1 Introduction

Food colorants play an important role in enhancing the aesthetic appeal of food. Synthetic colorants typically consist of bright colors, however, their use leads to a potential carcinogenicity occurring after their azoreduction to carcinogenic metabolites by intestinal microbiota [1, 2]. It is also shown that the synthetic colorants may contribute to the development of attention deficit hyperactivity disorder in children [3,4,5,6,7]. Consequently, the food industry is increasingly adopting for natural colorants as alternative to the synthetic colorant used in foods [2, 8].

Natural colorants used in food are obtained from renewable resources such as plants, microbes and insects [9]. Red sorghum (Sorghum bicolor) is a crop which provides good sources of nutrients and bioactive compounds [10, 11]. The leaf sheaths of red sorghum contain high and valuable bioactive compounds that are widely used as a natural colorant in processed foods in West Africa [12, 13]. These bioactive compounds exhibit several health-promoting effects [14,15,16,17,18]. In West Africa, the sorghum leaf sheaths are traditionally used in foods and as dyes [15]. Due to these benefits, the extraction methods of the pigments obtained from sorghum leaf sheaths should be standardized and these pigments can be presented in different forms in order to facilitate their access to the global market. The red color of sorghum leaf sheaths is associated with the presence of 3-deoxyanthocyanidin derivatives [16, 18,19,20,21]. These anthocyanidins are more stable colorant than the common anthocyanins found in fruits and vegetables [21]. Recently, Akogou et al. [21] proved that the watery extract of sorghum leaf sheath colorants is temperature-sensitive but showed further resistant to light.

Dye and food industries are looking for the production of suitable natural pigments’ sources due to the limited amounts of pigments in those sources. The leaf sheaths of red sorghum could be a suitable alternative thanks its stable pigments. The current review article aims at overviewing the taxonomic classification of red sorghum and the traditional uses, nutritional and bioactive compositions of red sorghum leaf sheaths. Furthmore, the stability of the pigments extracted from red sorghum leaf sheaths is highlighted and the prospectives are discussed.

2 Data collection methodology

An online thorough, comprehensive literature search was carried out by combining the relevant keywords, including pigments red sorghum, Sorghum bicolor, botanical characteristics, nutritional values, uses and economic values, bioactive compounds and stability. During the searching approaches “AND”/“OR” were also included. Regarding the search engine, Google, Google Scholar, ScienceDirect, Web of Science, Directory of Open Access Journals, Scopus, SpringerLink, Pubmed, and Wiley Online Library were independently scrutinized. In addition, articles published in both English and French from 1998 to 2024 were considered. The final search was performed on May 10, 2024. References were managed with Mendeley software.

3 Taxonomic classification and geographical distribution of red sorghum leaf sheath

Red sorghum, a plant native to Africa, is widespread in tropical and subtropical regions of Asia, America and Europe [22, 23]. Its wide geographical distribution is explained by its ability to adapt to a variety of climates and soils [24, 25], and it is cultivated in many parts of the world for its grains, stalks and leaves. Among the less valued parts of the plant, red sorghum leaf sheaths are attracting increasing attention for their content of stable natural pigments [13, 19, 26, 27]. These pigments, mainly anthocyanidins and flavonoids, show promising antioxidant, anti-inflammatory and coloring properties. First, we will explore the taxonomic classification and geographical distribution of red sorghum. This classification enables us to better understand its links with other grasses and to identify the different varieties of red sorghum, which vary in terms of grain color, plant size and disease resistance [28]. Secondly, we look at the traditional uses of red sorghum leaf sheaths. Red sorghum leaf sheaths have many traditional uses, which vary from one region of the world to another. These include human and animal food, traditional medicine and handicrafts [13]. Techniques for preparing and using leaf sheaths depend on the end use [29]. Finally, we turn to the prospects offered by red sorghum leaf sheath pigments. The prospects for red sorghum leaf sheath pigments are promising. Their antioxidant, anti-inflammatory and coloring properties offer potential exploitation in the food, cosmetics, textile and pharmaceutical industries [30]. Ongoing research is exploring new pigment extraction and purification technologies, as well as their application in various products [31]. This study aims to highlight the potential of red sorghum leaf sheaths as a source of stable natural pigments. By exploring the various aspects of this promising raw material, we hope to contribute to the sustainable development and increased valorization of this plant with its many assets.

