Fermentation of Cereals and Legumes: Impact on Nutritional Constituents and Nutrient Bioavailability

Fermented food products, especially those derived from cereals and legumes are important contributors to diet diversity globally. These food items are vital to food security and significantly contribute to nutrition. Fermentation is a process that desirably modifies food constituents by increasing the palatability, organoleptic properties, bioavailability and alters nutritional constituents. This review focuses on deciphering possible mechanisms involved in the modification of nutritional constituents as well as nutrient bioavailability during the fermentation of cereals and legumes, especially those commonly consumed in developing countries. Although modifications in these constituents are dependent on inherent and available nutrients in the starting raw material, it was generally observed that fermentation increased these nutritive qualities (protein, amino acids, vitamins, fats, fatty acids, etc.) in cereals and legumes, while in a few instances, a reduction in these constituents was noted. A general reduction trend in antinutritional factors was also observed with a corresponding increase in the nutrient bioavailability and bioaccessibility. Notable mechanisms of modification include transamination or the synthesis of new compounds during the fermentation process, use of nutrients as energy sources, as well as the metabolic activity of microorganisms leading to a degradation or increase in the level of some constituents. A number of fermented products are yet to be studied and fully understood. Further research into these food products using both conventional and modern techniques are still required to provide insights into these important food groups, as well as for an overall improved food quality, enhanced nutrition and health, as well as other associated socioeconomic benefits.


Introduction
Fermented food products are notable all around the world and are sometimes categorized as "functional foods" due to their purported health benefits. These food products have been in existence since the arrival of the human civilization and are likely to be with us far into the future. Fermentation is, thus, an age-long food processing technique used to transform food products [1,2], with different food crops (cereals, legumes, as well as fruits and vegetables) used as starting raw materials. Cereals and legumes are notable and major staple crops around the globe and are frequently fermented to obtain a number of food products [3][4][5]. The fermentation of cereals and legumes, as with other food crops, can be classified into three categories, viz., natural (also referred to as spontaneous), back slopping and controlled fermentation. Natural or spontaneous fermentation occurs through the sequential and competitive action of a plethora of microorganisms, with the best-adapted Alcoholic beverage Bacteria and yeast Zimbabwe Blandino et al. [3] Enturire Sorghum Alcoholic beverage Lactiplantibacillus plantarum, Saccharomyces cerevisae, Weissela confusa Uganda Mukisa et al. [40] Gowe  [63]; N'guessan et al. [64] [3]; Nout [71] Umqombothi Sorghum/maize Beverage Lactobacillus spp. and Saccharomyces cerevisiae Southern Africa Katongole [72]; Van Der Walt [73] [78] * Name of all Lactobacillus species have been modified according to novel classification of Zheng et al. [126].     Legumes are excellent sources of good-quality proteins and are rich in essential AAs. Fermentation increases the amount of free AA contents in legume-based products, depending on the legume species and cultivars [141], and such an increase could be of advantage in supplementing the nutrients obtained from other food crops and assisting people suffering from protein deficiency attributed to the maintenance and growth of the body. The fermentation of Bambara groundnuts into unhulled dawadawa (a fermented condiment) increased the protein content by approximately 18%, and this was attributed to the release of proteins initially bound to the antinutritional factors [142]. The mechanism of the protein increase in this study was also ascribed to an increase in the microbial mass resulting in an extensive hydrolysis of the protein molecules to AAs and other simple peptides. Additionally, in the same study, fermentation was observed to significantly increase all the essential AAs except for lysine and histidine. The trend observed for histidine and lysine was attributed to their distinct basic side chains (which contain nitrogen and resemble ammonia), possibly causing them to have reacted differently during fermentation [142]. Peas (Pisum sativum) fermented with Aspergillus niger NRRL 334 and Aspergillus oryzae NRRL 5590 for 6 h at 40 • C to obtain fermented pea protein-enriched flour through SSF led to an increase in protein (0.5-15%) and AA (1.8-29%) levels [11,143]. It was postulated that the increase in the level of protein was due to the fungi utilizing lipids and starch as well as the ability of these fungi species to produce proteins [11,143]. An increase in the protein (3-25%) through the SSF of legume flours has also been previously reported [144][145][146][147][148][149], with these studies ascribing such increases to the synthesis of new proteins during fermentation, yeast proliferation, the loss of dry matter, net synthesis of protein by fermenting seeds, increase in fungal biomass that was produced from the fermenting microorganism and partial protein denaturation and pH decrease during fermentation. The mechanism of an increase in the protein content of lupin flours fermented with Aspergillus ficuum, Aspergillus sojae and their co-cultures could be linked to the microorganisms using the substrate as carbon and energy sources during SSF to produce fungal protein [150]. The formation of soluble products and monomers after fermentation, as well as the interconversion of AAs, was reported to have also enhanced AA levels by up to 13%, though an AA decrease of between 0.3% and 16% was equally reported during the fermentation of African yam bean flour [149]. The increase in AAs might also be attributed to transamination or synthesis taking place during the SSF process [11,143]. Some anabolic processes leading to the build-up of polymer or microbial cell proliferation were also reported to have increased the protein content (5-94%) of soymilk from soybeans [151].
