In the conventional method to isolate vanillin from the oxidized solution, the remaining lignin is precipitated by acidification adding carbon dioxide or a mineral acid like sulfuric acid. A liquid-liquid extraction with organic solvents, such as benzene, toluene, or ethyl ether enables the recovery of vanillin from the acidified liquid fraction [98]. The vanillin is co-extracted for a sodium bisulfite aqueous solution in the form of vanillin-bisulfite complex insoluble in the organic solvent. Finally, the vanillin complex in aqueous fraction must be acidified to recover free vanillin [162]. The neutralization and isolation of the vanillin from the lignin precipitated are remarkable cost factor and can present technical problems. A large amount of acidic solution is required and, eventually, the precipitation of the high molecular weight compounds causes losses of vanillin. A direct liquid-liquid extraction to obtain sodium vanillate from the oxidized solution was suggested by Sandborn and Howard [162] and Bryan [163], applying solvents, or a mixture, immiscible with water (alcohols as n-and iso-butanol [162] and iso-propanol [163]). In this case, besides sodium bisulfite method, the vanillin can also be recovered from organic phase by carrier-steam distillation [164].

Although the sodium bisulfite method provides high selectivity, the bisulfite derivative of vanillin is not sufficiently stable to carry out one-stage stripping requiring, therefore, the use of multiple extraction steps [165]. Kaygorodov et al. [165] have reported data on possible extractants for vanillin recovery and discussed on related disadvantages: difficulties in vanillin stripping or solvent recovery, toxicity, price, and solubility in water of some solvents. The aliphatic alcohols of the series C6-C8 were evaluated to extract vanillin from weakly alkaline media, which could eliminate problems related to the emulsification of the extraction system and vanillin sorption by the precipitated when acidification is applied.