Sorghum in foods: Functionality and potential in innovative products

Abstract

Sorghum grain is a staple food for about 500 million people in 30 countries in Africa and Asia. Despite this contribution to global food production, most of the world’s sorghum grain, and nearly all in Western countries, is used as animal feed. A combination of the increasingly important ability of sorghum crops to resist heat and drought, the limited history of the use of sorghum in Western foods, and the excellent functional properties of sorghum grain in healthy diets, suggests a greater focus on the development of new sorghum-based foods. An understanding of the structural and functional properties of sorghum grain to develop processes for production of new sorghum-based foods is required. In this review, we discuss the potential of sorghum in new food products, including sorghum grain composition, the functional properties of sorghum in foods, processing of sorghum-based products, the digestibility of sorghum protein and starch compared to other grains, and the health benefits of sorghum. In the potential for sorghum as a major ingredient in new foods, we suggest that the gluten-free status of sorghum is of relatively minor importance compared to the functionality of the slowly digested starch and the health benefits of the phenolic compounds present.

Keywords:

• grain components
• grain processing
• phenolics
• sorghum
• starch

Introduction

Sorghum is a genus in the tribe Andropogoneae, the subfamily Panicoideae and the family Poaceae (the grasses) (Spangler et al. 1999). Cultivated sorghum (Sorghum bicolor (L.) Moench) is a C4 and self-pollinated plant with high photosynthetic efficiency (Gomez-Martinez and Culham 2000; Goyal et al. 2020). Sorghum is a drought-tolerant crop (Wagaw 2019) cultivated in arid and semi-arid regions of sub-Saharan Africa, Asia, the Middle East, as well as Central and North America and Australia. This cereal can also survive cool weather (12–15 °C), water-logged conditions (for short periods) and grow in high-rainfall areas (Lim 2012). Sorghum grain is a staple food in parts of Africa, Asia and other low-income regions, as well as a fodder and feed crop for livestock (Proietti, Frazzoli, and Mantovani 2015), with an annual world production (as of January 2021) of 61.62 million metric tonnes (MMT) (USDA-FAS 2021).

The major sorghum-producing countries are USA, Nigeria, Ethiopia, Sudan, Mexico, and India, followed by China, Argentina, Brazil and Niger (Table 1). In Africa and Asia, whole sorghum grain, decorticated grain and sorghum flour are used to make a large range of foods. These include thin or thick fermented or unfermented porridge, boiled products similar to those made with maize or rice grits, popped sorghum, flat bread and various types of deep-fried foods. Sorghum has been used in the production of beverages such as nonalcoholic fermented drinks and (alcoholic) beer in Africa and Mexico, as well as spirit and vinegar in China. Sorghum is also used as a wheat substitute to produce gluten-free foods (Chávez et al. 2018; Pineli et al. 2015) including pastas, breads, biscuits and porridges (Table 2).

Table 1. Sorghum grain production in the 10 leading sorghum-producing countries globally from 2018 to 2021 (USDA-FAS 2021).

Country	Sorghum production ('000 metric tonnes)
	2018–2019	2019–2020	2020–2021
USA	9271	8673	9474
Nigeria	6721	6665	6900
Ethiopia	4932	5200	5200
Sudan	4953	4000	5000
Mexico	4700	4300	4500
India	3475	4733	3850
China	2909	3600	3550
Argentina	2500	2500	3200
Brazil	2177	2254	2100
Niger	2100	1970	1900

Table 2. Sorghum foods from around the world.

Food type	Food name	Country of origin	References
Part of the main meal	Couscous Annam (Sora) Kaoliang mi fan Lehata wagen Nufro Okababa	India & Sahel India China Botswana Ethiopia Nigeria	Ratnavathi and Patil 2013; Young et al. 1993; Worku, Woldegiorgis, and Gemeda 2015
Confectionary or sweet	Rawa laddu Kesari	India India	Ratnavathi and Patil 2013
Bakery products	Roti Bhakri Dosa Kisra Injera Mantou	India India India Sudan Ethiopia China	Ratnavathi, Seetharama, and Chavan 2008; Ratnavathi and Patil 2013; Bai and Liu 2010
Porridge	Sankati (Mudda or Kali) Kanji (Ambali) Bogobe Ogi Tô Nasha Aceda Ugali	India India Botswana Nigeria West Africa Sudan Sudan Uganda, Rwanda & Tanzania	Ali, Merra, and Malleshi 2003; Boling and Eisener 1981; Ratnavathi, Sailaja, et al. 2008; Tunde Obilana 1981; Mukuru, Mushonga, and Murthy 1981
Beverages	Rabadi Pito Baijiu Obiolor	India Nigeria & Ghana China Nigeria & Ghana	Hussain, Garg, and Pal 2014; Modha and Pal 2011; Djameh et al. 2015; Ajiboye et al. 2014
Breakfast	Upma Idli Noodles	India India China	Ratnavathi and Patil 2013
Snack foods	Popped sorghum	India	Mishra et al. 2015

Sorghum is a cereal crop of incredible genetic and phenotypic variability, both among S. bicolor and its wild relatives (Mace et al. 2021). Sorghum grain quality, appearance and color are affected by genes controlling the presence (B1 and B2) or absence (b1 and b2) of a testa layer, pericarp color (R and Y) and thickness (Z), as well as endosperm color and structure (wx) (Rooney and Miller 1982). A single recessive gene, wx, controls the production of waxy endosperm in sorghum. Sorghum with ∼25% amylose in the endosperm is termed normal (WxWxWx) while grain with almost 100% amylopectin is termed waxy (wxwxwx). Grain with an intermediate amount of amylose (∼15%) is known as heterowaxy (WxWxwx or Wxwxwx) (Sang et al. 2008).

Sorghum varieties have been classified in several different ways (Shin 1986):

1. Based on distinguishing plant characters (taxonomy): A: leaves, glumes, florets B: kernel size, shape, pericarp, endosperm and testa.

2. Based on plant responses to environmental factors such as drought, temperature, humidity and photoperiod.

3. Based on use: A: Sorgo or sweet sorghum (for biofuel and sorghum sirup) B: Broomcorn (for brooms and whiskbrooms) and C: Grain sorghum (for biofuel, human foods and animal feed), which is classified in five sub-groups based on growing region: 1) Durra: predominantly in the Mediterranean and Middle East; 2) Shallu: in India and tropical Africa; 3) Kafir: mainly in South Africa; 4) Kaoliang: predominant in Eastern Asia; 5) Milo: in Eastern Africa.

