Roti, which is commonly consumed in Western and Central India, is an unleavened bread typically made with pearl millet or sorghum flour ^[1][2][3][22,23,24]. Sorghum roti has various names in different parts of India: chapati (Hindi), bhakri (Marathi), rotla (Gujarati), rotte (Telugu), etc. ^[4][25]. To produce roti, sorghum flour and the optimum amount of water are mixed until they reach the preferred consistency. The kneaded dough is divided into small balls which are flattened with the help of a rough wooden or metal surface, before then being dusted with flour and baked on both sides on a hot plate ^[3][24].

To evaluate the quality of roti, two different aspects were used: (a) dough handling properties, or how easily the dough stretches to make thin pancakes, and (b) sensory attributes such as softness, chewing properties, dryness, and absence of bitterness ^[5][44].

Pericarp color, endosperm type, and texture significantly affect the quality of roti. The most preferred grains in roti preparation are white/pale yellow, dense, and round grains ^[6][38]. Murty et al. ^[6][38] indicated that roti made from sorghum with 100% corneous endosperm had a firm texture and lower storage quality, whereas those with a floury endosperm had unfavorable dough properties with poor flavor and storage quality. The use of corneous grain up to a certain content improved dough and roti quality. With regards to endosperm texture, corneous grains had higher grain density, grain-breaking strength, and less water absorption. Good roti-producing grain varieties had a colorless thin pericarp, 60–70% corneous endosperm, less than 24% grain water absorption, and a flour particle size index of around 65 ^[6][38]. The particle size index of sorghum flour varied between different varieties and was linked to the endosperm texture of the grain. Grinding/milling techniques also had significant impacts on sorghum flour characteristics, including particle size and starch damage (Table 1).

Grain structure (degree of corneous endosperm, kernel weight, breaking strength and water absorption), dough properties (water required, kneading quality, rolling spreading), and roti sensory characteristics (color, taste, texture, aroma, storage) were all affected by the environment, season, year, and genotype and year interactions ^[6][38]. Roti quality was not significantly impacted by nitrogen fertility, but soil moisture stress had a significant effect on dough characteristics. Wet weather, which promoted grain deterioration, had the most significant influence on roti quality ^[6][38].

In addition to relationships between grain physical traits and roti quality, there were significant correlations between roti quality and the chemical properties of sorghum grain reported. Though it is not bread, Aboubacar and Hamaker ^[7][45] showed that sorghum couscous hardness correlated positively with the amylose content of flour. A positive correlation between the amylose content and overall acceptability of rotis made from sorghum flour was found ^[8][9][29,46]. Subramanian and Jambunathan ^[10][40] found a strong positive correlation between the texture of rotis and protein/total amylose of sorghum flour as well, along with a negative correlation between soluble sugars of sorghum flour and the texture of roti. In sorghum grain, it is known that more protein is found in the harder, more corneous outer layers of the grain (e.g., ^[11][47]). Fine flour fractions which comprised particles from the inner floury endosperm of sorghum grain have less total protein ^[10][40], thus resulting in an unwanted textural quality of roti ^[10][40]. This reflects the close relationship between physical properties, chemical properties, and milling of sorghum grains and how such relationships can influence the final quality of sorghum-based food products ^[10][40]. Chavan et al. ^[12][32] conducted a study on the nutritional quality of grain sorghum (post-rainy season) genotypes, which were developed through a systematic breeding program and compared with traditional genotypes. Many nutritional components including crude protein, total soluble sugars, soluble proteins, and soluble amylose content were found to be primarily important for high-quality roti. Nandini and Salimath ^[13][33] reported that arabinoxylans were important in the quality of roti as they were responsible for the entrapment of gas in the dough; as a result, the final product had a softer texture. Although arabinoxylans from various cereals have the same basic chemical structure, the differences in the molecular features of arabinoxylans, which include the degree of branching, the spatial arrangement of arabinosyl substituents along the xylan backbone, and the ferulic acid content, can affect the viscoelastic properties of gels, resulting in changes in their conformation and causing arabinoxylan molecules to interact with each other and with other polysaccharides. The crispier texture of rotis, which are made from sorghum, compared to rotis prepared with wheat flour may be related to the highly branched nature of sorghum arabinoxylans, which form an inflexible matrix ^[13][33].