3.1 Taxonomic classification

Sorghum is a monocotyledonous plant in the Poaceae family subfamily Panicoideae, tribe Andropogoneae [31]. This family comprises over 12,000 species in more than 700 genera, including wheat, rice, barley and maize [31,32,33,34,35]. The bicolor breed is often recognized as the first domesticated sorghum breed due to its relatively primitive characteristics [33]. Bicolor sorghums are distinguished by their small, elliptical kernels enveloped by tough glumes, generally with a high tannin content. Their panicles are often loose, reminiscent of wild sorghums, and they have a potentially high tillering capacity. In addition, many bicolor sorghum varieties have a sweet stem. There are special types, such as broom sorghums, paper sorghums and forage sorghums. They are found in all sorghum-growing areas, but generally on small surfaces [36]. Sorghum bicolor is the most widely cultivated species of the Sorghum genus. It is an important cereal plant, widely cultivated for its edible seeds, used for human consumption [37], animal feed [38] and bioethanol production. The red variety of sorghum (Sorghum bicolor var. rouge) is distinguished by the reddish color of its stems, leaves and sometimes even leaf sheaths. The leaf sheath, specifically, is an anatomical structure that surrounds the base of the leaf where it joins the stem. In red sorghum, the leaf sheath may be reddish or show shades of this color, according to the characteristics of the variety.

3.2 Geographical distribution

Red sorghum is native to Africa, more specifically to the continent's tropical and subtropical regions [39, 40]. Sorghum is thought to have first appeared in East Africa, in what is now Ethiopia and Sudan. From there, it spread across the African continent, adapting to a wide variety of climatic and soil conditions. However, while there is consensus on sorghum's African origin, its period of domestication is a matter of debate [40]. The cultivation of “Sorghum bicolor” began 8500 to 4000 BC [40, 41]. Over time, sorghum was introduced to many parts of the world, particularly in tropical and subtropical zones, where it adapted perfectly to a variety of environmental conditions. Today, red sorghum is grown in over 100 countries, mainly in tropical and subtropical zones [28]. The main exporting countries are the USA, Argentina, Australia and the main importing countries are China, Japan and Mexico [42]. According to USDA NASS [43], Africa's share rose from 32% of the global harvest to 37% in the 2016/2017 season. This was made possible by an increase in yields from 1.35 t/ha to 1.5 t/ha over one farming season. Major producing countries include the USA, India, China, Nigeria and Sudan [28] (Table 1). Sub-Saharan Africa is one of the most important regions for red sorghum cultivation. Nigeria is still Africa's leading sorghum producer with 6.5 million tons per year, followed by Burkina Faso (1.90 million tons), Mali and Niger (1.30 million tons) [33]. India is one of the world's largest sorghum producers, and red sorghum is widely grown. The states of Maharashtra, Karnataka, Andhra Pradesh and Telangana are among India's leading sorghum-producing states [25]. China is another major sorghum producer, where red sorghum is also grown. The northern regions of China, such as Hebei, Henan and Shandong, are important sorghum-growing areas [44,45,46]. The United States is the largest producer of sorghum in the Western world. Red sorghum is grown in states such as Kansas, Texas, Nebraska and South Dakota [43]. Brazil is the main sorghum producer in Latin America, and red sorghum is grown in various states, including Mato Grosso, Goiás and Minas Gerais. Red sorghum adapts to a wide variety of climates and soils, which explains its wide geographical distribution [44, 46]. It can be grown in arid and semi-arid regions, where other crops cannot survive [43, 45, 46].