4. Based on grain color and presence/absence of a pigmented testa (USDA-FGIS 2009):

Sorghum – Low in tannin due to the absence of a pigmented testa. (Tannin consists of specific phenolic compounds with molecular weights between 500 and 3,000.) This class includes around 98% white sorghum grains and 2% tannin-containing sorghum grains among the grain traded. The pericarp colour of this class may be white, yellow, pink, orange, red or bronze.

Tannin Sorghum – Contains more than 90% tannin-containing grains as judged by the presence of a pigmented testa. The pericarp colour is usually brown but may also be white, yellow-pink, orange, red or bronze.

White Sorghum – Low tannin content due to the absence of a pigmented testa. The pericarp colour is white or translucent. This class may include sorghums containing dark spots that cover less than 25% of the kernel.

Mixed Sorghum – This class of sorghum does not meet the definitions for any of the other three classes.

Grain sorghums are also categorized into type I, II, and III based on their tannin content (Price and Butler 1977). Type I sorghum is tannin free, while type II and III sorghums contain low and high levels of tannins, respectively. Due to their resistance against birds and insects, extreme weather and their high yield potential, tannin-containing sorghums may be preferred by farmers (Lubadde et al. 2019). However, tannin is one of the major nutrient-limiting components of the grain for animal or human consumption. Tannin is reported to reduce starch and protein digestibility and to lower weight gain in monogastric animals such as poultry and rabbit (Liu et al. 2015; Muriu et al. 2002). Other studies have confirmed through in vitro tests the reduction in the digestibility of starch and protein as a result of interaction with tannin (Barros, Awika, and Rooney 2012, 2014; Dunn et al. 2015).

Sorghum grain structure

The general composition of a sorghum grain has been reported to be 3–6% pericarp, 84–90% endosperm, and 5–10% germ (Figure 1) (Rooney, Rooney, and Serna-Saldivar 2016).

Figure 1. Sorghum kernel structure. Reprinted from Earp, McDonough, and Rooney (2004) with permission from Elsevier.

Pericarp

The pericarp is the outermost structure of the sorghum grain and is divided into three sub-tissues: epicarp, mesocarp and endocarp. The epicarp is made of elongated, rectangular cells covered by a thin waxy layer, and is composed of two sub-layers: the epidermis and hypodermis. Pigments are often present in the epidermis. Compared to the epidermis, the hypodermis has slightly thinner cells and is composed of one to three cell layers (Rooney, Rooney, and Serna-Saldivar 2016).

The mesocarp (the middle layer) is the thickest layer of the sorghum pericarp; however, its thickness differs greatly among genotypes. Unlike the mesocarp of most cereals, sorghum mesocarp contains starch granules (Waniska and Rooney 2000). The endocarp, the innermost layer, is composed of cross cells and tube cells.

The thickness of the pericarp is an important factor affecting grain processing. A thick pericarp along with a corneous endosperm is required to generate a high yield of sorghum flour (Shin 1986). A hard and thick pericarp is much easier to remove by pounding, which is a common practice in Africa, whereas a thin pericarp requires more time to remove, mainly achieved via a dehulling method (Guindo et al. 2016). However, sorghum varieties with a thick pericarp have some disadvantages, such as sensitivity to mold, grain weathering in the field during maturation, and rapid deterioration of grains during storage (Earp, McDonough, and Rooney 2004; Guindo et al. 2016).

The color of sorghum grain varies greatly due to pericarp pigmentation and thickness, as well as the presence of a testa. Phenolic compounds, particularly tannin, contribute to the pigmentation of the pericarp and testa (Sedghi et al. 2012).

Ferulic acid is the dominant phenolic acid in sorghum grain, as in many small-grained cereals (Boz 2016). Sorghum grain also contains flavonoids; e.g., flavonols (quercetin and kaempferol), flavanone (naringenin), and flavones (apigenin and luteolin) (Przybylska-Balcerek, Frankowski, and Stuper-Szablewska 2019).

Sorghum grain color is determined by the genes that control pericarp color (R and Y gene), pericarp thickness (Z gene), presence of pigmented testa (B1 and B2 genes), spreader (S gene), testa color (Tp genes), and the color intensifier (I gene) (Rooney and Waniska 2000; Xiong et al. 2019). According to the phenolic profile and genotype, sorghum can have different colors such as white (RRyy and rryy), yellow (rrYY), red (RRYY), brown (B1B2_), and black (RY_ and/or B1B2_) (Dykes et al. 2011; Dykes and Rooney 2006; Dykes, Rooney, and Rooney 2013; Waniska and Rooney 2000).

Dykes et al. (2009) reported that the composition of flavonoids in red sorghum is related to its R_Y_ genes for red pericarp and that the composition varies among different sorghum genotypes. The authors reported that red sorghums, especially those with a black pericarp, have higher 3-deoxyanthocyanin levels and also stated that the pericarp thickness is controlled by the Z gene.

Testa

Beneath the endocarp is the testa layer or seed coat. The testa among sorghum genotypes can be pigmented or non-pigmented. The B1B2 gene combination generates a pigmented testa layer, in which tannin is present, while B1b2, b1B2, or b1b2 lead to an absence of pigmented testa. Tp genes control the specific color of the testa (Wu et al. 2012). A purple testa is found when Tp genes are homozygous recessive (tptp), while TpTp or Tptp will give rise to a brown testa. The highest level of condensed tannin is in sorghums containing dominant B1BB2S genes; the S gene controls the presence of brown pigments, as well as tannins in the epicarp and endocarp when a pigmented testa is present (Blakeley, Rooney, Sullins, & Miller, 1979). The B1B2 genes appear to exert control over the polymerization of flavans (anthocyanidins) to flavan-3-ols (building blocks for tannins).

Endosperm

As in other cereals, the endosperm is the storage tissue and major part of the sorghum grain. It consists of the aleurone layer, as well as the peripheral, corneous and floury zones. The aleurone lies just under the testa, and consists of cells with thick cell walls. The aleurone contains minerals, hydrolytic enzymes (mainly synthesized upon imbibition of the grain) and lipid (Waniska and Rooney 2000). The periphery of the endosperm (not including the aleurone layer) is composed of several layers of dense cells containing starch granules and protein.