Nandini and Salimath ^[14][31] studied the carbohydrate profiles of wheat, sorghum, and bajra varieties with good chapati/roti-making quality and their isolated fractions such as water-soluble polysaccharides, barium hydroxide extract, hemicellulose A, hemicellulose B, and alkali-insoluble residue. In addition, the content of total sugar, uronic acid, rhamnose, fucose, arabinose, xylose, mannose, galactose, glucose of wheat, sorghum, and bajra varieties and their fractions were also measured. However, the researchers did not report on the effect of these components on the quality of the chapati/roti. To obtain optimal rolling quality, Chandrashekar and Desikachar ^[15][48] recommend choosing grains with a low gelatinization temperature, high peak viscosity, setback, and high water absorption ^[15][48]. Sorghum dough made with pre-gelatinized starch or flour from puffed grains had higher water absorption, and as a result, the dough handling and sensory properties of roti were improved compared to the control.

In terms of the flavor characteristics of roti, a positive correlation was found between flavor and protein, amylose, or ash content ^[10][40]. Carbohydrate composition and isolated fractions of sorghum, which are used for the good quality of roti, were investigated to determine relationships between taste and reducing sugars, water-soluble flour fraction, or flour swelling capacity, as well as the positive correlation between taste and water-soluble protein ^[10][40]. Unfortunately, mechanisms behind the correlated parameters, as well as why the chosen parameters were important for roti quality, were not explained. More studies need to be conducted to show the importance and mechanism of this research.

Due to the properties of sorghum flour and its impact on starch digestibility ^[16][49], there have been several research studies conducted on the glycemic index values or the digestive effects of sorghum-based foods. However, there have been only a few studies on the glycemic index of sorghum roti. Prasad et al. ^[17][43] studied the glycemic index and glycemic load of different sorghum foods and compared them with those of wheat/rice-based foods. Interestingly, Prasad et al. ^[17][43] found that all sorghum-based products tested, with the exception of sorghum roti, had a lower glycemic index than their respective wheat/rice-based foods. The authors related the high glycemic index of the sorghum roti to the disruption of the outer layer of starch granules due to the processing of the roti. Nambiar and Patwardhan ^[18][50] studied the glycemic index and glycemic load data of the traditional millet-based recipes of India and found that cooking techniques such as shallow frying, roasting, and steaming significantly impacted the glycemic index. Similar to the findings of Prasad et al. ^[17][43], roti had a higher glycemic load compared to other food products tested. Roti has a short shelf life and will become dry within 10–15 h of cooking ^[19][36]. The shelf life of sorghum roti was studied in order to improve the storage of roti ^[4][25]. Both ascorbic acid and potassium sorbate solutions were used in the formulation of roti to improve shelf life. Sorghum roti stored in polyethylene bags that contained either 0.5 g of potassium sorbate or 100 ppm of ascorbic acid in the roti formulation could be stored at room temperature for 6 days without any significant changes in sensory properties.

The impact of sorghum grain components on roti quality has been the subject of a wide range of studies, but the impact of processing methods on roti quality has received less attention. More research must be conducted on the use of various approaches to improve the quality of roti because there has only been a small amount of research on various processes and techniques.

Tortillas with alkali-cooked corn are commonly consumed in Mexico and Central America. However, sorghum or sorghum and corn blends are often used to bake tortillas because sorghum has a higher grain yield in hot and dry climates and is often a less expensive ingredient in certain Latin American countries ^[20][21][51,52]. Moreover, sorghum has a comparable nutritional value to corn ^[1][20][21][22,51,52].