Table 1 Top producing countries of red sorghum
Full size table

3.3 Red sorghum varieties

Red sorghum comes in a wide range of varieties, each distinguished by its morphological, agronomic and nutritional characteristics [27]. Among the main varietal characteristics is grain color, which can vary from dark red to light red, brown and purple. This variation is determined by anthocyanin pigment composition [47]. Plant size is also a variable aspect, ranging from 1 to 3 m in height, influenced by genetics and environmental conditions [28]. In addition, the precocity of red sorghum, whether early (short cycle of 11 weeks) or late (long cycle of 17 weeks), is of crucial importance for its adaptation to different growing zones [28]. Some varieties are more resistant to leaf diseases such as rust and anthracnose [29]. Finally, the content of nutrients such as proteins, lipids and dietary fiber can differ significantly from one variety to another [48].

3.4 Environmental factors influencing red sorghum growth

Red sorghum growth is influenced by various environmental factors, the most important of which are climate, soil, light, and pests and diseases. Climate plays a crucial role, as red sorghum is a tropical and subtropical plant that adapts to a wide range of temperatures (from 10 °C to 40 °C) and rainfall (from 400 to 1200 mm per year) [45, 46]. However, it is sensitive to frost and excess water [45]. Recent studies have also highlighted the impact of climate change on sorghum distribution and productivity, underscoring the need for agronomic adaptations to maintain production stability in the face of current climatic challenges [49]. For instance, MacCarthy and Vlek [49] reported a 20% reduction in grain yield as a result of the effect of climate change. Furthermore, Yan et al. [46] found out that the red sorghum growth duration was highly influenced by the ecological type, accounting for 87.2% of its total variance. Secondly, soil type has a significant influence on red sorghum growth. Preferring well-drained, fertile soils with a pH between 5.5 and 8.0, it can nevertheless tolerate poor, acidic soils, making it an interesting crop for marginal areas. Recent research has also highlighted the importance of soil health for sorghum productivity, highlighting the benefits of sustainable farming practices in maintaining soil fertility and promoting optimal growth [45, 50]. For instance, MacCarthy and Vlek [49] reported that the yield of the red sorghum increased of 4% with the application of fertiliser of 40 kg N, 30 kg P ha−1. Studies have shown that light availability can affect not only growth but also the pigment content of red sorghum leaves [51]. Finally, pests and diseases represent a challenge for red sorghum cultivation, as it is susceptible to various pests such as stem borer caterpillar, sorghum aphid and rust [29]. Recent research is focusing on the development of disease- and pest-resistant varieties, as well as integrated pest management strategies to reduce yield losses and improve the sustainability of red sorghum crops [52].

4 Traditional usages of red sorghum leaf sheath

4.1 Techniques for preparing and using leaf sheaths

For human consumption, in Africa, the leaf sheaths of red sorghum are usually sun-dried before being processed into flour (Fig. 1). This flour can then be used to prepare porridges, cakes and sauces [29]. Red sorghum leaf sheaths are also used as an ingredient in traditional dishes such as couscous and tajine in North Africa. They can also be used for better preservation [21, 53]. In addition to porridges, patties and sauces [26], red sorghum sheath flour can be used to make pasta, bread and cookies. In Asia, red sorghum leaf sheaths can be eaten raw in salads or cooked as a side vegetable for their flavor and nutritional value [45, 46]. They can also be used to make tea and infusions. In traditional medicine, in Africa and Asia, red sorghum leaf sheaths can be used fresh or dried to prepare decoctions, infusions or poultices [54]. They have antioxidant, antimicrobial, anti-inflammatory and analgesic properties and are traditionally used to treat a variety of illnesses, including diarrhea, dysentery, malaria, respiratory infections and joint pain [26, 30]. In Africa, red sorghum leaf sheaths are generally woven into baskets, hats and mats [29]. They can also be used to build roofs and walls [29]. They can also be used to dye textiles and fibers red, orange or yellow. Fibers extracted from sheaths are used to produce durable fabrics and ropes. They can also be used as filling material in the manufacture of cushions and mattresses. The leaf sheaths of red sorghum can also be used to produce bioethanol, biogas and biodiesel, as well as fodder for livestock.

