Green and Non‐conventional Extraction of Bioactive Compounds from Olive Leaves: Screening of Novel Natural Deep Eutectic Solvents and Investigation of Process Parameters

ABSTRACT: Olive leaf as an agricultural waste contains valuable bioactive compounds that are mainly used for pharmaceutical and cosmetic industries. Lately the major component, oleuropein, has gained extra attention due to the anti-viral activity against SARS-CoV-2 that causes Coronavirus disease (Covid-19). In this study, extraction of the bioactive compounds from olive leaves was conducted using a non-conventional and green method. New generation green solvents, natural deep eutectic solvents (NADES) were used in combination with ultrasound assisted extraction. Screening of NADES type, temperature, and particle size were investigated using one-pot-at-a-time method while, NADES amount and liquid-to-solid ratio were optimized using experimental design. The results were evaluated in terms of total polyphenol yield (Y(TP)), total flavonoid yield (Y(TF)) and antiradical activity (A(AR)). At the optimized conditions, the highest total polyphenol yield and the highest total flavonoid yield were achieved with choline chloride–fructose–water (CFW) (5:2:5) as 187.31 ± 10.3 mg gallic acid equivalent g(−1) dw and 12.75 ± 0.6 mg apigenin equivalent g(−1) dw, respectively. The extracts were also analyzed for oleuropein, caffeic acid and luteolin contents. The highest amount of oleuropein and caffeic acid were extracted by glucose–fructose–water (GFW) (1:1:11) as 1630.80 mg kg(−1) dw and 112.77 mg kg(−1) dw, respectively. GRAPHIC ABSTRACT: [Image: see text] SUPPLEMENTARY INFORMATION: The online version of this article (10.1007/s12649-021-01411-3) contains supplementary material, which is available to authorized users.