ABSTRACT
Precipitation of proteins from biological fluid, e.g., the precipitation of casein from milk by dilute acid, is known since old times. For some time, precipitation was the only practical way of separating different types of proteins by causing part of a mixture to precipitate through altering some property of the solvent. Precipitation still remains an important operation for the laboratory and industrial scale recovery and purification of proteins-often used in the early stages of downstream processing for both product stream concentration and fractionation. Though chromatography is the workhorse of protein recovery and purification operations, precipitation remains an indispensable unit operation. The efficiency of chromatographic separations is largely dependent on initial steps involving precipitation procedures (Englard and Seifer 1990). Up to 80% of published protein purification protocols include at least one precipitation step, ranging from primary isolation [separating cellular debris from protein product in recombinant cell cultures (McGregor 1983; Nakayama et al. 1987)] to finishing operations [producing solids prior to drying and formulation (Hoare et al. 1983; Paul and Rosas 1990; Niederauer and Glatz 1992)], to yield a partially purified product of reduced volume (Englard and Seifer 1990; Chen et al. 1992). Selective precipitation of target protein from a crude mixture is more attractive as the separate protein-enriched phase forms from a homogeneous solution during the process, and the mechanical separation of the proteinenriched phase (pellet) from the protein-depleted phase (superna-tant) is easily achieved by well-established and simple techniques such as filtration or centrifugation. The main function of precipitation is to concentrate the target protein. The protein precipitated from a large volume of the crude extract could be dissolved afterward in a small buffer volume. The technique also allows rapid isolation of protein products from denaturing conditions and proteolytic enzymes (Chen et al. 1992). It is easily adapted to large scale, uses simple equipment, has a large number of inexpensive alternatives, and can be done without denaturation of biological products (Glatz 1990).