The endosperm corneous and floury zones are composed of starch granules, a protein matrix, protein bodies and cell walls rich in glucuronoarabinoxylans (Hoseney 1994). The starch granules and protein bodies are trapped in a protein matrix in the peripheral and corneous regions of the endosperm. The floury endosperm is located in the core of the grain and has a very low abundance of protein bodies compared to the corneous and peripheral layers. The proportion of floury to corneous endosperm largely determines the endosperm texture, which is a property linked to milling yield of the grain and digestibility. The higher the ratio of corneous to floury endosperm, the easier it is to decorticate the sorghum grain, resulting in a higher yield of flour (Reichert, Youngs, and Oomah 1982).

Germ

The germ is divided into two major parts: the embryonic axis and scutellum. The embryonic axis forms the new plant and is subdivided into a radicle and plumule. The radicle forms primary roots, whereas the plumule forms leaves and stems. The scutellum is the single cotyledon of the sorghum seed. It contains large amounts of oil (in spherosomes), proteins (including enzymes) and minerals, and serves as the connection between the endosperm and germ (Rooney, Rooney, and Serna-Saldivar 2016; Waniska and Rooney 2000).

Sorghum grain composition

Carbohydrates

The carbohydrates of sorghum grain are the polysaccharides starch, which is dominant, and fructosan, the trisaccharide raffinose, the disaccharides sucrose and maltose, and the monosaccharides glucose and fructose (Anglani 1998). Starch is the major component and is mainly located as granules in the endosperm; however, some starch granules can be found in the pericarp of the grain (Taylor & Emmambux, 2010). Khan et al. (2013) reported that the range of starch content is 55.6–70.0% of dry grain weight depending on the cultivar.

Like starches from other sources, sorghum starch is made up of two biopolymers, amylose and amylopectin, with the ratio determining many physicochemical, thermal and rheological properties of the starch (Sang et al. 2008). Amylose and (to a lesser degree) amylopectin strongly affect starch gelatinization, retrogradation, paste viscosity, gelation and α-amylase digestibility (Dadnia, Mojaddam, and Ayran 2018; Sang et al. 2008; Yanagida et al. 2006). Typically, sorghum starches display higher gelatinization temperatures (68–78 °C), a higher degree of retrogradation, but slower enzymatic hydrolysis rates than starches from other cereals, such as maize (gelatinization at 62–72 °C), wheat (58–64 °C) and barley (51–60 °C) (Collar, 2017; Hoseney 1994). These properties could be due to the smaller number of short-branch chains in the amylopectin of sorghum compared to other cereals (Ai 2013). The high gelatinization temperature of sorghum starch may have an adverse impact on the quality of baked products (Taylor and Dewar 2001).

High peak paste viscosity, water uptake and set back, as well as low gelatinization temperature, have been shown to be advantageous in making flat breads such as Roti from sorghum, while low peak paste viscosity and high gelatinization temperature are desirable for making stiff porridge such as Indian Sankhati and African Tô (Ratnavathi and Patil 2013).

The free sugar content of sorghum grain is reported to be in the range 1–2% (Shin 1986). Non-starch polysaccharides (NSPs) are generally located in the pericarp and endosperm cell walls of sorghum grain. NSPs are the main cause of the insolubility and resistant nature of sorghum endosperm carbohydrates (Hoseney 1994). Wheat endosperm contains arabinoxylans, which are water-soluble; in contrast, sorghum endosperm cell walls contain high levels of glucuronoarabinoxylans, which are water-insoluble and may be disadvantageous in bread making and brewing (Taylor, Schober, and Bean 2006a; Taylor & Emmambux, 2010).

The in vitro digestibility of starch isolated from sorghum is reported to be 33–48%, which is considerably lower than that of corn starch at 53–58% (Sikabbubba 1989). Floury sorghum grains with their amorphous structure are more prone to enzymatic digestion as compared to the more ordered structure of corneous grains. Thus, starch from relatively floury sorghum grain after milling has a higher digestibility than starch from relatively corneous grains. This is because the floury grain particle size is lower, which facilitates enzymatic reactions required for digestion. Normal (non-waxy) sorghum is reported to have lower starch digestibility compared to waxy sorghum (Hibberd et al. 1982).

Proteins

Proteins make up a key component (on average 7–15%) of the sorghum grain, and are located mostly in the endosperm. Sorghum proteins are divided into albumins, globulins, glutelins and kafirins. The kafirins, which are hydrophobic, protease-resistant prolamins, represent 50–80% of total protein, with the glutelins being the second most abundant (Duodu et al. 2003; FAO 1995; Hamaker et al. 1995; Oria, Hamaker, and Shull 1995). The kafirins are divided into four subgroups based on differences in structure, solubility and molecular weight: α-kafirins (23 and 25 kDa, 80%), β-kafirins (20 kDa, 5%), γ-kafirins (28 kDa, 15%) and trace amounts of δ-kafirins (Belton et al. 2006; Izquierdo and Godwin 2005; Jambunathan, Mertz, and Axtell 1975; Taylor et al. 2007).

Amino acid analysis indicates that all three types of kafirins contain low amounts of lysine, an essential amino acid in the human diet. β-Kafirins and γ-kafirins are high in cysteine, and δ-kafirins are fairly high in methionine, another essential amino acid (Shull, Watterson, and Kirleis 1992). Protein bodies in both peripheral and central endosperm contain mainly α-kafirins, whereas the central endosperm contains greater amounts of β- and γ-kafirins than the periphery (Lim 2013).

Research on the structure of sorghum protein bodies shows that both γ- and β-kafirins cover the more digestible α-kafirins in a disulfide bond polymer network that shields the α-kafirins from exposure to proteases (Shull, Watterson, and Kirleis 1992).

In vivo and in vitro tests on sorghum protein digestibility have produced values ranging from 30 to 72%, which are lower than for wheat (81%) and maize (73%) (Axtell et al. 1981; Nawar et al. 1970). However, whole-grain sorghum with 46% protein digestibility showed a 35% increase in protein digestibility when the grains were decorticated and an extruded sorghum food was produced (MacLean et al. 1983).

Sorghum proteins are unable to form a gas-entrapping, viscoelastic dough, which is a basic demand for leavened bread making and for the production of many other baked products (Taylor and Dewar 2001). The other major issue with sorghum proteins is that their digestibility can be reduced if the grains are cooked during processing (Arbab and El Tinay 1997).

