NOVELTY - Controlling bacterial contamination in a fermentation process comprising a Zymomonas biocatalyst, comprises: (a) providing a growth medium; (b) adding stabilized chlorine dioxide to the growth medium to form a stabilized chlorine dioxide mixture; (c) inoculating the mixture with Zymomonas cells at a temperature suitable for Zymomonas cells to produce an inoculated broth; and (d) fermenting the inoculated broth under conditions suitable for growth of the Zymomonas cells.    USE - The method is useful for controlling bacterial contamination in a fermentation process using stabilized chlorine dioxide (claimed), for producing ethanol.    ADVANTAGE - The stabilized chlorine dioxide ensures effective control of bacterial contamination in a fermentation process, thus increasing the yield of ethanol, and reducing the process time.    DETAILED DESCRIPTION - Fermentation method for controlling bacterial contamination in a fermentation process comprising a Zymomonas biocatalyst, comprises: either (a) providing a growth medium having the potential for being contaminated by a bacterial species, (b) adding stabilized chlorine dioxide to the growth medium to form a stabilized chlorine dioxide mixture, where the temperature of the stabilized chlorine dioxide mixture is greater than 33 degrees C, (c) inoculating the mixture with Zymomonas cells at a temperature suitable for Zymomonas cells to produce an inoculated broth, and (d) fermenting the inoculated broth under conditions suitable for growth of the Zymomonas cells, where bacterial contamination is controlled during fermentation; or (a1) providing a seed medium lacking cellulosic biomass hydrolysate, (b1) inoculating the seed medium with Zymomonas ethanologen cells to form a seed culture, (c1) growing the Zymomonas cells in the seed culture, (d1) inoculating a fermentation medium with the seed culture to produce an inoculated fermentation medium, (e1) providing a saccharification reaction slurry, (f1) mixing the inoculated fermentation medium with the saccharification reaction slurry to form a fermentation beer, and (g1) growing the Zymomonas ethanologen cells in the fermentation beer under conditions, thus producing ethanol. The fermentation beer or the saccharification reaction slurry contains cellulosic biomass hydrolysate, and the stabilized chlorine dioxide is added to the seed medium, to the saccharification reaction slurry, or to the fermentation medium, (A) if the stabilized chlorine dioxide is added to the seed medium, the seed medium is maintained at a temperature of at least 33 degrees C for at least 6 hours prior to inoculation with the Zymomonas ethanologen, (B) if the stabilized chlorine dioxide is added to the saccharification reaction slurry of step (e1), the saccharification reaction slurry is maintained at a temperature of at least 33 degrees C for at least 8 hours prior to mixing with the inoculated fermentation medium, (C) if the stabilized chlorine dioxide is added to the fermentation medium of step (d), the fermentation medium is maintained at a temperature of at least 33 degrees C for at least 8 hours prior to the mixing with the saccharification reaction slurry, and the seed culture, the inoculated fermentation medium or the fermentation beer containing Zymomonas cells contain less than 5 g/l of lactic acid during the period of the growth of the Zymomonas cells. INDEPENDENT CLAIMS are included for:    (1) a fermentation medium composition comprising fermentation medium comprising cellulosic biomass hydrolysate, and stabilized chlorine dioxide; and    (2) a saccharification reaction slurry composition comprising cellulosic biomass, at least one cellulase enzyme, and stabilized chlorine dioxide.