Antioxidant recovery from hydrodistillation residues of selected Lamiaceae species by alkaline extraction

Aqueous solution of potassium hydroxide (KOH) was used in order to obtain rich in antioxidant activity extracts from Origanum dictamnus, Origanum hirtum, Origanum onites, Rosmarinus officinalis and Satureja thymbra, after removal of the essential oil via hydrodistillation. The increase of extraction time induced lower values of total phenolic content and DPPH radical scavenging ability of the extracts, while lower alkaline concentrations, i.e. 1% and 3%, proved to be more effective than 5% (v/w) KOH. The antioxidant activity of the extracts in sunflower oil was also measured by the oxidative stability index (OSI) method. OSI values decreased as extraction time increased, while the effect of KOH concentration at short extraction time was not significant. S. thymbra, O. hirtum and R. officinalis extracts showed high total phenolic content and good antiradical and antioxidant activity in just 30 min of extraction with KOH 1% (w/v). LC-MS analysis showed that all extracts were rich in phenolic acids, such as caffeic acid and rosmarinic acid, while R. officinalis was also rich in carnosic acid.