Stirred tank bioreactors have been designed for

$\text{Correct answer: C}$ 

$\text{Option (A):}$ 

$?$ Purification occurs after downstream processing.

$?$ It can only take place after the end products are collected from the tank and are subjected to chemical and physical alterations. 

$\text{Option (B):}$ 

$?$ Anaerobic condition is maintained in a culture vessel if the system is isolated from the atmosphere so that the internal culture does not have any contact with the outer atmosphere. 

$?$ This type of condition is maintained in those microorganisms that grow in an anaerobic environment.

$\text{Option (C):}$ 

$?$ The use of stirred tank bioreactors is to provide aeration in the culture media. There are large blades that keep rotating inside the culture.

$?$ Due to the movement of the blades, the air gets to mix with the culture leading to proper aeration. 

$?$ The availability of more air in the culture increases the availability of oxygen for the given biochemical process. 

$\text{Option (D):}$ 

$?$ Preservatives are added to the product only if it is necessary to be stored for a longer period of time.

$?$ But the constant rotation of the bleed in the bioreactor is not related to the addition of preservatives.

Hence, the correct answer is option (C).