DEVELOPMENT OF ANTIOXIDANT SKIN GEL USING COFFEE SILVERSKIN 319 in the extraction temperature above 40°C did not result in a signifi cant increase in the antioxidant activity of the extract (p 0.05). This result indicated that all phenolic com- pounds were completely extracted from the coffee silverskin at an extraction temperature of 40°C and an extraction time of 60 min. This is very interesting because an extraction temperature of 40°C is appropriate for the industrial scale extraction process because of the lower energy consumption for heating. COFFEE SILVERSKIN EXTRACT POWDER Based on the aforementione d results, coffee silverskin extract in powder form was prepared by drying the coffee silverskin extract solution using a spray dryer. First, the extract solution was prepared by extracting the coffee silverskin at 40°C for 60 min because this extrac- tion condition resulted in a high total phenolic content and a high antioxidant activity. Then, the fi ltered extract solution was dried using a spray dryer at an inlet temperature of 170°C and an outlet temperature of 125°C. Figure 5 shows the picture of the extract solution before drying and the extract powder after drying. The measurement of the moistu re content showed that the dried coffee silverskin extract powder had a moisture content of 5.6%. The analysis of the total phenolic content using the Folin–Ciocalteu method with GA as a standard showed that the coffee silverskin Figure 2. Effect of extraction time on the antioxidant activity of the coffee silverskin extract solution (ex- traction temperature: 40°C).

JOURNAL OF COSMETIC SCIENCE 320 extract powder had a total phenolic content of 188.83 mg GAE/g solid extract, which is slightly lower than that of the extract solution before drying of 210.27 mg GAE/g solid extract, as shown in Table II. The decrease in the total phenolic content after drying occurred because the extract was treated at a high temperature of 170°C during the spray drying process. The heating of the extract degraded some phenolic compounds, resulting in a lower phenolic content of the dried extract. A similar phenomenon was also reported by Orphanides et al. (19) who studied the effect of drying on the phenolic content and anti- oxidant capacity of spearmint. The effect of drying on the antioxidant activity can be seen in Table II. The antioxidant activity of the extract is expressed as IC50 value, which means the concentration of the sample needed to inhibit 50% of DPPH as the free radical. Thus, a low value of IC50 corresponds to a high antioxidant activity. As can be seen, the IC50 value of the extract powder was 433.64 ppm, higher than that of the extract solution of 358.15 ppm. This result is in accordance with the decrease in the total phenolic content after the drying process as described earlier. SKIN GEL CONTAINING COFFEE SIL VERSKIN EXTRACT The basic skin gel was prepare d using the formulation and the method described previ- ously. The coffee silverskin extract powder was then added to the basic skin gel and mixed Figure 3. Effect of extraction temperature on the total phenolic content of the coffee silverskin extract solu- tion (extraction time: 60 min).