Rhamnolipids have recently attracted considerable attentions because of their excellent biosurfactant performance and potential applications in agriculture, environment, biomedicine, etc., but severe foaming causes the high cost of production, restraining their commercial production and applications. To reduce or eliminate the foaming, numerous explorations have been focused on foaming factors and fermentation strategies, but a systematic summary and discussion are still lacking. Additionally, although these studies have not broken through the bottleneck of foaming, they are conducive to understanding the foaming mechanism and developing more effective rhamnolipids production strategies. Therefore, this review focuses on the effects of fermentation components and control conditions on foaming behavior and fermentation strategies responded to the severe foaming in rhamnolipids fermentation and systematically summarizes 6 impact factors and 9 fermentation strategies. Furthermore, the potentialities of 9 fermentation strategies for large-scale production are discussed and some further strategies are suggested. We hope this review can further facilitate the understanding of foaming factors and fermentation strategies as well as conducive to developing the more effective large-scale production strategies to accelerate the commercial production process of rhamnolipids.

The rhamnolipid production of Pseudomonas aeruginosa has been impeded by its severe foaming; overcoming the bottleneck of foaming has become the most urgent requirement for rhamnolipid production in recent decades. In this study, we performed rhamnolipid fermentation under weakly acidic conditions to address this bottleneck. The results showed that the foaming behavior of rhamnolipid fermentation broths was pH-dependent with the foaming ability decreasing from 162.8% to 28.6% from pH 8 to 4. The “non-foaming” rhamnolipid fermentation can be realized at pH 5.5, but the biosynthesis of rhamnolipids was significantly inhibited. Further, rhamnolipid yield rebounded from 8.1 g/L to 15.4 g/L after ultraviolet and ethyl methanesulfonate compound mutagenesis. The mechanism study showed that the species changes of rhamnolipid homologs did not affect the foaming behavior of the fermentation but had a slight effect on the bioactivity of rhamnolipids. At pH 8.0 to 5.0, increased surface tension, decreased viscosity and zeta potential, and aggregation of rhamnolipid molecules contributed to the “non-foaming” rhamnolipid fermentation. This study provides a promising avenue for the “non-foaming” rhamnolipid fermentation and elucidates the mechanisms involved, facilitating the understanding of pH-associated foaming behavior and developing a more efficient strategy for achieving rhamnolipid production.

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.