Introduction

Rice serves as a primary cereal food crop and staple diet in many developing nations. While white rice remains a predominant choice worldwide, various specialty rice varieties, including colored and aromatic types are also cultivated. Compared to standard white rice, these specialty variants boast a richer nutritional profile. Emphasizing the cultivation and utilization of specialty rice and its derivatives can not only help preserve these unique varieties but also advance the nations nutritional security, given their abundant vitamins, minerals, and polyphenols content. Increased income levels and self-sufficiency induced rice availability for consumption has brought a shift in the consumer as well as market preferences for better grain quality. With sharp increase in lifestyle-related health problems, rice consumers are shifting to brown rice for increased micronutrient content. Recently, pigmented rice varieties have been receiving increased attention from health conscious consumers for their high bioactive compounds which possess potential nutraceutical benefits to human health.

Black rice (Oryza sativa L. indica), is a special cultivar of rice which contains remarkably high anthocyanin pigments in the aleurone layer than white and red rice which account for it’s violet or dark purple color1. Owing to several health-promoting impacts associated with anthocyanins, such as anti-oxidative, anti-inflammatory and anti-carcinogenic effects2, colored rice is considered as a functional food and food ingredient in many Asian countries3. The functional properties of anthocyanins in red and black rice varieties have been verified in numerous nutritional cases1. Due to increased awareness and demand for coloured rice, farmers from Andhra Pradesh and Telangana states of India are growing the glutinous black rice varieties like Burma black, Kalabhatt and Chakhao amudi which are popular in North eastern India. But these varieties are tall in stature, low yielding, possess bold grain and had low amylose content which are not preferred by the consumers in South India. Purwanto et al.4 stated that only few farmers were willing to cultivate black rice, because of their longer duration and low grain yield/plant (16.8–23.94 g grain weight/plant). Somsana et al.5 also reported that farmers of Nepal mostly grow local genotypes of black glutinous rice with low yield potential. The black glutinous rice is mainly used for the preparation of sweet snacks and desserts in Asia which may not be included in daily diet. The bold and glutinous black rice varieties popular in north eastern states are not preferred in South India as the consumers prefer medium slender grain type with soft and flaky texture of cooked rice. Hence, a breeding programme was initiated at Agricultural Research Station, Bapatla of Acharya N.G. Ranga Agricultural University, Guntur, Andhra Pradesh to develop a medium duration high yielding improved black rice variety with good grain & cooking quality possessing resistance to major pests and diseases as an alternative to traditional black rice varieties.

Materials and methods

BPT Rice 2841 (IET 27,434) was developed from a three-way cross involving MTU 7029 (Swarna), IRGC 18,195, and MTU 1081 developed through the pedigree breeding method. After thorough testing at station level and in farmers fields of all over state, BPT Rice 2841 was released through State Variety Release Committee of Andhra Pradesh state during the year 2023. In all the trials conducted over years and also in the present study, the local popular glutinous black rice variety Burma black and the popular quality rice variety, Samba mahsuri (BPT 5204) were used as checks. The dehusked samples of BPT 2841, Burma black and BPT 5204 grown during kharif 2022 and 2023 were utilized for estimation of grain quality, nutritional and biochemical quality parameters along with grain yield.

Estimation of total starch content (%)

Total starch content was quantified following the protocol described by Hodge and Hofrieter6. Briefly, 100 mg of powdered rice sample was hydrolyzed with 2.5 mL of 3.2 M hydrochloric acid by heating in a boiling water bath for 2 h. After neutralization with sodium hydroxide, the resulting solution was subjected to colorimetric analysis using anthrone reagent, and the absorbance was measured at 620 nm. The starch content was calculated against a standard curve prepared with known concentrations of glucose.

Determination of amylose content (%)

Amylose content was determined using the iodine-binding method as described by Juliano7. Approximately 100 mg of rice flour was defatted with ethanol, followed by the addition of 1 mL 0.1 M sodium hydroxide. The suspension was diluted and treated with iodine-potassium iodide solution. Absorbance was recorded at 620 nm, and amylose content was calculated using a standard curve prepared with amylose standards.

Protein content analysis (%)

The total protein content was estimated using the Lowry method8. A 100 mg sample of rice flour was homogenized with phosphate buffer (pH 7.4) and centrifuged. The supernatant was treated with Folin-Ciocalteu reagent and alkaline copper sulfate solution. After 30 min of incubation, the absorbance was measured at 750 nm. Bovine serum albumin (BSA) was used as a standard for calculating protein content.

