Green extraction and partial purification of roselle ( <i>Hibiscus sabdariffa</i> L.) extracts with high amounts of phytochemicals and in vitro antioxidant and antibacterial effects

The aim of this study is to develop roselle (Hibiscus sabdariffa L.) extracts with high amounts of phytochemicals and in vitro antioxidant and antibacterial effects using green extraction technique. Roselle extract prepared by ultrasonic extraction with 80% ethanol and 1% hydrochloric acid (RSUEH) promoted the highest yield and high total phenolic (5.21 ± 0.05 g% gallic acid equivalent of dried extract), total flavonoid (1.29 ± 0.06 g% quercetin equivalent of dried extract), and total anthocyanin contents (0.21 ± 0.00 and 0.44 ± 0.00 g% cyanidin 3-glucoside equivalent of dried extract determined by pH-differential method with standard calculation and pH-differential method with standard curve of cyanidin 3-glucoside, respectively). From the validated high-performance liquid chromatography analysis, delphinidin-3-sambubioside, delphinidin-3-glucoside, and cyanidin-3-glucoside were found to be the major anthocyanins in roselle extract. This extract exhibited moderate 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging effect (EC₅₀ 289.61 ± 0.16 µg/mL) and strong inhibitory effects against Escherichia coli, Staphylococcus aureus, Staphylococcus intermedius, Proteus mirabilis, and Pseudomonas aeruginosa with the inhibitory concentration (IC₅₀) values in the range of 0.15 ± 0.05 to 4.64 ± 0.07 mg/mL and the minimum inhibitory concentration (MIC) values ranged from 1.66 ± 0.06 to 7.11 ± 1.05 mg/mL. From partial purification, fraction 28 (F28) contained higher amounts of all analyzed phytochemicals than roselle extracts and promoted stronger DPPH scavenging effects (EC₅₀ 54.88 ± 2.23 µg/mL). F28 showed antibacterial activity with IC₅₀ values against all tested bacteria in the range of 0.19 ± 0.07 to 4.69 ± 0.09 mg/mL, and the MIC values ranged from 2.46 ± 0.08 to 6.24 ± 0.09 mg/mL.