Abstract
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.
Highlights
INTRODUCTIONIn Brazil, research regarding ethanol fermentation was encouraged through the National Alcohol Program in the
In Brazil, research regarding ethanol fermentation was encouraged through the National Alcohol Program in the1980s, which aimed to promote the use of ethanol as a fuel in place of petroleum
The ethanol yield of Z. mobilis in immobilized cells and batch fermentation system, with 120 h, was about 9% higher compared to free cells system (Figure 1)
Summary
In Brazil, research regarding ethanol fermentation was encouraged through the National Alcohol Program in the. Studies have shown that the cell immobilization method operates more efficiently in continuous processes, because it eliminates the use of filters, decanters or centrifuges to recover the cells that should be returned to the fermentation vats, resulting in better ethanol yields in experimental conditions (Black et al, 1984; Shafaghat et al, 2011). Aiming to overcome these limitations and increase fermentation yields, studies were carried out using mixed inoculum of Saccharomyces uvarum IZ1904 and Z. mobilis CP4 (Morais et al, 1992) In this experiment, microorganism association increased ethanol production. Considering the findings of other researchers who used yeasts and bactérial cells, both immobilized for ethanol production, the present study aimed, in laboratory context, to compare fermentation performance of free and immobilized cells; to compare alcohol yields in batch and continuous fermentation processes; and to evaluate possible synergistic effects of using these techniques de Almeida combining S. cerevisiae and Z.mobilis
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