3.2.1. Reducing power
Reducing ability to convert Fe3+ to Fe2+ is an indirect evidence for the antioxidant activity of an extract or a compound (Roy et al.,2011). In the reducing power assay, the antioxidants, i.e. the reducing species present in the solution causes the reduction of the Fe3+/ferricyanide complex to form Fe2+ ions. This reaction was monitored spectrophotometrically by recording the absorbance of the reaction mixture at 700 nm (Barros et al., 2008). According to the mentioned principle, the higher absorbance value at 700 nm is gained, the stronger reducing power of the assayed sample shows. Reducing power characteristics of ascorbic acid, astaxanthin and astaxanthin/HPCD complex are shown in Fig. 5. Reducing powers of the three samples were all enhanced along with the increase of concentration among the tested range. The reducing power of
ascorbic acid was stronger than astaxanthin under the same concentration. It has reported that astaxanthin was poorly soluble and assembled in water (Köpsel et al., 2005). The assembled astaxanthin molecules could not fully close and react with the Fe3+/ferricyanide ions in the test. Nevertheless, the complexation made that astaxanthin was completely dissolved in water. Hence, the reducing power of the complex was stronger than that of native astaxanthin. Due to the limits of solubility and sample quantity, the complex was only tested at the concentration of 50 mol/L.