## 1 Introduction  
Oleuropein is a phenolic secoiridoid glycoside, derived from ligstroside, and found in all parts of the olive tree, mainly in the fruit and especially in the leaf. It was discovered in 1908 by [PERSON] and [PERSON] [1] who described it as a non-crystalline, intensely bitter substance that is easily soluble in alcohol, and fairly soluble in water [2]. Its concentration in the young olive fruit (up to 140 mg/g of dry matter) and in the leaves (60-80 mg/g dry matter) caused a very strong perception of bittermess [3, 4]. Degradation products of oleuropein are hydroxytyrosol, elenolic acid, and glucose. The olive leaf has been used in Mediterranean folk medicine since ancient times to treat diabetes, hypertension, and hypercholesterolemiaaemia [5]. More recently, olive leaf extract (OLE) has become one of the most valuable by-products of the olive oil industry due to its diverse biological potential (antimicrobial, antioxidant, antidiabetic, anticarcinogenic, immunomodulatory) [6; 7; 8]. Valuable biological potential is associated with the presence of phenolic compounds, especially phenolic secoriidoids such as oleuropein, hydroxytyrosol, verbascoside, oleuropein aglycone, ligstroside aglycone [10]. The conditions for the extraction of oleuropein from olive leaves have been demonstrated in many studies [1; 11; 12]. The choice of the optimal extraction method, solvent, and extraction time are decisive factors. Oleuropein from olive leaves can be extracted by various methods such as Soxhlet extraction, cold solvent extraction, ultrasound-assisted extraction (UAE), supercritical fluid extraction (SFE), microwave-assisted extraction (MAE), pressurized liquid extraction, and novel techniques using membranes [1]. In this study, low-frequency ultrasound was used for extraction. The effects of cavitation of ultrasound extraction are described in the literature [1], leading to mechanical stress on the cells and rupture of the cells. Ruptured cells release water, which is necessary for the hydration of the plant material and the mass transfer between the plant material and the solvent. The frequency of the device is a decisive factor in ultrasound extraction. It is assumed that low frequencies of around 20 kHz are more effective for plant materials such as olive leaves, as the bubbles generated by cavitation can implode more easily than those generated at high frequencies [1]. The highest yield of oleuropein (812.9 mg/g) extracted in methanol:water (80:20) with an ultrasonic bath of 25 kHz was reported by [PERSON] et al. [11]. MAE extraction is generally considered an environmentally friendly technique characterized by a lower solvent requirement and a short extraction time. [PERSON] et al. [13] compared three extraction methods for total phenols from olive leaves (MAE, maceration, and ultrasound-assisted extraction). They found that MAE with water as solvent was the most efficient (82% recovery of total phenols). [PERSON] et al. [12] investigated the total phenolic content in olive leaves of five Italian cultivars extracted with MAE under different extraction conditions and found that the cultivar and harvest period had a greater influence than the MAE conditions. Supercritical fluid extraction is also considered an environmentally friendly technique due to the use of non-toxic solvents [14]. The extraction of oleuropein from olive leaves with supercritical fluid has been described in several studies [15; 16; 17]. Some authors reported very low amounts of OLE after SC-CO\({}_{2}\) extraction [18]. The highest yield of OLE was obtained by SC-CO\({}_{2}\) at 300 bar, 100 \({}^{\circ}\)C and using methanol 20% \(v/v\)[1]. It has already been mentioned that oleuropein is known for its various pharmacological and biological properties (antioxidant, antimicrobial, antiviral, anticancer, hypolipidemic, cardioprotective, anti-inflammatory), which puts oleuropein in the focus of numerous in vivo and in vitro studies [19; 20; 21].