Leveduras Não-Saccharomyces para produção de etanol em mostos de cana-de-açúcar

Abstract

The main yeast used in the industrial ethanol production process is Saccharomyces cerevisiae. However, non-Saccharomyces yeasts may offer some advantages, such as greater adaptation to the fermentation environment and greater genetic diversity. Alcoholic fermentation is an essential step in fuel ethanol production, where factors such as must composition, process conduction system, and yeast treatment affect process efficiency. Therefore, this study aimed to evaluate the potential of non-Saccharomyces yeasts (Wickerhamomyces anomalus, Meyerozyma guilliermondii, and Scheffersomyces stipitis) for ethanol production from sugarcane musts compared to industrial strains of S. cerevisiae (PE-2 and CAT-1). Initially, the stress tolerance (pH, temperature, glucose, and ethanol concentrations) of the yeasts was evaluated in microplates and spot assays. There was a difference between the yeasts S. cerevisiae CAT-1 and PE-2. Among the non-Saccharomyces, W. anomalus and M. guilliermondii presented tolerance profiles closer to the industrial ones, with W. anomalus being similar to PE-2 and M. guilliermondii to CAT-1. Both non-Saccharomyces strains demonstrated promising characteristics for the ethanol fermentation process under typical industrial stresses. Next, the growth and ethanol yield of the yeasts were evaluated in three different sugarcane-derived musts (molasses, broth, and mixed) at a concentration of 40 g/L total reducing sugar, under agitation, at 30 °C. Molasses promoted an increase in cell number for S. cerevisiae and non-Saccharomyces strains. Strain PE-2 had the highest ethanol yield in 12 h, with little variation between media, while CAT-1 achieved its best yield in broth after 24 h. For S. stipitis and W. anomalus, the best yields occurred in broth, with M. guilliermondii showing the highest growth in molasses, but with similar yields in broth and molasses media. Considering molasses as a medium for cell propagation and fermentation, fermentation tests were performed in a fed-batch system with cell recycling (six cycles) in molasses, also evaluating the effect of treating the cells with sulfuric acid between fermentation cycles. The yeasts S. cerevisiae CAT-1 and M. guilliermondii were selected, and cell viability and fermentation parameters were evaluated. Cell recycling reduced the fermentation rate and viability of both yeasts. For M. guilliermondii, acid treatment decreased viability but maintained yield (50% of theoretical), indicating tolerance to acid stress and a limitation related to its fermentation capacity. For S. cerevisiae CAT-1, acid treatment increased yield, despite reducing viability and budding. M. guilliermondii was not competitive with S. cerevisiae in terms of fermentation yield, although it demonstrated that even with acid treatment and cell recycling, it can maintain cell viability at levels that do not impact its ethanol yield, although much lower than that demonstrated by S. cerevisiae CAT-1.