Md. Forshed DEWAN,Md. AHIDUZZAMAN,2, Md. Nahidul ISLAM,2, Habibul Bari SHOZIB

(1Department of Agro-Processing, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur 1706, Bangladesh;

2Institute of Food Safety and Processing, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur 1706, Bangladesh;

3Grain Quality and Nutrition Division, Bangladesh Rice Research Institute, Gazipur 1701, Bangladesh)

Abstract: Traditional rice varieties have been widely cultivated and popularly consumed by Asian people for a very long time and have recently garnered increased consumer concern. Traditional or indigenous rice varieties are known to be rich in a wide range of bioactive compounds, particularly phenolic compounds, flavonoids, tannins, anthocyanins, proanthocyanidins, phytic acids, and γ-oryzanol. We have identified 32 phenolic acids, including hydroxycinnamic acid derivatives, and 7 different flavonoids in rice varieties. These bioactive compounds have unique physiological effects on human health. Additionally,rice grains exhibit nutraceutical potential for antidiabetic, antiarthritic, anti-inflammatory, antibacterial, and antitumor activities. In this review, we critically analyzed the bioactive components of traditional rice and their nutraceutical potential in protecting against harmful microbial activities. To ensure that future generations have access to these beneficial substances, it is crucial to preserve traditional rice varieties.

Key words: antioxidant activity; health promoting compound; indigenous rice; pigmented rice

Rice, a staple food for more than half of the world’s population, exhibits remarkable genetic diversity,boasting hundreds of distinct varieties cultivated worldwide (Ito and Lacerda, 2019). Approximately 80%of rice production stems from Asia, where it serves as the primary staple for 90% of Asians (Bhat and Riar,2015; Chaudhari et al, 2018). Rice is an affordable energy source that is rich in macro- and micronutrients (Ghanghas et al, 2020).

Aside from these essential nutrients, rice grains contain numerous other bioactive compounds, which play diverse biological roles despite being present in small quantities (Huang and Lai, 2016). These compounds commonly refer to as bioactive compounds, contribute significantly to human growth and well-being (Verma and Srivastav, 2020).

As time passes, people are becoming increasingly concerned about their quality of life and seek out healthy and health-promoting foods (Bhat and Riar,2015). Consequently, there appears to be a reversal in the trend towards traditional rice varieties. These traditional variants are gaining popularity both domestically and internationally (Thennakoon and Ekanayake, 2021), offering consumers the choice of colored and non-colored grains (Fig. 1).

Fig. 1. Traditional unpolished pigmented (A) and non-pigmented(B, C, and D) rice.

Traditional rice varieties are enriched in amino acids, antioxidants, phytochemicals, vitamin E, and other nutrients (Devraj et al, 2020). Due to the adverse effects associated with synthetic antioxidants such as butylated hydroxyanisole and butylated hydroxytoluene,researchers and nutritionists have turned their attention to exploring antioxidants derived from natural sources(Bhat and Riar, 2015). In addition to their antioxidant activity, traditional rice varieties offer antidiabetic,anti-inflammatory, and other health benefits (Kariyawasam et al, 2016b). These cereals play essential roles in various functional activities since ancient times and have made significant contributions to traditional Asian medicines. In the Philippines, specific traditional rice varieties are employed as medicinal therapies within the healthcare system (Cabanting and Perez,2016). Furthermore, Ayurveda considers traditional whole rice grains as nourishing food and medicine(Rathna Priya et al, 2019). The presence of phytochemicals, phytonutrients, oryzanols, tocols, neurotransmitters, and various other bioactive components enables traditional rice to possess these beneficial properties (Devraj et al, 2020). Table 1 showcases some popular traditional rice varieties.

However, limited studies have provided a comprehensive overview of the important bioactive nutrients, phytochemical compositions, and functional attributes of traditional rice. Therefore, this study aimed to provide information on the bioactive compounds and potential health benefits of traditional rice varieties in Asia. We conducted a review of recently published scientific papers in specialized journals,focusing on the phytochemicals, health-promoting compounds, and functional attributes of traditional rice varieties in the South and Southeast Asian regions.

Bioactive compounds in rice

Bioactive compounds, including phenolics, flavonoids,carotenoids, phytosterols, and others, have garnered significant interest in recent years due to their potential roles in preventing various chronic illnesses(Jaime and Santoyo, 2021). These chemical compounds are found in small quantities in foods and plants, such as fruits, vegetables, nuts, and oils. Epidemiological investigations have shown that consuming foods rich in bioactive chemicals is beneficial for one’s health and may reduce the risk of developing various illnesses,including cataracts, diabetes, Alzheimer’s disease, heart disease, cancer, and age-related impairments (Siriwardhana et al, 2013). Additionally, recent studies have demonstrated that foods with antiviral properties can enhance immunity and bolster the body’s defense mechanisms against viral diseases (Azam et al, 2023).Fig. 2 provides a general schematic representation of bioactive compounds, their types, and distributions.

