Foam reduction through stimng was studied in 20 L bioreactors with artificial media and with an actual biotransforma-tion process. For a given stirrer configuration and within a certain range of the broth mass, the foam height was correlated with the broth mass, i.e. the distance between upper stirrer and dispersion surface, and with the superficial gas velocity. Increasing the stirrer speed often resulted in reducing the foam height. A mechanistic model was developed for the Stirring As Foam Disruption (SAFD) technique, relating the foam height to the horizontal liquid velocity near the dispersion surface. The model illustrates the general applicability of the SAFD technique and points to foam entrainment as the major mechanism for the foam disruption.