High-performance Fermentation Of Biosurfactant Rhamnolipid

Rhamnolipids are biosurfactants produced by Pseudomonas aeruginosa,which show excellent surface/interfacial activity,low toxicity,environment-friendliness and biodegradable property.So they can be potentially applied in fine chemicals,petroleum industry,agriculture,environmental industry,bio-medicine and other fields.Although the rhamnolipid production has been partially industrialized,there are still some problems on foam controlling,low yield and abnormal fermentation.Therefore,this study will try to establish a method to control the foam,explore the fermentation process to improve the productivity of rhamnolipid and investigate several factors that influence fermentation.First,we controlled the fermentation foam by adjusting the pH of the fermentation broth.There were two different methods to control the pH:1)to adjust the pH of the fermentation broth by adding the dilute acid solution;2)to adjust the pH by microbial metabolism when the composition of the medium was changed.The former strategy can not only control the pH between 6.2-6.5 without harming the cell growth,but also promote rhamnolipid synthesis,whereas the foam was effectively controlled.Nevertheless,this method required the continuous addition of dilute acid solution to maintain the pH,which will not be appropriate for industrial operation.The second strategy could sustain pH ranging with 6.5-6.6 by adding sodium nitrate and ammonium sulfate compound solution.This could not only ease the foaming control problem,but also improve the fermentation productivity.Secondly,the sequential fed-batch fermentation based on a fill-and-draw strategy was investigated.In shaking flasks,the rhamnolipid productivity was as high as 0.366gL-1h-1 after fill-and-draw of the sterilizated medium and reached 0.406gL-1h-1 after using the non-sterilizated medium,either of which were significantly higher than that in the control group(0.138 gL-1h-1).Then the optimal fill-draw volume was found to be 30 mL(a half of the initial volume),and the sequential operation could be run at least 4 times.TLC analysis showed no significant difference in the proportion of mono-rhamnolipid and di-rhamnolipid after fed batch culture.The rhamnolipid productivity was found to be 0.355 gL-1h-1 at an industrial scale of 20T by using the non-sterilized medium.It seems that this strategy could not be suitable for the shaking flask culture,but also for the industrial fermentation.Finally,the individual effect of nitrate,iron and dissolved oxygen level on Pseudomonas aeruginosa were examined.The increasing nitrate concentration at initial culture was beneficial to the growth of the cells and the synthesis of rhamnolipid in normal culture.The excessive iron ions promoted the cell growth but inhibited the productivity of rhamnolipids.The appropriate iron usage ranged 0.5-1 fold of the normal Fe concentration,and the stainless steel will release iron ions into the fermentation broth,enhancing the Fe concentration.The enhanced dissolved oxygen level could benefit the cell growth and rhamnolipid production.As interestingly found,a low oxygen level as well as the high iron environment could inhibit the rhamnolipid production.,accompanied by an enhanced comsumption of the nitrate.The increase of dissolved oxygen level can overcome the negative effects caused by iron ions and increase the yield even at a relative high concentration of iron.In summary,this study figured out some problems in the fermentation of rhamnolipid,and further promoted the industralized fermentation of rhamnolipid.