The elemental composition of rice remains nearly uniform across all varieties (Verma and Srivastav,2020). However, indigenous or traditional rice varieties are rich in bioactive compounds, specifically phytochemicals, compared with modern rice varieties(Devraj et al, 2020). These rice varieties abundantly contain numerous groups of phytochemicals, including phenolic compounds (phenolic acids, flavonoids, and tannins), anthocyanins, proanthocyanidins, γ-oryzanol,and phytic acids (Sivamaruthi et al, 2018; Nadini Thushara et al, 2019; Rathna Priya et al, 2019). The phytochemicals present in these rice grains exhibit potent antioxidant scavenging activity and potentialhealth benefits (Nadini Thushara et al, 2019). They are primarily stored in the pericarp and bran of rice kernels (Rathna Priya et al, 2019). During the parboiling process, a portion of these compounds migrates into the starchy endosperm of rice grains (Lee, 2017).

Table 1. List of popular traditional rice varieties in South and Southeast Asia.

Fig. 2. Schematic representation of types of important bioactive compounds and their distributions.

Phenolic acids

Phenolic acids, which consist of a phenolic ring and organic carboxylic acid, are one of the major phytochemical constituents of rice (Goufo et al, 2014;Sivamaruthi et al, 2018). Some pigmented traditional rice varieties from the Kashmiri temperate regions have been reported to contain syringaldehyde, coniferaldehyde, phloretic acid, oleanolic acid, carnosic acid, and ellagic acid. These rice varieties also contain hydroxycinnamic acid derivatives, including 1-Ocaffeoylquinic acid, 3-O-p-coumaroylquinic acid,tricaffeoyl-hydroxyferulic acid, caffeoyl-coumaroylquinic acid, 5-O-feruloylquinic acid,p-coumaric acid,(Z)-ferulic acid, caffeic acid, chlorogenic acid, 4-Ocaffeoylquinic acid, and coumaroylquinic acid. The bound fractions of these phenolic acids range from 2.38% to 26.00% (Bhat and Riar, 2017). A higher proportion of bound phenolic acid provides comparatively greater benefits to human health (Lum and Chong,2012). Some Indian landraces also claim to contain 3,4-xylenol,p-cresol,O-cresol, ferulic acid,p-coumaric acid, syringic acid, caffeic acid, vanillic acid,4-hydroxybenzoic acid, protocatechuic acid, and gallic acid (Ray et al, 2021). Eight Sri Lankan indigenous red rice grains containp-coumaric acid, ferulic acid,and sinapinic acid as major phenolic acids. Caffeic acid is a minor type of phenolic acid detected only in these rice samples and not found in any improved rice.The total phenolic acids in these traditional rice varieties range from 1.54 to 4.05 mg/g in unpolished rice(Gunaratne et al, 2013). Moreover, three traditional Indian rice varieties have been found to contain vanillic acid along withp-coumaric acid (Kotamreddy et al, 2020). An indigenous brownish-black rice, Kavuni,has been reported to have 15 phenolic acids, with caffeic, cinnamic, coumaric, ferulic acids, and their derivatives being predominant (Valarmathi et al, 2015).Additionally, eight Indian rice varieties have been observed to contain 3,5-di-tert-butylphenol, 2,4-di-tertbutylphenol,p-cresol, ionol, and 2,6-di-tert-butylphenol as additional phenolic acids. Among these five phenolic compounds,p-cresol is predominantly found in traditionally colored rice types (Ashokkumar et al,2020).

Flavonoids

Flavonoids, which are ubiquitous phenolic compounds in the plant kingdom, are well-known for their healthpromoting and disease-preventing properties (Babu and Liu, 2009; Bone and Mills, 2013). Traditional rice varieties have been found to contain seven different flavonoids, including quercetin, cirsimaritin, kaempferide,kaempferol, 5-hydroxy-3,3′,4′,7-tetramethoxyflavone,3,3′,4′,7-tetramethoxyflavone, and flavone. These flavonoids can be classified as flavones, flavonols,andO-methylated flavonoids (Valarmathi et al, 2015).Some pigmented rice varieties grown in the temperate regions of Kashmir, India, have been found to contain apigenin, luteolin, quercetinuronic acid, and myrecitin as the main flavonoids (Bhat and Riar, 2017). Colored and non-colored Indian landraces have been found to contain quercetin, myricetin, and rutin as identified flavonoid compounds (Ray et al, 2021).

