Improved ethanol production from sucrose by a mutant of Zymomonas mobilis lacking sucrases in immobilized cell fermentation

Abstract

Ethanol production from sucrose by Zymomonas mobilis LS1A, a mutant strain lacking levansucrase (SacB) and intracellular sucrase (SacA), was studied in an alginate-immobilized cell fermentation. Since this mutant strain does not produce levan from sucrose, the immobilized cells were stable in sucrose fermentations and therefore produced higher amounts of ethanol (73.5 g l −1) than that of its parent strain, B-806 (65.2 g l −1) in an 18-h fermentation with 150 g sucrose l −1. The absence of levansucrase and the decreased sucrose hydrolysis rate of this strain improved ethanol production without the formation of the by-products levan and sorbitol; however, an increase in the initial concentration of sucrose (150 to 250 g l −1) in the fermentation medium favored the formation of sorbitol as a by-product. Coimmobilization of invertase with cells decreased the fermentation time from 18 to 12 h, thereby resulting in increased ethanol productivity from 4.08 g l −1 h −1 to 6.22 g l −1 h −1 and also increased the level of sorbitol. Decreased fermentation time (from 18 to 12 h) and increased ethanol productivity (6.3 g l −1 h −1) without any by-products were also achieved with the use of repeated batch fermentations of sucrose by immobilized cells of this mutant strain.