Zinc and iron content (ppm)

Zinc and iron concentrations were determined using the Atomic Absorption Spectrophotometer (AAS) method, following the protocols of the Indian Institute of Rice Research9. Samples were digested with a tri-acid mixture (nitric acid, sulfuric acid, and perchloric acid in a 9:2:1 ratio) at 180 °C until clear solutions were obtained. The digested samples were diluted and analyzed for zinc and iron content.

Total phenol content (mg/100 g)

Total phenol content was determined using the Folin-Ciocalteu (FC) reagent method10. A 0.5 g sample was extracted with 80% methanol, followed by centrifugation. The supernatant was mixed with FC reagent and sodium carbonate solution. The reaction mixture was incubated for 30 min in the dark, and the absorbance was recorded at 765 nm. Gallic acid was used as a standard.

Total antioxidant activity (mg AAE/100 g)

Total antioxidant activity was measured using the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay11. A 0.1 g sample was extracted with methanol, and 1 mL of DPPH solution was added to the extract. The mixture was incubated in the dark for 30 min, and absorbance was measured at 517 nm. Results were expressed as mg ascorbic acid equivalent (AAE) per 100 g.

Total flavonoid content (mg QE/100 g)

Total flavonoid content was estimated using the aluminum chloride colorimetric method12. Rice extracts (0.5 mL) were mixed with 0.1 mL of 10% aluminum chloride, 0.1 mL potassium acetate, and 4.3 mL distilled water. After incubation for 30 min, absorbance was measured at 415 nm. Quercetin was used as a standard.

Glycemic index (GI)

The glycemic index was determined using an in vitro enzymatic digestion method as described by Goni et al.13 with slight modifications. Cooked rice samples were digested with a combination of pancreatic alpha-amylase and amyloglucosidase. Glucose release was quantified at regular intervals using a glucose oxidase–peroxidase assay, and the glycemic index was calculated as the incremental area under the curve relative to a standard glucose solution.

Rapidly digestible starch (RDS) and slowly digestible starch (SDS)

The contents of RDS and SDS were estimated using the enzymatic digestion method described by Englyst et al.14. Cooked rice samples were treated with pancreatic alpha-amylase and amyloglucosidase at 37 °C. Glucose released after 20 min (RDS) and 120 min (SDS) was measured, and the results were expressed as a percentage of total starch.

Resistant starch content

Resistant starch content was determined following the method of Goni et al.15. After enzymatic digestion with alpha-amylase and amyloglucosidase, the undigested residue was treated with sulfuric acid to hydrolyze resistant starch into glucose. The glucose content was measured colorimetrically at 540 nm and expressed as a percentage of total starch.

Amino acid profiling

Amino acid profiles of unpolished rice samples were analyzed using High-Performance Liquid Chromatography coupled with Diode-Array Detection (HPLC–DAD). Rice flour (0.5 g) was hydrolyzed with 6 N hydrochloric acid at 110 °C for 24 h in vacuum-sealed vials. Post-hydrolysis, the samples were neutralized, derivatized with phenylisothiocyanate, and separated on a C18 column using a gradient of acetonitrile and phosphate buffer. Individual amino acids were quantified based on retention times and peak areas compared to known standards.

Textural properties

For observing the textural properties of cooked rice, dry-milled head rice was washed with distilled water twice to eliminate residual contaminants and then soaked for 10 min in a test tube. The rice was prepared in a water bath for 20 min with a rice-to-water ratio of 1:2 by weight, thereafter, maintained at a warm temperature for 10 min. The cooked rice was removed and covered with Whatman Filter paper No. 1 at ambient temperature for 5 min. Upon reaching room temperature, the rice kernel was centrally placed under a probe on the base plate. Textural properties were assessed using a TA-XT plus Texture Analyzer (Stable Microsystems, UK) equipped with a 10 mm diameter Delrin probe (P/10) using a two-cycle, force-versus-distance compression protocol. The TPA settings were as follows: Pre-Test Speed, 1 mm/s; Test Speed, 5 mm/s; Post-Test Speed, 5.00 mm/s; Strain, 90%; Time, 5.00 s; Trigger Force (auto), 0.029 N16.