Nixtamalization is the traditional process used to produce corn flour used in tortilla making. In this process, corn is boiled in water with a low (1–5%) level of calcium hydroxide, soaked overnight, washed, and then ground in a stone grinder. The obtained flour is called “corn masa”. The masa can be wet or dried to obtain dry masa flour, which is also called an “instant” tortilla mix ^[21][52]. Masa is formed into dough balls, and the dough balls are flattened into thin layers and baked on both sides on a hot plate ^[22][68]. The alkaline treatment used during nixtamalization damages cell walls, allowing the pericarp to be removed more easily. The alkaline conditions also solubilize cell walls in the peripheral endosperm, induce swelling, partially destroy the starch granules, and change the physical appearance of protein bodies. In tortilla baking, cell walls are further degraded, starch crystallinity is lost, and protein bodies are partially destroyed ^[21][52].

Nixtamalization also reduces the total phenolic content of sorghum grains. Luzardo-Ocampo et al. ^[23][69] compared the influence of cooking or nixtamalization (alkaline cooking) on the bioaccessibility and antioxidant capacity of phenolic compounds of two sorghum varieties grown in Mexico (white/red). Variety, thermal treatment, and digestion phases all had a role in the changing bioaccessibility of phenolic compounds. Total phenolics and flavonoids became more bioaccessible after nixtamalization as well as after cooking. However, only nixtamalization resulted in a substantial decrease in the condensed tannins, and reduced tannin content by ~74%. The washing phase in the nixtamalization process resulted in the loss of pericarp and therefore phenolic compounds, which was stated as the reason for the lower phenolic content values in the processed samples. Importantly, nixtamalization impacted the bioavailability of the phenolic compounds present. For example, flavonoids from white sorghum showed strong absorption in the small intestine in the nixtamalized sample ^[23][69]. To find the best parameters for nixtamalization, different lime concentrations (0, 1, and 2%) and cooking times (20, 30, and 40 min) were tested by Gaytan-Martínez et al. ^[24][70] for white and red sorghum. Significant negative correlations between total phenol, flavonoid, and antioxidant capacities and nixtamalization parameters were found. The optimal conditions for maintaining antioxidant potential with low tannin content were 1.13% lime and 31.11 min of cooking time ^[24][70].

The majority of research on sorghum nixtamalization has focused on antinutrients and phenolic content. However, nixtamalization also improves sorghum protein bioaccessibility in tannin-containing sorghum by depolymerizing condensed tannins and disrupting protein–tannin complexes ^[25][71]. Interestingly, the bioaccessibility of phenolic compounds differed considerably throughout the digestive process, indicating that the release of condensed tannins from the food may also alter protein bioaccessibility and digestibility ^[23][69]. Cabrera-Ramírez et al. ^[25][71] conducted a study to evaluate how the nixtamalization method affects protein bioaccessibility in white and red sorghum varieties during in vitro gastrointestinal digestion. The results of this study showed that the nixtamalization process had no impact on the oral and gastric bioaccessibility of sorghum protein from both white and red sorghum, but it improved protein bioaccessibility during the intestinal digestion step, thus making nixtamalization a useful processing method for improving the nutritional quality of sorghum tortillas ^[25][71]. In addition to affecting digestibility, nixtamalization can impact protein composition. For example, Ali et al. ^[26][72] discovered that soaking sorghum in NaOH solutions at room temperature (27 °C) reduces the concentration of albumins and globulins.

Different levels of sorghum bran (0, 5, or 10%) have been added to corn tortillas before or after the extrusion process to improve the nutritional quality of extruded nixtamalized corn flour tortillas ^[27][28][60,61]. Total phenolic compounds and antioxidant activity levels in tortillas were lowered by the thermal technique used during baking. Corn flour with 10% sorghum bran added before extrusion retained almost 82 and 90% of the total phenolic compounds and antioxidant activity, while tortillas prepared with corn flour with 10% sorghum bran added after extrusion preserved more than 92 and 76% of the total phenolic compounds and antioxidant activity. The best texture, as well as the highest amount of total phenolic compounds and antioxidant activity, were obtained from tortillas prepared with sorghum bran added before extrusion. The retention of total phenolic compounds and antioxidant activity from flour to tortillas was higher for tortillas made with corn flour with 10% sorghum bran added after extrusion than for tortillas made with extruded nixtamalized corn flour ^[27][60].

