Abstract
BackgroundRhamnolipids are the best known microbial-derived biosurfactants, which has attracted great interest as potential ‘‘green” alternative for synthetic surfactants. However, rhamnolipids are the major contributors to severe foam problems, which greatly inhibit the economics of industrial-scale production. In this study, a novel foam-control system was established for ex situ dealing with the massive overflowing foam. Based on the designed facility, foam reduction efficiency, rhamnolipids production by batch and repeated fed-batch fermentation were comprehensively investigated.ResultsAn ex situ foam-control system was developed to control the massive overflowing foam and improve rhamnolipids production. It was found that the size of individual bubble in the early stage was much larger than that of late fermentation stage. The foam liquefaction efficiency decreased from 54.37% at the beginning to only 9.23% at the end of the fermentation. This difference of bubble stability directly resulted in higher foam reduction efficiency of 67.46% in the early stage, whereas the small uniform bubbles can only be reduced by 57.53% at the later fermentation stage. Moreover, reduction of secondary foam is very important for foam controlling. Two improved designs of the device in this study obtained about 20% improvement of foam reduction efficiency, respectively. The batch fermentation result showed that the average volume of the overflowing foam was reduced from 58–640 to 19–216 mL/min during the fermentation process, presenting a notable reduction efficiency ranging from 51.92 to 73.47%. Meanwhile, rhamnolipids production of batch fermentation reached 45.63 g/L, and the yield 0.76 g/g was significantly better than ever reported. Further, a repeated fed-batch fermentation based on the overall optimization was carried out. Total rhamnolipids concentration reached 48.67 g/L with the yield around of 0.67–0.83 g/g, which presented an improvement of 62% and 49% compared with conventional batch fermentation by using various kinds of defoamers, respectively.ConclusionsThe ex situ foam-control system presented a notable reduction efficiency, which helped greatly to easily solve the severe foaming problem without any defoamer addition. Moreover, rhamnolipids production and yield by repeated fed-batch fermentation obtained prominent improvement compared to conventional batch cultivation, which can further facilitate economical rhamnolipids production at large scales.
Highlights
Rhamnolipids are the best known microbial-derived biosurfactants, which has attracted great interest as potential ‘‘green” alternative for synthetic surfactants
Xu et al Biotechnol Biofuels (2020) 13:80 by repeated fed-batch fermentation obtained prominent improvement compared to conventional batch cultivation, which can further facilitate economical rhamnolipids production at large scales
Construction of foam‐control system for improving rhamnolipids batch fermentation production As stated, an ex situ foam-control system was developed to collect the in situ foam accumulation data and to control the massive overflowing foam in rhamnolipids fermentation (Fig. 1)
Summary
Rhamnolipids are the best known microbial-derived biosurfactants, which has attracted great interest as potential ‘‘green” alternative for synthetic surfactants. Rhamnolipids contain one or two l-rhamnose units linked to one or two β-hydroxy fatty acids, and it can accumulate at the interface to reduce the surface tension between two phases [2]. Due to their advantages over synthetic chemical surfactants (e.g., lower toxicity, better biodegradability, high specificity, constant effectiveness over wider range of pH and temperature), it possesses great potential in various fields ranging from cleaning agent in cosmetic industry, emulsifier or solubilizer in food-processing, oil-displacing agent for oil recovery or even cosolvent in the pesticide production [3]. Massive foam overflowing from the gas outlet of bioreactor would have several detrimental impacts on the fermentation, such as loss of culture medium, rhamnolipids and cells, and high risk of contamination [8, 9]
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