Nisin incorporation enhances the inactivation of lactic acid bacteria during the acid wash step of bioethanol production from sugarcane juice

Bioethanol fermentation is continuously exposed to contamination by Lactic Acid Bacteria (LAB). In this study, the effect of adding nisin (250 mg l⁻¹) to the acid wash on the viability of five bacterial contaminants were evaluated both alone and in co-incubation with Saccharomyces cerevisiae. Additionally, fed-batch fermentation was performed using an acid or acid/nisin wash for S. cerevisiae alone and cocultured with the LAB strains. Parameters such as ethanol production, sugar consumption and lactic acid production were monitored. Four model LAB were more susceptible to the acid/nisin wash than the acid wash, and were most susceptible when incubated with yeast. A fifth model LAB was very sensitive to both treatments regardless of the presence of yeast. The addition of nisin to the acid wash lowered the required time for adequate washing and resulted in a higher ethanol production (54·5 ± 0·1 g l⁻¹) than the acid wash alone (52·6 ± 0·1 g l⁻¹) in a subsequent fermentation. These results indicate the potential benefits of supplementing with nisin to improve the acid wash step of bioethanol fermentations. Significance and Impact of the Study: Acid washing by the bioethanol fermentation industry reduces yeast efficiency and selects for contaminant bacteria that are resistant to acid treatments. This study demonstrates that the incorporation of nisin into the acid wash step results in a more potent removal of lactic acid bacteria while significantly shortening the length of time needed for the acid wash.