Some studies have reported both an increase and a decrease in protein and AA levels during the fermentation of legumes. Difo et al. [146] recorded both an increase (12%) as well as a decrease of 10% in protein in fermented Vigna racemose flour. Such a decrease was suggested to have been due to the metabolism of Aspergillus niger with respect to other compounds present in V. racemosa, and such a metabolism might have produced some compounds capable of interfering with the protein content. The decrease in AAs in a study by Kumitch et al. [143] over the fermentation time (6 h) could have been due to the fungi utilizing these AAs and reducing the essential AAs further. Another study was conducted on the common bean (Phaseolus vulgaris) fermented with Limosilactobacillus fermentum for 72 h at 37 • C to obtain fermented bean powder through SmF, leading to an increase in protein (1%) as well as an increase (1-20%) and decrease (3-7%) in AAs [152]. While the increase in AAs was linked to the synthesis of substances by bacteria present in the substrate, the decrease suggested their utilization by the bacteria [152]. The modification of nutritional constituents usually occurs simultaneously with one another. For example, the slight decrease in the crude protein of Aspergillus ficuum fermented lupin was suggested to be interrelated to the observed increase in soluble carbohydrate and starch [150]. Noting that food constituents exist together in a food matrix, it could be postulated that a greater dissolution of carbohydrate and starch led the "exposed" proteins to the fermenting organisms, leading to this reported decrease. Asensio-Grau et al. [153] attributed the modification of protein levels to the bioconversion of some carbohydrates into protein. The differences in the trend of modification (increase/decrease) of protein and AA compositions in fermented cereals and legumes could be associated with factors such as the fermentation conditions used (which differs), growth rate and metabolic capabilities of the microbiota, initial protein content and AA composition of the grains as well as the solubility and molecular structure of the inherent protein and AAs.

Carbohydrate, Energy and Starch Fractions
Fermentation is an exothermic metabolic process which involves the consumption of food nutrients through the activities of microorganisms (either native or deliberately introduced) that serve as fermenters. These organisms rely on the different nutrients of foods and favourable environmental conditions for their growth and metabolic activities, leading to their survival, proliferation and synthesis of by-products. Fermentation enriches cereal-based food in protein by removing part of the carbohydrates and helping in energy reduction during cooking [153,200,201]. The effect of fermentation on the carbohydrate, energy and starch contents of some cereal-based foods are presented in Table 3. Nnam and Obiakor [137] reported a progressive increase (1.1-2.4%) and decrease (0.3%) in carbohydrate contents of spontaneously fermented rice for 72 h (24 h interval) at 28 • C and ascribed this to changes in the population of the fermenting organism, which could be as a result of continuously changing the fermentation environment, enabled through changes in acidity and chemical balances. A significant increase in the carbohydrate content was also reported in fermented pearl millet flour (3%) [128], fermented oat flour (1%) [130], fermented sorghum flour (0.9%) [139] and ogi (5-6%) [140]. Decreases in carbohydrate levels have also been reported in fura (0.7%) [129], fermented sorghum flour (0.3-1%) [132], fermented rice flour (0.5-7%) [134], fermented sorghum flour [136], fermented maize flour (4%) [156] and fermented pearl millet flour (3%) [158], with the studies attributing these to the metabolic activity of microorganisms degrading carbohydrates into simple sugars for their growth, as well as hydrolyses of starch by α-amylase. Increases in energy levels have been reported in fermented pearl millet flour (2%) [128] as well as decreases in fermented maize dough (1.6%) [138] and fermented sorghum flour (1.6%) [139] with no mechanisms reported. An increase in the total starch of a fermented cereal starter (from barley and pea) through SSF was ascribed to the decline in amylase activity and the release of trapped starch granules from the fibrous cell wall structure (by crude multienzyme composed of non-starch polysaccharide-hydrolysing enzymes) [18]. Decreases in resistant (20.6-72.9%) and total (12.2-16.8%) starches in fermented sorghum flour were also associated with the natural fermentation of sorghum that led to increased enzymatic reactions [166]. Table 3. Influence of fermentation on the nutritional composition of some cereal-based products.  Protein increase attributed to increase in AA synthesis. Decrease in fibre due to enzymatic action. Decrease in tannin was due to action of a tannase.

