Effect of Saccharomyces cerevisiae and Zymomonas mobilis on the co-fermentation of sweet sorghum bagasse hydrolysates pretreated under varying conditions

•We fermented pentose/hexose sugars using Saccharomyces cerevisiae and Zymomonas mobilis as single or mixed cultures at varying inoculum ratios.•We monitored the changes in total sugar concentration, microbial cell growth and ethanol concentration over a period of 24 h.•Single cultures produced lower concentrations of ethanol whilst the co-cultures showed significant increase in yield.•Increasing the ratio of inoculum for S. cerevisiae to Z. mobilis gave the maximum ethanol concentration.•Overall the initial total sugar concentration has a significant influence on the final ethanol concentration.

Hydrolysates from sweet sorghum bagasse pretreatment normally contains hexose and pentose sugars, and this complex mixture of sugars presents a challenge for a single microorganism to effectively ferment all sugars to ethanol. In this study, synergistic effects on the co-fermentation of the hydrolysates using Sacchromyces cerevisiae and Zymomonas mobilis ATCC31825 at different ratios were studied. An inoculum of mixed cultures (1:3 and 5:10 g/L, Z. mobilis to S. cerevisiae ratios) was investigated. Each mixed culture was added to the hydrolysates at pH 4.8 and incubated 32 °C for 24 h. The mixture of Z. mobilis to S. cerevisiae at 5:10 g/L showed the highest synergistic effect with ethanol yields of 0.5 g/g. Since the yield for co-culture was significantly higher than the sum of yields from each microorganism, the improvements can be directly related to co-fermentation of hydrolysate by S. cerevisiae and Z. mobilis.