A method of converting liquid micro-biological substrate is disclosed, as is an apparatus for carrying out the method. The liquid substrate is caused to flow along a duct having a cross-sectional area which alternately decreases and increases along the flow path to cause the pressure of the substrate to alternately decrease and increase. Preferably, gas is introduced into the substrate to promote the formation of bubbles of the gas in the substrate. The gas may be so introduced as to cause the substrate to flow through the duct and/or an impeller pump may be employed for this purpose.