Lipids, vitamins and minerals

Sorghum grain contains nearly 3% lipid, which is lower than in millet and maize but higher than in rice and wheat (Dykes and Rooney 2006; Serna-Saldivar and Rooney 1995). Sorghum grain lipids are mainly found in the scutellum, and have a typical fatty acid profile of 49% linoleic (18:2), 31% oleic (18:1), 14% palmitic (16:0), 2.7% linolenic (18:3) and 2.1% stearic (18:0) acids (Dykes and Rooney 2006; Glew et al. 1997). Sorghum lipid would have an impact on the flavor and shelf life of sorghum-based foods (Shin 1986).

Sorghum grain also contains a considerable abundance of β-carotene (the provitamin of vitamin A) and tocopherols, and is a source of lipid-soluble vitamins such as A, D and K, as well as water-soluble vitamins such as thiamin, riboflavin, and pyridoxine (Martino et al. 2012).

Sorghum grain pericarp, aleurone layer and germ are rich in minerals such as magnesium, iron, zinc, copper, calcium, phosphorus and potassium (Anglani 1998; Glew et al. 1997). Vitamins and minerals in sorghum grain are mainly located in the aleurone and germ, while lipids are concentrated in the germ. For this reason, decortication of sorghum grain will result in the loss of important nutrients.

Phenolic compounds

Phenolic compounds represent the largest group of phytochemicals and are found throughout the Plant Kingdom. The basic structural feature of phenolic compounds is an aromatic ring bearing one or more hydroxyl groups (Chirinos et al. 2009). Plant phenolic compounds are classified as simple phenols or polyphenols based on the number of phenol units in the molecule. Thus, plant phenolics comprise simple phenols, coumarins, lignins, lignans, condensed and hydrolyzable tannins, phenolic acids and flavonoids (Soto-Vaca et al. 2012).

Phenolic compounds are synthesized in plants partly as a response to ecological and physiological pressures such as pathogen and insect attack, UV radiation and wounding (Diaz Napal et al. 2010; Kennedy and Wightman 2011; Zulak et al. 2006). These phytochemicals can act as antioxidants and are thought to contribute to the prevention of heart disease (Dai and Mumper 2010; Wijngaard, Rößle, and Brunton 2009), reduce inflammation (Zhang et al. 2011), lower the incidence of cancers (Sawadogo et al. 2012) and diabetes (Scalbert et al. 2005; Thompson et al. 1984), as well as reduce rates of mutagenesis in human cells (Gomez-Cordoves et al. 2001; Sawadogo et al. 2012). The disease protection afforded by the consumption of plant products such as fruits, vegetables and legumes is largely associated with the presence of phenolic compounds.

The chemo-protective nature of phytochemicals and the limitations on the addition of synthetic antioxidants to food has attracted worldwide attention on phytochemicals as good sources of antioxidants and food/beverage colorants (Fu et al. 2011; Zhang et al. 2015). Sorghum grain is an excellent example of a food ingredient that has been the subject of this interest. Sorghum phenolics comprise of three major categories: flavonoids, phenolic acids and tannins (Khoddami, Mohammadrezaei, and Roberts 2017; Khoddami et al. 2015). The phenolic compounds of sorghum are well known to act as antioxidants in in vitro assays (Awika 2003) and some are known to be more potent antioxidants than vitamins found in other plant species (Rhodes and Price 1997).

Sorghum phytochemicals, particularly anthocyanins and other flavonoids, are claimed in some studies to exhibit anti-inflammatory (Bralley et al. 2008; Burdette et al. 2010; Shim et al. 2013), anti-cardiovascular disease (Mellen, Walsh, and Herrington 2008), anti-diabetic (Farrar et al. 2008; Hargrove et al. 2011), anti-cancer (Chen et al. 2011; Massey, Reddivari, and Vanamala 2014; Wu et al. 2011), anti-lipidemic (Chung, Yeo, et al. 2011), immunomodulatory (Chung, Yeo, et al. 2011), anti-anemic (Oladiji, Jacob, and Yakubu 2007), neuroprotective (Oboh, Akomolafe, and Adetuyi 2010), anti-diarrheal (Nwinyi and Kwanashie 2009), anti-microbial (Mohamed et al. 2009; Svensson et al. 2010) and anthelminthic (Iqbal et al. 2001) activities. While this list of activities is compelling, it is not possible in this review to provide a detailed evaluation of the evidence for each of these claims. Consumption of whole-grain sorghum is reported to lead to lower plasma glucose in diabetic individuals compared to the consumption of whole wheat and decorticated sorghum grain (Lakshmi and Vimala 1996; Thompson et al. 1984).

The potential health effects of sorghum grain are not just due to the content of phytochemicals. Non-tannin sorghum lipid extract was found to lower the level of cholesterol absorption (in a hamster model), which might be due to the presence of policosanols (waxy, high-molecular-weight aliphatic alcohols with chain-lengths typically 24- to 34-carbons) and sterols in the grain lipid fraction (Carr et al. 2005).

Properties of sorghum grain and food quality

Protein and starch digestibility

Sorghum is often regarded as a low-value crop for food uses due to low digestibility of its protein and starch relative to other cereals (Duressa et al. 2018). Duodu et al. (2003) reported that the low digestibility of sorghum protein is most pronounced in wet cooked foods where protein digestibility decreases by up to 50%.

The different digestibility of proteins among sorghum cultivars is likely to be related to many factors such as differences in genotype, grain structure, protein body structure, protein cross-linking, and the complement of phenolic compounds (Dunn et al. 2015; Kaufman et al. 2013; Lemilioglu-Austin 2014; Mertz et al. 1984; Moraes, Queiroz, et al. 2012; Wong et al. 2009).

Regardless of protein digestibility, starch digestibility in raw and processed sorghum grain is also low. This might in turn be related to starch and protein cross-linking or binding of starch to tannin, which could limit accessibility of starch granules to amylolytic enzymes (Barros, Awika, and Rooney 2012; Dunn et al. 2015; Ezeogu, Duodu, and Taylor 2005; Lemlioglu-Austin et al. 2012; Taylor & Emmambux, 2010; Wong et al. 2009a; Zhang and Hamaker 1998).

It is common to consider low starch digestibility as a negative attribute; however, since many companies employ energy- and resource-intensive processes to create resistant starches, sorghum-based foods with this property may be valuable in countries with a high incidence of obesity (Awika and Rooney 2004).