Fig. 1
figure 1

Different uses of red sorghum leaf sheath

Full size image

4.2 Properties and benefits of leaf sheath pigments

Red sorghum leaf sheaths contain a variety of natural pigments, including anthocyanidins, flavonoids and phenolic acids [26, 30], which give them their distinctive red color as well as health benefits. These pigments in the leaf sheaths of red sorghum have a higher antioxidant activity, displaying FRAP value of 350.0–1060.1 μmol/g, DPPH value of 78.1–86.8% and TEAC of 3760.6–5580.9 μmol/g [26]. They can thus protect cells against oxidative damage, responsible for aging and many chronic diseases [26]. The pigments including luteolinidin and apigeninidin in red sorghum leaf sheaths could inhibit the enzymes responsible for inflammation [30]. They can thus help reduce inflammation in the body and relieve the symptoms of chronic inflammatory diseases [30]. In addition, they possess antimicrobial properties that could inhibit the growth of bacteria and fungi [30]. They can thus help prevent infections and strengthen the immune system [30]. Studies have also demonstrated that thanks to the presence of 3-deoxyanthocyanidins and several other phenolic compounds in red sorghum leaf sheaths, they provided the ability to improve cardiovascular health by reducing cholesterol and blood pressure, as well as preventing diabetes and cancer by inhibiting the growth of cancer cells [26, 30, 55]. In addition to their medicinal properties, red sorghum pigments are also used in a variety of industrial applications, notably as natural food colorants and additives in cosmetics. The stability of pigments in red sorghum leaf sheaths makes them a promising material for the development of natural colorants in the food and cosmetics industries. This stability is influenced by several factors, including pH, temperature, light and the presence of oxygen. Pigment stability can be enhanced by microencapsulation and liposome encapsulation techniques.

4.3 Limits and potential risks associated with the use of leaf sheathing

The use of red sorghum leaf sheathing comes with certain limitations and potential risks to consider. As far as limitations are concerned, the availability of red sorghum leaf sheath is seasonal and can vary according to region and time of year [29]. In addition, the composition of pigments and other nutrients in the leaf sheath can be variable depending on sorghum varieties, growing conditions and harvesting and drying techniques [13, 26]. In addition, further research is needed to validate the beneficial properties and potential risks of the red sorghum leaf sheath. Such studies could include in-depth analyses of the chemical composition of red sorghum leaf sheaths under different growing conditions, as well as assessments of their stability and bioavailability. As for potential risks, it should be noted that red sorghum leaf sheaths may contain cyanides (0.46–1.01 mg HCN g−1), which are toxic substances in high doses [56]. It is therefore essential to prepare and consume the leaf sheath in moderation. Pre-treatment methods, such as soaking and boiling, can be used to reduce the cyanide content in red sorghum leaf sheaths prior to consumption [57]. In addition, cyanide testing should be carried out regularly to ensure the safety of products derived from red sorghum leaf sheaths intended for human consumption [57]. In addition, some people may develop allergies to the pigments contained in red sorghum leaf sheaths [26]. Allergic reactions can vary in severity and may include cutaneous, respiratory or gastrointestinal symptoms. Individuals with a history of food allergy are advised to consult an allergist before incorporating red sorghum leaf sheaths into their diet. Finally, it's important to consider possible interactions with medications, and it's advisable to consult a healthcare professional before consuming leaf sheaths if taking medication. Certain substances present in red sorghum leaf sheaths may interact with certain drugs, affecting their efficacy or leading to adverse effects. Particular attention should be paid to interactions with anticoagulant, hypoglycemic and anti-hypertensive drugs, as the bioactive compounds in red sorghum leaf sheaths could potentiate or inhibit the effects of these drugs [57].

