Solvent extraction utilizes the selective dissolving of one or more constituents of a solution into a suitable immiscible liquid solvent. It has been widely used for refining petroleum products, chemicals, vegetable oils, and vitamins. When applying solvent extraction to remove inhibitors, Wilson et al. (1989) found that ethyl acetate extraction was more effective than roto-evaporation in removing the inhibitors. The roto-evaporation removed furfural and most of the acetic acid but did not reduce lignin-derivative levels. The ethyl acetate extraction removed all the inhibitory compounds, except acetic acid, which was not completely removed by the ethyl acetate extraction process.

Aqueous Two-Phase Extraction

Aqueous two-phase systems (ATPS) are clean alternatives for traditional solvent extraction systems. ATPS are formed when two polymers, or one polymer and one salt are mixed together at appropriate concentrations and at a particular temperature. The two phases are mostly composed of water and nonvolatile polymers, thus eliminating the use of volatile organic solvents. ATPS are normally performed under mild conditions, for example, 250C, which do not harm or denature unstable/labile biomolecules or microorganisms. In ATPS, the interfacial stress (at the interface between the two layers) is lesser (400-fold less) than that in water-organic solvent systems used for solvent extraction, causing less damage to the molecules to be extracted. The separation of the phases and the partitioning of the compounds occur rapidly. The ATPS have been tested for a number of years in biotechnological applica­tions as a benign separation method. In addition, ATPS have been investigated for extractive fermentation (HahmHagerdal et al. 1981. Jarzebski et al. 1992. Banik et al. 2003) and removal of inhibitors (Hasmann et al. 2008) from lignocellulosic hydrolysates during biofuel production from biomass.

Major disadvantages of ATPS include the relatively high cost of polymer, recycle of polymer(s), and poor selectivity, although specialized and efficient systems may be devel­oped by varying factors such as temperature, degree of polymerization, and presence of certain ions.