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Oxygen is the enemy of finished beer, and most producers of standard beers invest a lot of time and money to reduce into reduinge oxygen exposure in their finished beer (referred to as "dissolved oxygen", or "total packaged oxygen"). Excessive oxygen exposure can negatively impact the beer either permanently through the formation of acetic acid and/or ethyl acetate, temporarily by increasing [[tetrahydropyridine]] (THP) formation, or through the [[Aging_and_Storage#General_Effects_of_Oxidation|oxidation of various compounds]]. For mixed fermentation beers where fresh bottling yeast is added, this may not be as significant of a concern, depending on how much oxygen is allowed to enter the packaging. These beers, especially barrel aged versions, generally have more oxygen exposure during fermentation and aging, and this contributes to the characteristics of the beer. ''Brettanomyces'' is known as an "oxygen scavenger", which helps to prevent oxidative reactions in the bottle over time (see [[Aging and Storage]]). Fresh bottle conditioning yeast can help prevent acetic acid and THP production by consuming the oxygen in non-purged bottles before it can have an effect on the mixed culture. Even without fresh bottling yeast, some brewers achieve good results without purging bottles before filling (for example, lambic producers and blenders), although this might be dependent on whether the strains present in the beer are big THP producers or if the brewer has limited time to store bottles long enough for the THP to age out. Others do take care to avoid oxygen exposure in the packaging process and report that purging bottles with CO₂ helps prevent THP formation. Due to these many variables, there is no easy answer in terms of whether it is better to carefully CO₂ purge as is standard practice for clean beers. CO₂ purging can require more expensive equipment, as well as require more time to package. Based on the characteristics of the microbes being used, experience, equipment, process, and preferences, the brewer can determine how much limiting O² exposure during packaging is necessary. The safest route is to treat them as you would a normal beer and take care to avoid O² where this is possible or affordable, though the experience of some will say that this extra work and cost might not be completely necessary like it is for clean beers.

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