Precipitation may be conducted at various stages of the product recovery process. It is a particularly useful process in that it allows enrichment and concentration in one step, thereby reducing the volume of material for further processing.
It is possible to obtain some products (or to remove certain impurities) directly from the broth by precipitation, or to use the technique after a crude cell lysate has been obtained.
Typical agents used in precipitation render the compound of interest insoluble, and these include:
(a) Acids and bases to change the pH of a solution until the isoelectric point of the compound is reached and pH equals pi, when there is then no overall charge on the molecule and its solubility is decreased.
(b) Salts such as ammonium and sodium sulphate are used for the recovery and fractionation of proteins. The salt removes water from the surface of the protein revealing hydrophobic patches which come together causing the protein to precipitate. The most hydrophobic proteins will precipitate first, thus allowing fractionation to take place.
(c) Organic solvents. Dextrans can be precipitated out of a broth by the addition of methanol. Chilled ethanol and acetone can be used in the precipitation of proteins mainly due to changes in the dielectric properties of the solution.
(d) Non-ionic polymers such as polyethylene gylcol (PEG) can be used in the precipitation of proteins and are similar in behaviour to organic solvents.
(e) Polyelectrolytes can be used in the precipitation of a range of compounds, in addition to their use in cell aggregation.
(f) Protein binding dyes (triazine dyes) bind to and precipitate certain classes of protein (Lowe and Stead, 1985).
(g) Affinity précipitants are an area of much current interest in that they are able to bind to, and precipitate, compounds selectively (Niederauer and Glatz, 1992).
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