Statistical analysis

Data were analyzed using SPSS software (version 30) to determine significant differences among rice varieties for various quality parameters. Results were expressed as mean ± standard error (SE). One-way ANOVA followed by Duncan’s Multiple Range Test (DMRT) was used for mean separation (p < 0.05). Textural properties were tested at p < 0.01, with different lowercase letters indicating significant differences.

Results and discussion

With a duration of 130–135 days, BPT 2841 exhibits a robust plant structure with green foliage, moderate tillering, dense panicle, straw glume and medium slender grains with 13.5 g test weight. The variety has moderate resistance to prevalent biotic stresses such as leaf blast, neck blast, gall midge, WBPH, BPH and yields up to 5.5–6.0 t/ha. BPT 2841 possess medium slender grain with black pericarp coupled with excellent cooking quality. The physico-chemical grain quality and nutritional parameters are presented in Table 1. BPT 2841 rice variety has medium slender grain with intermediate amylose content (23.4%) and alkali spreading value (4.5) which determines the soft and flaky texture of cooked rice. Li et al.17 also stated that the amylose content of the rice variety has culinary implications because it has an influence on the organoleptic qualities of rice once cooked. Unlike the traditional black rice varieties of North Indian states, BPT 2841rice variety has excellent cooking quality, which makes it to be suitable as raw rice and can be included in daily diet also. In comparison, popular desi black rice varieties like Burma black has long bold grains and low amylose content (4.5%) and high alkali spreading value (7.0) resulting in a sticky texture when cooked, rendering it less suitable for raw rice applications. When compared with the non pigmented quality check variety, BPT 2841 possessed similar physico-chemical quality parameters which make it more suitable for use as raw rice. Significant differences were observed for crude fibre content between pigmented and non pigmented varieties. BPT 2841 recorded 1.16% fibre content while Burma black and BPT 5204 manifested 1.39% and 1.0% crude fire content respectively. For carbohydrate content significant difference was observed between black rice and white rice varieties but significant differences were not observed for fat content and energy between the genotypes under study. In addition, BPT 2841contains 24.3 ppm of zinc, 15.6 ppm of iron, and a protein content of 11.2%. When compared with BPT 5204(non pigmented rice), BPT Rice 2841 possess significantly high protein content, high micronutrient content, hence consumption of BPT Rice 2841 can help in addressing malnutrition concerns.

Table 1 Physico-chemical and nutritional quality parameters of BPT Rice 2841 unpolished rice in comparison with popular desi variety Burma black and quality check BPT 5204.
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Black rice, being a potent source of anthocyanin which imparts a master role in inflammation and cholesterol reduction18. In this study, no significant difference was observed between the two black rice varieties for anthocyanin content (Table 2) indicating that the improved black rice possess similar nutritional profiles as that of traditional black rice. The phenolic compounds are also known as antioxidants and are likely to have functional effects against oxidative damage and associated with reduced risk of chronic diseases such as diabetes and cardiovascular diseases19. Unpolished BPT 2841 rice variety showcases a total phenol content of 238.17 mg/100 g and an antioxidant activity of 110.5 mg AAE/100 g, which is beneficial for neutralizing free radicals and enhancing human immunity (Table 2). Flavonoid content was highest in BPT 2841 ( 566.66 mg/100 g) followed by Burma black ( 495.19 mg/100 g) and non pigmented BPT 5204 (144.4 mg/100 g). Total antioxidant activity (AOA) and total flavonoid contents (TFC) are higher in colored rice genotypes when compared with non pigmented varieties20 which is in confirmation with the present findings. If the consumer prefers to add the unpolished BPT 2841 rice in diet, the total nutrition will be available, but for improving the palatability, minimum polishing of 1–2% is needed for any rice. Hence, the antioxidant activity of BPT 2841 was estimated for 2% polished rice also. The results revealed that after 2% polishing also BPT 2841 recorded 98.71 mg/100 g AOA while Burma black had 70.98 mg/100 g AOA. The difference in AOA content of two black rice varieties at 2% polishing may be attributed to the loss of pericarp in Burma black when compared with BPT 2841. Even during dehusking process, the pericarp of Burma black has slightly removed while the pericarp is tightly intact in case of BPT 2841. The 2% polished BPT 2841 rice will be cooked as normal white rice without soaking and no extra time and water needed for cooking. The health effects of antioxidants present in colored rice suggest potential uses of the cereal anthocyanins for positive human nutrition21 and these colored rice reduce atherosclerotic plaque by 50% more than white rice in rabbits22. BPT Rice 2841 recorded 36.75% slowly digestible starch and a glycemic index of 62.5, placing it in the medium GI range. The widely recognized glutinous desi black rice variety, Burma black, has a glycemic index of 65.8. Furthermore, the population-specific glycemic response can be influenced by genetic, dietary, and metabolic factors, which necessitates region- or ethnicity-specific evaluations23.

