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.