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The peculiarities of Brazilian fuel ethanol fermentation allow the entry of native yeasts that may dominate over the starter strains of Saccharomyces cerevisiae and persist throughout the sugarcane harvest. The switch from the use of baker's yeast as starter to selected budding yeasts obtained by a selective pressure strategy was followed by a wealth of genomic information that enabled the understanding of the superiority of selected yeast strains. This review describes how the process of yeast selection evolved in the sugarcane-based bioethanol industry, the selection criteria and recent advances in genomics that could advance the fermentation process. The prospective use of genetically modified yeast strains, specially designed for increased robustness and product yield, with special emphasis on those obtained by the CRISPR (clustered regularly interspaced palindromic repeats)-Cas9 (CRISPR-associated protein 9) genome-editing approach, is discussed as a possible solution to confer higher performance and stability to the fermentation process for fuel ethanol production., (© The Author(s) 2020. Published by Oxford University Press on behalf of FEMS.)

Dekkera bruxellensis is a spoilage yeast in wine and fuel ethanol fermentations able to produce volatile phenols from hydroxycinnamic acids by the action of the enzymes cinnamate decarboxylase (CD) and vinyphenol reductase (VR) in wine. However, there is no information about this ability in the bioethanol industry. This work evaluated CD and VR activities and 4-ethylphenol production from p-coumaric acid by three strains of D. bruxellensis and PE-2, an industrial Saccharomyces cerevisiae strain. Single and multiple-cycle batch fermentations in molasses and sugarcane juice were carried out. Dekkera bruxellensis strains showed similar CD activity but differences in VR activity. No production of 4-ethylphenol by S. cerevisiae in any fermentation system or media was observed. The concentrations of 4-ethylphenol peaked during active growth of D. bruxellensis in single-cycle fermentation but they were lower than in multiple-cycle fermentation. Higher concentrations were observed in molasses with molar conversion (p-coumaric acid to 4-ethylphenol) ranging from 45% to 85%. As the first report on 4-ethylphenol production in sugarcane musts by D. bruxellensis in industry-like conditions, it opens up a new avenue to investigate its effect on the viability and fermentative capacity of S. cerevisiae as well as to understand the interaction between the yeasts in the bioethanol industry.

The alcoholic fermentation for fuel ethanol production in Brazil occurs in the presence of several microorganisms present with the starter strain of Saccharomyces cerevisiae in sugarcane musts. It is expected that a multitude of microbial interactions may exist and impact on the fermentation yield. The yeast Dekkera bruxellensis and the bacterium Lactobacillus fermentum are important and frequent contaminants of industrial processes, although reports on the effects of both microorganisms simultaneously in ethanolic fermentation are scarce. The aim of this work was to determine the effects and interactions of both contaminants on the ethanolic fermentation carried out by the industrial yeast S. cerevisiae PE-2 in two different feedstocks (sugarcane juice and molasses) by running multiple batch fermentations with the starter yeast in pure or co-cultures with D. bruxellensis and/or L. fermentum. The fermentations contaminated with D. bruxellensis or L. fermentum or both together resulted in a lower average yield of ethanol, but it was higher in molasses than that of sugarcane juice. The decrease in the CFU number of S. cerevisiae was verified only in co-cultures with both D. bruxellensis and L. fermentum concomitant with higher residual sucrose concentration, lower glycerol and organic acid production in spite of a high reduction in the medium pH in both feedstocks. The growth of D. bruxellensis was stimulated in the presence of L. fermentum resulting in a more pronounced effect on the fermentation parameters than the effects of contamination by each microorganism individually.

Unlabelled: Dekkera bruxellensis is an important contaminant yeast of fuel ethanol fermentations in Brazil, whose system applies cell repitching between the fermentative cycles. This work evaluated the addition of potassium metabisulphite (PMB) on yeast growth and fermentative yields in pure and co-cultures of Saccharomyces cerevisiae and D. bruxellensis in two situations: addition to the acidic solution in which the cells are treated between the fermentative cycles or to the fermentation medium. In the range of 200-400 mg l(-1) , PMB was effective to control the growth of D. bruxellensis depending on the culture medium and strain. When added to the acidic solution (250 mg l(-1) ), a significant effect was observed in mixed cultures, because the inactivation of SO2 by S. cerevisiae most likely protected D. bruxellensis from being damaged by PMB. The physiological response of S. cerevisiae to the presence of PMB may explain the significant decrease in alcohol production. When added to the fermentation medium, PMB resulted in the control but not the death of D. bruxellensis, with less intensive effect on the fermentative efficiency. In co-culture with the addition of PMB, the fermentative efficiency was significantly lower than in the absence of PMB., Significance and Impact of the Study: This study is the first to evaluate the action of potassium metabisulphite to control the growth of Dekkera bruxellensis in the fermentation process for fuel alcohol production. As near as possible of industrial conditions, the study simulates the addition of that substance in different points in the fermentation process, verifying in which situation the effects over the starter yeast and alcohol yield are minimal and over D. bruxellensis are maximal. Co-culture fermentations were carried out in cell-recycled batch system. The feasibility of using this substance for this specific fermentation is discussed in light of the possible biological and chemical interactions., (© 2014 The Society for Applied Microbiology.)