Table 2 Functional properties, anti-nutritional and Biochemical quality parameters of BPT Rice 2841 unpolished rice in comparison with popular desi variety Burma black and non-pigmented quality rice check BPT 5204.
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The primary concern regarding the presence of phytic acid is the anti-nutritional properties resulting from strong negative charges under gastro-intestinal conditions24. This is often reported in mineral availability due to the formation of stable phytate complexes and the inhibition of phytase enzymatic actions25. Additionally, the binding of proteins by phytic acid in insoluble binary and ternary structures makes proteins unavailable for digestion26. The ability of phytic acid to bind proteins also leads to inhibition of α-amylase enzyme, which leads to the incomplete digestion of starch, and thus reduces starch digestibility27. The popular mega rice variety, BPT 5204 with excellent grain quality and high consumer preference recorded 169.50 mg/100 g of phytic acid. When compared with normally consumed white rice, black rice contains more phytic acid and significant differences were observed for this trait between the genotypes under study. Among the two black rice varieties, BPT 2841 contains less amount of phytic acid (348.94 mg/100 g) when compared with the traditional black rice variety Burma black (406.126 mg/100 g). Hence, after intake, the absorption of nutrients will be high in BPT 2841 when compared with Burma black rice. The results indicated that the non pigmented rice variety BPT 5204 recorded more oxalate content (8.662 mg/100 g) when compared with black rice.

Even though rice protein content is slightly lower than other cereals, with respect to protein quality, the rice protein amino acid profile is better balanced compared to other cereals such as wheat and maize28. Therefore, the impact of improving the protein content in rice would be enormous in combating the protein energy malnutrition which is prevalent among rice consumers. Unpolished BPT 2841 contains 11.2% protein content and the amino acid profile of BPT 2841 is compared with traditional black rice Burma black and the popular non pigmented mega rice variety, BPT 5204 (Table 3). Previous studies indicated that lysine is the major limiting essential amino acid in rice protein which is crucial for regulating hormones, antibodies and enzymes. In the present investigation, improved black rice variety BPT 2841 recorded maximum lysine content (6.16%) followed by the traditional black rice Burma black (5.44%) and BPT 5204 (2.860%). Threonine, the second most limiting amino acid was also highest in BPT 2841 (6.42%) followed by Burma black (6.0%). Yang29 reported that lysine is considered the main limiting essential amino acid required for human health supplementation with lysine raised biological value of rice protein markedly. Hence, addition of black rice in diet will improve both the quality and quantity of rice protein. Methionine another essential amino acid ranged from 1.64% in Samba Mahsuri to 4.24% in BPT 2841. Two non essential amino acids viz., Glycine and tyrosine contents were high in black rice varieties when compared with non-pigmented check variety. For Serine, valine and Isoleucine contents not much difference was observed between pigmented and non-pigmented rice varieties studied in the present investigation. While non pigmented check, BPT 5204 recorded maximum values for 4 amino acids viz., Alanie (11.00), Arginine (5.770), Leucine (9.710) and Hydroxy proline (4.360) than the black rice varieties. The improved black rice variety BPT 2841 exhibited the highest concentrations of all three major limiting essential amino acids in rice protein viz., lysine, threonine, and methionine, indicating a superior biological value of it’s protein content compared to other genotypes studied. According to Ke et al.30, protein is an important modulator in glucose homeostasis by increasing gluconeogenesis and preventing insulin resistance, hence genotypes possessing high protein content digest slowly and aids in slow release of blood glucose.

