Microwave-assisted extraction of phenolic compounds from tea residues under autohydrolytic conditions

Phenolic compounds were extracted from three kinds of tea residues (green, oolong and black tea residues) by microwave-assisted extraction in water under autohydrolytic conditions without using any catalyst or organic solvent. Productions of phenolic compounds were enhanced by microwave heating at 230 °C. The main phenolic constituent in the extract from green tea residue was pyrogallol (24.6%) which was estimated to be originated from degradations of catechins. Derivatives of guaiacyl units of lignin such as dihydroconiferyl alcohol (10.3%) and vanillin (8.1%) were, however, the main constituents in oolong tea residue. In the case of black tea residue, derivatives of both catechins and lignin were comparably extracted. These phenolic compounds are interesting as a bio-based chemical feedstock such as phenolic precursors and antioxidants.