Changes

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 will also help minimize the surface area of the beer that can be exposed to air. Avoid oversampling the beer (once every 3 months at the very most). 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]) that allow gases to escape the fermenter but not enter from the environment. Topping up barrels with fresh beer every 3-6 months might help reduce acetic acid and ethyl acetate, and humidity and temeprature temperature control can help reduce evaporation (see [[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.

Different airlocks and vessel materials diffuse oxygen at different rates. For example, a set of experiments published by Dr. Enrich L. Gibbs at BetterBottle™ showed that rubber stoppers prevented oxygen transfer more effectively than silicone stoppers, plastic stoppers, and both the "3 piece" airlock and "S" airlock. Solid bungs, however, can build up pressure inside the fermenter as the beer slowly ferments, and can pop off due to the pressure (and can cause messes if the vessel becomes pressurized too much). Weekly degassing for a few months while the beer ages is one option with solid bungs. Another option is to rack the beer to a keg and age it in a sealed environment, however, pressure can build up in kegs as well so they should occasionally be partially degassed (some gas should remain in the keg to maintain the seal for corny kegs). See the [http://www.better-bottle.com/pdf/ClosuresOxygenPassageStudy.pdf BetterBottle™ paper for more information]. Raj Apte found that HDPE buckets let in far more oxygen than carboy setups, and taking into account the high surface area to volume ratio in homebrew setups versus full-size barrels, oxygen exposure over time on the homebrew level can be a difficult issue to solve. For example, Apte attempted using wooden dowels as stoppers in carboys and found that it let in about the same amount of oxygen as wooden tanks at Rodenbach. However, the swelling of the wooden dowels led some people to crack or destroy glass carboys (therefore this method is not recommended) <ref name="Apte">[http://web.archive.org/web/20100410025103/http://www2.parc.com/emdl/members/apte/flemishredale.shtml "How to Make Sour Ale: an inquiry". Raj B Apte. 2004. Retrieved 1/1/2016.]</ref>. If signs of oxygen exposure appear (the growth of a [[pellicle]], the smell of acetic acid or ethyl acetate, etc.), it might be wise to package the beer sooner rather than later, assuming the gravity is stable.