5 Chemical composition of sorghum leaf sheath

5.1 Nutritional composition

Sorghum grain is the most important edible part of sorghum plant, highly used for food and feed applications due to its high nutritional value (Table 2). Interestingly, the leaf sheath of sorghum plant contains significant high nutritional value which may be similar, higher or slightly lower to the one of the grains, depending on the nutritional component (Table 2). The leaf sheaths contain higher level of all the reported individual minerals and certain vitamins such as vitamin C and vitamin B6 than that of the grains (Table 2). The majority of fat present in the leaf sheaths may be comprised of omega-3 and omega-6 fatty acids in the ratio of 1:2 as reported by Abugri et al. [15, 58]; corresponding to the recommended level required for optimum nutritional and anti-inflammatory benefits [59].

Table 2 Nutritional content of sorghum leaf sheaths and sorghum grains
Full size table

Due to its significant nutritional value, the leaf sheaths are used as ingredient to improve nutritional quality of foods, as food coloring additive in cooking meals or as medicinal beverage through infusion [60, 61]. For instance, the sorghum stem sheath beverage known in Nigeria as Poporo and in Benin as Kpokpo drink is reported to have low content of crude protein (0.005–0.047%) and crude fat (0.009–0.140%), while exhibiting high mineral contents including magnesium (145–200 mg/100 ml), sodium (25.00–25.20 mg/100 ml), potassium (23.03–26.68 mg/100 ml), calcium (50.00–54.54 mg/100 ml), iron (4.00–5.91 mg/100 ml), zinc (2.50–3.51 mg/100 ml) and copper (0.041–0.079 mg/100 ml) [62]. When prepared with Sorghum bicolor leaves, Waakye, a popular traditional cereal-based dish widely consumed in Ghana, depicted high content of essential fatty acids, polyphenols and antioxidant which could contribute to the health and biological activities [59].

5.2 Bioactive composition

The leaf sheath of red sorghum is a rich reservoir of phytochemical compounds. They include catechol tannins, phenolic acids, flavonoids, carotenoids, steroids, chlorophylls, cardenolides and coumarins [15, 17]. The leaf sheath contains higher concentrations of bioactive compounds including 28.89–1036 mg/mg, 0.20–0.36 mg/g, 0.99–11.78 µg/mg, 0.64–13.86 µg/mg and 194.50–995.72 mg/g for total phenolic, total flavonoids, total carotenoids, total chlorophyll and total tannins, respectively [13, 15, 58].

5.2.1 Anthocyanidins/Anthocyanins

Anthocyanidins are responsible of the red color of leaf sheath of red sorghum. Kayodé et al. [13] reported that the extraction parameters such as temperature and solvent content affect the extraction rate of the natural colorant pigments from leaf sheath of red sorghum as the apigeninidin content was three times higher in the cool and hot alkaline extracts than in the aqueous extract. The total anthocyanin content of leaf sheath extracts obtained with different extraction techniques such as cool alkaline, hot alkaline and hot aqueous extractions was found in the range of 43.9–228.5 µg/ml [12, 21]. Kayodé et al. [26] reported a total anthocyanin content ranged from 13.7 to 35.5 mg of cyanidin 3-glucoside equivalent/g of dry matter. The red sorghum contains large quantities of the rare 3-deoxyanthocyanidin which are pigments with the red–orange hues in mildly acidic to neutral solutions (Table 3) [19, 27]. Only 3-deoxyanthocyanidins including luteolinidin and apigeninidin are the major pigments in red sorghum [19, 27]. Awika et al. [27] detected luteolinidin and apigeninidin in the red sorghum bran (Fig. 2). The major pigment detected in the red sorghum leaf sheath is apigeninidin followed by apigeninidin-flavene dimer, apigeninidin-7-o-methylflavene dimer and 7-o-methylapigeninidin [19]. Moreover, Petti et al. [20] emphasized that luteolinidin (68%) is the dominant 3-deoxyanthocyanidin compound in the red sorghum leaf sheath, followed by apigeninidin (16%) and 7-o-methylluteolinidin (13%), respectively. They detected low concentrations of 7-o- and 5-o-glucosides and diglycosides of luteolinidin and apigeninidin, as well as 7-o and 5-o-glucosides and diglycosides of o-methyl derivatives of luteolinidin and apigeninidin. They added that low amounts dimers of luteolinidin and its 7-o-methyl derivative were also identified [20]. It has been shown that luteolinidin and its derivatives are more than more than 80% of total 3- deoxyanthocyanidins, whereas apigeninidin and its derivatives accounted for less than 20%. Similarly, many studies reported apigeninidin and luteolinidin as the predominant pigments in red leaf sheath of S. bicolor [16, 18, 19, 21].

