Objective  Extracting polyphenols from peels of coffee beans (Coffea arabica L.) was optimized, and antioxidant activity of the extract determined.

  Method  Single factor and response surface experiments were conducted to optimize the ultrasound-assisted extraction process. Antioxidant capacity of the polyphenols obtained was determined by an in vitro method.

  Result  The optimized process applied a solid-to-liquid ratio of 1 ∶ 54 (g ∶ mL) and the ethanol concentration of 56% to extract at 69^oC for 42 min. The polyphenols content in the extract reached 34.68 mg·g⁻¹. The IC₅₀ of the extract was 2.10 μg·mL⁻¹ on 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radicals, 314.97 μg·mL⁻¹ on hydroxyl free radicals, and 322.02 μg·mL⁻¹ on superoxide anion free radicals. The scavenging effect of the extracted polyphenols was 0.99 times of that of L-ascorbic acid on DPPH free radicals, 0.52 times on hydroxyl free radicals, and 0.12 times on superoxide anion free radicals.

  Conclusion  The optimized polyphenol extraction process produced extract with significant antioxidant activity and could be applied to utilize the byproduct for the coffee industry.