Ethanol and co-product generation from pressurized batch hot water pretreated T85 bermudagrass and Merkeron napiergrass using recombinant Escherichia coli as biocatalyst

Pretreatment of grasses is required to maximize ethanol yield during fermentation. T85 bermudagrass and Merkeron napiergrass (Pennisetum purpureum Schumach.) were either left untreated or were pressurized batch hot water (PBHW) pretreated for 2 min at 230 °C at 5% w/v whole grass solids loading. Following a 24 h enzymatic digestion, untreated and PBHW pretreated grasses were evaluated for ethanol production and co-product generation including potential fermentation inhibitors. Fermentations of PBHW pretreated grasses with E. coli LY01 produced twice the ethanol of their untreated counterparts. PBHW pretreated Merkeron napiergrass produced 224.5 mg/g grass ethanol (73% maximum theoretical yield) and PBHW pretreated T85 bermudagrass reached 213.0 mg/g grass (70% maximum theoretical ethanol yield). Pretreatment by PBHW resulted in increased solubilization of hemicelluloses. PBHW pretreatment also produced potential fermentation inhibitors such as acetic, formic, cinnamic acids, and aldehydes. Despite some of these inhibitors remaining with the solids after PBHW pretreatment, there was more efficient hydrolysis of the cellulose and remaining hemicellulose during the enzymatic digestion of the grasses prior to fermentation when compared to the untreated grasses. This increase in digestibility observed with enzymes prior to fermentation resulted in increased ethanol yields during bioconversion using E. coli LY01 as the biocatalyst.

► Hot water pretreatment study on T85 bermudagrass and Merkeron napiergrass.
► Simultaneous analysis of sugars, ethanol and 41 potentially inhibitory compounds.
► Pressurized batch hot water pretreatment increased digestibility of both grasses.
►  Fermentation of pretreated grasses by E. coli, LY01 produced increased ethanol.
► Acetic acid and formic acids constituted 70–80% of the total degradation products.