## 1 Introduction  
In recent years, the nutritional awareness of consumers has increased significantly [1; 2]. Nowadays, grocery customers are more frequently looking for healthy food rich in bioactive compounds. Phenolic compounds are a well-known and large group of bioactive substances, which are commonly spread in nature. Phenolic compounds regulate the number of free radicals formed during metabolomic transition, inhibit the growth of microorganisms, and extend the freshness and acceptance of products [3; 4; 5]. Therefore, manufacturers are looking for new natural sources to be used in functional foods [1; 2]. _Eutere olerca_ Mart. (acai) is a plant assigned to the family areaceae, originating from Amazonia and characterized by a high number of phenolic compounds. Acai is considered a \"superfruit\" because it contains many secondary metabolite compounds (it was identified as having about 90 bioactive compounds: 31% are flavonoids, 23% phenolic compounds, 9% anthocyanins, 11% lignoids) [1; 6]. Thanks to its composition, acai has a proven antioxidant activity against hydroxyl, peroxyl and 2,2-diphenyl picrylhydrazyl (DPPH) radicals and it also inhibits liposome oxidation [7; 8]. Moreover, among Amazonian fruits such as bacuri, abiu, cupuacu and graviola, acai berries have the highest activity against 2,2-anizobis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS) radicals [9]. Acai is currently used as an anti-inflammatory agent, reducing the damage caused by oxidative stress and working against neurodegenerative diseases, even cancer [6; 10; 11]. Acai, due to its unique and broad-spectrum usage, drew the attention of both consumers and food producers [12], which, in turn, increased the demand for acai on the market [13]. Therefore, it seems to be of great importance to find and optimize the best method of acai extraction. Two alternative groups of extraction methods are used:  
* Conventional. * Green (Table 1) [14]. Conventional methods use toxic solvents such as dichloromethane and long periods of heat exposure, which result in high energy requirements and the degradation of thermally labile molecules [15]. Green extraction reduces the energy consumption, time and/or the amount of organic solvents used in the procedure. Moreover, green extraction is also more efficient than the conventional method in maintaining the high amount and quality of isolated compounds [16]. Ultrasound breaks down the biological cellular structure through cellular sonoporation, which causes the formation of cell pores, and increases the extraction surface area [17; 18]. This extraction method might be used at various stages (for raw materials and final products) and in various industries (food, biotechnology or agricultural technology) [19; 20]. In turn, microwaves are used for extraction to separate components from the solid and create the liquid fraction. microwaves are a form of radiation with an electromagnetic spectrum in the frequency range from 300 MHz to 300 GHz. The influence of microwaves on the processes of chemical synthesis and extraction is closely related to the conversion of electromagnetic energy into heat. Microwaves have electric and magnetic field components perpendicular to each other. Rapid dielectric heating is caused by the formation of a phase difference between the orientation of the field and the dipole [21]. Until now, microwaves and ultrasound have been utilized for extraction in different types of raw materials [22; 23; 24; 25] and acai berries [26; 27]. The study conducted by [PERSON] (2020) proved that the extraction of phenolic compounds and acai anthocyanin might be optimized using microwaves and ultrasound. These studies mainly focused on obtaining as many of these compounds as possible. However, the most important characteristic of phenolic compounds is their antioxidant activity, which plays an important role in the \"scavenging of radicals\" [5; 28; 29].