Anthocyanins

Anthocyanins, which are responsible for the watersoluble pigments ranging from purple to blue in black rice and other pigmented cereal grains, have been studied (Limtrakul (Dejkriengkraikul) et al, 2020;Mbanjo et al, 2020). Table 2 provides the total anthocyanin contents of various types of traditional rice. Pigmented rice varieties are found to have a higher amount of anthocyanins compared with nonpigmented rice (Saikia et al, 2012; Pathak et al, 2017;Nayeem et al, 2021).

Black rice varieties contain a significantly rich amount of anthocyanins (Maisuthisakul and Changchub,2014; Rajendran et al, 2018; Wongsa et al, 2018;Agustin et al, 2021). Some pigmented traditional black rice varieties have been found to contain quercetin-3-O-galactoside, 5-pyranopelargonidin-3-O-glucoside,cyanidin-3-O-rutinoside, pelargonidin-3-O-diglucoside,and cyanidin-3-O-galactoside, with cyanidin-3-glucoside being the major anthocyanin in rice (Bhat and Riar,2017). Additionally, pelargonidin-3-O-glucoside has been assumed to exist in traditional black rice(Agustin et al, 2021). Cyanidin-3-glucoside has beenfound to exhibit antidiabetic activity through α-amylase inhibition (Sui et al, 2016). Therefore, the anthocyanin pigment is suspected to be one of the vital causes of variation in phytochemicals and antioxidant properties in traditional colored and non-colored rice grains(Devraj et al, 2020). Indian black rice, Chak hao poreiton, which contains a very high amount of anthocyanin, possesses probiotic and antidiabetic activities. Anthocyanin interacts with two human enzymes (pancreatic α-amylase and β-glucosidase)and one transmembrane protein (glucose transporter 1)(Rajendran and Chandran, 2020).

Table 2. Total anthocyanin contents in different traditional rice varieties.

Red rice varieties also contain a considerable amount of anthocyanins (Maisuthisakul and Changchub, 2014;Rajendran et al, 2018; Wongsa et al, 2018; Agustin et al,2021). Anthocyanins from red and black rice varieties show significant antioxidant activity (Agustin et al,2021). Surprisingly, anthocyanins are quantified in a few white rice varieties along with pigmented varieties(Pathak et al, 2017; Rajendran et al, 2018; Devraj et al,2020; Agustin et al, 2021; Nayeem et al, 2021).Researchers have found that the anthocyanin content of aerobically grown traditional rice is lower than that of wetland-grown varieties in 11 purple rice varieties.Therefore, traditional rice production under wetlands can be beneficial for obtaining more health benefits(Jaksomsak et al, 2021). The extraction methods of anthocyanins also affect the quantity (Maisuthisakul and Changchub, 2014). The availability of anthocyanins may decrease with processing steps. For example, the cooking process considerably reduces the anthocyanin content by 50% in pigmented rice grains and by 40%in non-pigmented rice grains. A significant portion of the anthocyanins remains in the strained cooking water (Wimalarathne and Ekanayake, 2021).

Proanthocyanidins

Proanthocyanidins are high-molecular-weight oligomers and polymers of flavan-3-ols (Limtrakul (Dejkriengkraikul) et al, 2020; Mbanjo et al, 2020). These flavonoids are responsible for the purple and red pigments in rice pericarps (Verma and Srivastav,2020). Traditional red rice varieties have been found to contain total proanthocyanidins ranging from 1.07 to 2.27 mg/g in whole grain samples (Gunaratne et al,2013). An Indian traditional black-colored rice called Samarkand has been identified to contain proanthocyanidin trimer, while three Indian traditional red rice varieties (Zag, Shel kew, and Kaw quder) have been recognized to contain proanthocyanidin dimer (Bhat and Riar, 2017). Due to the presence of various phenolic hydroxides, proanthocyanidins have been found to have a more potent free radical scavenging capacity and antioxidant activity than anthocyanins(Cai et al, 2006).

