The three popular modes of operation of mechanically agitated reactors for cell cultivation are batch, fed-batch and continuous operations. The mechanically agitated reactors are equipped with capabilities for on-line and off-line sensing of a variety of culture characteristics, such as pH, temperature, concentrations of dissolved gases (such as CO2 and O2), biomass concentration, cell morphology, concentrations of various components of the nutrient medium, total protein content of cells, activities of certain proteins, and concentrations of certain metabolites, and tighter control of some of these using appropriate controllers. The classification of the reactor operation is based on culture [suspension of cells in a liquid medium or a composite of liquid medium and cells attached to a suitable solid support (immobilized cells)]. Besides the liquid and solid phases, these reactors also have a gas phase, to provide oxygen to liquid culture in an aerobic bio-process, to provide a blanket of an inert such as nitrogen in an anaerobic bioprocess, and to remove carbon dioxide (from the culture) generated as a product of cellular metabolism. Irrespective of the mode of operation with respect to culture, the bioreactors are always operated in continuous mode with respect to gas phase (gas phase continuously entering and leaving the reactor).
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