Health benefits

The chemical composition of sorghum grain is comparable with that of maize, and its nutrient profile is similar to that of wheat (Dicko et al. 2006). The nutrients in sorghum grain have a positive role in human health and nutrition, especially in people suffering from disorders such as celiac disease, diabetes and obesity (Pontieri et al. 2013).

Consumption of sorghum-based food products might help control the body’s glycemic response, leading to a lower risk of metabolic diseases such as diabetes (Zhang and Hamaker 2009). A proportion of the starch in sorghum could also act as resistant starch (dietary fiber), lowering the incidence of some diseases such colon cancer and diabetes (Birt et al. 2013). Sorghum-based food products, including bread, biscuits, porridge, pasta and pastry, are excellent for sufferers of Celiac disease due to the absence of gluten in the grain protein profile (Lemilioglu-Austin 2014).

Sorghum offers exciting opportunities as a food ingredient and source of compounds that can be used to enhance the health properties of foods. The array of phenolic compounds in sorghum are not commonly found in other cereal grains (Awika 2011; Awika and Rooney 2004). Sorghum polyphenols have higher potential to suppress cancer cell proliferation than their analogs found in other food sources (Awika 2017). Yang et al. (2015) reported that phenolics and other compounds in sorghum have important and unique bioactive properties relevant to cancer prevention, cardiovascular health, and reduced chronic inflammation and oxidative stress, among others.

Recent evidence also suggests polymeric sorghum polyphenols (tannins) may be useful as natural ingredients to reduce the caloric impact of starch (Amoako and Awika 2016). The starch and protein in sorghum take a longer time to digest than other similar products. This slow digestion results in low glycemic index and is thus particularly helpful for individuals with diabetes (Hargrove et al. 2011; Zhang and Hamaker 2009).

Monomeric flavonoids in sorghum have been reported to have varying levels of anti-inflammatory activity (Moraes, Natal, et al. 2012). It also reduces the risk of or delay the development or recurrence of cancer (Park et al. 2012). Sorghum extracts have anticancer bioactivities higher than other grains like wheat, millet, and panicum, indicating that sorghum can serve as an effective and inexpensive edible supplement in cancer management (Xie et al. 2019).

Sorghum is rich in potassium and phosphorus. It has good levels of calcium with small amounts of iron and sodium. It is also high in fiber and protein, making it one of the healthier options for those who cannot consume gluten and therefore cannot eat traditional forms of flour. Malted sorghum flour can be used to make anti-constipation drinks and formulations of other foods. Lawan et al. (2020) tested malted and milled sorghum cultivars (pelpeli white and chakalari white) to make different formulations with cowpea. They reported that a ratio of 70:30 sorghum:cowpea (Pelembe, Erasmus, and Taylor 2002) was associated with a slight increase of vitamins in the malted formulations than in formulations without malt. The increase or decrease in vitamins varied with the nature of the raw material, processing methods and supplementations.

Sorghum is a also a good source of vitamins. Egbujie and Okoye (2019) reported that complementary foods formulated with sorghum, yam bean and crayfish had higher levels of ascorbic acid, niacin, thiamin, vitamin A and folic acids than the non-substituted controls. The increase in the vitamin content of the formulations confirms the beneficial effect of complementation.

Eze et al. (2020) reported that when extruded healthy snacks were made from sorghum and charamenya flour blends, extrusion affected vitamin and antinutrient content. The content of vitamins (A, B₁, B₂, B₆, B₁₂, C, and D) and antinutrients (phytate, oxalate and tannin) was reduced post extrusion when compared with the flour blends prior to extrusion. They were significantly (p < 0.05) affected by the feed composition (10–30%), feed moisture content (25–30%) and exit barrel temperature (110–130 °C).

Sorghum-based products

Research on the pasting quality of sorghum indicates that a high setback viscosity is advantageous for porridge making while a low setback is advantageous for bread making (Shin 1986). Microscopy showed that waxy grain (high in amylopectin) had larger starch granules with less protein than non-waxy grains (Shin 1986).

The main reason that sorghum based-food products are not in favor (globally) compared to other grain products is due to their inferior sensory qualities. For instance, foods based on tannin sorghum have a bitter and astringent taste (Khan et al. 2014; Kobue-Lekalake, Taylor, and De-Kock 2007; Rose et al. 2014). Sorghum containing tannin is reported to have a relatively soft endosperm texture, while tannin-free sorghum is reported to have a hard endosperm with sweet and maize-like flavor (Kobue-Lekalake, Taylor, and De-Kock 2007). Decortication of sorghum grain to make sorghum flour is reported to be preferred by consumers because of the lower astringency, lighter color of the food products, and higher digestibility (Taylor and Dewar 2001). However, decortication reduces the level of healthy secondary compounds such as phenolics (Buitimea-Cantúa et al. 2013; Chiremba, Taylor, and Duodu 2009; Dlamini, Taylor, and Rooney 2007).

Sorghum is the basis for relatively few food products in the Western world. For example, in contrast to oat, rice, maize and wheat, sorghum grain has not been widely used in the production of ready-to-eat breakfast cereals (Serna-Saldivar and Rooney 1995; Welch 2011). There are few research publications on producing ready-to-eat sorghum-based foods that have indicated positive feedback from consumers. In a study comparing whole-grain sorghum flakes to wheat bran flakes, consumers reported a better texture and almost the same flavor scores for the sorghum-based product (Shin 1986). In other research, a product made from flaked sorghum was compared with rolled oats. The panelists favored the sorghum-based product over the oat-based product (Celis, Rooney, and McDonough 1996). Dlamini, Taylor, and Rooney (2007) described the production of extruded sorghum cereals from both tannin-based sorghum (reported to have high antioxidant activity) and non-tannin sorghum. The results indicated that processing in general could decrease the antioxidant activity of sorghum. However, this antioxidant level was lower in extruded sorghum than in conventionally cooked porridge. A combination of sorghum/corn/defatted soy flour combined with whey protein has been shown to improve the sensory qualities of cereal products (Devi et al. 2013).

