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====Secondary Fermentation====
After primary fermentation, the mostly attenuated beer is sometimes moved to a secondary fermentation vessel (and sometimes not; read below). In commercial production secondary fermentation is often conducted in wine barrels (mostly because it is messy to conduct primary fermentation in barrels), however, home brewers can accomplish this phase in glass or plastic carboys with low oxygen permeability. A mixed culture of ''Brettanomyces'', ''Lactobacillus'' and ''Pediococcus'' is then introduced to the beer. If barrels are being used then these microbes may simply come from the walls of the barrel, originating from a previous batch. Alternatively, the brewer might inoculate the wort with a mixed culture directly, either with a house culture or by introducing the dregs of bottled sour beer. Upon their introduction, these new microorganisms begin converting the longer chain sugars left over from the primary fermentation. These sugars are primarily converted into alcohol and lactic acid, increasing the degree of attenuation and lowering the pH of the beer. This also corresponds with a decrease in ''S. cerevisiae'' cell counts, and the release of amino acids and vitamins from yeast autolysis helps feed lactic acid bacteria and ''Brettanomyces'' <ref name="Hubbe">[https://www.facebook.com/groups/MilkTheFunk/1407620505932826/ Effect of mixed cultures on microbiological development in Berliner Weisse (master thesis). Thomas Hübbe. 2016.]</ref>. Other flavor-impacting secondary metabolites are also produced, depending on the strains used. For example, if the beer contains ''Brettanomyces'' this often results in the production of a high amount of fruity esters such as ethyl acetate and ethyl lactate, as well as "funky" phenols and other flavor compounds specific to ''Brettanomyces'' (see [[Brettanomyces#Secondary_Metabolites|''Brettanomyces'' secondary metabolites)]]. In the presence of oxygen, acetic acid is also produced by ''Brettanomyces'' (and acetic acid bacteria if they are present) which in low amounts can be complementary, adding to the complexity of the beer. In another study, researchers found that diacetyl that was formed around month 2 in mixed fermentation beers had disappeared after another 4 months of aging, indicating that diacetyl, if present in earlier stages of fermentation, will age out of mixed fermentation beer. They also reported that the antioxidant phenol guaiacol was present above flavor threshold during all stages of aging from 2-12 months, and [[Isovaleric Acid]] was formed after 12 months of aging <ref>[https://www.mdpi.com/2076-2607/11/7/1681 Postigo, V.; García, M.; Arroyo, T. Study of a First Approach to the Controlled Fermentation for Lambic Beer Production. Microorganisms 2023, 11, 1681. https://doi.org/10.3390/microorganisms11071681.]</ref>. These flavor compounds are essential to the flavor profile of mixed fermentation sour beer. For example, traditional Berliner Weisse was fermented with a mixed culture containing ''Brettanomyces'', and this was considered the most important aspect of achieving the fruity ester character of that beer style historically (see [https://docs.google.com/spreadsheets/d/1CNrO46TPSFpjhO3HX1-CbKK5rhFd7uGWdpONf7AJAlU/edit#gid=0 Benedikt Koch's table comparing esters of traditional Berliner Weisse versus kettle soured Kindl Weisse and Belgian gueuze]).
Some brewers (including homebrewers and professional brewers) do not find it necessary to move the mostly attenuated beer from the primary fermentation vessel to a secondary aging vessel. Instead, the mixed culture is pitched directly into the primary fermenter. While yeast autolysis is a concern in regular brewing, it is arguably not a cause for concern in mixed fermentations that contain ''Brettanomyces''. Lambic brewers, for example, perform a primary fermentation in barrels and leave the beer in the barrels during the beer's entire aging process, which is usually 1-3 years <ref>[http://www.lambic.info/Brewing_Lambic#Barrels Lambic.info Wiki. Brewing Lambic. Retrieved 6/8/2015.]</ref>. Yeast autolysis releases trehelose, acids, and other compounds, which are metabolized by ''Brettanomyces'' <ref>[http://www.mbaa.com/districts/michigan/events/Documents/2011_01_14BrettanomycesBrewing.pdf Brettanomyces in Brewing the horse the goat and the barnyard. Chad Yakobson. 1/14/2011.]</ref>. Maintaining a [[Solera]] may be an exception to this (see the [[Solera]] page for details). The advantage of not moving the beer into a secondary vessel is that less overall oxygen is introduced into the beer (oxygen exposure will contribute to more acetic acid and then ethyl acetate production), and might be the best option if the brewer does not have a closed/CO2 system to prevent exposure to oxygen during transferring. Some [[Brettanomyces#Nitrogen_Metabolism|evidence suggests]] that the nutrients released by yeast autolysis are beneficial to ''Brettanomyces'', so leaving the beer on the yeast cake might even be more desirable than not. Some sour beer brewers strive to achieve autolysis in their beers with the belief that it could improve mouthfeel and react with other compounds to produce favorable flavors, similar to how autolysis is sometimes desired in winemaking in the form of [https://www.thekitchn.com/wine-words-lees-aging-179813 lees aging] or [https://www.thekitchn.com/wine-words-btonnage-191331 bâtonnage ]<ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/3177292242298968/ Lars Meiner, Richard Preiss, and Alex Seitz. milk The Funk Facebook group thread on autolysis. 01/03/2019.]</ref>.