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.

Prefermentation: an approach to improve co-fermentation in SSF

Prefermentation was reported as a possible means to overcome the problem of competitive
inhibition fron glucose on xylose: i.e. the free hexoses initially present in the slurry, were fermented before adding enzymes.

Corn steep liquor: a sole cheap nutrient for fermentation

In the lab, expensive yeast and peptone extracts have been used to provide these nutrients for fermentation. However, the cost of the fermentation medium is also one of the principal factors that determines the economic viability of the ethanol production, it is very necessary to find low cost medium components that can supply all the nutritional requirements for good growth and fermentation activity.

Corn steep liquor (CSL) is a liquid by-product of wet milling process of maize-starch industry.It is a sole inexpensive source of nitrogen, vitamins, amino acids and other nutrients, which has been used in the lab study of fermentation and demonstrated the favorable effects compared with traditional complex media formulations for the fermentation.

Recirculation of process streams from fermentation

When SSF fermentation is conducted via yeast, it is suggested to recirculate the process stream. The purpose is to maintain a high concentration of ethanol and dissolved solids and therefore to reduce the energy requirements in distillation and evaporation units ( expensive steps) and reduce fresh water usage. Based on the study by Alkasrawi et al (2002) for SSF process,when 60% of the fresh water was replaced by stillage, ther was no changed for ethanol yield and productivity but ethanol production cost reduced by 17%. When the liquid after SSF was partially recycled, 40% fresh water was replaced;final ethanol yield was not affected with initial productivity decreased; ethanol cost reduced 12%.

However, the disadvantage is that ccumulation of compounds released, or formed in the hydrolysis and fermentation steps may be toxic to enzymes or yeast.

So, we at least have effective enzymes and robust microorganisms for fermentation for this initial consideration of stream liquor circulation, right?