Post-Distillation By-Products of Aromatic Plants from Lamiaceae Family as Rich Sources of Antioxidants and Enzyme Inhibitors

There is currently no use for the vast quantities of post-distillation by-products, such as spent plant materials and residual waters, produced by the essential oil (EO) industry of aromatic herbs. In this study, the EOs of three Lamiaceae species (thyme, oregano, and basil) and their total, spent, and residual water extracts were phytochemically characterized and biologically assessed. The collected information was put through a series of analyses, including principal component analysis, heatmap analysis, and Pearson correlation analysis. Concerning the EOs, 58 volatile compounds were present in thyme (e.g., p-cymene, thymol), 44 compounds in oregano (e.g., thymol, carvacrol), and 67 compounds in basil (e.g., eucalyptol, linalool, estragole, (E)-methyl cinnamate). The LC-HRMS/MS analysis of the total, spent, and residual water extracts showed the presence of 31 compounds in thyme (e.g., quercetin-O-hexoside, pebrellin, eriodictyol), 31 compounds in oregano (e.g., rosmarinic acid, apigenin, kaempferol, salvianolic acids I, B, and E), and 25 compounds in basil (e.g., fertaric acid, cichoric acid, caftaric acid, salvianolic acid A). The EOs of the three Lamiaceae species showed the highest metal-reducing properties (up to 1792.32 mg TE/g in the CUPRAC assay), whereas the spent extracts of oregano and basil displayed very high radical-scavenging properties (up to 266.59 mg TE/g in DPPH assay). All extracts exhibited anti-acetylcholinesterase (up to 3.29 mg GALAE/g), anti-tyrosinase (up to 70.00 mg KAE/g), anti-amylase (up to 0.66 mmol ACAE/g), and anti-glucosidase (up to 1.22 mmol ACAE/g) effects. Thus, the present research demonstrated that both the raw extracts (EOs and total extracts) and the post-distillation by-products (spent material and residual water extracts) are rich in bioactive metabolites with antioxidant and enzyme inhibitory properties.