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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 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, or temporarily by increasing [[tetrahydropyridine]] (THP) formation. For mixed fermentation/sour/funky beerswhere fresh bottling yeast is added, this may not be as significant of a concern. These beers, especially barrel aged versions, generally have more oxygen exposure during fermentation and aging , and this contributes to the characteristics of the beer. Some ''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), and others 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 processand report that purging bottles with CO² helps prevent THP formation. There Due to these many variables, there is no definite easy answer in terms of whether it is better to carefully CO2 CO² purge as is recommended in more normal standard practice for clean beers or if the yeasts present in mixed-fermentation beers are better able to quickly scavenge O2 before it . CO² purging can damage the beerrequire more expensive equipment, though excessive oxygen exposure can hurt your beer (either permanently through formation of acetic acid and/or ethyl acetate, or temporarily by influencing [[tetrahydropyridine]] (THP)as well as require more time to package. Based on your the characteristics of the microbes being used, experience, equipment, process, and preferences, you the brewer can determine how much to limit O2 limiting O² exposure in your beersduring packaging is necessary. The safest route is to treat them as you would a normal beer and take care to avoid O2O² where this is possible or affordable, though the experience of some will say that this extra work and cost is not completely necessary.  If force carbonating the beer instead of bottle conditioning with fresh yeast, or if bottle conditioning without adding fresh yeast, it might be best practice to purge bottles and kegs with CO² in order to limit the production of both acetic acid and THP. Again, experience should drive this decision one way or the other.