ANTIOXIDANT ABILITY AND STABILITY STUDIES OF 3-O-ETHYL ASCORBIC ACID 237 contour map were drawn to point out the main effects of temperature and pH on 3-O- ethyl ascorbic acid. RESULTS AND DISCUSSION DPPH FREE RADICAL SCAVENGING ABILITY Figure 2A shows the DPPH radical scavenging ability. When the concentration was 0.04 g/L, the DPPH radical scavenging ability of ascorbic acid and 3-O-ethyl ascorbic acid was 82.91% and 55.52%, respectively. Ascorbic acid had better radical scavenging ability Fi gure 2. (A) DPPH radical scavenging ability. (B) First-order kinetic model (black fi lled circle: ascorbic acid white fi lled circle: 3-O-ethyl ascorbic acid).

JOURNAL OF COSMETIC SCIENCE 238 because it has more hydroxyl groups on the ring structure. However, when oxygen was presented, ascorbic acid easily degraded and resulted in a reduced biological ability. The ethyl group in the 3-O-ethyl ascorbic acid prevented it from being oxidized. Because of its better stability, this ascorbic acid derivative may be an ideal cosmetic ingredient. Based on equation 4, linear equations were achieved from the regression of -ln(1 - XA) plots (Figure 2B). The slope of the linear regressions represents the reaction rate con- stant (k). For ascorbic acid and 3-O-ethyl ascorbic acid, the k values were 59.70 and 30.03 L/g, respectively. The half-inhibitory concentration (IC50) was calculated from equation 4. The IC50 values were 0.014 and 0.032 g/L for ascorbic acid and 3-O-ethyl ascorbic acid, respectively. A higher k value represented a better DPPH radical scaveng- ing ability. Although the radical scavenging ability of the derivative was lower than that of ascorbic acid, 3-O-ethyl ascorbic acid was still a good antioxidant and could be used in cosmetics. REDUCING ABILITY ANALYSIS The ability of chemical compounds to provide electrons was indicated as the reducing ability. In this study, a higher absorbance measured at 700 nm wavelength represented a stronger reducing ability. Figure 3 shows the reducing ability of 3-O-ethyl ascorbic acid and BHA. Because BHA is a good electron donor and has the ability to reduce free radi- cals, it is often used as a standard for this analysis. As seen in Figure 3, when the concen- tration was higher than 1.5 g/L, the reducing ability reached a plateau. On the other hand, when the concentration was lower than 0.75 g/L, there was no signifi cant difference between 3-O-ethyl ascorbic acid and BHA. Although the reducing ability of the ascorbic Fig ure 3. Reducing ability analysis of 3-O-ethyl ascorbic acid and BHA (black fi lled circle: BHA white fi lled circle: 3-O-ethyl ascorbic acid black fi lled triangle: ascorbic acid).