Tandem Production of Levan and Ethanol by Microbial Fermentation

Tandem production of levan and ethanol by microbial fermentation using sucrose substrate was investigated. The tandem process involves fermentation of Bacillus subtilis (natto) Takahashi in a sucrose medium to produce levan, separation of the levan product from glucose by-product by ultrafiltration and fermentation of the glucose remnant from levan production by Zymomonas mobilis to produce ethanol. After cultivation of B. subtilis (natto) Takahashi for 48 h, 56.0 +/- 0.6 g l(-1) of levan was produced in a medium containing 250 g l(-1) sucrose, which was 45 +/- 0.5% yield on available fructose. After removing the cells, the fermentation broth was concentrated by ultrafiltration through a membrane of 5 kDa cutoff. The filtrate which contained 179 +/- 3 g l(-1) of hexose was diluted, supplemented with yeast extract (5 g l(-1)), (NH(4))(2)SO(4) (1 g l(-1)), MgSO(4)center dot 7H(2)O (0.5 g l(-1)), KH(2)PO(4) (1 g l(-1)) and used for alcohol fermentation by Z. mobilis. Incubation of Z. mobilis in the medium containing glucose remnant at 30 degrees C, pH 5.5, 175 rpm for 120 h gave from 21.1 +/- 0.3 to 26.5 +/- 0.2 g l(-1) of bio-ethanol depending on the dilution. The tandem process developed in this study is an eco-friendly process in that the sucrose substrate was fully utilized without wasting any by-products in the process; in addition, two invaluable environmentally-friendly biomaterials (levan and ethanol) were produced. Furthermore, the amount of alcohol required for levan recovery could be reduced to a quarter of that generally used in the conventional precipitation.