Extraction is a process in which two phases come into contact with the objective of transferring a solute or particle from one phase to the other. For the separation and purification of biological products, the phases are most commonly immiscible liquids, and the solute is in soluble form. In certain instances, however, one phase is a liquid and the other phase is a solid; the extraction of caffeine from coffee beans is one example. Although most extractions in biotechnology involve the transfer of soluble bioproducts, organelles and cells have at times been transferred between phases. An organic solvent is often used as the extracting liquid when the solute to be extracted is stable in the organic solvent, typical examples being low molecular weight antibiotics. It is usually not feasible to extract proteins with organic solvents, since proteins are often denatured or degraded as a result of contact with the organic solvent. Proteins can often be successfully extracted by means of two immiscible liquid phases that consist of solutions of two water-soluble but incompatible polymers, or one polymer plus a high concentration of certain salts. Extraction usually comes early in the purification process for a bioproduct and typically would precede a high-resolution step such as chromatography. Extraction is often advantageous because it can bring about a significant reduction in volume and/or can separate the desired product from cells or cell debris. It is desirable to reduce the volume as soon as possible in the process, since large volumes typically lead to large costs. The extractions of interest in the purification of biotechnological and pharmaceutical products are mainly liquid-to-liquid, and this is the emphasis in this chapter. The basic definitions and principles of extraction are developed first, followed by an explanation of scale-up and design procedures for the extractors most commonly used for bioproducts. After completing this chapter, the reader should be able to do the following: • Define and use key constants such as the partition coefficient, solvent-to-feed ratio, and extraction factor. • Explain the factors that affect the partitioning of biomolecules.
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