Mechanically Activated Extraction of Bioactive Substances from Populus tremula L. Bark: Yield and Functional Properties

A more efficient extraction of bioactive plant substances is a major biotechnological and pharmaceutical challenge. The composite structure of wood fiber creates a major diffusion barrier to the extraction of valuable secondary metabolites. This research established some reliable correlations between the mechanical activation modes, the structural and physical profile of tree bark, and the bioactive extraction kinetics. The study involved aspen (Populus tremula L.) bark harvested in the Tomsk Region, Russia. The mechanical activation was performed in an AGO-2S planetary mill. At a centrifugal acceleration of 300 m/s2 for 5–10 min, the yield of water-alcohol fractions increased by 9.5–19%; when it was raised to 600 m/s2, the yield increased by 40–44%. The yield of extracted polyphenols reached 34.6% at 1,395 rpm (7–10 min) and 61.5% at 1,820 rpm. The highest yield of salicin occurred at 1,820 rpm for 1–3 min and was between 0.80 ± 0.01 and 1.00 ± 0.02% by weight. A three-minute dispersion of aspen bark at 1,395 rpm increased the share of the carbohydrate component D1080/D1460, alcohol D1140/D1460, carboxyl groups D1280/D1460, -C=C- D1600/D1460, and OH groups D3400/D1460 while the number of alkyl substituents D2920/D1460 and carbonyl groups D1720/D1460 decreased. The mechanical activation of raw material during one-minute grinding at 1,395 rpm raised the antioxidant content to 1.15 ± 0.02 mol/kg and led to the formation of second type inhibitors. When the reactor speed remained 1,820 rpm for 5 min, the increase reached 1.66 ± 0.03 mol/kg, with an induction period of 90 min. The identified patterns of mechanical pretreatment increased the profitability and sustainability of producing valuable bioactive substances from renewable plant raw materials.