Solvent-dependent metabolomic profiling of Acacia mearnsii bark: an untargeted metabolomics approach

With great social and economic importance, black wattle bark (Acacia mearnsii De Wild.) stands out due to its high content of condensed tannins. Although it is a known and exploited source of proanthocyanidins, little is known about the other compounds that may be present in this raw material and that may further contribute to its bioactive properties. Given this context, this work focused on applying untargeted metabolomics to investigate the metabolic diversity of black wattle bark using different extraction solvents. Furthermore, it aimed to investigate the proanthocyanidin profile of the raw material cultivated in southern Brazil. It was observed that the use of different solvents influenced the extraction and composition of metabolites, with the main differences observed between aqueous and organic solvents. Thus, 50% and 70% acetone, as well as 70% methanol, achieved the highest relative abundances in terms of metabolites. Eighty different compounds were annotated, including organic acids and phenolic compounds, among others, many of which were detected for the first time in black wattle bark. These results demonstrate the richness of the extracts in terms of their metabolic composition, exhibiting chemical complexity far beyond the condensed tannins. Regarding proanthocyanidins, the presence of oligomers up to pentamers was observed. The monomers observed were varied, with (epi)catechin and (epi)gallocatechin acting as polymer chain initiators, while (epi)robinetifinol and fisetinidol are the extender units. The metabolic diversity verified in black wattle bark justifies the potential use of its extracts as natural additives to be explored by the food and feed industries.