2.3.1. Ultrasound-Assisted Extraction

The extractions of phenolic compounds from black locust flowers were performed in an ultrasonic cleaning bath (Sonic, Nis, Serbia) with dimensions: 30 × 15 × 20 cm. The bath was filled with distilled water up to one third of its total volume (about 3.0 dm3). The operating frequency was 40 kHz, while the total power was 3 × 50 W. The powder of black locust flowers (2 g) was transferred into a flask of 100 cm<sup>3</sup> and extracted with 20 cm<sup>3</sup> of the ethanol solution at the defined temperature. The liquid-to-solid ratio of 10 cm<sup>3</sup> g<sup>−</sup><sup>1</sup> was the same during extractions, because the evaporation of the used solvents was prevented using the reflux condenser. After extraction, the flasks were immediately cooled to room temperature using chilled water. The extracts were separated from the solid matrix by vacuum filtration and subjected to further analysis. In order to determine the concentration of extracts, 1 cm<sup>3</sup> of the aliquot was dried at 105 ◦C in a laboratory oven.