Antinutrients

Antinutrients get their name because they interfere with the uptake of nutrients (Astley and Finglas, 2016).They can be found in various animal and plant-based diets. These chemicals are produced by plants to protect themselves against bacterial diseases and are consumed by insects (Peumans and van Damme, 1995).Despite their adverse effects, they may also have health benefits on the human body (Thompson, 1993).Phytic acids, tannins, and saponins are important antinutrients present in cereal grains (Samtiya et al,2020). Indigenous Indian rice varieties from different areas contain varying amounts of phytic acids, ranging from 1.72 to 5.59 g/kg, regardless of grain category based on color or size (Chanu et al, 2016; Prasad et al,2018; Dhaliwal et al, 2020; Muttagi and Ravindra,2020). The highest amount of phytic acids was found in a medium-sized grain of traditional rice variety Doddabyranellu, while the lowest amount was found in an aromatic traditional rice Lalat (Prasad et al, 2018;Muttagi and Ravindra, 2020). Some Indian red rice varieties contain a significant amount of tannins(703.4-763.7 mg/kg) (Dhaliwal et al, 2020). Tannic acid is detected in whole grain and significantly differs among long-grain rice varieties, with the lowest value at 257.4 mg/kg and the highest value at 541.3 mg/kg.Medium-grain rice varieties range from 299.8 to 568.8 mg/kg for tannic acid, and short-grain rice varieties range from 286.5 to 422.9 mg/kg (Muttagi and Ravindra, 2020). This antinutrient is also identified in a Bangladeshi genotype (Mannan et al, 2013). Saponin is qualitatively identified in one pigmented and two non-pigmented Bangladeshi local traditional rice grains. The study only reveals the presence of saponin in these varieties (Mannan et al, 2013).

Tocols (tocopherol and tocotrienol)

Tocols are lipid-soluble compounds derived from plants and are important for human health due to their antioxidative effects. These compounds are also known as vitamin E (Obermuller-Jevic and Packer,2004; Verma and Srivastav, 2020). There are eight biologically active forms of tocols, including four tocopherols (α, β, δ, and γ) and four tocotrienols (α, β,δ, and γ) (Gentili and Caretti, 2017). In Indian and Thai traditional rice, both colored and non-colored whole grains, contain vitamin E content ranging from 0.47 to 53.74 mg/kg (Gunaratne et al, 2013; Rajendran et al, 2018; Sudtasarn et al, 2019; Lichanporn et al,2020). Researchers have shown that non-parboiled rice grains have higher percentages of vitamin E compared with parboiled rice grains. Parboiling may be responsible for the significant loss of this lipid-soluble vitamin content (Khatoon and Gopalakrishna, 2004).

The average tocopherol content in eight Sri Lankan red-pigmented rice varieties is 15.22 mg/kg for the entire grain. The most significant contribution to total tocotrienols is γ-tocopherol (7.16 mg/kg), followed by α-tocopherol (5.75 mg/kg) and δ-tocopherol (0.89 mg/kg), with β-tocopherol present in negligible amounts (Gunaratne et al, 2013). The total tocopherol content of local Basmati rice was found to be 31.19 mg/kg, consisting of 12.09 mg/kg α-tocopherol, 14.41 mg/kg β- + γ-tocopherol, and 4.69 mg/kg δ-tocopherol(Khatoon and Gopalakrishna, 2004). Lalat is another traditional rice variety that contains 2.9 mg/kg of α-tocopherol and 0.6 mg/kg of γ-tocopherol (Prasad et al,2018). Tocopherol is primarily detected as γ-tocopherol and α-tocopherol in 85 local glutinous and 16 local non-glutinous varieties in Thailand. γ-Tocopherol ranges from 0.47 to 9.78 mg/kg among the glutinous varieties and from 0.67 to 5.21 mg/kg among the nonglutinous varieties. α-Tocopherol is identified in 40 glutinous and 5 non-glutinous varieties, ranging from 1.02 to 6.29 mg/kg and 1.92 to 2.83 mg/kg, respectively(Sudtasarn et al, 2019). From the results, it is evident that glutinous varieties have higher tocopherol content compared with non-glutinous varieties.

The whole grains of some Sri Lankan red-pigmented rice contain 28.97 mg/kg of tocotrienols. The largest quantity is γ-tocotrienol (23.29 mg/kg), trailed by α-tocotrienol (4.28 mg/kg) and δ-tocotrienol (0.81 mg/kg), with β-tocotrienol present at negligible levels(Gunaratne et al, 2013). δ-Tocotrienol is detected in 85 varieties (8.00-22.10 mg/kg), γ-tocotrienol in 81 varieties (0.57-7.00 mg/kg), and α-tocotrienol in 39 varieties (1.52-9.94 mg/kg) in Thai glutinous varieties.Only α-tocotrienol is found in 9 non-glutinous Thai rice varieties, ranging from 4.56 to 8.93 mg/kg.γ-Tocotrienol and δ-tocotrienol are observed in all 16 Thai non-glutinous rice varieties, ranging from 8.80 to 178 mg/kg and 1.64 to 8.87 mg/kg, respectively(Sudtasarn et al, 2019). A highly significant variation was observed in the levels of tocotrienols in two Indian varieties (Basmati and Lalat). One variety contains 21.30 mg/kg of α-tocotrienol, while the other contains 0.50 mg/kg. Additionally, the proportion of γ-tocotrienol is quite low compared with α-tocotrienol in both varieties (Khatoon and Gopalakrishna, 2004;Prasad et al, 2018).

