Ethanol production from sugar cane syrup using Zymomonas mobilis

Abstract

Since Zymomonas mobilis cannot be made available to industry in dried or lyophilized form, cell biomass was compared with liquid culture for rapid initiation of ethanol formation. The latter gave a more constant production rate at 0.5 g l −1 dry weight equivalent. Of twelve different carbon source combinations in the pre-culture, only those containing sucrose at > 10% (w/v) secured and maintained rapid and complete overall sucrose hydrolysis (97–100%). Of all the nutrient combinations tested, the presence of 1–2 g l −1 MgSO 4 and 0.5 g l −1 diammonium phosphate gave close to 20% improvement in conversion efficiency. The addition of 0.06–0.125 g l −1 invertase overcomes sucrose hydrolysis problems owing to pre-culture conditions. Sorbitol formation was reduced to 0.1–0.3% (w/v) using a continuous feeding train cultivation technique at 110 ml h −1 [ D = 0.18 h −1]. Batch and continuous processes exhibit ethanol production rates of 3 g l −1 h −1 and final sucrose to ethanol conversion efficiencies in excess of 90%.