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Aging is generally required for mixed fermentations that include ''Brettanomyces''. The necessary/ideal amount of aging time will depend on many factors including the microbes pitched, the pitching rate, wort composition, storage temperature, and the desired final beer. Keep in mind that the beer will also continue to develop once packaged. For more straightforward beers with highly attenuative primary strains (like tart saisons), a reasonable final product with tartness and ''Brettanomyces'' character can be reached in a few months. For more complex and/or acidic beers (such as Flemish reds or beers inspired by lambics) you may expect an aging time of at least 9 months, but quite possibly as long as 12-18 months or longer. In general longer aging will allow more complex expression of the spectrum microbes present. Some brewers will package a beer after the finishing gravity has stabilized (see [[Packaging]]), and allow the beer to fully develop in the bottle. Keep in mind that some volatile flavor compounds, such as sulfur-based compounds, may volatilize off at a faster rate in a fermenter (especially a shallow fermenter such as a barrel) than they would in a sealed bottle, and bottling too early can result in over-carbonation.
Sour beer should be aged in an environment that minimizes high temperatures and exposure to oxygen. Avoid temperatures over 85°F (29.5°C) and under 55°F (13°C). Drastic temperature fluctuations and changes in atmospheric pressure will cause a vacuum inside of the fermentation vessel causing water airlocks to "suck back" air into the fermenter. This could potentially contribute to [[Acetic Acid]] and [[Ethyl acetate]] (nail polish aroma in high concentrations) production by ''Brettanomyces'', and these off-flavor metabolites are considered permanent. Filling the carboy to the neck or topping up carboys or barrels after primary fermentation will also help minimize the surface area of the beer that can be exposed to air. Topping up and flushing with CO<sub>2</sub> might also help reduce the risk of mold growth <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2640657275962470/?comment_id=2641402822554582&reply_comment_id=2642128842481980&comment_tracking=%7B%22tn%22%3A%22R%22%7D Lars Meiner. Milk The Funk Facebook group thread on preventing mold growth on krausen. 05/02/2019.]</ref> on any krausen material that has dried on the sides of the fermentation vessel after primary fermentation. Avoid oversampling the beer (once every 3 months at the very most). Using an "S airlock" has the benefit of showing if there is positive, negative, or equalized pressure in the fermenter, which could possibly assist in showing whether suck-back is a problem(see the Mark Trent YouTube video below). One way ventilated silicone bungs can be used for barrels or other waterless type airlocks (such as the [http://www.better-bottle.com/products_master.html BetterBottle "DryTrap"] or kegs with a [http://seanterrill.com/2015/06/25/build-a-better-spunding-valve/ spunding valve]; see also [[Sanke Fermenter]]) that allow gases to escape the fermenter but not enter from the environment, and [https://beer.thegremlyn.com/2019/09/04/grems-the-gas-reducing-multivessel-system/ Colin Burton's homebrew setup for connecting multiple cornelius kegs to a single spunding valve]. Topping up barrels with fresh beer every 3-6 months might help reduce acetic acid and ethyl acetate, and humidity and temperature control can help reduce evaporation (see [[Barrel#Using_Barrels_for_fermentation_and.2For_aging|Barrel]]). It should also be noted that micro-oxygenation is helpful for creating certain flavors in sour beer, and many homebrewers have reported not having any issues with overexposure to oxygen using water-based airlocks. For example, a small amount of oxygen helps [[Brettanomyces]] growth, and a small level of acetic acid is desirable for the complexity of long-aged sour beers <ref>[https://www.milkthefunk.live/podcast Richard Preiss. Interview on Milk The Funk "The Podcast" Episode #000. 12/13/2017.]</ref> (~30 minutes in). Higher levels of acetic acid are sometimes desirable for [[Flanders Red Ale]] style beers.
:Mark Trent's demonstration of how easy it is for temperature changes to cause a vacuum and suck-back air into a vessel: