ANTIOXIDANT ABILITY AND STABILITY STUDIES OF 3-O-ETHYL ASCORBIC ACID 241 shows the 3-D mesh plot of the RSM model. The surface curve of the fi gure represented the HPLC peak area ratio of 3-O-ethyl ascorbic acid and methylparaben. A larger peak area ratio indicated that more 3-O-ethyl ascorbic acid was detected. The highest calcu- lated point of the surface curve in Figure 5 was 3.133, which occurred at 36.3°C and pH 5.46. When cosmetics were stored at the optimal conditions, 3-O-ethyl ascorbic acid had the best stability. Because this compound was very sensitive to changes in pH, buffers would be required for cosmetics to maintain the optimal pH (5.46). On the other hand, the stability of 3-O-ethyl ascorbic acid was not signifi cantly affected by temperature. These results would be useful for cosmetic manufacturers who could use 3-O-ethyl ascor- bic acid in their products. CONCLUSIONS Based on the reducing and DPPH radical scavenging ability analysis results, 3-O-ethyl ascorbic acid is a good antioxidant. Moreover, this compound inhibited the activity of tyrosinase to prevent the formation of melanin. Therefore, it could be used in cosmetics Figur e 5. Response surface plot showing the effect of temperature (°C) and pH on the stability of 3-O-ethyl ascorbic acid. Table III Regression Coeffi cients of the 3-O-Ethyl Ascorbic Acid Stability RSM Model Regression coeffi cient Estimate Intercept, β0 -3.8093 β1 0.1063 β11 -0.0004 β2 1.8378 β22 -0.1222 β12 0.0139

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