Biomass hydrolyzate fermentation performance:

An economically-attractive cellulosic technology usually requires the strain to achieve ethanol yield, titer and rate higher than 90%, 40 g/L (5.1%v/v), 1.0 g/L/h, respectively. Many metabolically-engineered ethanologens  have been developed to hit the metrics. A research paper published recently by Lau et al compared the fermentation performance of the three engineered microorganisms such as S. cerevisiae 424A(LNH-ST), Z. mobilis AX101 and E. coli KO11, demonstrating that the difficulty to achieve the targets when fermenting  milled AFEX pretreated corn stover using CSL as nitrogen source. 

In a large  commercial scale with a  un-milled biomass biomass at a solids content >= 15%, it will be much harder to achieve the target fermentation conversion through one-pass hydrolysis and fermentation. To avoid harsh pretreatments at extreme conditions, a process with partial recycling  of solid stream from fermentation might be helpful to achieve the target. Without harsh pretreatments and expensive capital cost, the overall cost will be low if the target metrics are achieve.