DEVELOPMENT OF ANTIOXIDANT SKIN GEL USING COFFEE SILVERSKIN 323 expressed by the IC50 value of the skin gel. The IC50 value indicates the concentration of the antioxidant, which is necessary to inhibit 50% of the DPPH as the free radical. The IC50 is given in ppm, which means milligram skin gel per liter solution. As can be seen in Figure 8, the addition of more coffee silverskin extract reduced the IC50 value, which means it resulted in a skin gel with a higher antioxidant activity. The increase in the antioxidant activity of the skin gel is correlated with the increase in the total phenolic content of the skin gel due to the addition of the coffee silverskin extract. The statistical analysis showed that there is a signifi cant difference of IC50 with the variation of the extract concentration (p 0.05). This result indicates that the coffee silverskin extract has great potential to be used as an additive for skin gels to obtain antioxidant-rich skin gel products. The pH value of a topical preparation should be within the skin pH range between 4.0 and 7.0 (20). The value of the pH should not be too acidic as it causes skin irritation and should not be too alkaline as it may cause scaly skin. The skin gels containing coffee sil- verskin extract showed pH values of 7.0, 6.63, 6.50, and 6.50, for the extract concentrations of 0.125%, 0.25%, 0.5%, and 1%, respectively. The addition of more coffee silverskin extract into the skin gel resulted in a slight decrease in the pH value. This is reasonable because the phenolic compounds of the coffee silverskin extract are mostly chlorogenic acids, which lead to an acidic condition of the gels. However, the pH values of the skin gels are all still within the pH range for topical preparations. Figure 7. Total phenolic content of skin gels with various concentrations of coffee silverskin extract (F1: 0.125%, F2: 0.25%, F3: 0.5%, F4: 1%).
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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