Bioconversion of Sweet Sorghum Stalk Sugars into Ethanol by Zymomonas mobilis Isolated from Palm Wine

Background and objective : Sweet sorghum is a sugar-rich crop with considerable potential as a feedstock for bioethanol production. This study aimed to characterize sweet sorghum juice and evaluate its alcoholic bioconversion by Zymomonas mobilis isolated from palm wine, using Saccharomyces cerevisiae as a reference microorganism. Material and methods : Z. mobilis was isolated from fresh palm wine on Wallerstein Laboratory (WL) medium, purified, and cultivated on a standard medium. Its ethanol tolerance was compared with that of S. cerevisiae at different ethanol concentrations. Sweet sorghum juice was extracted by pressing and characterized for dry matter, ash, total and reducing sugars, nitrogen, and selected mineral elements. Fermentation was performed under non-aerated conditions at 30°C in a 2-L fermenter using a Z. mobilis preculture corresponding to 10% of the final volume. Results : Colonies attributed to Z. mobilis on WL medium were lenticular and appeared after 4-5 days of incubation at 30°C. The isolate showed growth at ethanol concentrations ranging from 2.5% to 10%. Sweet sorghum juice had a mean dry matter content of 25.97 g/100 mL and mean total and reducing sugar contents of 18.99 and 19.12 g/100 mL, respectively. Fermentation with Z. mobilis did not result in a significant decrease in °Brix, which declined from 17°B initially to 16.5°B after 3 days and 16.3°B after 6 days. In contrast, the S. cerevisiae control produced an alcohol content of 5% with a reported yield of 50%. Conclusion : Under the experimental conditions investigated, the Z. mobilis strain isolated from palm wine did not efficiently ferment sweet sorghum juice. This failure may be associated with the nutritional characteristics of the substrate, preculture preparation conditions, or the physiological properties of the strain. Further studies involving more precise microbiological identification, optimization of the culture medium, and analytical monitoring of fermentation metabolites are warranted.