A widely recognized heuristic for categorizing downstream processing operations divides m into four groups which are applied in order to bring a product from its natural state as a component of a tissue, cell or fermentation broth through progressive improvements in purity and concentration.
Removal of insolubles is first step and involves capture of product as a solute in a particulate-free liquid, for example separation of cells, cell debris or or particulate matter from fermentation broth containing an antibiotic. Typical operations to achieve this are filtration, centrifugation, sedimentation, precipitation, flocculation, electro-precipitation, and gravity settling. Additional operations such as grinding, homogenization, or leaching, required to recover products from solid sources such as plant and animal tissues, are usually included in this group.
Product isolation is removal of those components whose properties vary considerably from that of desired product. For most products, water is chief impurity and isolation steps are designed to remove most of it, reducing volume of material to be handled and concentrating product. Solvent extraction, adsorption, ultrafiltration, and precipitation are some of unit operations involved.
Product purification is done to separate those contaminants that resemble product very closely in physical and chemical properties. Consequently steps in this stage are expensive to carry out and require sensitive and sophisticated equipment. This stage contributes a significant fraction of entire downstream processing expenditure. Examples of operations include affinity, size exclusion, reversed phase chromatography, ion-exchange chromatography, crystallization and fractional precipitation.
Product polishing describes final processing steps which end with packaging of product in a form that is stable, easily transportable and convenient. Crystallization, desiccation, lyophilization and spray drying are typical unit operations. Depending on product and its intended use, polishing may also include operations to sterilize product and remove or deactivate trace contaminants which might compromise product safety. Such operations might include removal of viruses or depyrogenation.
So correct option is 'Expression'.
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