The effects of dried oregano leaves (Mediterranean and Mexican oregano) extracted using different extraction techniques, solvent types, and six different ratios of each solvent to distilled water on total phenolic (TP) content and antioxidant properties were examined. The Folin-Ciocalteu and 1,1-Diphenyl-2-picryl-hydrazyl (DPPH) assays were performed to assess the antioxidant capacity. The different species of oregano had a significant effect on TP content (107.6 vs. 34.5 mg GAEg-1 in Mexican vs. Mediterranean oregano, respectively) (P<0.05). Comparing extraction techniques, the vortex procedure significantly increased the measured TP content compared to sonication or shaking (P<0.05); however, its effectiveness was sample species and solvent type dependent. Solvent type also had a significant impact on TP content of extracts in decreasing order of acetone, methanol, ethanol, and water (P<0.05). The solvent:water ratio on TP content of each extract was significant (P<0.05); higher TP content was measured for 40:60 and 60:40 acetone:water ratios for Mediterranean and 60:40 and 80:20 acetone:water ratios for Mexican oregano. The antioxidant capacity had a strong relationship with total phenolic contents. The current findings indicated that the species, extraction techniques, solvent type and the ratio of solvent:water had a significance influence on the TP content of two different species of dried oregano leaf, which may be a possible reason behind most variability reported on TP compounds of herbal and medicinal plants.
