Immobilization and association of microorganisms to improve fermentation performance for ethanol production

Alcohol production in industry is carried out almost exclusively by means of batch or continuous fermentation processes. In both cases, cells are found spread throughout the fermenting substrate and have to go through a centrifugation cycle in order to be recovered. Investment in equipment is very high and performance is often lower than expected. According to available information, ethanol fermentation performance could be better in conditions where the cells are immobilized in continuous processes.  In this experiment, it was possible to affirm, based on statistical data that fermentation with immobilized cells improves alcohol yields. Using a free cell batch fermentation process, Zymomonas mobilis reached 59.95% of the theoretical yield. Immobilized cells reached 68.53% using a batch and 74.49% using a continuous fermentation process. Under the same conditions, Saccharomyces cerevisiae reached respectively 70.03, 77.10 and 78.47% of the theoretical yield. Higher yields were achieved for both microorganisms using mixed culture fermentation, compared to pure cultures. Under the same conditions for both pure cultures, mixed cultures reached respectively 70.86, 79.07 and 80.86% of the theoretical yield. Findings suggest that association and immobilization cultures of S. cerevisiae and Z. mobilis result in better yields for batch as well as continuous fermentation processes. Key words: Cell immobilization, mixed cultures, ethanol fermentation, Saccharomyces cerevisiae, Zymomonas mobilis.