In this section, the focus is on downstream processes for recovering vanillin from the oxidation media. The extraction and purification of vanillin from the reaction mixture have been matters of great concern, leading to the development of several technologies on chemical engineering separation methods such as solvent extraction, distillation, acidification/precipitation, bisulfitation, membrane, crys­tallization, supercritical extraction, adsorption, and ion exchange.

After the chemical alkaline oxidation of the lignin several low molecular weight aromatic compounds besides vanillin are present in the solution as water-soluble sodium phenolates. Clearly, the full composition of the complex mixture derived from the oxidation process depends on the lignin-based raw materials, operating conditions and applied chemicals. It is expected a mixture containing lignin oligomers, simple phenolics as aromatic aldehydes, and respective ketones and acids, and other secondary products in minor amount as lactones and also guaiacol and syringol [160, 161].