Evaluation of the polyphenol content, antioxidant activity, and tyrosinase inhibitory activity of ethanol/water extracts of rhizomes of Vietnamese Zingiber officinale, Curcuma longa, Alpinia officinarum, and Boesenbergia pandurata†

This study was conducted to determine the total polyphenol content, antioxidant activity and tyrosinase inhibitory activity of ethanol/water extracts of rhizomes of Vietnamese Zingiber officinale, Curcuma longa, Alpinia officinarum Hance, and Boesenbergia pandurata (Roxb.) Schltr. Total polyphenol content was determined by the Folin–Ciocalteu method, antioxidant activity was evaluated through the DPPH (2,2-diphenyl-1-picryl hydrazyl) free radical scavenging method, and tyrosinase inhibitory activity was estimated using the substrate of 3,4-dihydroxy-L-phenylalanine (L-DOPA). Qualitative results showed that all ethanol/water extracts contained phenolic compounds. The total polyphenol content of the extracts was higher than that determined in previous research, of which the Zingiber officinale extract had the highest value (188.52 ± 2.52 mg GAE per g) and the Boesenbergia pandurata (Roxb.) Schltr extract (61.32 ± 1.72 mg GAE per g) had the lowest value. Compared with the positive control substances used, of the four extracts, the Zingiber officinale extract also showed the best ability to inhibit DPPH free radicals with IC₅₀ = 35.04 ± 2.82 μg mL⁻¹, and the Curcuma longa extract showed the best tyrosinase enzyme inhibition ability with IC₅₀ = 26.07 ± 0.79 μg mL⁻¹.