A sorghum-cowpea composite has been developed to make an instant porridge (Pelembe, Erasmus, and Taylor 2002). The authors found that a mixture of 50% sorghum and 50% cowpea extruded at 130 °C had functional properties closest to those of a commercial instant maize–soya composite porridge. In another study, cookies made from a combination of cocoyam flour and fermented sorghum flour (50:50) had a high ‘biological value’ (the efficiency of utilization of absorbed nitrogen) and net protein utilization (a measurement of biological value and digestibility of amino acid mixture absorbed from food) (Okpala and Okoli 2014).

Production of a low-digestibility pasta that could maintain adequate cooking quality and consumer acceptability was the aim of a study by Khan et al. (2014). Results showed that adding 20% red sorghum flour or 30% white sorghum flour to pasta could reduce starch digestibility without major impacts on preset criteria (the pasta was considered to be acceptable if the mean sensory score for overall acceptability was equal to or higher than 6.0, corresponding to “like slightly” on the 9-point hedonic scale). In another study, incorporation of 40% sorghum flour into flat wheat bread significantly decreased in vitro starch digestibility while maintaining sensory acceptability (Yousif, Nhepera, and Johnson 2012).

Processing of sorghum grain

Milling

Milling is the process of grinding, crushing, or pulverizing grains into fine particles with the aim of maximizing the yield of endosperm flour (Taylor and Dewar 2001). The kernel size of sorghum is similar to wheat and the pericarp of sorghum is more friable than other cereals (Zhao and Ambrose 2018). The friable nature of the sorghum pericarp is probably related to its unique starch (Earp, McDonough, and Rooney 2004). The thickness of the pericarp, which is determined by the mesocarp, affects milling properties. Since the pericarp breaks at the mesocarp layer, a thick pericarp holds together better and is removed in larger pieces during dehulling, thus requiring less decorticating time (Zhao 2016).

Sorghum kernels are primarily decorticated and milled into flour, or flaked for further processing. Milling sorghum to flour is not as well developed as wheat and traditional milling methods are still used in many countries (Dahir et al. 2015). Milling is performed mainly by using a roller mill (similar procedure used for wheat and corn), producing a gritty and speckled flour (Zhao 2016).

Dry milling

Methods of sorghum dry milling include various combinations of milling processes. They can involve removing the bran and germ and then reducing the endosperm to fine particles, breaking the kernel and then scraping the endosperm from the bran, or using abrasive decortication to remove the bran, followed by hammer milling. Removal of the germ from sorghum grain to give a clean endosperm is difficult in all milling methods (Shin 1986), and failure to remove the germ can substantially reduce the shelf life of sorghum flour as compared to wheat flour.

In grain processing generally, a large germ in proportion to the whole seed is potentially problematic because of the high fat content of the germ, leading to rancidity and thus a relatively low shelf life of the milled product if the endosperm flour is contaminated with germ.

In addition, the sorghum pericarp is highly fragile and, since most of the sorghum polyphenolic compounds (some of which are colored) are located in the pericarp, use of the milled product in foods can lead to an appearance undesirable to the consumer (Taylor and Dewar 2001).

Application of the breaking kernel method in sorghum flour production has adverse consequences on the final flour. As mentioned earlier, sorghum bran is highly fragile, and breaking the kernel can result in discoloration of the flour. Using decortication + milling is the most common method in sorghum milling. It involves abrading off the testa and pericarp, thereby reducing unfavorable discoloration due to polyphenolic compounds as well as the antinutrient impact of tannin on the flour (Serna-Saldivar and Rooney 1995). In an attempt to overcome these problems, sorghum grain was conditioned to reach 26% moisture at 60 °C for more than 6 hours. The results showed a good level of bran and germ separation but still a low flour yield (Cecil 1992). In other research, conditioning to about 16% moisture followed by milling using double rollers gave a higher yield and lower germ contamination in sorghum flour (Gomez 1993).

A study on dry grit milling on the milling performance of various sorghum grains from floury to corneous endosperm was performed by Maxson et al. (1971). The floury endosperm grain shattered in the milling process, resulting in a low yield of grit and a high level of lipid, whereas the corneous sorghum grain gave the highest yield of grit with lower levels of ash and lipid.

Wahyuningsih et al. (2019) reported that when dry and wet milling methods were compared, dry milled sorghum flour had a higher protein concentration than wet milled sorghum flour, except for the cross-linked kafirin fraction. Dry milled sorghum flour had more cross-linked kafirin fraction, whereas wet milled sorghum flour only had a small number of protein fraction ribbons from the cross-linked kafirin.

Wet milling

Clean sorghum grain can be soaked in water for about 2 days and then crushed to loosen the embryo. The starch/protein suspension is then centrifuged to separate the protein from the starch. The separated starch is then washed, and either dried for food production or hydrolyzed to make glucose sirup (Shin 1986).

The aim of wet milling is mainly to separate and isolate various grain components such as protein, starch and fiber from grains. Wet milling is mostly practised for wheat and maize, and tends not to be applied to sorghum due to difficulties in further processing to separate clean and pure starch, protein and oil (Taylor and Dewar 2001).

Amoura, Mokrane, and Nadjemi (2020) reported that when whole sorghum grain flour was wet milled, the protein content and water binding capacity decreased. The authors also reported that gelling concentration, oil binding capacity and the foam capacity increased but in the meantime foam stability and emulsifying stability decreased in a dough made from sorghum flour.

Malting

Malting can be defined as the process of steeping, germination and drying (kilning) of cereal grains to advance the development of hydrolytic enzymes (responsible for converting starch into simple sugars, and other hydrolytic activities), which are absent in un-germinated grains (Taylor and Dewar 2001). During steeping, grain moisture content reaches around 45% (Contreras-Jiménez et al. 2019; Vinje, Duke, and Henson 2015). In germination, metabolism is very active, and enzymes such as hemicellulases, amylases, proteases, oxidases, among others, are involved. The germination process is stopped by kilning. This stage is key to stop the metabolic activity and maintain the quality characteristic of the malt, including the flavor, aroma, and obtaining a malt with a long shelf life (Vinje, Duke, and Henson 2015).

Malt (particularly from barley) acts as a crucial ingredient with multiple functions in many beverages and food products, including as a dough leavening agent; in browning of bakery crust; softening and moisture retention of bakery crumb; color, aroma and flavor enhancement; texture improvement and nutrition enrichment, as well as in commercial beverages products like Milo® and Ovaltine®, which are a dry blend of malt extract and milk powder, and Horlicks®, a blend of malt extract, milk powder and wheat flour mashed together and dried (Joe White Malting System 2002; Taylor and Emmambux 2008).

