This study examines dispersions of rapeseed oil (RSO) in water by mechanical agitation under conditions mimicking those found in certain antibiotic fermentations; for example, in the presence of air, antifoam, and finely divided CaCO3 particles. A problem with residual oil has been reported for such fermentations, and it has been suggested that the use of pre-emulsified oil can reduce this problem. Hence, the dispersion of a preemulsified oil produced by the "phase inversion temperature (PIT) method" has been evaluated. In both cases, the volume fraction of oil was 2%. For the RSO systems, a relatively high agitation speed was required to disperse the oil, especially in the presence of the particles and, when the agitation was stopped, separation occurred rapidly. The Sauter mean drop diameters depended on the system, being at an average energy dissipation rate of approximately 0.9 W kg(-1), 180 mum for RSO/water, 130 mum for RSO/water(antifoam)/air, 580 mum for RSO/water/CaCO3, and 850 mum for RSO/water(antifoam)/air/CaCO3. For the same four systems, the PIT emulsion, once dispersed, was very stable and the drop size was essentially independent of the operating conditions, with a Sauter mean diameter of similar to0.3 mum. The implications of these findings for fermentations in which oil is used as a carbon source are assessed. (C) 2003 Wiley Periodicals, Inc.