Levan-ethanol biosynthesis using Zymomonas mobilis cells immobilized by attachment and entrapment

Fermentation of sucrose by Zymomonas mobilis cells attached to stainless steel wire spheres (WS) and to Al₂O₃ granules was compared with sucrose fermentation by cells entrapped in Ca-alginate. Similar amounts of cell biomass were applied at the beginning of the immobilized fermentation systems. The immobilization of Z. mobilis cells to the carrier surface was checked by scanning electron microscopy (SEM). Most cells were present in holes and pores of Al₂O₃ surfaces following colonization. Observation of the carrier after repeated fermentation cycles showed that a surface of Al₂O₃ granules was partly covered by levan. Alginate beads were extended in volume and partly disrupted. Changing the medium every 48 h established that free Z. mobilis cells in the culture liquid exceeded many times of cells attached to the carrier or incorporated in it. Ethanol and levan production did not differ significantly in these fermentation systems - maximal ethanol concentration at the end of second and third fermentation cycles reached 2.3-2.7% and levan 1.3-1.5%. Fermentation resulted in destruction of Ca-alginate beads. The conclusion was reached, that WS and Al₂O₃ with immobilized bacteria can be used as an inoculum generator for sucrose fermentation but that the general producer of levan and ethanol is the free suspended cell biomass.