Fermentation Kinetic Modeling And Process Optimization Of Sophorolipid Production By Starmerella Bombicola O-13-1

Sophorolipids is a glycolipid biosurfactant produced by the non-pathogenic fungi Starmerella bombicola O-13-1 using renewable resources.It is the most productive biosurfactant in the world.Due to its excellent properties such as good wettability,foaming,antitumor,antibacterial,antiviral,non-toxic,and biodegradable,sophorolipids are now widely used in agriculture,food,biomedicine,bioremediation,oil extraction,cosmetics,etc.However,its large-scale production is limited because of its high fermentation cost,low fermentation efficiency and difficulty in product separation.In this article,we focused on the pilot-scale study of the fermentation of sophorolipids,developed a fermentation kinetic model for the batch fermentation and fed-batch fermentation of sophorolipids,and applied the model to evaluate the applicability of experimental data on other glycolipid surfactants.With glucose and soybean oil as carbon sources,the fed-batch fermentation was carried out in a 10L fermentor using Starmerella bombicola O-13-1.Firstly,the effects of different stirring speeds and aeration rates on the yield of sophorolipids（SLs）were studied by observation between groups and data monitoring,so as to determine the aeration rates and stirring rates suitable for industrial fermentation.Then,the calculation and analysis of 100 L fermentor conditions were carried out using the empirical scaling method with three scaling principles of Reynold’s number（Re）,ratio of non-aerated stirring power to fermentation broth（P₀/V_L）and oxygen transfer rate（K_La/k）,respectively.When the stirring speed was 400 rpm,the aeration volumes of0.3vvm and 0.6vvm had significant differences on the SLs yield,and SLs yield started to decrease when the aeration volume reached 1.5vvm.Therefore,0.6vvm and 1.5vvm were chosen as the lower and upper limits for aeration calculation during the scaling-up process.The maximum yield of SLs（184.67 g/L）was obtained when the stirring speed was 750 rpm,while it was only 128.22 g/L at 400 rpm.Therefore,750 rpm and400 rpm were selected as the upper and lower limits for the calculation of the stirring rate during the amplification process.Then,the constant K_La/k criterion was selected for 100 L fermentation,and the SLs yield reached 108 g/L.The results indicated that this method was more suitable for the pilot-scale amplification calculation of SLs.An unstructured kinetic model for the bacth fermentation of SLs production by S.bombicola（S.bombicola）was proposed based on the characteristics of the immiscible dual-substrate fermentation broth,which incorporates biomass,oleic acid,glucose,and SLs yields.The equation for the variation of droplet volume of oleic acid droplets in the fermentation broth with increasing surfactant was proposed.The accuracy of the models reached 0.9583,0.9925,0.9612,and 0.9918 after being fitted by MATLAB software.Bothαandβin the product generation equation are above 0,indicating that the fermentation is a partial growth coupled fermentation.The applicability of rhamnolipid data was evaluated using this model,and the model accuracy reached0.8971,0.9891,0.9970,and 0.9845.The values of physiological metabolic parameters of S.bombicola at 150 rpm,300 rpm and 450 rpm were investigated by applying the model,and it was shown that the values of maximum specific growth rate and maximum cell concentration of S.bombicola at 450 rpm were the highest and more suitable for cell growth.When the rotational speed was 300 rpm and 450 rpm,the rate of SLs production per unit of glucose consumed was basically equal,while the rate of SLs production per unit of oil-soluble carbon source consumed was larger for the latter,which means that the oil concentration in the fermentation system could be appropriately increased when increasing the rotational speed.According to the requirements of industrial fermentation,SLs are mostly fermented by fed-batch fermentation in industry,therefore,this paper further proposes an unstructured kinetic model of fed-batch fermentation for sophorolipid production by S.bombicola,which contains biomass,soybean oil,glucose,and sophorolipid yields.After fitting by MATLAB software,the model accuracy reached 0.9724,0.9202,0.9644,and 0.9905,respectively.The relationship between the rate of SLs ratio synthesis and the concentration of each substance during the entire fermentation process was calculated and analyzed according to the model.