Effect of extraction conditions on the concentration of phenolic compounds in Mexican oregano (Lippia graveolens Kunth) residues

Introduction: Mexican oregano (Lippia graveolens Kunth) leaves are marketed for use in food and for essential oil extraction. Phenolic compounds with antioxidant properties can be obtained from the residues (leaves without oil and stems) under appropriate extraction conditions. Objective: To evaluate the effect of extraction solvent and mass/volume ratio on phenolic compounds concentration and their antioxidant capacity in oregano residues. Materials and methods: Residual leaf (without oil) and stems of oregano were used to obtain extracts with 30, 50 and 80 % aqueous ethanol (ET30, ET50, ET80, respectively) and solvent mass/volume ratios (1:10, 1:20 and 1:30). Yield in solids, total phenols concentration and flavonoids, and antioxidant capacity were evaluated in the extracts. Similarity analysis between extracts was performed by HPLC-DAD and the main phenols were identified by UPLC-MS. Results and discussion: Extract yields, phenol concentrations and flavonoids and antioxidant capacity for leaves were higher than for stems. The maximum concentration of phenols was obtained with ET80 and 1:30 ratio, which indicates better dissolution in ethanol than in water. The extracts ET50 and ET80 had chromatographic similarity of phenols in both residues; naringenin, taxifolin, eriodictyol, caffeic acid and luteolin were the major compounds. Conclusion: Ethanol-water concentrations and mass/solvent ratios are feasible for obtaining flavonoid and non-flavonoid phenolic compounds with antioxidant capacity from oil-free leaves and stems of oregano.