DPPH radical scavenging activity
The DPPH antioxidant assay is based on the characteristic of DPPH a stable free radical which centered on nitrogen atom, to decolorize in the presence of antioxidants. The DPPH radical contains an odd electron, which is responsible for the strong absorbance at 517 nm, it is, a visible deep purple color. When DPPH accepts an electron donated by an antioxidant, the DPPH is decolorized which can be quantitatively measured from the changes in absorbance (Ara & Nur, 2009). DPPH radical scavenging activity of astaxanthin and astaxanthin/HPCD complex were evaluated and compared with that of the ascorbic acid, a control compound (Fig. 6). Scavenging activities of ascorbic acid and astaxanthin were enhanced with the increase of concentration among the evaluated range. The scavenging effect of astaxanthin was agreed with the previous reports (Chen, Wang, Ma, Zheng, & Li, 2007; Liu & Osawa, 2007). However,the scavenging activity of astaxanthin was weaker than ascorbic acid (about a half) under the same concentration. The DPPH radical scavenging activity of the complex degraded with the increase of concentration. Nevertheless, it also has a considerable scavenging effect at lower concentration, scavenging rate reached 54.49% at 2.5 mol/L, and the result was higher than that of ascorbic acid and astaxanthin, 10.50% and 4.43%, respectively. When the concentration was raised to 7.5 mol/L, scavenging activity of the complex was equal to ascorbic acid, and then became lower. It indicates that the complexation was helpful to enhance the antioxidant activity of astaxanthin in aqueous solution. However, at a high concentration, the increase of HPCD made that the release of astaxanthin became difficult from the complex and the activity obviously Fig. 6. DPPH radical scavenging activity of ascorbic acid, astaxanthin and astaxanthin/HPCD complex. decreased. Rao et al. (2007) reported that astaxanthin exhibited about 50% of antioxidant activity at the 20 ppm level of carotenoid by using the DPPH method and hydroxy radical scavenging activity and which clearly indicated that astaxanthin is stable in its ester form in almost all the edible oils at room temperature rather than in aqueous form. Those result coincides with our present study.