Monday, April 30, 2007

Fermentability improvement of dilute-acid hydrolyzate and high ethanol production rate by encapsulated S. cerevisiae

Farid Talebnia Rowshan III and Mohammad Taherzadeh III. School of Engineering,University College of Borås and Chalmers University of Technology, Allegatan 1, Borås, 501 90, Sweden

Fossil fuels have been used dominantly over the last century, but nowadays interest to find a substitute fuel is increasing due to global warming caused by excessive emissions of CO2, shortage of oil reservoirs and instability of the price of fossil fuels. Ethanol as a suitable substitute for fossil fuel can be produced from the renewable resources like wood residuals. However, despite its availability and low price, lignocellulosic materials need hydrolysis and pre-treatment prior to fermentation step.

Fermentation of lignocellulosic material to ethanol is still a big challenge due to simultaneous formation of many known/unknown toxic components during the hydrolysis. Since, the yeast cells have capacity for in situ detoxification, providing of high-cell concentration in the bioreactor could be a possible solution to overcome the toxicity problem. However, free-cell cultivation even at high concentration could not fairly improve the fermentability of the hydrolyzate. Instead, we have applied this fact, by using of encapsulated cells, where the yeast cells were confined in a semi-permeable membrane.
Using undetoxified hydrolyzate, continuous cultivation with free-cells can be carried out only at D=0.1(h-1), while encapsulated yeast assimilated sugars by more than 75% (±5) at D=0.5(h-1).

Investigation on encapsulated yeast over 20 sequential batches showed that the cells gradually accumulate glycogen and trehalose while protein and RNA content showed a reverse trend. The volumetric ethanol production rate however, remains almost unchanged.

In conclusion, highly toxic hydrolyzate can be fermented to ethanol without any pre-treatment and at higher dilution and production rate.

Web Page: