The influence of small bubbles on the behaviour of airlift reactors during citric acid fermentation was studied. The small bubbles formed up to 70% of the total gas hold-up, so their distribution strongly influenced the predicted liquid circulation velocities. A periodically changing air flowrate could be used to significantly improve the oxygen transfer capacity of airlift reactors. This could be achieved by the effective utilisation of the oxygen content of small bubbles, which originated from the foam. A procedure based on the evaluation of transient pO(2) and gas hold-up profiles was suggested for the determination of the period of sparging.