Cellulosic ethanol production from AFEX-treated corn stover using Saccharomyces cerevisiae 424A(LNH-ST)

Abstract

Current technology using corn stover (CS) as feedstock, Ammonia Fiber Expansion (AFEX) as the pretreatment technology, and Saccharomyces cerevisiae 424A(LNH-ST) as the ethanologenic strain in Separate Hydrolysis and Fermentation was able to achieve 191.5 g EtOH/kg untreated CS, at an ethanol concentration of 40.0 g/L (5.1 vol/vol%) without washing of pretreated biomass, detoxification, or nutrient supplementation. Enzymatic hydrolysis at high solids loading was identified as the primary bottleneck affecting overall ethanol yield and titer. Degradation compounds in AFEX-pretreated biomass were shown to increase metabolic yield and specific ethanol production while decreasing the cell biomass generation. Nutrients inherently present in CS and those resulting from biomass processing are sufficient to support microbial growth during fermentation. This platform offers the potential to improve the economics of cellulosic ethanol production by reducing the costs associated with raw materials, process water, and capital equipment.

Fig. 1.

Effect of (A and B) pH, (C) temperature and (D) initial OD on fermentation using hydrolysate from enzymatically-digested AFEX-treated CS using S. cerevisiae 424A(LNH-ST). (Glucose consumption profiles are not shown in this graph). Note: Ethanol production from 0 to 25 g/L shown in (B) is largely due to glucose fermentation

Fig. 2.

Fermentation of (A) complex media; (B) CS-Hydrolysate I; (C) CS-Hydrolysate II using S. cerevisiae 424A(LNH-ST). Fermentations were initiated with 1.1 g (dry-wt.)/L of 424A(LNH-ST) inoculum, carried out at 30 °C, pH 5.5, and 150 rpm under largely anaerobic condition. The hydrolysates were neither detoxified nor externally nutrient-supplemented.

Fig. 3.

Effect of washing and nutrient supplementation on xylose consumption in the fermentation of hydrolysates from enzymatically-digested AFEX-treated CS. (Glucose consumption profiles are not shown in this graph.)

Fig. 4.

Effect of AFEX-CS wash stream concentration on cell growth, metabolic yield, and specific xylose consumption rate under (A) nutrient-rich and (B) nutrient-limiting conditions. Fermentations were initiated with 0.3 g (dry-wt.)/L of 424A(LNH-ST) inoculum, carried out at 30 °C and 150 rpm. Data points presented were at 24 h.

Fig. 5.

CS to ethanol mass balance analysis. The analysis was based on AFEX as feedstock pretreatment technology and S. cerevisiae 424A(LNH-ST) as the ethanologenic strain; enzymatic hydrolysis was conducted at 6.0% glucan loading (equivalent to 17.6% wt/wt solids loading). Carbohydrate contents in CS are expressed as the hydrated monomers. Glc: Glucose; Xyl: Xylose; Mo: Monomeric; Olig: Oligomeric.