Ultrasound-assisted extraction of bioactive birch (Betula sp.) bark triterpenoids using hydrophobic natural deep eutectic solvents

The simultaneous extraction of high-value bioactive triterpenoids, betulin (Bet) and betulinic acid (BAc), from birch (Betula sp.) bark has been optimized and analytically characterized to provide an improved approach in terms of efficacy and sustainability. First, in silico screening of numerous conventional organic solvents, biosolvents and hydrophobic natural deep eutectic solvents (h-NADESs) via the COSMO-RS theory revealed thymol:1-octanol (molar ratio of 4:1) as the optimal extractant (Bet + BAc solvent capacity, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>C</mml:mi> <mml:mrow><mml:mtext>Bet</mml:mtext> <mml:mo>+</mml:mo> <mml:mtext>BAc</mml:mtext></mml:mrow> <mml:mi>∞</mml:mi></mml:msubsup> </mml:math> = 21.7). Because ultrasound-assisted extraction (UAE) was found to outperform conventional solid-liquid extraction, a central composite experimental design was used to determine the UAE operating conditions that maximized the extraction yield of both target bioactives (61 °C for 24 min and 1 extraction cycle). Precise (RSD < 2.3%) and efficient extraction of Bet and BAc (recovery > 80% and 96%, respectively) was achieved by this new h-NADES UAE method, whose greenness was confirmed by an AGREEprep score of 0.76. A wide variability in the concentration of lupane-type triterpenoids (Bet: 12.09-32.17 mg g^-1; BAc: 0.69-1.43 mg g^-1), which is not fully dependent on the birch species considered, was observed in its application to different Betula sp. samples. The combination of UAE with h-NADES is a new and potential green approach for the enhanced extraction of bioactive birch bark triterpenoids for application in different fields.