Gamma (γ)-oryzanol

γ-Oryzanol is a bioactive chemical found in rice bran that promotes growth (Tuncel and Yilmaz, 2011). It contains 4-fold the antioxidant potential of vitamin E(Srikaeo, 2014). In unpolished rice grains, the total γ-oryzanol content of unimproved paddies is found to be between 0.231 and 0.638 mg/g (Khatoon and Gopalakrishna, 2004; Gunaratne et al, 2013). The polished rice yields a relatively minor quantity of this bioactive compound (Khatoon and Gopalakrishna,2004), as it remains in the outer layer of rice. In red-grain rice varieties, four major components of γ-oryzanol are easily recognized as cycloartenyl ferulate,2,4-methylene cycloartenyl ferulate, campesteryl ferulate,and sitosteryl ferulate. Three minor constituents of γ-oryzanol have also been discovered as Δ7-campesteryl ferulate, campestanyl ferulate, and sitostanyl ferulate(Gunaratne et al, 2013). Several local Thai glutinous rice varieties (0.276-0.638 mg/g) show comparably higher γ-oryzanol levels than non-glutinous rice varieties (0.272-0.469 mg/g) (Sudtasarn et al, 2019).

Gamma aminobutyric acid (GABA)

GABA, an inhibitory neurotransmitter that is vital in the mammalian central nervous system, has various health impacts (Guha et al, 2015). The GABA content significantly varies among traditional rice varieties,ranging from 3.6 to 51.0 mg/kg (Karladee and Suriyong,2012; Siddiquee et al, 2017; Ghosh and Roychoudhury,2018; Liyanaarachchi et al, 2021). This non-proteinogenic amino acid in germinated grains appears to be at least 3.5 times higher than that in non-germinated grains(Karladee and Suriyong, 2012; Ghosh and Roychoudhury,2018). GABA is largely generated during the germination process by the decarboxylation of L-glutamic acid mediated by α-glutamate decarboxylase. Hence,germinated paddy rice shows a higher GABA content than brown rice (Guha et al, 2015). Additionally, rice varieties with red pericarps have significantly lower levels of GABA compared with white pericarp varieties(Liyanaarachchi et al, 2021). Moreover, this essential neurotransmitter has been identified in three Indian traditional rice varieties (Kotamreddy et al, 2020).

Extraction of bioactive compounds

The recovery of bioactive compounds from rice can be accomplished using a variety of extraction methods,which must be chosen based on the chemistry and distribution of the desired components in the plant matrix (Goufo and Trindade, 2014). Rice extracts have been prepared using a range of procedures, from conventional methods such as maceration and Soxhlet extraction to modern techniques (Verma and Srivastav,2020). Solvent extractions are the most widely used conventional approach for removing bioactive chemicals from traditional rice samples due to their simplicity, efficiency, and wide range of applications.Several variables, including the solvent employed,solvent-to-matrix ratio, residence time, pressure, and temperature, impact the extraction yield, efficiency,and selectivity (Noori et al, 2018). The steps of solvent extraction are illustrated in Fig. 3.