Raw
Espinosa-Páez et al. [130]        Fibre decrease attributed to partial solubilisation of cellulose and hemicellulose type of materials by microbial enzymes. Fat decrease due to grain variety, fermentation conditions and other processing steps. Vitamin decrease ascribed to mechanical loss during other process and lipid solubilisation.
Onwurafor et al. [192]  Increase in protein attributed to activities of extracellular enzymes.
Lai et al. [195]  Decrease in fibre linked to secretion of cellulose/hemicellulosedegrading enzymes by yeasts.
Rashad et al. [196]  Decrease in TIA and phytate due to enzymatic activities. Protein increased attributed to enzyme synthesis and compositional change following degradation of other constituents. Fat increase due to increased activity of lipolytic enzymes that led to production of more fatty-related compounds. CHO reduction linked to their use as carbon source (substrate) in order to synthesize cell biomass.
Oluseyi and Temitayo [198]  Reduction in ANFs attributed to degradation by microorganisms. Decrease in mineral contents ascribed to leaching of the minerals into fermentation water and mineral utilization by fermenting microbiota. Raffinose and stachyose reduction could be due to their utilization as energy sources.
In legumes, fermentation has been observed to lead to both a decrease and increase in carbohydrate or starch contents (Table 3). A previous study on the determination of available starch contents of two fermented Vigna sinensis seed varieties revealed a reduction in the starch content from 24.3% to 22.33% in the orutico variety and from 29.7% to 22.9% in the tuy variety, [180] with the authors attributing this to the degradation of available starch by microbial and enzymatic activities. This trend was also reported by Doblado et al. [179] evident with the reduction in total starch, though with a corresponding increase in sugar contents of fermented Vigna sinensis (var carrila) samples. In contrast, an 8% increase in the starch content and a corresponding 0.5% decrease in the carbohydrate content was reported in fermented bean powder (using L. fermentum) [152]. Olagunju et al. [197] also reported a reduction in carbohydrate contents of tamarind seeds fermented for 3 days, with values of 1.04-42%. The study related this decrease in carbohydrate content to the decrease in the carbohydrate ratio in the total mass, resulting in the redistribution of nutrient percentages [197]. A 3% decrease in the carbohydrate content reported during the fermentation of red bean (Phaseolus angularis) was attributed to the use of carbohydrate as the energy source for fungal growth [148]. Different authors [130,144,146,149,153,178,190,191] have all equally reported reductions in carbohydrate levels during the fermentation of African oil bean (7%), tempeh (0.7%), cowpea (3%), mahogany bean (up to 61%), kidney bean (17%), lentil (6%), African yam bean (4%) and Lyon bean (up to 26%), and ascribing such reductions to the use of carbohydrate-related compounds as the energy source by fermenting microorganisms for growth and metabolism as well as the conversion of oligosaccharides to simple sugars. The observed varying decreases in the carbohydrate values of these legumes could be due to differences in the inherent composition (e.g., amylose, amylopectin and the structural composition of carbohydrates), plant varieties, species as well as fermenting microorganisms present during the fermentation process. Furthermore, Obadina et al. [151] reported a progressive reduction in carbohydrate contents (10-99%) of fermented soymilk at 72 h as the fermentation time increased, attributing this to the activities of the fermenting microorganisms which transformed and utilized them into energy for growth and other cellular activities. According to Olagunju et al. [197], protein fermentation is mostly facilitated by Bacillus spp., and these organisms are notable producers of enzymes such as amylase, glucosidase, fructofuranosidase and lactanase, which could break down different components of carbohydrates in fermenting legumes, leading to their reduction.
Increases in carbohydrate levels of fermented cowpea (up to 5%) [181], fermented Bambara groundnut (0.3%) [175], fermented black bean (146%) [130], fermented lima bean (3%) [186] and fermented pigeon pea (up to 8%) [193] were reported with such trends linked to activities of enzymes during fermentation that must have led to the conversion of resistant starches to available starches; subsequently, increasing the carbohydrate contents. Different studies have reported increases and decreases in the energy content during the fermentation of legumes (Table 4). An increase in the energy content of fermented pigeon pea flour (50.6-57.4%) [193] and fermented lentil flour (15%) [154] has been previously reported. Decreases in energy contents of fermented African oil bean flour (26%) [144] and fermented pigeon pea flour (0.5-3%) [145] have also been observed (Table 4). Obadina et al. [151] recorded both an increase (0.1-7.4%) as well as a decrease of 7.5-15.5% in energy value in fermented soymilk. While most of these aforementioned studies did not describe the mechanisms of such modifications in energy values, Adebowale and Maliki [145] linked the decrease in the energy value of fermented pigeon pea flour to the decrease in both the nitrogen-free extract and fat values of the samples.

