JOURNAL OF COSMETIC SCIENCE 324 CONCLUSION This study demonstrated the utilization of coffee silverskin through an optimized extraction process to produce an extract with high total phenolic content and high antioxidant activity. It was observed that the extraction time and temperature strongly affected the total phenolic content and the antioxidant activity of the coffee silverskin extract. The extraction at 40°C and 60 min resulted in an extract with a high total phenolic content of 31.15 ± 2.77 mg GAE/g coffee silverskin and a high antioxidant activity of 68.44 ± 0.76%. It was found that the addition of the coffee silverskin extract powder to a basic gel resulted in a gel having antioxidant property. The antioxidant activity of the gel increased with increasing coffee silverskin extract concentration in the gel. The result of this study showed that coffee silverskin has great potential to be used as a source of antioxidant and the extract can be applied as an additive for various skin care products. ACKNOWLEDGMENT This work has been fi nancially supported by the Ministry of Research, Technology and Higher Education of the Republic of Indonesia through the Penelitian Terapan Unggulan Perguruan Tinggi research grant program 2018. Figure 8. IC50 of skin gels with various concentrations of coffee silverskin extract (F1: 0.125%, F2: 0.25%, F3: 0.5%, F4: 1%).

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