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After primary fermentation the mostly attenuated beer is moved to a secondary fermentation vessel. Often in traditional commercial production secondary fermentation is conducted in wine 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 these "bugs" 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 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. In the presence of oxygen acetic acid is also produced which in low amounts can be complementary, adding to the complexity of the beer.
Some brewers (mostly homebrewers) do not find it necessary to move the mostly attenuated beer into a secondary vessel. Instead, the mixed culture is pitched directly into the primary fermenter. While yeast autolysis is a concern in regular brewing, it is less so in mixed fermentations that contain Brettanomyces. Lambic brewers, for example, perform a primary fermentation in barrels, and continue to leave the beer in the barrels during their maturation 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). Not The advantage of not moving the beer into a secondary vessel has the advantage of not introducing is that less overall oxygen is introduced into the beer(oxygen exposure will contribute to more acetic acid production), and may be the best option if the brewer does not have a closed/CO2 system to prevent exposure to oxygen during the transfertransferring.
====Aging====