Objective: Achillea species are endowed with multiple biological activities including antioxidant properties. However, no study has yet investigated the impact of extraction method and pH on the biological activities of these plants. The present study aimed to investigate the antioxidant and antimicrobial effects of methanol extracts from the aerial parts of the species Achillea biebersteinii Afan and Achillea wilhelmsii C. Koch (Asteraceae). In addition, the impact of extraction method and pH on these biological activities was evaluated.
Materials and methods: Methanol extracts of A. biebersteinii and A. wilhelmsii were prepared using classical maceration and high-intensity ultrasound methods. Ultrasound-assisted extraction was performed at three different pH values: 5.7, 6.3 and 6.9.
Results: Total phenolic compounds (range: 20.16-108.54 vs. 17.18-59.61 mg gallic acid equivalent/g sample in A. biebersteinii and A. wilhelmsii, respectively), total flavonoids (range: 8.33-12.97 vs. 7.79-9.41 mg catechin equivalent/g sample), 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity (IC₅₀: 40.63-346.34 vs. 84.02-462.41) and reducing power (IC₅₀: 504.44-4406.67 vs. 1710.00-5501.67) were significantly higher in A. biebersteinii vs. A. wilhelmsii and ultrasound-assisted vs. classical maceration extracts of both species. The aforementioned items were higher at pH = 6.3, followed by pHs of 6.9 and 5.7, respectively. Overall, A. biebersteinii extracts were more active against all of the tested microorganisms than A. wilhelmsii. Sensitivities of Gram-positive bacteria were higher for both Achillea extracts than the Gram-negative bacteria. No observable inhibitory activity was found from different extracts against Aspergillus niger.
Conclusion: The findings of the present study suggest that methanol extracts of A. biebersteinii and A. wilhelmsii possess antioxidant and antimicrobial activity, being higher in the former. Ultrasound-assisted extraction and pH of 6.3 have significant augmenting impact on the total phenolic and flavonoid content as well as antioxidant activities of both species.