The influence of nitrogen sources on ethanol production by yeast from concentrated sweet sorghum juice

Ethanol production from concentrated sweet sorghum juice was carried out under very high gravity (VHG) fermentation conditions using Saccharomyces cerevisiae yeast strain at 30 °C. Different nitrogen sources (CO(NH2)2 or (NH4)2SO4), with the concentration of 0.8 g nitrogen L−1, were added to promote the ethanol yields and prevent the byproducts formation (no additional phosphorus). With an initial sugar concentration of 300 g L−1, the maximum ethanol concentration and cell number of the fermentation medium with CO(NH2)2 as nitrogen source (135 g L−1 and 5.5 × 108 mL−1, respectively) were higher compared to those of the control one (120 g L−1 and 4.7 × 108 mL−1, respectively). However, no positive effects of (NH4)2SO4 were observed compared with the control. The yield of main byproducts (glycerol and acetic acid) obtained was 0.027 g g−1 of consumed sugar when CO(NH2)2 was used as a nitrogen source, which was clearly lower than those for ammonium-grown culture (0.035 g g−1 consumed sugar) and the control (0.037 g g−1 consumed sugar). The lower values of glycerol and acetic acid yields indicate that CO(NH2)2 succeeds in the efficient balancing of the redox potential and reduces the formation of byproducts during VHG fermentation compared with that when, for example, ammonium salt is the only available nitrogen source. It was concluded that production of high levels of ethanol could be achieved by supplementing urea as nitrogen source during ethanol fermentation from concentrated sweet sorghum juice.