Generally, before extracting bioactive compounds,rice samples undergo various processing steps such as milling, grinding, pulverization, and homogenization.These steps help retain higher levels of bioactive compounds in traditional rice samples (Saikia et al,2012; Balakrishnan et al, 2019; Devraj et al, 2020).The conventional method frequently employs a variety of organic solvents. Bioactive chemicals from traditional rice samples have been extracted using several solvents,including acetone, ethanol, hexane, methanol,chloroform, phosphate buffer solution, water, and their combinations in certain proportions (Dutta et al, 2013;Gunaratne et al, 2013; Rahman et al, 2015a; Valarmathi et al, 2015; Pathak et al, 2017; Balakrishnan et al, 2019;Lichanporn et al, 2020; Thennakoon and Ekanayake,2021). Even concentrated solvents are employed to extract bioactive components from traditional rice(Asaduzzaman et al, 2013; Ashokkumar et al, 2020;Kotamreddy et al, 2020), and the solvents are utilized in various concentrations (Maisuthisakul and Changchub,2014; Mudoi and Das, 2019; Devraj et al, 2020).Solvents are occasionally acidified to improve efficiency(Rajendran et al, 2018; Nayeem et al, 2021). The sample-solvent solutions are often stirred or shaken intermittently for a specific period (15 min to 7 d)(Saikia et al, 2012; Asaduzzaman et al, 2013; Mannan et al, 2013; Thennakoon and Ekanayake, 2021) and sometimes maintained at a certain temperature to obtain superior extracts or sustain metabolite intake(Saikia et al, 2012; Devraj et al, 2020; Kotamreddy et al,2020). The supernatant is then collected. Filtration and centrifugation are occasionally used to obtain the supernatant. Different grades of filter paper can be used for filtration (Thennakoon and Ekanayake, 2021).Supernatants are sometimes concentrated using a water bath (Ashokkumar et al, 2020), rotary evaporator(Maisuthisakul and Changchub, 2014), or speedvac(Valarmathi et al, 2015). The extraction procedure is repeated using residue to guarantee effective extraction(Rajendran et al, 2018; Mudoi and Das, 2019). Finally,the extracts are kept at low temperatures (-4 °C)(Gunaratne et al, 2013; Rajendran et al, 2018; Devraj et al, 2020) or extremely low temperature (-20 °C)before further use (Saikia et al, 2012; Nayeem et al,2021).

Nutraceutical potential of traditional rice

Bioactive chemicals can be found in a variety of natural sources, including cereal grains, fruits, and vegetables. According to the literature, traditional rice has a wide range of phytochemicals, including phenolic and flavonoid compounds. These compounds have recently gained attention for their nutraceutical potential,such as antioxidant, antidiabetic, anti-inflammatory,and other benefits. The potential health benefits of traditional rice grains are presented in Fig. 3 (Devraj et al, 2020; Verma and Srivastav, 2020; Thennakoon and Ekanayake, 2021).

Antioxidant activity

Antioxidants play a vital role in the body’s defense mechanisms against several life-threatening diseases.In vivoandin vitrostudies have shown that phytochemicals have antioxidant properties. It has been demonstrated that consuming foods rich in phytochemicals increases the antioxidant capacity of serum or plasma (Zhang et al, 2015). The consumption of antioxidant-enriched rice can boost the antioxidant state in the body, as demonstrated by a rat model(Shozib et al, 2015). Various types of traditional rice have been investigated for their antioxidant potential using differentin vitromethods such as the 1,1-diphenyl-2-picrylhydrazine radical scavenging (DPPH)assay, ferric reducing antioxidant power (FRAP) assay,total antioxidant capacity (TAC) assay and 2,2′-azinobis-3-ethylbenzothiazoline-6-sulfonic acid radical scavenging (ABTS) assay (Table 3).

Several synthetic and natural antioxidants, including ascorbic acid, butylated hydroxytoluene, trolox, curcumin,mannitol, gallic acid, ferulic acid, catechol, quercetin,and rutin are used as standards. Along with the standards, multiple solvents are used to conduct the assays. Antioxidant activities vary significantly based on the solvents used (Maisuthisakul and Changchub,2014; Valarmathi et al, 2015; Shin et al, 2016).

Numerous studies suggest a linear relationship between total phenolic and flavonoid contents and antioxidant capability. Phenolic and flavonoid molecules play crucial roles as antioxidant components by providing hydrogen atoms to deactivate free radicals.Additionally, they provide perfect structural qualities for free radical scavenging (Aryal et al, 2019). The total phenolic and flavonoid contents of different traditional rice varieties are given in Table 4.

Fig. 4. Effects of traditional rice bioactive compounds on human body.

Rice color and fragrance contribute to its unique quality, and these characteristics are associated with variations in its antioxidant capabilities (Verma and Srivastav, 2020). Pigmented rice has been observed to have a comparatively higher antioxidant capacity than non-pigmented rice (Saikia et al, 2012; Mannan et al,2013; Maisuthisakul and Changchub, 2014; Chanu et al,2016; Shin et al, 2016; Bhat and Riar, 2017; Pathak et al,2017; Rajendran et al, 2018; Anuprialashmi et al, 2019;Ashokkumar et al, 2020; Agustin et al, 2021).Anthocyanins, along with phenolics and flavonoids,play a significant role in the antioxidant activity of pigmented rice. Surprisingly, an indigenous nonpigmented hill rice variety Vandana shows no antioxidant activity (Pathak et al, 2017). It is evident that consuming colored rice varieties quickly enhances antioxidant activity and reduces plasma malondialdehyde levels in a healthy population (a marker for free radicals).These changes may be attributed to the polyphenols found in rice varieties with color (Callcott et al, 2019). Some traditional aromatic rice varieties exhibit satisfactory antioxidant activity and contain a good amount of phenolic and flavonoid compounds(Dutta et al, 2013; Rahman et al, 2015a).Volatile compounds present in fragrant rice can also contribute to antioxidant activity along with other bioactive compounds(Ashokkumar et al, 2020; Hu et al, 2020).