In another study, researchers found that sorghum bran addition (5 or 10%) prior to extrusion increased free phenolic acid content and in vitro antioxidant activity ^[29][73]. Tortillas with sorghum bran added had higher total starch, amylose, and total soluble and insoluble dietary fiber content compared to the control (nixtamalized corn flour) due to the higher water absorption capacity of sorghum bran and its interaction with starch molecules from nixtamalized corn flour. Tortillas with sorghum bran had better texture and flexibility compared to those prepared without sorghum bran ^[30][31][64,65]. The addition of sorghum bran in corn flour tortillas also caused an increase in ferulic acid, cellular antioxidant activity, flavones, and total phenolic compounds ^[29][32][59,73]. Sorghum bran is a byproduct of sorghum dry milling and, in addition to containing fiber and phenolic compounds, also contains protein ^[33][74]. Thus, the addition of sorghum bran to corn tortillas could improve the protein content.

Tortillas prepared with whole white sorghum had a softer texture and darker color than those made with white corn ^[34][75]. Bedolla et al. ^[34][75] suggested that sorghum could be used alone or in combination with corn to make tortillas that were satisfactory in color, flavor, and texture. Quintero-Fuentes ^[35][55] reported that the use of sorghum provided more rollable and flexible tortillas compared to those prepared without heterowaxy sorghum. This might be more of a water-binding effect of waxy samples. In addition, grain from sorghum cultivars with a tan plant, white pericarp without sub-coat, intermediate endosperm texture, and low levels of color precursors is used to make tortillas with acceptable color and texture ^[35][55]. Research has demonstrated the benefits of using micronizing, which is a dry heat technique. Substituting micronized sorghum flour for commercial corn flour improved the palatability of the masa and the rollability of the tortilla. If micronizing is used to make tortilla flour, the process will be speedy and cost-effective because it will eliminate the existing method’s extensive heating and soaking times as well as the costly drying procedures ^[30][36][62,64].

The kernel size, texture, and structure of sorghum grain affected the quality of tortillas as much as they impacted the quality of roti. Khan et al. ^[37][56] reported that the lightest-colored tortillas with the highest potential quality were produced from sorghum with white or colorless pericarps and no testa, while colored sorghum produced tortillas with an undesirable color and poor taste. Sorghum flour from decorticated kernels was tested at varying levels of substitution for wheat flour ^[38][66]. There were three varieties of decorticated soghum: medium and fine. Sorghum flour was used to replace wheat flour at a rate of 15% to 30%. Mixing standards were followed for preparing composite doughs. When sorghum flour replaced 30% of wheat flour in wheat sorghum dough, the maximum stress peak, stress during relaxation, stress at a given strain, and dough viscosity all increased ^[38][66].

Sorghum hybrids and commercial sorghum flour were also evaluated to make gluten-free sorghum tortillas ^[39][57]. Tortillas with commercial sorghum flour had the highest sensory scores due to a smaller particle size and higher starch damage, which resulted in higher water absorption. Sorghum flour with smaller particle size and higher starch damage produced softer and more extensible tortillas ^[39][57]. The findings revealed that sorghum hybrids varied in kernel and flour quality, which could aid in the prediction of sorghum flour quality for gluten-free products.

Injera is an important food staple in Ethiopia, Eretria, and portions of Somalia ^[40][41][76,77]. Injera is a thin round bread with the top surface containing “eyes” and an overall soft and fluffy texture that can be rolled without breaking ^[37][39][56,57]. Injera is typically made from teff, and teff-based injera is said to have the best quality compared to other cereals ^[41][42][77,78]. In addition to teff, sorghum is also a common cereal grain for making injera ^[41][43][77,79]. The three main steps used in making injera are the preparation of a batter with flour and water, the addition of batter from the previous injera batter, and then fermentation with dough at an ambient temperature for almost 48 h. Then, a small amount of batter is poured onto hot clay and baked. A standard procedure for making injera was described by Yetneberk et al. ^[42][44][78,80], and the recent review of Neela and Fanta ^[41][77] details the history and traditional production of injera.

