USE OF SULFITE AND HYDROGEN-PEROXIDE TO CONTROL BACTERIAL-CONTAMINATION IN ETHANOL FERMENTATION

Lactic acid bacteria isolated from an industrial-scale ethanol ferment ation process were used to evaluate sulfite as a bacterial-contaminati on control agent in a cell-recycled continuous ethanol fermentation pr ocess. The viabilities of bacteria were decreased by sulfite at concen trations of 100 to 400 mg liter(-1), while sulfite at the same concent rations did not change the viability of the Saccharomyces cerevisiae s train used in this process, Sulfite was effective only in the presence of oxygen. Bacteria showed differences in their susceptibilities to s ulfite, Facultatively heterofermentative Lactobacillus casei 4-3 was m ore susceptible than was obligatory heterofermentative Lactobacillus f ermentum 7-1. The former showed higher enzyme activities involved in t he production and consumption of hydrogen peroxide than did the latter , The viability of L. fermentum 7-1 could be selectively controlled by hydrogen peroxide at concentrations of 1 to 10 mM. Based on these fin dings, it is hypothesized that the sulfur trioxide radical anions form ed by peroxidase in the presence of hydrogen peroxide are responsible for the control of contaminating bacteria, Sulfite did not kill the ye ast strain, which has catalase to degrade hydrogen peroxide, A cell re cycled continuous ethanol fermentation process was run successfully wi th sulfite treatments.