The antioxidant capabilities of the final product can be influenced by the grain type and processing methods (Ragaee et al,2014). Milled and polished traditional rice grains generally have lower (sometimes zero) antioxidant potential and fewer bioactive compounds compared with unpolished rice. The milling and polishing processes primarily remove the bran layer,which is the major source of bioactive compounds in rice. Therefore, this specific class of rice exhibits lower antioxidant potential (Mannan et al, 2013; Mudoi and Das, 2019; Thennakoon and Ekanayake,2021). Cooked traditional rice grains have lower antioxidant activity than raw rice grains (Saikia et al, 2012; Nayeem et al,2021; Thennakoon and Ekanayake, 2021)because a portion of the phenolic, flavonoid,and anthocyanin contents remains in the strained cooking water, ranging from 60% to 70% (Nayeem et al,2021). Additionally, parboiled traditional rice grains exhibit relatively lower antioxidant potential as a portion of the phenolic compounds is lost during the thermal treatment (Thennakoon and Ekanayake,2021).

Antiarthritic activity

Millions of people worldwide, including children,suffer from arthritis, a type of pain that is typically not severe enough to cause screams or tears. The two most prevalent types of arthritis are rheumatoid arthritis and osteoarthritis (Reginster, 2002). In bovine serum protein denaturation and egg albumin denaturation procedures,an Indian indigenous aromatic rice variety Joha Rice shows considerable antiarthritic efficacy (Rahman et al,2015b). Another Indian rice inhibits bovine serum denaturation by 43.6%, 64.3%, and 77.6% at contents of 250, 500, and 1 000 mg/mL, respectively, compared with diclofenac’s 51.7% inhibition at 250 mg/mL(Boominathan and Bakiyalakshmi, 2016). Conventional medicines and rice extracts inhibit thermally-induced protein denaturation in a dose-dependent manner.Some findings suggest that the creation of auto-antigens in certain rheumatic disorders may be related to protein denaturation within the body, which is one of factors contributing to rheumatoid Arthritis (Uttra and Alamgeer,2017). The extracts of these rice varieties inhibit bothprotein denaturation assays, suggesting that it might be one of the reasons for their antiarthritic action owing to the abundance of bioactive substances, particularly flavonoids (Rahman et al, 2015b; Boominathan and Bakiyalakshmi, 2016).

Antidiabetic activity

Diabetes mellitus is a worldwide epidemic, with a global incidence of 2.8% in 2000 and is anticipated to rise to 4.4% by 2030 (Neeland and Patel, 2019). Two traditional Indian brown rice varieties demonstrate strong antidiabetic activity, with inhibitory concentration 50 (IC50) values for α-amylase and α-glucosidase ranging from 2.00 to 2.70 g/mL and 2.12 to 2.23 g/mL,respectively (Balakrishnan et al, 2019). Another traditional brownish-black rice variety, Kavuni, suppresses both of these enzymes more effectively (0.10 and 0.04 g/mL) than normal (0.30 and 0.15 g/mL) (Valarmathi et al, 2015). These enzymes are crucial for glucose metabolism. An Indonesian red rice variety also inhibits α-amylase, indicating potential antidiabetic effects (Agustin et al, 2021). Inhibiting pancreatic α-amylase and intestinal α-glucosidase can effectively control type II diabetes (Zhou et al, 2004). The anthocyanin of an Indian black rice variety, Chak hao poreiton, exhibits a significant antidiabetic effect. The bioactive compound of this rice inhibits hydrolytic enzymes (pancreatic α-amylase and β-glucosidase) as well as human glucose transporter-GLUT1 (Rajendran and Chandran, 2020). It seems that pigmented rice varieties are good at inhibiting hydrolytic enzymes.An aromatic rice and a red rice exhibit a low glycemic response (Prasad et al, 2018; Rohitha Prasantha, 2018).This low value may be due to other supporting elements, such as amylose and resistant starch (Prasad et al, 2018; Rohitha Prasantha, 2018; Mondal et al,2021). Resistant starch provides no calories during digestion (Mondal et al, 2021). Additionally, various traditional rice varieties show moderate glycemic index and resistant starch (Nicholas et al, 2014; Abeysekera et al, 2018; Rohitha Prasantha, 2018; Saragih et al,2019; Mondal et al, 2021). Low glycemic-index foods raise blood glucose levels steadily (Saragih et al,2019). Ultimately, all these factors contribute to health benefits for diabetes patients.