The production of injera from sorghum has been conducted by several research groups, with reasons for this typically cited as desirable due to the cost of teff relative to sorghum and the low yield of teff ^[41][42][45][77,78,81]. Research efforts have been made to identify sorghum genotypes that have the potential to produce high-quality injera using both the instrumental characterization of injera and consumer preference sensory panels ^[45][46][47][81,82,83]. Yetneberk et al. ^[42][78] studied the influence of different cultivars on injera quality using different sorghum cultivars that varied in kernel characteristics. The findings showed that the quality of injera depended on sorghum cultivar and identified three sorghum cultivars that produced injera with positive sensory attributes and quality traits. The chemical (total starch, amylose, and tannin contents), image, and/or sensory analysis results of the study identified several sorghum genotypes that were superior to teff in making injera ^[48][84]. The imaging analysis also allowed for a more objective and quick evaluation of injera ^[48][84]. Thus, the selection of sorghum lines with quality attributes specific to the production of injera may be one avenue to enhance the utilization of sorghum for injera production.

In addition to cultivar selection, other methods have been investigated to improve the quality of injera made with sorghum. Decortication and the use of sorghum-teff blends were also evaluated as techniques to enhance the quality of injera ^[44][80]. Decortication enhanced the color and other quality characteristics of injera by lowering the level of non-starch components in the grain. Both decortication and the use of a sorghum-teff blend were found useful to improve the quality of injera since the tannin content of sorghum was reduced. However, composing sorghum and teff rather than decorticating was found to be a better option since higher grain loss occurred during decortication ^[44][80]. Abraha and Abay ^[45][81] also reported that 50:50 blends of teff and sorghum produced acceptable injera. Ghebrehiwot et al. ^[40][76] also investigated blends of teff and a closely related grass species, Eragrostis curvula (Shrad.) Nees, with low levels of sorghum (5 and 10%), and reported that the texture, taste, and appearance of the teff injera were improved with the addition of 5% sorghum.

Thermal processing has an impact on the nutritional content of food products ^[49][85]. Accordingly, changes in the chemical and nutritional properties of sorghum flour, processed from flour into batter and into cooked injera, have been reported ^[50][86]. The content of protein, ash, and fat were reduced in injera compared to that of the flour and batter. The levels of antinutrients (polyphenols, phytate, and tannins) were reduced, and levels of Ca, Fe, Cu, and protein digestibility were higher in injera than in flour and batter ^[50][86].

Kisra is a Sudanese bread made from sorghum, which is similar to injera but is smaller, thinner, and lacks a spongy texture ^[51][112]. Overall, the baking steps are similar to those of injera. The formulation for kisra can vary ^[52][87], but several procedures have been described in the literature (e.g., ^{[52][53][54][55]}[87,88,100,102]). The processing of sorghum flour into kisra can significantly impact the final composition. For example, during the fermentation process of kisra production, the tannin, starch, total, and non-reducing sugars decreased while the protein, glutelin, crude fiber, tyrosine, and methionine content increased ^[55][56][57][98,102,106].

As with other sorghum-based foods, the sorghum cultivar used for kisra production has a significant effect on the final quality of the product. The evaluation of Sudanese sorghum cultivars during cooking and by sensory evaluation found that kisra quality varied among the sorghum samples ^[52][87]. Sorghum types with a floury endosperm were preferred over those with a corneous endosperm for cooking quality. For final production quality, kisra color and texture were found to be the most important traits, and kisra made from white sorghum types was rated the highest ^[52][87]. Awad-Elkareem and Taylor ^[53][88] also reported that non-tannin white sorghum types were the best for kisra production and could be a good option for the production of gluten-free wrap.

Research has been conducted to combine sorghum flour with other flours to improve the nutritional quality of kisra. For example, kisra made with sorghum flour plus bean protein isolates had higher protein and lysine content compared to kisra made only with sorghum ^[58][90]. The use of lactic acid bacteria starter cultures to reduce the fermentation time required for kisra production has also been investigated ^[54][100].