The most commonly used bioreactors are of stirring type, which are shown in figure.

(a) Simple stirred-tank bioreactor
(b) Sparged stirred-tank bioreactor through which sterile air bubbles are sparged

A stirred-tank reactor is usually cylindrical or with a curved base to facilitate the mixing of the reactor contents. The stirrer facilitates even mixing and oxygen availability throughout the bioreactor. Alternatively air can be bubbled through the reactor.

If you look at the figure closely you will see that the bioreactor has an agitator system, an oxygen delivery system and a form control system, a temperature control system, pH control system and sampling ports so, that small volumes of the culture can be withdrawn periodically.

Small volume cultures are usually employed in laboratories in a flask for research and production of less quantities of products. However, large scale production of the products is carried out in bioreactors.