Effect of Lactic Acid Bacteria and Some Bacterial Contaminant on Non-Sterile Yeast Cell Production

Lactobacillus plantarum LF2, previously isolated from sugarcane juice, was used to assess its potential to reduce and prevent bacterial contamination in non-sterile yeast cell production using sugarcane based medium. The results showed that LF2 and Saccharomyces cerevisiae M30 coexisted, and that the maximum specific growth rate of the yeast was not affected by the presence of LF2, neither in sterile nor non-sterile batch cultures. The yeast specific growth rate was also not affected by the deliberate addition of gram-negative aerobic bacterial culture at the level of 10⁷ CFU.ml^-1 at the start of batch culture. However, compared to the pure yeast culture, the maximal cell dry weight decreased in all mixed cultures. During batch culture, growth of the bacterial contaminant was suppressed either when the pH fell below 4.0 or in the presence of LF2. During the prolonged growth phase of cyclic-fed batch culture (CFBC), the number of yeast M30 and LF2 remained coexistent. However, the bacterial contaminant exhibited growth competition during CFBC, resulting in a rapid increase in the cell number from approx. 10⁵ to 10⁸ CFU.ml^-1 over the period of 24 hours while the number of LF2 tended to decrease slightly.

Keywords: Lactic acid bacteria, yeast, fermentation, contamination

Corresponding author: E-mail: kcsasivi@kmitl.ac.th