Lignin – Derived antioxidants as value-added products obtained under cavitation treatments of the wheat straw processing for sugar production

Most of biorefinery technologies are focused on the utilization of plant polysaccharides into chemicals, whereas lignin, the second most abundant natural polymer, is commonly used as a resource to generate power. This is the first report on flavones-bound lignin obtained by wheat straw (WS) pretreatment under low-frequency ultrasound (US) and hydrodynamic cavitation (HC) in sugar production processing. US and HC procedures carried out in aqueous 0.2–0.4% NaOH solutions and various liquid/biomass ratios strongly decreased the lignin content in the solid fraction by 30–45% and 25%, respectively, compared to non-treated WS. Moreover, НС pretreatment increased (approximately tenfold) the portion of ordered cellulose in residual biomass that could hinder the further enzymatic hydrolysis of WS. By means of sequential HC and US pretreatment the ordered cellulose content in solid fraction significantly decreased, compared to the HC alone. Furthermore, it was demonstrated that, after US pretreatment, hemicelluloses and oligomeric fragments of lignin and ligno-carbohydrate complexes were transferred into the liquid phase. In particular, tricin derivatives were detected in liquid fractions with a concentration of 30–35% on o.v.d. ash free sample, and more than 50 compounds associated with low-molecular-weight lignin fragments. These compounds were tentatively identified using high-resolution mass spectrometry (HR-MS) based fragmentation (sequencing) and a self-made database, showing that 16 out of 50 identified compounds belonged to the fragments of model lignin structural units. Tricin and low molecular weight lignin derivatives may find applications as strong antioxidants (i.e. lipidic systems). These findings proved that waste lignin streams obtained after cavitational processes could be a source of value-added products from WS conversion.