Repeated batch fermentation for surfactin production with immobilized Bacillus subtilis BS-37: Two-stage pH control and foam fractionation

BACKGROUND: Surfactin, a lipopeptide biosurfactant produced by Bacillus strains, is one of the most surface-active biosurfactants, and has the potential to enhance oil recovery or environmental remediation. However, low yield and high production costs limit its use in high-volume applications. In this work, Bacillus subtilis BS-37 was immobilized on a piece of cotton towel in a 5 L stirred tank reactor; the fill-and-draw process and foam fractionation were used for repeated batch fermentations. A two-stage pH control operation was developed for surfactin fermentation: for repeated batch fermentation, the pH value was 5.0 for the biomass accumulation in the previous 6 h; and then the pH value was raised to 7.5 for surfactin production over the following 18 h of fermentation. RESULTS: The immobilized Bacillus subtilis BS-37 cells were efficiently reused for 7 cycles, producing 7887.3 mg surfactin in total. CONCLUSIONS: The repeated batch fermentation, low-cost foam fractionation, reusable immobilized cells, and energy-efficient open (nonsterile) fermentation could greatly promote the development of industrial production of surfactin.