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When bottling or priming a keg at packaging time, the brewer should consider re-yeasting. In a long aged beer, especially an acidic beer, the ''Saccharomyces'' will mostly be dead already. This leaves ''Brettanomyces'' in charge of conditioning the beer. Although some lactic acid bacteria are capable of producing CO2, their contribution is probably negligible. It is also possible that homofermentative LAB will consume a portion of the sugar before the ''Brett'' has a chance to produce CO2. Re-yeasting is a very effective way to ensure proper CO2 levels in an aged sour beer. Another benefit of re-yeasting is that it tends to help avoid (or minimize) [[Tetrahydropyridine]] production.
Commercial producers and MTFers have had success re-yeasting with their mixed culture, wine yeast, and champagne yeast. The specific yeast you choose is up to you, and we recommend that you try a couple different yeasts out to find the one you prefer (MTFers have reported success with Lalvin EC-1118 Champagne, Red Star Premier Cuvée, Red Star Pasteur Blanc, Danstar CBC-1, Fermentis T-58, Scott Laboratories DV10 wine yeast, and fresh cultures of ''Brettanomyces'' <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1477763322251877/ Conversation on MTF regarding bottle conditioning yeast. 11/21/2016.]</ref>). When re-yeasting at bottling, it is not recommended to add new highly attenuative yeast to make sure that the bottling yeast you add cannot ferment additional carbohydrates remaining in the beer <ref name='Sour Hour episode 6'/> (~41 minutes in). Re-yeasting with a yeast that beer has already seen should eliminate the possibility of continued attenuation as long as the beer is already at final gravity. Brewers who are having difficulty carbonating sour beer or need to ensure that the beer will carbonate properly should grow their yeast first in a yeast starter that has yeast nutrients (Fermaid K + DAP, for example) and diluted 1:1 with the sour beer itself. This will acclimate the yeast to the harsh conditions of the sour beer, and has shown to be more effective for ensuring bottle carbonation <ref>[http://www.sciencedirect.com/science/article/pii/S0740002016301605 Terminal acidic shock inhibits sour beer bottle conditioning by Saccharomyces cerevisiae. Cody M. Rogers, Devon Veatch, Adam Covey, Caleb Staton, Matthew L. Bochman. 2016.]</ref>. Not all sour beers will need the priming yeast to be acclimated, but sour beers with darker malts, acidic fruits, and high ABV (8%+) might require this step. See [[Saccharomyces#Fermentation_Under_Low_pH_Conditions|Fermenting in low pH conditions]] for more information on what can cause this, and [[Packaging#Acclimatizing_Bottling_Yeast|Acclimatizing Methods of Acclimating Packaging Yeast Methods]] below.
The yeast required for carbonation is very little. A good rule of thumb to use is to use 10% of the yeast that you would normally use for a primary fermentation (approximately 1 million cells per mL). For example, for dried yeast use ~2 grams of yeast for 5 gallons of beer <ref name="priming_calc">[http://jeffreycrane.blogspot.com/2015/06/blending-calculator-ph-abv-and.html Blending Calculator - pH, ABV and Carbonation. Jeff Crane. Blending Calculator - pH, ABV and Carbonation. Bikes, Beer, & Adventures Blog. June 12, 2015.]</ref>. Rehydrating the yeast is recommended. See [http://jeffreycrane.blogspot.com/2015/06/blending-calculator-ph-abv-and.html Jeff Crane's "Blending Calculator" (extension of Michael Tonsmeire's "Blending Calculator")] for a re-yeasting and priming calculator.