Development of a novel assay to measure the antioxidant activity of Hawaiian naupaka leaf infusion

Plant foods provide nutrients necessary for the maintenance of life. They also produce other secondary metabolites, or phytochemicals, that may be helpful in promoting optimal human health. An important benefit of these phytochemicals is their antioxidant potential. Several different methods have been developed to measure antioxidant activity, most of which require expensive analytical equipment. In the current study, an inexpensive antioxidant assay was developed to evaluate an infusion made from leaves of an underutilized food plant, naupaka (Scaevola taccada), collected along the North Shore region of Oahu. This novel method utilizes a smartphone camera and colorimeter app to measure the degree of ferric to ferrous ion reduction upon addition of antioxidants and displays excellent linearity with ascorbic acid serving as an antioxidant standard. An infusion made from rosemary leaves (Rosmarinus officinalis) served as a positive botanical reference for the development of this assay. The average percent reduction of ferric ions by naupaka infusion ranged, depending on the smartphone and app used, from 21.64% to 24.15%. This demonstrates acceptable inter-device robustness of this novel antioxidant assay. Also, for the first time, antioxidant activity was demonstrated for a hot water infusion made from naupaka leaves.