Polyphenolic contents and antioxidant activities of two medicinal plant species, Mentha piperita and Stevia rebaudiana, cultivated in Iran

Mentha piperita (peppermint) and Stevia rebaudiana (stevia) are now widespread in cultivation worldwide. However, reports about the contents of these two plant species are scarce. To establish the polyphenolic contents and antioxidant activity of these plants by ferric ion-reducing activity (FRAP) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) methods, local plants were harvested in flowering stage. Samples without any previous treatments were analyzed using high-performance liquid chromatography (HPLC) to determine the polyphenolic contents. Also, their antioxidant properties were determined using FRAP and DPPH methods. Six phenolic compounds including quercetin, p-coumaric acid, trans-ferulic acid, hesperidin, hesperetin, and rosmarinic acid were identified and quantified in peppermint. However, hesperidin, eugenol, coumarin, and vanillin were the most component in stevia. Peppermint showed better antioxidant properties than stevia in both FRAP and DPPH assessments. Our findings demonstrated that these two plants contain high polyphenolic compounds and are potent antioxidant species. Further studies on the therapeutic properties of these plants in animal and human models of diseases are recommended.