Ethanol Production from Sugars and Complex Biomass by Thermoanaerobacter AK5: The Effect of Electron-Scavenging Systems on End-Product Formation

Abstract

The ethanol production capacity from sugars and lignocellulosic biomass hydrolysates by Thermoanaerobacter strain AK5 was studied in batch cultures. The strain converts various carbohydrates to acetate, ethanol, hydrogen, and carbon dioxide. Maximum ethanol yields on glucose and xylose were 1.70 and 1.35 mol (mol sugars)−1, respectively. Increased initial glucose concentration inhibited glucose degradation and end-product formation leveled off at 30 mM. Cultivation of the strain with decreased liquid–gas (L-G) ratios on glucose resulted in a shift to more acetate and less ethanol. End-product formation from glucose was further manipulated by adding extracellular electron acceptor (thiosulfate) or using a coculture of hydrogenotrophic methanogen. In both cases, the hydrogen-scavenging systems resulted in a dramatic shift from ethanol to acetate. Ethanol production from 4.5 g L–1 of complex biomass hydrolysates (grass, hemp, wheat straw, newspaper, and cellulose) pretreated with acid (0.50% H2SO4), alkali (0.50% NaOH), and without acid/alkali (control) and the enzymes Celluclast and Novozymes 188 (0.1 mL g–1 dry weight (dw); 70 and 25 U g–1 of Celluclast and Novozyme 188, respectively) was investigated. Highest ethanol yields (27.5 mM; 5.5 mM (g biomass)−1) were obtained on cellulose but lowest on hemp leafs (5.1 mM; 0.8 mM g–1). Chemical pretreatment increased ethanol yields substantially from lignocellulosic biomass but not from cellulose. The most-pronounced increase was on straw hydrolysates, where ethanol production increased from 5.5 mM to 15.2 mM on alkali-pretreated biomass. Ethanol yields were increased from 5.5 mM to 7.7 mM g–1 on cellulose by decreasing the hydrolysate concentration to 2.25 g L–1, because of incomplete glucose degradation in the higher-loaded system.