Entrainment of oxygen

Shake flasks have a limited culture volume based on the transfer of oxygen to the medium they contain. The concept of how much of a substance it is possible to move from one phase to another in this case gaseous to aqueous in a system is known as mass transfer. Through trial and error it has been established that an aerobic organism will only grow optimally if the Erlenmeyer flask has a volume 10 times that of the medium it contains. Thus a 250 ml Erlenmeyer flask should ideally contain only 25 ml of medium. When growing Escherichia coli this volume may be raised to as much as 50 ml but this is only possible as E. coli is facultatively anaerobic. Practically the maximum size of standard Erlenmeyer flask that can be used is 5 liters so if culture volumes of more than 500 ml are required alternatize methods such as a simple stirred tank reactor must be used.

The oxygen mass transfer rate of shake flasks is limited because the system relies on the agitation of the surface of the medium to allow air to form small bubbles in the medium and the oxygen to diffuse from those bubbles to the bulk medium. This entrainment of air can only happen at the surface of the medium so if the level of the medium is too far up the conical flask the surface area available is too small for mass transfer of oxygen to the medium beneath it. Entrainment of air can be increased through making the mixing of the liquid more vigorous frequently through increasing the speed of agitation of the flask. Those organisms with a very high demand for oxygen can demand adapted flasks in which baffles in the sides of the flasks disrupt the smooth mixing of the medium further still.