Fats and Fatty Acids
Most studies on fermented cereal, such as pearl millet and maize-based products [128,156,157], reported a reduction (6-34%) in the fat content. The decrease in the fat content has been associated with the metabolism of lipids by the fermenting organisms and the leaching of soluble inorganic salts. In the study conducted by Ejigui et al. [138], a decrease of 11% in the fat content of fermented maize at 30 • C for a period of 4 days was attributed to a variety of grains, fermentation conditions and steps, such as washing and sieving, which was involved in the production of dough. In addition, Nnam and Obiakor [137] reported about an 81% reduction in rice fermented for 72 h, whereas in another study on fermented stale rice, Zhang et al. [171] reported an increase (252%) in the fat content. Nnam and Obiakor [137] attributed the reduction in the fat content of fermented rice to an increase in the lipase activity in the fermenting medium. A decrease in the lipid of 6.1-49% was reported in rice bran fermented for 120 h at 30 • C, and this was presumably due to the use of fat-related components for mycelial synthesis [162].
Additionally, in Table 3, most of the studies on the fermentation of legumes, such as African oil bean, African yam beans, black beans, cowpea, kidney beans and lima beans, revealed that fermentation reduced the fat content between 0.63% and 58.7% [130,144,146,149,178,184,186]. Some of the authors attributed these reductions to the metabolism of microorganisms in the fermentation medium, the breakdown of lipids by lipase, the use of lipids as the food source by fermenting organisms, the loss of total solids during soaking and the denaturation of the fat by heat processing as well as the leaching of fat-related components into the processing water. Onwurafor et al. [192] also reported that fermenting mung bean flour using spontaneous and back-slopping methods for 72 h reduced the fat content by 33-38%, and this was due to the activities of lipolytic enzymes during fermentation. A similar mechanism for the decrease in fat contents was reported by Adebowale and Maliki [145] in fermented pigeon peas and fermented soybeans [151], and was also attributed to increased activities of the lipolytic enzymes during fermentation, which hydrolysed fat components into fatty acid and glycerol. In contrast, increases in fat levels of fermented chickpea (1.8%) [147], fermented lupin (3-11%) [150], fermented African yam bean (86%) [174], fermented Bambara groundnut (2%) [175], fermented cowpea (100-133%) [181], fermented mahogany bean (3-39%), [191] and fermented tamarind (17-48.9%) [198] have been reported ( Table 4). The mechanisms involved in the increase in the fat content might be linked to the increased activity of lipolytic enzymes that may have produced more fatty acids during the fermentation, the extensive breakdown of large molecules of fat into simple fatty acids, the fat from dead microflora and/or the assumption that fermenting microflora did not use the fat as a source of energy [174,191,198]. In their study, Barampama and Simard [152] reported that fermentation reduced fatty acids (linoleic and linolenic fatty acids) in common bean by about 20%. A decrease of 2-18% and an increase of 2-24% were also observed in fatty acids of ugba (fermented African oil bean), and the concentrations of some fatty acids did not change during fermentation. An observed increase as well as a decrease in these fat-related constituents after fermentation suggest selective lipase activities. While these lipolytic enzymes could have contributed to the lipid dissociation and increased the extractability of fat-related constituents, same enzymes could also have selective reductive activities, perhaps using these fat-related components as carbon sources [70,202,203]. Equally important and not highlighted in these studies are the role of other microorganisms involved in fermentation that could have promoted lipid hydrolysis [204,205].

