Yeast strains ethanol-resistant

Seven S. cerevisiae strains were characterized relative to their resistance to sulfur dioxide (since it is a desirable feature in the fermentative yeast strains), ethanol (where tolerance is an indispensable property due to the high concentrations reached by the end of fermentation (Carrasco et al., 2001)), and osmotic stress (due to the high osmotic potential of mead at the commencement of fermentation). Pereira (2008) and Pereira et al. (2009) verified that significant differences did not exist between the strains. S. cerevisiae strains isolated from honey were similar to commercial and reference strains—all appearing to be suitable for mead production. 

Aranda, A., Querol, A., del Olmo, M. (2002). Correlation between acetaldehyde and ethanol resistance and expression of HSP genes in yeast strains isolated during the biological aging of sherry wines. Arch. Microbiol., 177, 304-312. 

In choosing a yeast strain, the yeasts should certainly be resistant to ethanol. Yeasts commercialized under the name S. bayanus could be recommended but they seem to have a propensity to form volatile acidity in these conditions. Commercially available S. cerevisiae yeasts (formerly S. bayanus), known for their resistance to ethanol and low probability of producing volatile acidity, are recommended for this purpose. 

The number of pasteurization units required to obtain a certain level of destruction can be calculated from yeast thermoresistance criteria in a given medium. Laboratory studies anticipated that 0.05 pasteurization units would be sufficient for the destruction of yeasts in a dry wine at 12% vol.ethanol. In practice, 0.5 pasteurization units are required to sterilize such a wine. The uneven heat supplied by industrial heating equipment and the existence of particularly resistant yeast strains at the final stages of fermentation can explain this difference. Table 9.5 shows the necessary conditions for the destruction of germs according to the constitution of the wine. 

In many countries, alcoholic fermentation is induced by inoculation with a yeast starter culture of Saccharomyces selected for its desirable winemaking qualities (Kunkee, 1984 Kunkee and Bisson, 1993 Rainieri and Pretorius, 2000 Reed and Chen, 1978 Reed and Nagodawithana, 1988). Starter cultures of S. cerevisiae strains are generally used because of to their increased ethanol and sulfur dioxide resistance and production of desirable aromas and flavors (Boulton et al., 1996 Ebeler, 2001 Nykanen, 1986 Reed and Chen, 1978 Reed and Nagodawithana, 1988). 

Notwithstanding recent progress, it remains to be demonstrated that ther-mophiles can produce economically viable titers under industrial conditions - for example, an affordable lignocellulosic feedstock, inexpensive growth media, and in the presence of potential inhibitors. It has been shown that thermophiles can be adapted to manifest increased resistance to inhibitors other than ethanol, in particular in the case of by-products resulting from Populus pretreatment [85]. We speculate that it may be difficult and perhaps not feasible to develop strains of thermophiles that are as resistant to chemical inhibition as yeast, and that the merits of thermophiles may be best realized in the context of processes that minimize formation of inhibitors in the first place. 

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