extraction which is used for the separations of many metabolites from fermentation such as alcohols, carbox~lic acids, amino acids, and antibiotic~.'.~ The other is the aqueous two-phase partitioning using water solublepolymers such as polyethylene glycol (PEG) and dextran, and salts such as potassium phosphate. The latter method is very attractive for the separation of biomolecules such as proteins and peptides including many e n ~ y m e s ~ - ~ may be denatured by solvents. As the scale of bioseparation that processes goes up, liquid-liquid partitioning becomes more and more competitive because it is easy to scaleup and it permits continuous steady-state pera at ion.^ The cost for liquid-liquid partitioning is much lower than other more sophisticated bioseparations method, such as liquid chromatography.
II. SOLVENT EXTRACTION FOR BIOSEPARATIONS
The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of liquid-liquid distribution over the traditional term solvent extraction."However, solvent extraction is still used prevalently in the literature. Solvent extraction utilizes the partition of a solute between two practically immiscible liquid phases: one solvent phase and the other aqueous phase.1' A separatory funnel can be used in a lab to carry out solvent extraction. Of course, a simple test tube can also be used in conjunction with a glass pipette. The organic phase(solvent ~ h a s e is usually the top phase and ) the aqueous phase bottom phase. However, some organic solvents are heavier than water (for example, methylene chloride's specific gravity is 1.33 at 20°C) and in such cases the organic phase becomes the bottom phase. Thousands of papers and dozens of books and book chapters have been published on solvent extraction. Most of them deal with extractioncompounds that are not biologically derived. The chemistry of solvent extraction is extensively investigated in a book edited by Sekine and ~ a s e g a w a " entitled "Solvent Extraction Chemistry: Fundamentals and Application." Many operational aspects of extraction are investigated in a book edited by Rydberg, Musikas, and Choppin13 entitled "Principles and Practices of Solvents Extraction."Various extraction equipment and their operations are discussed in a book edited by Godfrey and Slater14 entitled "Liquid-Liquid Extraction Equipment." For practitioners, the "Handbook of Solvent Extraction" by Baird et al.ls should be very helpful. Schiigerl' wrote a book entitled "Solvent Extraction in Biotechnology: Recovery of Primary and Secondary Metabolites." This is a book exclusivelydealing with solvent extraction for bioseparations. There are some book chapters dealing with solvent extraction for the separation of biomolecules. Thornton's book of "Science and Practice of Liquid-Liquid Extraction" contains a chapter dealing with solvent extraction of pharmaceuticals (such as antibiotics) and a chapter on extraction of food products.2 Aires-Barros and Cabrall' contributed a chapteron liquid-liquid extraction to a book edited by Kennedy and Cabral17 entitled "Recovery Processes for Biological Materials." Weatherley18 discussed the operational consideration of extraction of fermentation broth in a chapter of the book entitled "Downstream Processing of Natural Products"
LIQUID- LIQUID PARTITIONING
edited by Verrall.19 wheelwright2' summarized solvent extractionbriefly in his downstream protein purification book. Some theoretical and practical aspects of solvent extraction were discussed by Scopes2' in his book on protein purification. Belter et a1.22 discussed the basics of solvent extraction in an easy to read chapter in their bioseparations textbook. They studied the following factors in solvent extraction.
A. Solvent Selection