Changes

In the presence of oxygen and alcohol, ''Brettanomyces'' species produce acetic acid as a byproduct of glucose fermentation. The more oxygen that is present, the more acetic acid is produced and the less ethanol is produced by ''Brettanomyces'' <ref>[https://link.springer.com/article/10.1007/s00253-002-1197-z Brettanomyces bruxellensis: effect of oxygen on growth and acetic acid production. M. G. Aguilar Uscanga, M.L. Délia, P. Strehaiano. 2003.]</ref><ref>[https://escarpmentlabs.com/blogs/resources/how-to-choose-a-brett-strain-for-beer "How to Choose a Brett Strain," Escarpment Labs blog post, 01/20/21.]</ref><ref name="Rozpędowska" /><ref>[https://academic.oup.com/femsyr/article/13/1/34/544881?login=true Fernanda Cristina Bezerra Leite, Thiago Olitta Basso, Will de Barros Pita, Andreas Karoly Gombert, Diogo Ardaillon Simões, Marcos Antonio de Morais, Jr, Quantitative aerobic physiology of the yeast Dekkera bruxellensis, a major contaminant in bioethanol production plants, FEMS Yeast Research, Volume 13, Issue 1, February 2013, Pages 34–43, https://doi.org/10.1111/j.1567-1364.2012.12007.x]</ref><ref>[https://link.springer.com/article/10.1007/BF00400180 Wijsman, M.R., van Dijken, J.P., van Kleeff, B.H.A. et al. Inhibition of fermentation and growth in batch cultures of the yeast Brettanomyces intermedius upon a shift from aerobic to anaerobic conditions (Custers effect). Antonie van Leeuwenhoek 50, 183–192 (1984). https://doi.org/10.1007/BF00400180.]</ref>. In an environment with oxygen present, sugar ''Brettanomyes'' switches to respiratory metabolism. Sugar is reduced to pyruvate within the cell and is then broken down into acetaldehyde which is then enzymatically oxidized into acetic acid or ethanol (dubbed the Custers effect). In an aerobic environment, acetate production is required for the cells to uptake certain types of sugars like galactose <ref>[https://link.springer.com/article/10.1007/s12010-023-04398-w Teles, G.H., Xavier, M.R., Da Silva, J.M. et al. The Metabolism of Respiring Carbon Sources by Dekkera bruxellensis and Its Relation with the Production of Acetate. Appl Biochem Biotechnol (2023). https://doi.org/10.1007/s12010-023-04398-w.]</ref>. This is thought to be a defensive tactic against competing microorganisms (e.g. ''Brettanomyces'' has been shown to produce more acetic acid when co-fermented with ''S. cerevisiae'', and ''S. cerevisiae'' has been shown to have less viability over time in the presence of acetic acid and ethanol) <ref>[https://link.springer.com/article/10.1023/A:1022592810405 Production of acetic acid by Dekkera/Brettanomyces yeasts under conditions of constant pH. S.N. FreerB. DienS. Matsuda. 2003.]</ref><ref name="Hubbe"></ref>. Depending on the brewer's palate and the degree of acetic production, this can be a desirable or undesirable trait. The degree of acetic acid production varies among different ''Brettanomyces'' species and strains, and it is limited by limiting oxygen exposure (see [[Mixed_Fermentation#Aging|aging mixed fermentation beer]] for practical tips on limiting oxygen exposure). For example, ''B. naardenensis'' and ''B. custersianus'' produce less acetic acid than other species of ''Brettanomyces'' <ref name="colomer_2020_genome" /><ref name="Tiukova_2019" />. Acetic acid produced by ''Brettanomyces'' is also used in the synthesis of [[Brettanomyces#Ester_Production|acetate esters]] such as ethyl acetate, perhaps as a mechanism to protect itself after hindering other microbes via the acetic acid precursor.