Ash and Mineral Composition
Varying effects of fermentation on the ash and mineral contents of cereals and legumebased food products have been reported, and these effects are independent of the forms of these foods. For fermented pearl millet, Adebiyi et al. [128] reported a decrease in total ash contents from 1.86% to 1.36% after fermentation for 3 days; however, they reported an increase in mineral elements such as Ca, Na, Cu, Fe, Zn and K. A reduction in ash was attributed to the leaching of soluble salts, while an increase in mineral elements was due to the improved extractability and availability of minerals as a result of fermentation. The study of Nnam and Obiakor [137] reported a reduction in the ash content of rice from 1.5% at 0 h of fermentation to 1% after 72 h, with irregular trends in the values of minerals such as Ca, P, K, Fe, Zn and Cu during time intervals. They attributed the loss in the ash content to a reduction in the dry matter, which was as a result of the breakdown of total solids during fermentation. A decrease (14-97.9%) and increase (3.8-100%) in minerals were also reported during the fermentation of rice [137]. Both opposing trends were linked to the metabolic activities of the fermenting microorganisms which hydrolyse the metal-phytate complexes to release free minerals for use and losses in dry matter, which led to apparent increases in minerals [137]. An increase of 0.5-14% in ash with a decrease of 0.5-31% were also reported during the fermentation of rice, and were attributed to the activation of phytase which reduces phytic acid [134]. The increase in mineral (13-34%) and ash contents (7%) of fermented rice was reported by Ilowefah et al. [133], and the increase in the ash content was due to the increase in the mineral solubility and bioavailability. In a similar study by the authors, an increase in mineral (13-34%) and ash contents (9%) of fermented rice at 6 h for 32 • C was reported as well as an increase in minerals linked to the reduction in phytic acid contents, which may have formed complexes with the minerals [161].
For legumes, an increase in ash contents (Table 4) was reported for fermented soymilk [151], mung beans flour [192] and tamarind seeds [197]. These products were subjected to different fermentation times and recorded a general progressive increase in ash contents as the fermentation time increased, except for the slight reduction in ash content from 0 to 24 h in fermented tamarind seeds [197]. With an increase in ash contents as the fermentation time increased, a corresponding increase in minerals, such as Ca, Mg, P, Zn, Cu, Mn and Fe, was also reported [197]. This was also similar to the findings of Obadina et al. [151] and Onwurafor et al. [192], who reported increases in Ca, Fe, Mg and Zn contents as the fermentation progressed. An increase in ash and mineral contents in these studies was ascribed to metabolic activities of microorganisms as well as the breakdown of complex chelated compounds within the fermenting lot, leading to an improved synthesis of minerals. On the other hand, Granito et al. [180,184] reported a significant decrease in ash and mineral contents during the natural and submerged fermentation of Phaseolus vulgaris and two varieties of Vigna sinensis, respectively. They attributed this decrease to the leaching of mineral elements into discarded fermentation water and the utilization of mineral elements for the proper growth of microorganisms during fermentation. The decrease (29.8%) in ash content in African yam bean fermented at 24 h was attributed to vegetative loss, leaching into the fermentation medium as well as the microflora which might have used the ash-related components for metabolism [174]. A decrease (12.1-66.7%) in some minerals present in fermented Bambara groundnut was attributed to their utilization by fermenting microorganisms for their physiological and metabolic activities, while an increase (2.3-43.8%) was linked to the reduction in phytic acids and other antinutritional factors [142].

Vitamins
Fermentation has been reported to exhibit varying effects on different vitamins such as B vitamins and vitamin E in cereals and legumes (Tables 3 and 4). In most of the studies, especially in the fermentation of maize, buckwheat, rice and sorghum using different starter cultures (LAB species, yeast and fungi), an increase in vitamin B1, B2, B3 and E were reported by up to 10 folds [133,157,167,171]. Ilowefah et al. [134] reported that vitamin B increased in fermented maize flour due to enzyme interactions with starch, protein and other key biosynthetic precursors, which stimulated their synthesis of bound forms of the vitamins. Contrary to these studies, Ejigui et al. [138] and Tamene et al. [172], on the fermentation of maize and tef, reported a reduction in vitamin B1, B2, β-carotene (as retinol equivalent) and the folate content of the resulting flour and their products. A decrease in vitamins was caused as a result of mechanical loss due to processes, fermentation and lipid solubilization, as well as consumption by other microorganisms or losses due to discarding the supernatant [138,172]. In some other studies, fermentation reportedly increased the vitamin B1, B2 and E (α-tocopherols) levels of fermented legumes (cowpea and kidney beans) by 17 to 94% [179,184]. Likewise, levels of vitamin A, B1, B2, B3, α-, γand δtocopherols reportedly reduced in fermented common bean, cowpea, lupin and mung bean by 5-106% [152,179,188,192]. The level of vitamins after fermentation seemed to be dependent on the fermenting strain and metabolic activity of these strains. This could have impacted the varying reported trends in the vitamin content.

