The sizes of air bubbles and castor oil drops were studied by image analysis as a function of the concentration of soluble protein (bovine serum albumin [BSA] and lipase, as model proteins) in a three-phase system using a simulated fermentation medium (aqueous salt solution, castor oil, and air). Small amounts of proteins (<0.02 g/L) caused an important decrease in oil drops and bubbles sizes, together with a pronounced decrease in surface tension. The extent and profiles of this decrease seem to be determined by the conformation of the protein at the interface. The k(perpendicular to)alpha value increased considerably for increasing concentration (up to 0.02 g/L) of the two proteins but was very different (2-fold higher for the lipase) at the highest concentrations tested (0.5 g[L), a phenomenon that can be caused by the extent to which bubbles are trapped within oil drops.