Antioxidant Properties of Greek O. dictamnus and R. officinalis Methanol and Aqueous Extracts—HPLC Determination of Phenolic Acids

This study determined the total phenol, phenolic acid content, the free radical scavenging activity, and ferric reducing antioxidant properties of methanol and aqueous extracts of R. officinalis and O. dictamnus species (Crete, Greece). The methanol extracts showed higher radical scavenging activity (P < 0.05) than the aqueous extracts (mean EC₅₀ value: 0.33 and 0.38 mg/mL, respectively). Rosemary was superior in activity to dittany (mean EC₅₀ value: 0.26 and 1.04 mg/mL, respectively). A number of phenolic acids were also tested for comparison reasons and showed stronger radical scavenging activity than the synthetic antioxidant BHT (mean EC₅₀ value: 0.0821 mg/mL). In the contrary, the aqueous extracts showed significantly higher ferric reducing antioxidant properties (P < 0.05) than the methanol extracts did (mean EC₅₀ value: 0.028 and 0.038 mg/mL, respectively). Rosemary was superior in reducing activity to dittany (mean EC₅₀ value: 0.035 and 0.107 mg/mL, respectively). Caffeic and rosmarinic acids were found to be stronger reducing agents (mean EC₅₀ value: 0.0034 and 0.0021 mg/mL, respectively) than the other phenolic acids as strong as ascorbic acid (mean EC₅₀ value: 0.0033 mg/mL). The aqueous plant extracts contained significantly higher total phenol (P < 0.05) than the methanol extracts did (mean value: 7.55 and 4.07 mg/g dry leaf). Rosemary was a better source of total phenol than dittany (mean value: 5.14 and 3.35 mg/g dry leaf). Rosmarinic and caffeic acids were identified in all the plant extracts and rosmarinic acid was the predominant phenolic acid ranging from 5574.6 to 7542.9 mg/kg dry leaf. The aqueous plant extracts were significantly higher in phenolic acid content than the methanol extracts (P < 0.05) (mean value: 9631.2, 7796.4 mg/kg dry leaf, respectively). Dittany was the highest source of phenolic acid content (P < 0.05) compared to rosemary (mean value: 9134.9 and 8292.7 mg/kg dry leaf, respectively). A positive linear correlation was found between total phenol content, radical scavenging activity and ferric reducing antioxidant properties of dittany extracts (r² = 0.9998).