Fibre
Studies on rice showed that fermentation increased the fibre content in their resulting flours [171] and, likewise, the insoluble and soluble fibre fractions at 22 • C for 72 h [133]. The increase in fibre in stale rice by Cordyceps sinensis was attributed to the transformation mechanisms of corresponding substances in the fermentation process, and some mycelia of Cordyceps sinensis possibly attached onto the surface of rice grain [171]. Jood et al. [167] reported about a 10% reduction in the total and insoluble dietary fibre, and the authors suggested that an increase in the activity of hydrolysing enzymes such as cellulase, α-galactosidase, etc., caused the rapid hydrolysis of the insoluble dietary fibre constituents, leading to their conversion into soluble dietary fibres. The mechanism of the decrease in fibre in fermented cereal was attributed to the partial solubilization of cellulose and hemicellulose type of materials by microbial enzymes [30]. Other authors ascribed the reduction in fibre of fermented maize flour (74%) to an enzymatic breakdown by LAB, which utilized the fibre as a carbon source [156]. In addition, the authors explained that due to the enhanced activity of β-glucanases and carboxypeptidases, insoluble β-glucan could be degraded into soluble β-glucan and, further, due to the fermentation activity of other enzymes such as β-glucosidases, cellobiose, etc., it could hydrolyse the soluble β-glucan into glucose. A 55% decrease in fibre was attributed to the enzymatic degradation of the fibre during the fermentation process of fermented pearl millet [158], while a decrease of 40% in fibre levels in fermented sorghum was attributed to the partial solubilisation of cellulose and hemicellulosic type of material by microbial enzymes [132]. Onyimba et al. [135] reported a decrease of 66-69% in fermented sorghum and ascribed this to the action of cellulolytic microorganisms present in the fermenting substrate [135]. Likewise, a 22% decrease in fermented oats was ascribed to the action of enzymes from Pleurotus ostreatus such as hemicellulase, xylanases, cellulase and laccase [130].
The fermentation of various legume seeds and their effect on fibre levels have also been reported (Table 3). In legume seeds, such as African yam beans and Lima beans, fermentation reduced the crude fibre content, with other studies equally reporting that fermentation reduced the insoluble and soluble fibres of pigeon pea and kidney beans [149,174,184,186]. The decrease in crude fibre was attributed to hydrolysis and leaching into the fermentation medium, or the microflora used the fibre-related components for its metabolism [174], while a decrease in insoluble fibre was ascribed to the use of cellulose and arabinoxilnase by the fermenting microorganisms [184]. The decrease in the fibre content (59%) in black bean fermented at 336 h (14 days) with the Pleurotus ostreatus CS155 strain was attributed to the action of enzymes from Pleurotus ostreatus, such as hemicellulase, xylanases, cellulase and laccase [130]. Additionally, a study on curd waste from soybeans fermented with two types of yeasts showed that a decrease in fibre (7.4-46%) was an indication of the secretion of cellulose/hemicellulose-degrading enzymes by the yeasts during fermentation, and the individual preparation of yeast may have different enzyme activities as well as being able to interact differently with soluble and insoluble fibre components [196]. In common beans and lupin seeds, Barampama and Simard [152] and Olukomaiya et al. [150] reported that due to microbial actions, the acid detergent fibre increased about 86%, and others, such as hemicellulose and lignin and cellulose, were approximately 2-14% of the fibre fractions. The increase in cellulose was ascribed to the build-up of acid, alkaline or neutral detergent-insoluble substances causing the fibre values to be overestimated [150].

Antinutritional Factors
Food fermentation has been shown to effectively increase the nutritional composition of foods as well as decrease the levels of antinutritional factors (ANFs) and toxic constituents, and might be a better alternative in minimizing the adverse effects of these compounds in diets [197,206]. The fermentation of sorghum flour reduced hydrogen cyanide by 52.3% [168], while Nivetha et al. [154] reported a 66% reduction in the cyanogenic glycosides content of a linseed (Linum usitatissimum) fermented beverage using Lactobacillus acidophilus [154]. The reduction in cyanogenic glycosides was due to the breakdown and degradation of the ANFs into smaller units by the action of the enzymes mobilized during the fermentation period [154]. The inherent phytase activity of sorghum activated by LAB during fermentation degraded phytates, while the decrease in tannin content was due to microbial activity and phytate acyl hydrolases [168]. Likewise, decreases between 30% and 98.7% in tannin levels were reported in ting (a fermented product from sorghum), and were attributed to the rearrangement and depolymerization of the tannin structure [163][164][165]. This can be linked to the acidic environment of the fermentation medium, reduced extractability, selfpolymerization, interaction of tannin with other macromolecules (such as starch and AAs) and the ability of LABs to possibly metabolize tannins [163][164][165]. Indications from these studies suggest that fermentation leads to the production of enzymes, such as tannase [130], that reduce and/or eliminate tannins during this process. In fermented rice, the decrease in tannin (50%) was attributed to milling, which removed most of the tannin-related fractions, while phytate (19-69%) was reduced due to the increased activities of phytases during fermentation [137], and the reduction in ANFs in sorghum fermented for 72 h at room temperature was due to the ability of microorganisms to use them up [169].
