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Polyphenols and flavonoid compounds are known to have antioxidant activity and can reduce blood glucose levels. Zingiber officinale var. Rubrum is one of the plants with high polyphenol content. This study aimed to extract phenolic and flavonoid compounds from this plant with various solvents primarily natural deep eutectic solvents (NADES), namely 70% ethanol, NADES1 citric acid: sucrose (1:1), NADES2 sucrose: glucose: fructose (1:1:1), NADES3 choline chloride: glycerol (1:2), and NADES4 glycerol: urea (1:1). The phenolic and flavonoid levels were determined using spectrophotometer, the antioxidant activity was measured using DPPH and Ferric Reducing Antioxidant Power (FRAP) methods, while the antidiabetic activity was evaluated using enzymatic assay of α -amylase and α-glucosidase enzymes. The findings indicated that all NADES outperformed 70% ethanol in extracting phenols, with NADES1 showing the highest phenolic content. Meanwhile, the highest flavonoid content was observed in extracts with 70% ethanol and NADES1. The results of antioxidant activity analysis using DPPH revealed that extracts with 70% ethanol, NADES1, and NADES4 exhibited considerable strength (IC50: 45-49 µg/mL), with the average values of the three solvents being not significantly different. From the results of the FRAP assay, NADES3 demonstrated the highest strength Antioxidants Activity Index (AAI: 29.24±3.08 mmol/g), whereas 70% ethanol showed the lowest strength (AAI: 22.76±5.23 mmol/g). The average AAI values for all solvents indicated no significant differences. Among the solvents, NADES1 exhibited very strong inhibitory activity against α-amylase (19.62±0.20 µg/mL), while for α-glucosidase, NADES1 was stronger (IC50: 57.36±6.08 µg/mL) than other solvents. In conclusion, NADES can be a promising tool for extracting secondary metabolites from Zingiber officinale var. Rubrum as it shows both antioxidant and antidiabetic activities.
Kartini et al. (Mon,) studied this question.