]. Millets also contain a considerable amount of phytochemicals that are important in reducing chronic diseases such as cardiovascular diseases, cancer and diabetes [61]. Bioavailability of nutrients is restricted due to the presence of anti-nutritional factors in millets, which include phytic acid, tannins, goitrogens, oxalic acid and trypsin inhibitors. These compounds interfere with mineral bioavailability, carbohydrates and protein digestibility through inhibition of proteolytic and amylolytic enzymes [51]. Anti-nutritional factors in millets interfere with the digestibility of proteins and starch along with reduction in bioavailability of minerals as a result of phenol–protein interactions and metal chelation.

4.1 Phytic acid

Phytic acid is present in the bran portion of cereals as a crystalline globoid. It is the organic form of phosphorous (myoinositol 1,2,3,4,5,6,-hexakis dihydrogen phosphate) occurring in plant constituents as the major portion of phosphorous. They are negatively charged and have a compound that attracts and binds positively charged substances like zinc, iron and calcium minerals, forming insoluble complexes that are unavailable for digestion and absorption [51]. Phytate is an anti-nutrient that impairs the bioavailability of some minerals such as copper, iron, zinc and calcium [62]. Phytic acid has a strong chelating ability and readily forms mono and multivalent cations of potassium, calcium, iron, zinc magnesium and other cations, reducing their bioavailability [63]. Chelates formed with the di- and trivalent metallic particles of minerals are insoluble compounds and are not absorbed in the gastrointestinal tract (GIT) [51]. Krishnan and Meera [26] reported that finger millet grains contain polyphenols and phytates which are known to influence the availability of minerals. Processing reduces the phytic acid concentration in millets, hence reducing the amount of positively charged compounds such as iron, zinc and calcium bound to it and making them bioavailable.

4.2 Phenolic compounds and tannins

Phenolic compounds are a class of secondary metabolites found in plants and are further divided into phenolic acids and polyphenols. Millets contain nine identified phenolic acids: gallic acid, vanillic acid, syringic acid and trans-cinnamic acid [64]. Phenolic compounds interfere with numerous enzymatic systems in humans, especially humans, especially those that control thyroid hormone synthesis [63]. Epidemiological evidence suggests that a diet based on millet, as a staple food, plays a role in the formation of goitre. In South Sudan, goitre prevalence in rural areas was high where 74% of diet is derived from millets causing iodine deficiency [63]. Tannins prevent protein from being digested and phenolic compounds reduce the digestibility of protein and carbohydrates as well as the bioavailability of vitamins. Hydrolysable tannins are susceptible to digestive hydrolysis, which can result in toxic substances [51]. Soaking dissolves phytates and hydrolysable tannins, thereby reducing their concentration and positively impacting feed value [24].

4.3 Effect of processing on anti-nutritional factors in millets

Millets have a high concentration of anti-nutritional factors, which reduce the bioavailability of nutrients. Processing of millet is important to reduce amounts of phytic acid and polyphenols and get the full nutritional benefits of the grain. Processing of the millets has shown promising results in their successful utilisation [65]. Dehulling reduces polyphenols and phytic acid in pearl millet. In vitro protein digestibility is increased due to the removal of anti-nutrients, which precipitates proteins when millets undergo decortication. Abrasive decorticated pearl millet showed decreased anti-nutritional compounds (fibre and iron binding phenolics compound); however, high phytate content after decortication might be associated with their occurrence in germ and endosperm [65]. Sharma et al. [66] reported that after soaking millet, protease and amylase activity increased significantly with the increased duration of sprouting. Protein solubility increased with soaking and sprouting, modifying the proximate composition of millet grain by enhancing the hydrolysis of complex insoluble organic compounds present in seeds. Sheethal et al. [67] state that the molar ratio of phytic acid to zinc and phytic acid to iron decreased after the application of different processing methods, especially those associated with fermentation.

5. Effects of processing on bioavailability and bioaccessibility of nutrients

Millets have been reported to be highly nutritious, having high amounts of polyunsaturated fatty acids and high dietary fibre, unavailable carbohydrates and a high satiety effect. They are a very good source of calcium, iron, phosphorus, zinc and potassium [68]. Millets have antimicrobial, anti-diabetic and anti-mutagenic properties due to the presence of polyphenols [69]. This makes them an ideal solution to health problems such as obesity, diabetes and hypertension. Millets, however, have tannins, phytates, polyphenols and trypsin inhibitors. These anti-nutrients influence the bioaccessibility and bioavailability of both micro and macronutrients. Shobana et al. [70] reported poor iron availability in some varieties of finger millet due to thigh tannin content.

The absorption of nutrients in the gastrointestinal tract is highly dependent on the release of nutrients from food. The released nutrients are then accessible for absorption in the small intestine. Bioaccessibility refers to the amount of nutrients available for intestinal absorption. The quantity of ingested nutrients, which is then absorbed and utilised is defined as bioavailability [71, 72], and it indicates nutritional effectiveness. Bioavailability is affected by the form in which the nutrient is incorporated, its chemical bonding and its interference with other nutrients, which can either enhance or inhibit absorption or post-absorption metabolism. The nutritional and health benefits of millets are, therefore, subject to bioaccessibility and bioavailability of the nutrients in millets.

The bioaccessibility and bioavailability of nutrients in millets is reliant on the mechanical breakdown of foods and the enzymatic hydrolysis of nutrients. Bioavailability is also influenced by the localisation of nutrients within the food matrix, physical and chemical breakdown of the food and the availability of the lipid phase. Millet processing affects the bioaccessibility and bioavailability of nutrients and the processing methods can result in the transformation of the food matrix and nutrient forms. Decortications, milling, germination, fermentation, malting and roasting have all been reported to improve the organoleptic and nutritional properties of millets. Table 1 summarises the effects of some processing methods on the bioaccessibility and bioavailability of millets.

