For tower fermentors, i.e., bubble columns (BCS), treating various foaming liquids, the critical disk rotational speed N(c) required for foam-breaking of rotating-disk mechanical foam-breakers (MFRDs) fitted to BCs was studied in relation to the foaming characteristics of the liquids. An empirical equation was presented for predicting N(c) in foam-breaking regions. With this equation, it was found that prediction of N(c) of MFRDs fitted to BCs treating various liquids was possible regardless of the type of foaming liquid or its concentration.