Effect of various factors on ethanol yields from lignocellulosic biomass by Thermoanaerobacterium AK ₁₇

The ethanol production capacity from sugars and lignocellulosic biomass hydrolysates (HL) by Thermoanaerobacterium strain AK ₁₇ was studied in batch cultures. The strain converts various carbohydrates to, acetate, ethanol, hydrogen, and carbon dioxide. Ethanol yields on glucose and xylose were 1.5 and 1.1mol/mol sugars, respectively. Increased initial glucose concentration inhibited glucose degradation and end product formation leveled off at 30mM concentrations. Ethanol production from 5gL ^-1 of complex biomass HL (grass, hemp, wheat straw, newspaper, and cellulose) (Whatman paper) pretreated with acid (0.50% H ₂SO ₄), base (0.50% NaOH), and without acid/base (control) and the enzymes Celluclast® and Novozyme 188 (0.1mLg ^-1dw; 70 and 25Ug ^-1 of Celluclast and Novozyme 188, respectively) was investigated. Highest ethanol yields (43.0mM) were obtained on cellulose but lowest on hemp leafs (3.6mM). Chemical pretreatment increased ethanol yields substantially from lignocellulosic biomass but not from cellulose. The influence of various factors (HL, enzyme, and acid/alkaline concentrations) on end-product formation from 5gL ^-1 of grass and cellulose was further studied to optimize ethanol production. Highest ethanol yields (5.5 and 8.6mM ethanol g ^-1 grass and cellulose, respectively) were obtained at very low HL concentrations (2.5gL ^-1); with 0.25% acid/alkali (v/v) and 0.1mLg ^-1 enzyme concentrations. Inhibitory effects of furfural and hydroxymethylfurfural during glucose fermentation, revealed a total inhibition in end product formation from glucose at 4 and 6gL ^-1, respectively.