Summary This study investigated both the anthocyanin content and the antioxidant capacity of a set of genetically related glutinous and nonglutinous Thai black rice varieties. The ethanol/water extracts of the brans of these black rice varieties showed relatively potent antioxidant activities compared with those of tocopherol. These antioxidant activities were determined by thiocyanate, H2O2‐scavenging chemiluminescence (XYZ), Cu++/bathocuproine colorimetry (PAO) and 1,1‐diphenyl‐2‐picrylhydrazyl radical‐scavenging assay. The structural identification and quantification of the black rice anthocyanins performed by high‐performance liquid chromatography coupled with electrospray ionisation and tandem mass spectrometry found cyanidin‐3‐O‐glucoside and peonidin‐3‐O‐glucoside as the major anthocyanins in the ranges of 16.01–34.40 and 2.43–7.36 μg mL−1, respectively. The comparative study in terms of quantity of these phytochemicals and antioxidant capacity of the black rice bran extracts suggested the contribution of overall phenolic components rather than that of the particular anthocyanin pigments.

A method employing supercritical fluid extraction (SFE) followed by high‐performance liquid chromatography‐electrospray ionization‐mass spectrometry (LC‐ESI‐MS) was developed and used for quantification of the antioxidants in groups of carotenoids and flavonoids in the bran of four Thai black rice cultivars: Kao Hom Nin BD, Kao Hom Nin BT, Kao Hom Nin BT no. 3 and 1000–11–2–26. Trans‐β‐carotene, quercetin and isorhamnetin were identified in the crude yellow extracts obtained from SFE using liquid chromatography‐tandem mass spectrometry (LC‐MS/MS). Their contents in the black rice bran samples were determined using LC‐ESI‐MS operated in the selected ion monitoring (SIM) mode. The response of the mass spectrometric detection in the SIM mode was linear, within the range of 1.00–40.00 µg/mL (r2 = 0.9997), 0.25–2.00 µg/mL (r2 = 0.9987) and 0.25–2.00 µg/mL (r2 = 0.9982); the limit of detection was at 0.10, 0.15 and 0.15 µg/mL, and the limit of quantitation was 0.80 g of rice bran for trans‐β‐carotene and 0.60 g of rice bran for both quercetin and isorhamnetin. Method precisions in terms of intraday and inter‐day coefficients of variation were under 3.67% (relative standard deviation). The accuracy of the method estimated by determination of known concentrations of trans‐β‐carotene in the certified reference material, Standard Reference Material 3276, was found to range from 92 to 101%. Trans‐β‐carotene, quercetin and isorhamnetin were present in the bran of all black rice cultivars within the range of 33.60–41, 1.08–2.85 and 0.05–0.83 µg/g, respectively. PRACTICAL APPLICATIONS Production and accumulation of trans‐β‐carotene, quercetin and isorhamnetin in the bran of black rice revealed in this study imply that more attention should be paid to the analysis for screening black rice that has high contents of these bioactive phytochemicals, especially those obtained in rice‐breeding programs. The developed supercritical fluid extraction and liquid chromatography‐mass spectrometry method for quantification of β‐carotene, quercetin and isorhamnetin in black rice bran samples proved to be selective, precise, linear and sensitive, and can efficiently be employed for such analytical purposes. Additionally, extraction of carotene components by supercritical CO2 prior to LC‐MS analysis not only makes the total analysis time much shorter than that of the conventional extraction methods but also eliminates a sample clean‐up step, thus offering a clean analytical procedure.

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