Table 3 Phytochemical composition of the red sorghum leaf sheath
Full size table
Fig. 2
figure 2

Structure of the major anthocyanidins of red sorghum leaf sheaths, A apigeninidin and B luteolinidin

Full size image

5.3 Other phytochemical compounds

Phenolic acids, flavonoids and tannins are widely reported in red sorghum leaf sheaths (Table 3). Ferulic, tannic and p-coumaric acids are the major phenolic acids detected in red sorghum leaf sheaths with ferulic acid (186.0–10,369.0 µg/g) being the predominant phenolic acid [15]. Apart from anthocyanidins, other flavonoids including naringenin, apigenin and luteolin are widely reported in leaf sheath [15, 63]. Abugri et al. [15] reported luteolin (133.0–58,434.0 µg/g) and apigenin (9.1–3066.3 µg/g) as the major flavone while catechin gallate (323.8–2135.2 µg/g) and epigallocatechin (184.9–1828.3 µg/g) are the major flavanols. Naringenin is a bioactive compounds of subclass of flavanone, recognized for its potential biological activities among them antioxidant, antitumor, antiviral, antibacterial, anti-inflammatory, antiadipogenic and cardioprotective effects [64]. Naringenin (130.0 µg/g) has been reported as the major flavanone in the red leaf sheath of sorghum [30].

6 Stability of pigments of red sorghum leaf sheath

Recently, anthocyanin derivatives attracted considerable interests mainly due to their important sources of natural pigments primarily used as food colorant and their significant health benefits [2]. Anthocyanidin derivatives extracted from red sorghum leaf sheaths are extensively used in coloring various food products such as cheese, biscuits, yogurt, porridge, waakye and other [12, 26, 65]. Pigments of red sorghum leaf sheaths are more resistant than the common anthocyanins to many factors as the lack of substitution at C-3 confers the 3-deoxyanthocyanins greater resistance to nucleophilic attack of the flavylium cation (Fig. 3) [66, 67]. A study reported that the sorghum pigment extract was very stable in the presence of ascorbic acid where this latter induced only 31% color loss at pH 2.0, but the color intensity increased by 1.9- and 1.3-fold at pH 3.2 and 5.0 respectively; in contrast, the color of red cabbage pigment has been lost up to 85% in the presence of ascorbic acid [66]. Sorghum derived 3-deoxyanthocyanidins were subjected to the microwave-assisted extraction (MAE) and they remained structurally stable to MAE conditions up to 1200 W/100 °C/30 min [67]. Complexation of 3-deoxyanthocyanidins with copigments such as tannic acid, ferulic acid, O-coumaric acid and rutin did not produce the striking hyperchromic shift typically observed with anthocyanins, likely because the colored species are predominant in the 3-deoxyanthocyanidins in the entire acidic to neutral pH range [65]. In the same study, it has been found that these copigments improved significantly the color stability of the 3-deoxyanthocyanidins in the presence of light, suggesting that the flavylium cation has been protected against photoexcitation or nucloephilic attack [66]. Similar findings have been reported when tannins have been used copigment in sorghum extracts, resulting in an increasing of recovery of 3-deoxyanthocyanidins, color intensity and stability [68]. In addition, Yagishita et al. [69] discovered that 3-deoxyanthocyanidins, specifically luteolinidin, apigeninidin and tricetinidin showed photochromic properties. Thermal stability tests of crude and pure 3-deoxyanthocyanidins revealed that the color retention (79–89% after 95 °C/2 h and 39–118% after 121 °C/30 min) was high compared to literature reports for anthocyanins under similar treatments [70]. According to Kayodé [13], anthocyanidins extracted from red sorghum constitute a natural alternative to synthetic colorants, providing higher stability to light and heat. Although 3-deoxyanthocyanidins are more stable than other common anthocyanins, they are affected by pH [70] and sulfites which cause the bleaching of 3-deoxyanthocyanidin-rich extracts [71].