Processing method	Millet type	Effect	Source
Decortication/Dehulling	Pearl millet and finger millet	Decreased in dietary fibre, minerals and antioxidant activity; Removal of phytate localised in the outer layer increases mineral bioavailability	[73]
Milling	Pearl millet and finger millet	Reduction in anti-nutrients and an enhancement in the bioaccessibility of minerals; Increase in protein and starch digestibility	[72]
Hydrothermal/germination	Pearl millet and finger millet; Sorghum	Enhancement of mineral availability by breaking down anti-nutrients; Enhances nutrient content and energy density while reducing phytic acid in food products. Increases in vitro bioaccessibility of minerals like calcium, zinc and iron	[71, 74]
Fermentation with lactic acid bacteria	Pearl millet, Finger millet, foxtail millet bran	improves protein digestibility and nutritional value and limits the action of the anti-nutrients, tannins, phenols, phytate and trypsin inhibitors	[71, 72, 75]
Soaking and germination	Finger millet, pearl millet and sorghum	Soaking grains before dehulling and milling reduces phytates tannin and phenolic content. Phytate content is reduced as soaking activates phytase. This in turn increases the availability of Ca, P, Zn, Fe, Cu, K and Mg	[64, 65, 66, 76]
Blanching	Pearl millet	Reducing inhibitory the factors phytic acid and polyphenol increases the iron and zinc bioaccessibility	[26, 77]
Malting (germination, milling and sieving)	Pearl millet	Improves the nutritional quality by decreasing tannin and phytic acid levels, making Na, Ca, P, Zn, Fe and Cu more bioaccessible	[78]
Roasting	Pearl millet	Roasting reduces polyphenols in finger millet; Increased Na, Ca, P, Zn, Fe and Cu availability when roasted at 70°C, above which Fe and Mn availability decreases	[24, 79]

Table 1.

Effect of processing methods on the nutrient content and the bioaccessibility and bioavailability of nutrients.

6. Value-added millets and their diverse use for food and feed products

Millets are being considered as a valuable and promising solution for the achievement of food security, in the face of population increases and adverse climatic conditions, especially in the developing world. Increased adoption of millets as a food source for all and not just for the poor rural communities will only be possible through the value addition of millet products, in the form of varying dishes such as pasta, baked, flaked and popped products as well as instant food recipes [80, 81]. Millets can totally replace wheat, maize and rice or be blended together with these cereals in the production of varying cakes, pasta, macaroni, vermicelli, noodles, spaghetti and flakes [2, 5, 19]. The processing of millets can result in a number of various products (Table 2) for human consumption. Millets are commonly used to make stiff or thin porridge or can be cooked like rice including soup or weaning food [82].

Value-added product	Processing technique
Porridge	Cooking [82]
Puffs	These are made from different puffing machines, giving expanded and crispy products [82]
Popped and flaked products	Flakes are flat round shaped products which are ready to eat and are made using an extruder and a roller flaker machine [81, 82]
Millet cookies and cakes	Baking has been through oven baking of different mixed flour of some of the following pearl millet, finger millet flour combined with usual baking ingredients such as sugar, milk, eggs, vanilla, fat and salt [82, 83]
Millet pasta	Sorghum, finger millet and pearl millet are mixed with a small portion of flour and extruded in a pasta-making machine [81, 82]

Table 2.

Various products from millets for human consumption.

Millets have also been used as livestock feed, including ruminants, pigs and poultry. They have been adopted in the production of animal feed as a source of energy and protein in place of corn. Ground or whole pearl millet can be mixed with maize up to 50% inclusion level. Hassan et al. [16] found that the inclusion of 40% pearl millet in livestock feed increased milk production with no adverse effect on milk quality in dairy cows. The inclusion of pearl millets in animal feed resulted in improved egg quality with higher omega-3 fatty acids and lower omega-6 [84].

A study looked into the effects on broiler chicken performance of using millet hulls in place of wheat offal (0–100%) [85]. The daily feed, weight gain, feed conversion ratio, feed efficiency ratio, gut contents and prime cuts were all found to be similar. Feed costs can be decreased by using millet hulls up to 100% of the time in broiler chicken diets. The main phenolic components in millet husk are 1-O-p-coumaroylglycerol, apigenin-C-pentosyl-C-hexoside and 1-O-feruloyl-3-O-p-coumaroylglycerol being [86]. Therefore, millet husks, in addition to their acetylcholinesterase and α-glucosidase inhibitory activities, are a valuable source of naturally derived antioxidants [85, 86].

7. Conclusion

Millets are considered to have high nutritional components and health benefits as they are sources of quality protein, dietary fibre, energy and essential minerals. The health benefits from millets are a result of the presence of antioxidant properties exhibited due to the presence of polyphenols such as tannins and some phenolic compounds that can protect the body from oxidative stress. These also act as anti-inflammatory and antiviral agents, thus boosting the immune system. Despite millets being known globally as a source of quality grains, they continue to lack established and improved processing techniques, hence hampering the extent of millet processing and utilisation. However, the potential for millet as a source of valuable health food and feed remains high. Prior to the inclusion of millets in food and feed formulations, it is necessary to address the issues of their digestibility, micronutrient availability and anti-nutritional factors. Millets’ food and feed inclusion is mainly dependent on innovation, value-added products, attractive branding and impressive marketing strategies supported by public awareness and an increased sense of ownership.

Conflict of interest

The authors declare no conflict of interest.