For legumes, the decrease in ANFs of fermented African oil bean (24-79%) was attributed to soaking (which caused some of the ANFs to leach out), as well as microflora enzymes which degraded organic complexes to release antinutrients and the subsequent leach out of these components into the surrounding medium [173]. Adebiyi et al. [142] observed significant reductions in ANFs in unhulled dawadawa samples from Bambara groundnut-phytic acid (18.06%), oxalate (59.12%) and tannin (34.16%)-, with the reduction in phytic acid attributed to the enzymatic activity of fermenting microorganisms that degrade phytic acid or the complex(es) formed by them. In fermented Bambara groundnut flour, a decrease of 16-42% in ANFs was also observed, and this was due to the effect of the biodegradation of chemicals involved during fermentation [175]. Similarly, the traditional fermentation of tamarind seed for the production of iru (daddawa) resulted in a significant reduction in ANFs, tannin contents (75%), phytic acid contents (50%) and trypsin inhibitor activity (86%), while Bacillus pumilus, B. subtilis and B. licheniformis were implicated as the organisms responsible for fermenting the legume [89]. A 29% decrease in phytic acid in fermented soymilk was ascribed to the action of phytase and β-glucosidase produced by fermenting microbes [197]. Olaleye et al. [190] reported an increased nutritional content as well as a significant reduction in oxalate (16.5-68%), phytate (13.7-26%) and tannin (9.2-25.7%), following the fermentation of beans for 72 h at 45 • C with no reported mechanism. As described in various studies, the fermentation of cereals and legumes reduces tannins via hydrolysis by tannase, which catalyses the hydrolysis of ester and depside bonds, yielding gallic acid and glucose [168,207,208]. This enzymic degradation of tannins is facilitated by a lower pH, such as that achieved during the fermentation of legumes and cereals. Some researchers have suggested that the reduction in tannins during fermentation may also be attributed to its water solubility; hence, leaching out into the fermenting media, just as all other polyphenolic compounds [207][208][209]. Elsewhere, the fermentation of tamarind seed for 72 h resulted in up to an 85.7% reduction in tannin, 89.4% reduction in trypsin inhibitor activity and 14.3% reduction in phytate [198]. The decrease in phytate was attributed to a wide range of microflora that is known to possess phytase activity and enzymatic hydrolysis that causes a decrease in trypsin inhibitor activity [198]. Some authors argue that phytate reduction during fermentation is a consequence of plant phytases activated during fermentation, although phytase activity is very variable depending on the plant species [210][211][212]. According to Licandro et al. [212], fermentation leads to the production of organic acids, decreasing the pH of the substrate and, thus, optimizing conditions for the activity of phytases.
A number of studies have reported reductions in oxalate concentrations after fermentation-27% in dawadawa [142], 62-77% in ugba [173], 36-52% in fermented Bambara groundnut flour [176], 67% in fermented horse gram flour [183] and 95% in fermented lima bean [186], with such reductions attributed to the utilization of oxalate as a carbon source of microbes and the microbial degradation of ANF-related components [183,186]. It has also been suggested that the reduction in oxalate content following fermentation can be attributed to the hydrolytic action of enzymes produced during fermentation [213].

Nutrient Digestibility and Bioavailability
Fermentation is known to enhance nutrient bioaccessibility, bioavailability and digestibility, mainly via the disruption of plant cell wall structures/tissues and the release of enzymes and other bioactive components. Additionally, lower pH values of the food medium attained during fermentation may improve the absorption of certain nutrients, as well as facilitate the decrease in some ANFs which interfere with nutrient bioavailability and bioaccessibility. The quality of protein should not only consider the composition of AAs, but also the digestibility as well as the absorption of the produced hydrolysis products in the human gastrointestinal tract [214][215][216]. For example, protein might have a very good AA profile, but are unable to absorbed well and/or be digested in the body. Some studies have reported an increase in in vitro protein digestibility (IVPD) during the fermentation of cereals and legumes. An improved protein digestibility during fermentation was attributed to the release of protein from plant tissues by the enzymatic breakdown of dietary fibres, with a simultaneous reduction in/degradation of polyphenols, tannins and phytic acid by the action of microbial enzymes [156,210,215]. Polyphenols are known to bind to recognition/receptive sites of digestive enzymes, or crosslink with proteins; hence, limiting the hydrolysis reaction [211]. Furthermore, during fermentation, insoluble/complex storage proteins undergo perturbations in structural configurations, which render them more accessible and susceptible to attack by pepsin and endopeptidase that breaks down proteins into smaller peptides that are more soluble. Ogodo et al. [134] suggested that lower pH values obtained during fermentation may well promote the enzyme activity of peptidases and activate endogenous proteases, which increases peptides and the free AA concentration; hence, increasing protein solubility.
