Enhancing rhamnolipids production and exploring the mechanism of low-foaming fermentation under weakly acidic conditions

Abstract

The commercial applications of rhamnolipids have seriously been impeded by the high cost of production caused by severe foaming. Effective low-foaming fermentation and mechanism study have become the most urgent requirement for the larger-scale rhamnolipids production. In this study, the low-foaming fermentation of rhamnolipids is realized by controlling fermentation at pH 5.5. Further, the rhamnolipids production is enhanced 0.9-fold by ultraviolet and ethyl methanesulfonate composite mutagenesis. To the best of our knowledge, this is the first report to enhance production of rhamnolipids by strain improvement under weakly acidic conditions. The mechanism exploration tests indicated that increasing surface tension and decreasing viscosity are conducive to reducing foaming ability of rhamnolipids fermentation. The decrease of negatively charged ions may weaken the electrostatic repulsion between charged substances adsorbed on membranes of bubbles such as rhamnolipids and cells, making liquid membranes prone to rupture. In addition, the vesicle or lamellar structures of rhamnolipids are formed under pH 5.0-6.0, weakening the foaming ability of rhamnolipids. The work reveals promising potentiality by genetic modification to enhance rhamnolipids production and is conducive to understanding the pH-associated foaming behavior as well as developing the more effective pH-associated control strategies for large-scale rhamnolipids production under weakly acidic conditions.