Enhancing rhamnolipid production and exploring the mechanisms of low-foaming fermentation under weak-acid conditions

Abstract

The commercialized applications of rhamnolipid has seriously impeded by high cost of production caused by severe foaming. Effective low-foaming fermentation and mechanism study have become the most urgent requirement for larger-scale production of rhamnolipid. In this study, the low-foaming fermentation was realized by controlling fermentation at pH 5.5. Further, the production of rhamnolipid was enhanced 0.9-fold by the ultraviolet and ethyl methanesulfonate composite mutagenesis. To the best of our knowledge, this was the first report to enhance production of rhamnolipid by strain improvement at weak-acid conditions. The mechanisms exploration tests indicated that increasing surface tension and decreasing viscosity were conducive to reduce foaming ability of rhamnolipid fermentation. The decrease of negatively charged ions may weaken the electrostatic repulsion between charged substances adsorbed membranes of bubbles such as rhamnolipid and cells, leading to the liquid membranes to rupture easily. In addition, the structure of vesicle or lamellar of rhamnolipid formed at pH5.0-6.0 may also weaken the foaming ability of rhamnolipid fermentation. The work revealed the promising potentiality for genetic modification to enhance the production of rhamnolipid and facilitated the understanding of pH-associated foaming behavior, as well as were conducive to develop a more effective pH-associated control strategies for large-scale production of rhamnolipid.