Effect of nutrients on fermentation of pretreated wheat straw at very high dry matter content by Saccharomyces cerevisiae.

Wheat straw hydrolysate produced by enzymatic hydrolysis of hydrothermal pretreated wheat straw at a very high solids concentration of 30% dry matter (w/w) was used for testing the effect of nutrients on their ability to improve fermentation performance of Saccharomyces cerevisiae. The nutrients tested were MgSO4 and nitrogen sources; (NH4)2SO4, urea, yeast extract, peptone and corn steep liquor. The fermentation was tested in a separate hydrolysis and fermentation process using a low amount of inoculum (0.33 g kg(-1)) and a non-adapted baker's yeast strain. A factorial screening design revealed that yeast extract, peptone, corn steep liquor and MgSO4 were the most significant factors in obtaining a high fermentation rate, high ethanol yield and low glycerol formation. The highest volumetric ethanol productivity was 1.16 g kg(-1) h(-1) and with an ethanol yield close to maximum theoretical. The use of urea or (NH4)2SO4 separately, together or in combination with MgSO4 or vitamins did not improve fermentation rate and resulted in increased glycerol formation compared to the use of yeast extract. Yeast extract was the single best component in improving fermentation performance and a concentration of 3.5 g kg(-1) resulted in high ethanol yield and a volumetric productivity of 0.6 g kg(-1) h(-1).