Rhamnolipids are among the best‐known biosurfactants. Severe foaming occurs in aerobic rhamnolipid fermentation and negatively affects operation and economics of the biosurfactant production. In this study the foaming properties were examined with samples taken along a Pseudomonas aeruginosa fermentation that produced 55 g l−1 rhamnolipids with a maximum volumetric productivity of 0.080 g l−1 h−1 and a maximum specific productivity of 0.013 g g−1 h−1. For a better understanding of the process, the broth samples were also centrifuged to prepare cell‐free supernatants and cell suspensions in water, and all samples were evaluated under fixed foaming conditions. In addition to the time profiles of foam rise, the initial foaming rates and maximum foam volumes were determined. Contrary to the general assumption, the cells, not rhamnolipids, were the main foaming agents in the fermentation. Soluble components including rhamnolipids had secondary roles. Supernatant foaming was higher after the culture entered the rhamnolipid‐producing stationary phase; however, the foaming appeared to decrease with increasing rhamnolipid concentrations at high concentrations (>15 g l−1). The pH effects on foaming of broths, supernatants, and cell suspensions were also studied. Broth foaming was 55 and 80 % less at pH 5.5 and 5.0, respectively, compared to that at pH 6.5. Cell growth and rhamnolipid production at lower pH should be included in future studies. In addition, strain selection or genetic engineering and medium modification to reduce cell hydrophobicity are suggested as useful strategies to address the foaming issue of rhamnolipid fermentation.

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As compared to static systems, the flow chamber systems significantly improved the reproducibility of initial attachment results. Flow chamber systems were more suitable for experimental investigations of bacterial attachment to surfaces. Rhamnolipids were found to be potent antifoulants for PAO1 attachment on glass. The initial cell attachment increased with increasing shear at the very low shear range (up to 3.5-5.0 mN/m(2)), but the attachment could be minimized with further increase of the shear.

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