Evaluation of Antioxidant Activity of Curcumin-Free Turmeric (Curcuma longa L.) Oil and Identification of Its Antioxidant Constituents

Abstract

Antioxidant capacity of curcumin-free turmeric (Curcuma longa L.) oil was evaluated by two different in vitro assays: the DPPH radical scavenging assay and reducing power assay. Results showed that the turmeric oil (TO) possessed strong free radical scavenging activity and reducing power when compared to standard antioxidants such as butylated hydroxytoluene (BHT) and α-tocopherol (VitE). An aliquot of 20 μL/mL TO showed 91% free radical scavenging activity in the DPPH* assay, which was comparable to 10 mM BHT (86%) and 10 mM VitE (96%) under the same conditions. In the reducing power assay, the absorbance at 700 nm of 20 μl/mL TO was 1.085, which was comparable to 10 mM BHT (1.164). Higher concentration of TO at 100 μL/mL reached an absorbance of 1.537, which had no significant difference with 10 mM VitE (1.530). Among the complex constituents in the crude TO, ar-turmerone, turmerone, curlone and α-terpineol that were found to be the major components responsible for the detected antioxidant activities were isolated and identified using various chromatographic techniques including silica gel open column chromatography, normal phase HPLC, and GC-MS. These results showed that TO and some of its inherent bioactive components could potentially serve as alternative natural antioxidants.