Wedad et al. [170] reported an IVPD increase of 0.49-31.3% in sorghum fermented with starter inoculum through SSF. Mohammed et al. [139] also reported an increase of 21% in fermented sorghum, and such an increase was due to the reduction in ANF during fermentation. An increase of 10% was reported in IVPD of African yam bean (Table 4), and this was attributed to proteolysis, an increased availability of AAs and reduced ANFs [149]. An increase of 15.2% was reported in IVPD of chickpea fermented into tempeh flour and the authors attributed this to the elimination of undesirable factors (i.e., tannins during soaking and phytic acid during fermentation) as well as protein hydrolysis during fermentation, which resulted in proteins that were more vulnerable to enzyme action [178]. Additionally, an increase of 4.4% in IVPD of chickpea fermented with Cordyceps militaris was ascribed to the unfolding of the proteins during fermentation; thus, making them more accessible and easier to be hydrolysed by proteases [147]. On the contrary, during the fermentation of lupins into fermented lupin flour, Olukomaiya et al. [150] reported a 16-32.5% decrease in IVPD, with the authors attributing this decrease to protein being locked within the fibre matrix and, thus, reducing the hydrolytic action of the enzymes as well as partial protein denaturation during drying, which might also lower protein dispersibility and solubility; thus, resulting in a reduced IVPD.
An increase in the in vitro bioavailability of iron (68.3-90.6%) and zinc (86.7-100.6%) was reported by Dhull et al. [185] in fermented lentils. The authors attributed this increase to the reduction in ANFs as well as compounds that formed complexes with zinc and iron in the unfermented flour. Significant increases in in vitro starch digestibility (IVSD) have been recorded for maize (Zea mays) flour fermentation with LAB-consortium from maize (10.68-49.32%), LAB-consortium from sorghum (10.68-58%) and natural fermentation (20.10-49.45%) [156]. The enhanced digestibility was due to changes in the endosperm protein which allowed starch to become more accessible to the digestive enzymes [156].
The increase in IVSD in fermented sorghum (1.6-54%) was equally attributed to changes in the endosperm protein fractions that allowed starch to become more accessible to the digestive enzymes [166].

Conclusions and Future Perspectives
It was evident from the various studies consulted in this review that fermentation, though being an ancient food processing practice, remains an important approach for increasing the level of nutrients, reducing antinutritional factors and enhancing nutrient bioaccessibility/bioavailability of cereals and legumes. Very often, fermentation does not only increase the availability and digestibility of nutrients, but also makes the food more appetizing and acceptable by improving its texture, aroma, flavour, etc., as well as rendering the food safer for consumption by reducing/degrading certain inherent toxins in the food crop. This established fermented foods an important part of diet and nutrition in many cultures around the world, particularly in developing countries, with limited access to sophisticated food processing techniques and infrastructure. Additionally, some of the microorganisms implicated in food fermentation have been linked with important health benefits. Based on inference from the reviewed literature, we see fermentation as an important process in the food production value chain. Indeed, fermentation is a complex process and food components do not necessarily exist in isolation, but as an entity. Accordingly, modifications in these constituents are influenced by the crop specie and cultivar, grain composition, fermenting microorganisms and the metabolism of these organisms. Additionally, important are the prior processing steps before and after fermentation. These intricacies tend to limit the understanding of food fermentation and insights into the mechanisms governing the modification in these components somewhat difficult. Hence, in order to fully exploit the benefits of fermentation, more research should be conducted, particularly focusing on modern microbial and biotechnological techniques, as well as the adoption of advanced techniques, including, but not limited to, metabolomics, metagenomics, metatranscriptomics, proteomics and artificial intelligence models to better optimize, standardize and describe the fermentation process for an overall improved food quality, enhanced nutrition and health as well as other associated socioeconomic benefits.