Fig. 3
figure 3

Stability of pigments of red sorghum leaf sheath

Full size image

7 Future prospectives

7.1 Exploitation potential of red sorghum leaf sheath pigments

The pigments present in the leaf sheath of red sorghum offer a multitude of potential uses in various sectors. Red sorghum leaf sheaths play an important role in food security. As an integral part of the sorghum plant, they represent an additional source of nutrients and fiber in human and animal nutrition. Color is an important factor contributing for the food choices since it enhances the appeal towards foods, thus influencing preference, pleasantness and acceptability of food products [72]. The pigments can be used as natural colorants for foods and beverages, providing a natural alternative to artificial colorants with better stability to light and heat [13]. What's more, due to their richness in antioxidants and other nutrients, they can also be used to enhance the nutritional value of foods [26]. Pigments extracted from the leaf sheaths of red sorghum can be used to color a variety of foods, including baked goods, dairy products, beverages and confectionery [73]. They are also of growing interest in medical and pharmaceutical research. Red sorghum is known for its health-promoting properties, notably its high content of antioxidants and bioactive compounds. The antioxidant properties of red sorghum leaf sheath pigments make them interesting for the development of drugs and dietary supplements [26]. Similarly, their anti-inflammatory properties can be exploited in the treatment of chronic diseases such as arthritis [30]. Extracts of red sorghum leaf sheaths are being investigated for their potential as active ingredients in the formulation of pharmaceutical products and dietary supplements aimed at improving joint health and combating oxidative stress [74]. This research is paving the way for the development of new plant-based drugs to treat various inflammatory conditions. In the field of cosmetics, these pigments can be used as natural colorants for cosmetic products [27], offering a safer alternative to artificial colorants. In addition, due to their antioxidant properties, they can be used in the formulation of anti-aging products [26]. Red sorghum pigment extracts are sought after for their ability to protect the skin against UV-induced oxidative damage, as well as for their potential to improve skin texture and elasticity [75]. These properties make them valuable ingredients in the formulation of natural, sustainable cosmetics.

