Seasonality modifies rosemary’s composition and biological activity

• Effects of seasonal variation on Rosmarinus officinalis extracts were evaluated.
• The highest value of carnosic acid was found in summer.
• Essential oil: camphor was found as the major compound; mainly in summer.
• Compounds extracted from plants collected in the summer had the best antimicrobial and antioxidant activity.

The rosemary (Rosmarinus officinalis L.) essential oil is widely used as an ingredient in food formulations and fragrance industry. However, there are few studies on the effects of seasonal variation on the composition and biological activities of this oil. Thus, the aim of present study was to evaluate the effects of seasonal variation (2012–2013) on composition and antioxidant and antimicrobial activities of rosemary essential oils and ethanolic extract. Composition of essential oil was determined by GC-MS and that of the extract by LC-MS/MS. Antioxidant activity was evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and beta-carotene/linoleic acid methods. Total phenolic compounds were determined by Folin–Ciocalteu method, and antimicrobial activity was evaluated by the determination of the minimum inhibitory concentration. Camphor was found as the major compound in essential oil (24.3–35.9%, mainly in January/2013), while carnosic acid was found in largest amount in January/2013 (summer) in the ethanolic extract, which presented the largest antioxidant activity, similar to butylated hydroxytoluene (BHT) and alfa-tocopherol. The results showed that essential oil and extracts harvested in summer exhibited strong activity against Staphylococcus aureus, even higher than sulfanilamide, due to the high amount of carnosic acid and camphor. Altogether, the essential oil and the extracts collected in summer showed better antioxidant and antimicrobial activities, which can be explained by the high levels of camphor and carnosic acid.