Table 3 Aminoacid profile (in %) of BPT 2841 in comparison with traditional black rice variety Burma black and and non-pigmented rice variety BPT 5204.
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The textural parameters hardness, stickiness, springiness, gumminess and chewiness for the three rice varieties namely, BPT 5204, Burma black and BPT 2841 was presented in Table 4. The hardness of cooked rice is the measure of the peak resistive force during the first compression cycle, and it defines the gel strength of the rice. The hardness of cooked Burma black rice was twofold and fourfold higher than the BPT 2841 and BPT 5204 respectively. The hardness was found to be significantly different at p < 0.01 for the three rice samples, shown in Table 3. The stickiness of cooked rice indicates the adhesiveness between the rice kernel and the TPA probe. Cooked Burma rice has a stickiness value of 4.13 g/cm2 whereas for the cooked BPT 2841 and BPT 5204 stickiness is approximately 36% and 77.5% lower than the cooked Burma rice stickiness respectively. Similar to hardness the stickiness was found to be significantly different for all the rice samples at p < 0.01. Springiness refers to the capacity of cooked rice to revert to its initial form after compression. The springiness of cooked Burma black rice, BPT 2841 and BPT 5204 was found to be 0.912 \(\times {10}^{-2}\), 0.801 \(\times {10}^{-2}\) and 0.665 \(\times {10}^{-2}\) respectively. Springiness of BPT 5204 and Burma black rice was found to be significantly (p < 0.01) different. Gumminess refers to the energy used to masticate the rice into a consumable state. The gumminess of cooked BPT 5204 was found to be 11.93 which is lower when compared with the Burma black rice (66.42) and BPT 2841 (30.54). There was significance difference was observed for the gumminess of three rice samples evident from Tukey’s post hoc test presented in Table 3. Chewiness is the product of gumminess and springiness, and it was observed that the chewiness is maximum for the Burma black rice sample with value 0.605 and minimum for the BPT 5204 with value 0.091, presented in Table 3. Cohesiveness refers to strength of the internal linkages inside the cooked rice and there was significant difference for cohesiveness between the three cooked rice samples at p < 0.05.

Table 4 Textural Properites of BPT 2841 in comparison with BPT 5204 and Burma Black.
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BPT 2841 was tested/evaluated extensively for grain yield for 8 years before its release which is mandatory for release of a variety by State Variety Release Committee (SVRC) of Andhra Pradesh State, India (Table 5). The average of location wise results submitted to SVRC, Andhra Pradesh state revealed that BPT 2841 (5788 kg/ha) recorded over 10% yield improvement when compared with the non-pigmented popular quality check variety BPT 5204 (5256 kg/ha). The traditional glutinous black rice variety Burma black had low yield potential and recorded an average grain yield of 3904 kg/ha.

Table 5 Grain yield potential of BPT Rice 2841 in comparison with the popular non pigmented quality rice variety (BPT 5204) and traditional black rice variety Burma black.
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Gopala Swamy et al.31, conducted a study on the response of stored grain pests involving 8 non-pigmented and 2 pigmented rice varieties and the results revealed that significantly higher number of lesser grain borer adults were found in BPT 2766 (336.67 no.s) while the red flour beetles were more in BPT 3111 (59.0 no.s) compared to BPT 2841 black rice which recorded 278.0 no.s lesser grain borer adults and 46.67 no.s red flour beetles. Eventhough, BPT 2841 possess high phenol content (272.33 mg GAE/100 g), other varieties recorded maximum number of stored grain insect pest adults, hence it may be concluded that normal management practices are enough for storage of coloured rice.

Studies on making rice based products like flakes, puffed rice, cakes, vermicelli, fryums etc. with traditional and improved black rice revealed that the glutinous black rice variety, Burma black is not suitable for making flakes (Fig. 1) due to it’s low amylose content (due to stickiness flakes were not separated) whereas BPT 2841 flakes were similar to other normal rice flakes. These products demonstrated favorable sensory attributes, which can be attributed to the intermediate amylose content of BPT 2841. Black and red rice bran can be an excellent ingredient to increase the nutritional value and antioxidant properties of noodles and bakery products32. Black rice powder extracted from the rice bran could be used as a healthy food coloring dye in sodas, functional foods, cosmetics, neutraceutical and other health products also.

Fig. 1
figure 1

Value added products prepared with BPT Rice 2841.

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Conclusion

BPT Rice 2841 is a lysine-rich, high-yielding black rice variety that combines superior nutritional attributes with excellent cooking quality, textural properties and agronomic resilience. Its high lysine content, moderate Glycemic index along with elevated levels of antioxidants, flavonoids, positions it as a functional food capable of addressing malnutrition and promoting health. The intermediate amylose content enhances it’s versatility for raw rice and value-added products, meeting diverse consumer demands. With moderate resistance to major pests and diseases, it offers sustainable cultivation potential, making BPT 2841 a significant contribution to food security, farmer’s income and health-focussed diets.