7.2 Current research and innovations

In these last decades, significant studies have been carried out in order to figure out the various possibilities for exploiting the pigments present in the leaf sheath of red sorghum. Significant efforts are being made to develop new natural food colorants from these pigments, offering a safer alternative to artificial colorants that are potentially harmful to health [13]. This research is crucial in a context where demand for natural colorants is increasing due to growing concerns about synthetic food additives and their effects on health [76]. Pigments extracted from the leaf sheaths of red sorghum can be used to color a variety of foods, including baked goods, dairy products, beverages and confectionery, meeting the food industry's demands for naturalness and product safety [13]. At the same time, researchers are working to perfect the extraction and purification processes for these pigments, aiming to make these techniques more efficient and economically profitable [26]. Advances in extraction methods, such as the use of innovative technologies like ultrasound and microwaves, are being explored to improve the yield and quality of pigments extracted from red sorghum leaf sheaths [77]. In addition, studies on the use of non-toxic, environmentally friendly solvents in the extraction process are underway to reduce the environmental impact of producing these natural dyes. In addition, there is active research into the development of dietary supplements and drugs exploiting the antioxidant and anti-inflammatory properties of red sorghum leaf sheath pigments [30]. These efforts aim to capitalize on the potential health benefits offered by natural pigments, particularly in the context of preventing chronic diseases such as cardiovascular disease and cancer [78]. Dietary supplements based on red sorghum pigments are being studied for their ability to boost the immune system and protect against oxidative stress, thus offering natural solutions to improve health and well-being. Finally, particular attention is being paid to the valorization of sorghum co-products, such as leaf sheaths, with the aim of creating high value-added products, opening new prospects for industry and agriculture [19]. These co-product valorization initiatives help reduce food waste and create new economic opportunities for farmers and agri-businesses [79].

7.3 Ways for the sustainable development of leaf sheath pigment exploitation

Despite the promising prospects offered by red sorghum leaf sheath pigments, several challenges stand in the way of their sustainable exploitation. Firstly, the availability and variability of the raw material is a major obstacle. As leaf sheaths are a seasonal resource, their composition can vary according to sorghum varieties, growing conditions and harvesting and drying methods [29], which can restrict both their availability and their quality for industrial use. These seasonal and geographical variations require adjustments in collection and processing strategies to ensure adequate quality and continuity of supply. In addition, production costs associated with pigment extraction and purification processes can be high, which can compromise the profitability of their exploitation. Research aimed at optimizing extraction and purification methods, as well as the use of more efficient and less costly technologies, is needed to meet this challenge. Moreover, green ways combined with emerging technologies should be applied the for extraction of pigments from the red sorghum. In addition, the extracting bioactive compounds and pigments of leaf sheaths of red sorghum can be encapsulated in order extend the storage duration and to facilitate their uses in food products. Another challenge is the lack of sufficient scientific data to validate the beneficial properties and assess the potential risks associated with red sorghum leaf sheath pigments. Although preliminary studies have revealed promising properties such as antioxidant and anti-inflammatory activity, further research is needed to fully understand the therapeutic potential and possible side effects of these pigments [27]. In-depth toxicological studies are also needed to assess the safe use of red sorghum pigments in various food, cosmetic and pharmaceutical applications [80]. In addition, compliance with the regulations in force in each country concerning the use of natural pigments in food and cosmetics is an important consideration. Manufacturers must comply with food safety and cosmetics quality standards, which requires thorough documentation on the origin, composition and safety of the pigments used [81]. International regulatory and standardization efforts are also needed to harmonize guidelines and facilitate international trade in these by-products. Finally, the development of markets for products derived from red sorghum leaf sheath pigments is essential to ensure the economic viability of their long-term exploitation. Marketing and awareness-raising initiatives are needed to promote the added value of red sorghum pigment products to consumers, the food, cosmetics and pharmaceutical industries, and potential investors [82]. Further investigations need to be carried out to explore the optimum extraction conditions and to implement storage studies.

8 Conclusion

In conclusion, the leaf sheaths of red sorghum (Sorghum bicolor) represent a promising source of stable pigments with multiple beneficial properties. Ongoing research is exploring the potential for exploiting these pigments in various fields, such as food, nutraceuticals and cosmetics. Sustainable development of red sorghum leaf sheath pigments requires research and innovation to overcome existing challenges and obstacles. Collaboration between sorghum industry players, researchers and public authorities will be essential to guarantee the economic and environmental viability of this exploitation. The future perspectives for red sorghum leaf sheath pigments is promising. By adding value to this natural and abundant raw material, it is possible to develop innovative and sustainable products that meet consumer needs and contribute to the diversification of agricultural production.