Anti-inflammatory activity

Inflammation is a tissue response to damage involving a complex series of enzyme processes (Vane and Botting, 1995). The indigenous aromatic rice variety,Joha Rice, exhibites significant membrane-stabilizing properties in the human red blood cell membrane stabilization method, indicating anti-inflammatory activity (Rahman et al, 2015b). When red blood cells are exposed to harmful substances, they rupture due to excessive fluid accumulation within the cell. This lysis causes secondary damage through free radicalinduced lipid peroxidation (Umukoro and Ashorobi,2006). Therefore, it is expected that compounds with membrane-stabilizing properties should offer protection to the cell membrane against harmful substances. It is also necessary to interfere with the release of phosphorlipases that trigger the formation of inflammatory mediators (Vadivelan et al, 2009). Rice may contain compounds that inhibit the release of phospholipases,which trigger the formation of inflammatory mediators(Rahman et al, 2015b). Polyphenol-rich rice can decrease pro-inflammatory cytokines that cause inflammation in the human body. These bioactive compounds and their metabolites may directly or indirectly target specific inflammatory pathways (Callcott et al, 2019).

Antibacterial activity

Microbial invasion of the skin and underlying soft tissues can result in skin and soft tissue infections,which may progress to life-threatening necrotizing fasciitis (Ki and Rotstein, 2008). These infections are commonly caused by gram-positive bacteria likeStaphylococcus aureus, methicillin-resistantS. aureus(MRSA), andStreptococcus pyogenes, as well as gramnegative bacteria likePseudomonas aeruginosaandEscherichia coli(Sukumaran and Senanayake, 2016).

Sri Lankan traditional un-parboiled rice varieties show remarkable antibacterial activity against grampositiveS. aureusand MRSA bacteria (Godakumbura et al, 2017). Among these, the rice variety Rathdal(crude extract) shows the highest inhibitory activity againstS. aureusand MRSA, including clinically isolated skin and wound bacteria. Additionally,Pokkali rice (crude extract) partially inhibitsS. aureusbacteria. The minimum bactericidal concentration for the un-parboiled rice extracts is 200 μg/mL, while for the parboiled rice extracts, it is reduced or shows no bactericidal activity (Godakumbura et al, 2017).Parboiling treatment reduces the antibacterial activity of rice varieties. Moreover, this study indicates that red pericarps possess higher antibacterial activity compared with white ones. This may be due to the presence of various bioactive compounds such as flavones, anthocyanins, tannins, phenolics, sterols,tocols, and γ-oryzanols mainly found in black, red,and purple pericarps (Chakuton et al, 2012; Deng et al,2013; Nadini Thushara et al, 2019).

Antitumor activity

‘Tumor’ is a medical term synonymous with ‘neoplasm’or ‘real tumor’. It has the potential to be both malignant and noncancerous (Bignold, 2015). Kalijira and Chinigura, two traditional aromatic rice varieties of Bangladesh, exhibit a notable antitumor effect. The unpolished grains of Kalijira manifest the highest tumor inhibition (57.43%), and the unpolished grains of Chinigura also show a similar extent of inhibition(55.53%) against the tumor. Both varieties demonstrate their antitumor properties using the standard potato disc bioassay against threeAgrobacterium tumefaciensbacterial strains (Mannan et al, 2014). This tumorsuppressive activity may be attributed to the presence of phenolic and polyphenolic compounds (quercetin,isoquercitrin, ferulic acid, and ferulate ester) in their grains and bran layers (Mannan et al, 2014; Phusrisom et al, 2021). One of these varieties contains a substantial quantity of total phenolic compounds(Asaduzzaman et al, 2013).

Concluding remarks

Indigenous rice varieties have always been an imperative part of the Asian country’s food chain.Nowadays, these varieties have unfolded new interest and have intensified their use in nutraceutical prospects with scientific evidence. Bioactive compounds like phenolic acids, flavonoids, anthocyanins, gamma oryzanols, tocols, and phytic acids are found to be abundant in traditional rice grains. Although the amount of bioactive compounds in indigenous rice varies with rice genotype types, each of these bioactive compounds plays a crucial role in human health. We have found tremendous nutraceutical potential in indigenous rice. It is therefore necessary to raise awareness among farmers and consumers about its cultivation and daily intake. The government and rice research institutes should take necessary steps to preserve these varieties. The scientific community should also come forward to uncover the potential mechanisms of action of these bioactive compounds on human health.