The malting process has been shown to result in higher in vitro sorghum protein digestibility (Wang and Fields 1978). The addition of malted sorghum flour to thick porridge could help to break down the complex carbohydrates present and thereby liquefy the porridge. This in turn could enhance vitamin and mineral availability, as well as the sweetness and flavor of the porridge (Nout et al. 1988).

Malting has an impact on the phytochemicals in sorghum, which in turn has an influence on the possible health effects of the finished product. Mixed results have been obtained on the effects of malting on the abundance of phenolics in sorghum grain, although most studies have found a decrease (Ahmed, Mahgoub, and Babiker 1996; Bvochora et al. 2005; Bvochora et al. 1999; Elmaki et al. 1999; Kayodé, Hounhouigan, and Nout 2007; Khoddami 2016; Nwanguma and Eze 1996; Taylor and Duodu 2015; Towo, Svanberg, and Ndossi 2003).

Oseguera-Toledo et al. (2020) reported that during malting of sorghum grain, the amount of reducing sugars increased due to the enzymatic action in amylose and debranching of amylopectin, decreasing the viscosity. Djameh et al. (2015) reported that germination time significantly influenced diastatic power, free amino nitrogen and yield of malt. It was found that optimal conditions for the malting process were 12 h steeping, 5 days of germination at 30 °C and drying at 40 °C.

Fermentation

Fermentation is a process in which a range of food properties are changed by the activities of lactic acid bacteria, a combination of both yeast and lactic acid bacteria, and in some cases specific species of molds. Fermented sorghum products comprise flatbread, doughs, dumplings, porridges, gruels, as well as alcoholic and nonalcoholic beverages (Anukam and Reid 2009; Elkhalifa, Ali, and El-Tinay 2007; Kalui, Mathara, and Kutima 2010; Nout 2009; Taylor and Duodu 2015).

Several authors have reported that fermentation processes have an impact on the levels of phenolic compounds and the functional properties of sorghum. Adebo, Njobeh, and Kayitesi (2018) reported a decrease in the level of phenolic compounds after fermentation. The drop in the level of these compounds could be due to polyphenol oxidase and peroxidase activities, which degrade polyphenolics, cleavage of proanthocyanidins into flavan-3ols, which are then oxidized to quinones, or polymerization of or interaction with proteins (Starzyńska-Janiszewska et al. 2019; Beta et al. 2000; Dlamini, Taylor, and Rooney 2007; Porter, Hrstich, and Chan 1985; Scalbert et al. 2000; Towo, Matuschek, and Svanberg 2006).

A decrease in total phenolic content and total tannin content has been associated with fermenting sorghum, with this effect attributed to the ability of tannins to bind with proteins and other components, reducing extractability as well as tannin degradation (Adebo, Njobeh, and Kayitesi 2018; Dlamini, Taylor, and Rooney 2007). In another study, treatment of tannin sorghum gruels by adding germinated sorghum flour (to 5%) and then fermenting the gruels resulted in a significant decrease in the level of both polyphenolics and phytates, with an increase in the level of accessible iron, compared to the untreated and unfermented gruels (Taylor & Duodu, 2019). The reduction in polyphenolics and phytates and the increase in iron accessibility were greater when the mixture was incubated with phytase and polyphenol oxidase (Towo, Matuschek, and Svanberg 2006).

Fermented sorghum products are microbiologically safe due to low pH (<4) preventing the growth of gram-negative pathogens such as Escherichia coli, Campylobacter jejuni, Shigella flexneri and Salmonella typhimurium. Moreover, the fermentation could inhibit the growth of gram-positive Staphylococcus aureus (Kingamkono et al. 1995; Sekwati-Monang and Gänzle 2011; Svanberg et al. 1992) to prevent infection that could cause serious diseases such as pneumonia.

Sorghum fermentation can have both probiotic and prebiotic activity. For instance, fermented sorghum sourdough inoculated with the probiotic gram-positive bacterium, Weisella cibaria, produced substantial levels of prebiotics (carbohydrates that cannot be digested by human enzymes), which can stimulate ‘good’ bacteria in the colon (Schwab et al. 2008).

Ojha et al. (2018) reported that fermentation of sorghum grain reduced bulk density, viscosity and antimicrobial factors in sorghum flour while its functional properties were improved compared to the unfermented control.

Thermal processing

One of the most common methods to process cereal grains is thermal processing, which can be performed by wet, steam and extrusion cooking, micronization (the process of reducing the average diameter of a solid material's particles), gun or oven puffing, baking and roasting (Niernberger and Taylor 2001; Taylor and Duodu 2015) and recently the use of microwave heat treatment (Sharanagat et al. 2019; Adebowale, Taylor, and de Kock 2020).

White non-tannin and red tannin sorghums have been compared to oats as main ingredients of breakfast cereals and processed using a twin screw extruder. The red sorghum had a higher level of protein while the white sorghum had more starch (p < 0.05). The red sorghum protein digestibility dropped compared to the oat reference and white sorghum grain, while the level of resistant starch was higher in the white sorghum than the red sorghum and oat cereal product (Mkandawire et al. 2015).

Al-Rabadi et al. (2011) reported that particle size and extrusion temperature had a significant impact on processing parameters including specific mechanical energy. Licata et al. (2014) reported for extruded sorghum-maize composite flour (red sorghum grain, of a tannin-free variety, ‘Alpha’) that slowly digestible starch increased with increasing sorghum level and decreased as the barrel temperature increased. The barrel consisted of four independent zones, electrically heated and cooled by water. The barrel temperature zone profiles used were 90/95/100/120 °C and 90/105/125/150 °C.

Duodu et al. (2002) reported that when a cooking process was performed in sorghum and in maize, a lower digestibility of the protein was observed in sorghum due to the formation of enzyme-resistant, disulfide-bonded oligomers. In another study, application of high-temperature, high-pressure extrusion led to protein-polyphenolic interactions that resulted in lower extractability of phenolic compounds (Dlamini, Taylor, and Rooney 2007). Therefore, phenolic compounds (mainly tannins) may interact with proteins to form a complex, which may lead to lower digestibility (Duodu et al. 2002, 2003; Taylor et al. 2007).

