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Questions often arise regarding if and when wort aeration should be done. It is well documented that ''[[Saccharomyces]]'' uses oxygen to biosynthesize lipids, which include fatty acids and sterols, for their cell membranes. The cell membrane regulates the flow of nutrients into the cell and waste out of the cell, and allows the yeast to reproduce. Each time a yeast cell doubles during growth, the parent cell gives approximately half of its lipids to the daughter cell. The more sugar available to the yeast, the more they will reproduce, and thus the more lipids they require. Therefore, without a healthy cell membrane and a build up of lipids, the cell can die or produce weak daughter cells, potentially resulting in a range of off-flavors, especially in higher gravity beers <ref name="Aquilla">[https://www.morebeer.com/articles/how_yeast_use_oxygen "The Biochemistry of Yeast," by Tracy Aquilla. Morebeer Website. 07/25/2013. Retrieved 04/13/2016.]</ref><ref name="danstar">[http://www.danstaryeast.com/articles/aeration-and-starter-versus-wort Aeration And Starter Versus Wort. Danstar Website. Retrieved 04/13/2016.]</ref>. In the brewing of non-mixed fermentation beers, aerating both the yeast starter and the wort before pitching the yeast is generally considered mandatory to the brewing process.
Brewers have historically had concerns about aerating wort that has either been pre-soured with lactic acid bacteria (if the lactic acid bacteria is still alive) or if it will receive a co-pitch of lactic acid bacteria, ''[[Brettanomyces]]'', and ''[[Saccharomyces]]'' (see [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|Reusing a Sour Yeast Cake]], [[Mixed_Fermentation#Multi-Stage_Fermentation|Multi-Stage Fermentation]] and [[Sour Worting]]). These concerns, however, are largely unfounded. Most species of ''[[Lactobacillus]]'' are either not effected by oxygen, or benefit slightly. [[Butyric Acid]] production by ''Lactobacillus'' is not a concern (see [[Lactobacillus#Effects_of_Oxygen|''Lactobacillus'', effects of oxygen]] for details). Some species/strains of ''[[Pediococcus]]'' might be inhibited by oxygen, but not all (see ''[[Pediococcus#Growth_and_Environment|Pediococcus]]'' for details). ''[[Brettanomyces]]'' creates acetic acid in the presence of oxygen, however in the presence of a healthy pitch of ''[[Saccharomyces]]'', which rapidly consumes the oxygen, this is probably also not a concern. Additionally, oxygen greatly improves the vitality and cell count of ''Brettanomyces'' and a small amount is required for effective ''Brettanomyces'' growth and fermentation (see [[Brettanomyces Propagation Experiment]]). Thus, as long as a healthy pitch of ''[[Saccharomyces]]'' is present, aerating wort for mixed fermentation should lead to a healthy fermentation and good results.
Many brewers, however, do not aerate their wort when either pre-souring the wort with a pure culture of lactic acid bacteria, pitching fresh wort on top of a mixed culture yeast cake, co-pitching a mixed culture such as Wyeast Roeselare, or pitching a custom mix of microbes from multiple sources <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1182597671768445/?qa_ref=qd Conversation on MTF about oxygenating wort for mixed culture fermentation. 11/22/2015.]</ref>. Many such brewers have reported success without aerating. We therefore recommend that the brewer investigate and experiment with their process in order to decide whether or not aeration is desired.