Phytate may also be another factor reducing sorghum protein digestibility (Carnovale, Lugaro, and Lombardi-Boccia 1988). Sorghum starch and protein bodies are closely associated with each other as large spherical protein bodies surrounding starch granules. By cooking and gelatinizing the starch, the accessibility of enzymes to the protein bodies is reported to be reduced, resulting in lower protein digestibility (Shull et al. 1990). Other researchers also postulate that low protein digestibility might be due to a significant increase in the level of resistant starch in cooled cooked porridge compared to sorghum flour. The authors assumed that protein may form a complex with resistant starch, making the protein less available for enzyme attack (Knudsen et al. 1988).

Arbab and El Tinay (1997) studied the impact of adding reducing agents (sodium bisulfite and ascorbic acid) on cooked and un-cooked low-tannin and high-tannin sorghum. The results showed an increase in in vitro protein digestibility with the addition of reducing agent; however, the high-tannin sorghum had lower digestibility than the low-tannin sorghum.

The effect of heat treatment on sorghum flour used to make sorghum-based gluten-free bread and cake was studied by Marston, Khouryieh, and Aramouni (2016). For both products, dry heating the sorghum flour before application for processing resulted in a greater number and volume of cells per slice area than in the unheated control. In addition, the acceptability scores for both the bread and cake made with heat-treated sorghum flour were higher than the control samples.

Benhur et al. (2015) reported that when different proportions of sorghum and wheat semolina were used to make pasta, applying a low temperature short-time (LTST) extruder process, optimal sensory tests and nutritional data were obtained when rates of 50/50, 60/40 and also 70/30 (sorghum/wheat semonila) was used.

The effects of extrusion temperature and grit moisture content on the physico-chemical and textural properties of whole-grain red sorghum have been investigated (Llopart et al. 2014). Water sorption and water solubility of the extrudate were shown to have a direct relationship to temperature and an inverse relationship to moisture. The results indicated that the highest degree of cooking was obtained at 200 °C with 14% moisture; however, the product that showed the greatest expansion properties was from extrusion at 182 °C with 14% moisture.

The impacts of sorghum milling extraction rate (60, 80 and 100%), pin milling (no pin milling, low and high speed) and particle size of flour on the quality of gluten-free bread have been investigated by Trappey et al. (2015). Higher specific volume, lower crumb firmness and better crumb properties were found for the 60% extraction rate compared to the higher rates.

Thermal processing methods have been reported to either increase or decrease the level of phenolic compounds and antioxidant activity in sorghum grain products. The variation between studies in the changes in levels of phenolic compounds and antioxidant activity after thermal processing are mainly due to polymerization and oxidation, thermal degradation, depolymerization, release of bound phenolics or the formation of Maillard reaction by-products (Awika et al. 2003; Chiremba, Taylor, and Duodu 2009; Dlamini, Taylor, and Rooney 2007; Emmambux and Taylor 2003; Itagi et al. 2012; Towo, Svanberg, and Ndossi 2003; Wu et al. 2013).

Morais Cardoso et al. (2014) studied the impact of dry and wet heating on sorghum antioxidant activity, vitamin E and phenolic compounds. Dry heat increased the level of vitamin E and reduced the abundance of carotenoids but had no impact on phenolic compounds, while wet heating resulted in a decrease in the levels of all antioxidants except the carotenoids.

The operating parameters for microwave puffing to produce a sorghum-soy ready-to-eat snack (90/10) have been optimized (Pawar et al. 2014). The optimal process was to apply convective heating at 210 °C for 240 s, then microwave heating at 80% power using a 1350-W oven for 60 s. In another study, the effects of moisture level, microwave power and time on puffed sorghum quality were that the grain with 21% moisture exposed to 100% microwave power for 3 min had the highest puffing yield, expansion ratio and flake size volume (Sharma, Champawat, and Mudgal 2014).

Possible future trends for sorghum

There are general challenges that influence the future production, demand, and use of sorghum as compared to other cereal crops such as corn, wheat and barley. One of the main challenges for farmers around the globe is adaptation to climate change and its impact on crop farming and production. Sorghum has high tolerance to drought and heat compared to many other kinds of cereal. Therefore, the selection of sorghum to grow in regions that are experiencing an increase in drought and heat will probably be a valuable option (Akinseye et al. 2020). To help farmers and food producers to have access and grow sorghum cultivars that give the best yield with high crop nutritional quality, several different approaches have been taken into account.

Breeding new varieties of cereal crops that are tolerant to climate change is another approach. Therefore, research to explore and exploit sorghum genetic variation by looking at sorghum’s wild relatives that potentially survived for centuries under severe climate conditions is of interest to many researchers (Mace et al. 2021). Future research into the genetic and physiological mechanisms underlying drought and heat tolerance will potentially avoid adverse effects on early vegetative, pre-, and post-anthesis on sorghum growth, which lead to reductions in yield (Akman, Zhang, and Ejeta 2021; Chiluwal et al. 2018).

The other approach is to use or incorporate sorghum into a range of food products, including most staple foods, especially in developing countries where sorghum has a comparative advantage. This can potentially help to improve the nutritional values of the product and play a preventive role against several diseases such as diabetes (Sharma and Arora 2020).

The other recent approach is to use sorghum to make degradable packaging and films to replace synthetic polymers. Mehboob et al. (2020) reported research on cross-linking and/or acetylation of sorghum starch that can create edible films to pack food. Sorghum starch is also used as shell material in combination with gum arabic to suppresses the degradation of flavoring ingredients in nutmeg oleoresin (Arshad, Ali, and Hasnain 2020).

The most challenging approach is the commercialization of sorghum-based food products for nontraditional consumers in Western countries. Understanding the supply chain limitations and the reason behind poor collaborations between key stakeholders (i.e., researchers, food manufacturers, marketers, government agencies, dietitians and consumers) is necessary to increase sorghum awareness (Stefoska-Needham and Tapsell 2020).

Conclusions

Global climate variability, water scarcity, increased frequency of heat waves and longer periods of drought will require recognizing sorghum as a valuable and resilient crop alternative for sustainable food production. Progress has already been made to use sorghum whole grain in functional foods, as well as incorporating sorghum whole grain or components in foods to improve quality and functionality for nontraditional consumers. However, the work is still in the early stages and further research is required.

With the collaboration of researchers, farmers, food manufacturers, and other stakeholders, a successful new era for sorghum-based food products is achievable.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Grains Research and Development Corporation (GRDC) [grant number UCS00025].