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updates to using dry yeast and rehydrating
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. Using rehydrated dry yeast has been shown to be more reliable than both slurries and even freshly propagated yeast probably due to their rich sterol reserves (this excludes freshly propagated yeast using acid shock starters - see below) <ref>[https://www.researchgate.net/publication/293337388_Effect_of_pitching_yeast_preparation_on_the_refermentation_of_beer_in_bottles an Landschoot, A., Vanbeneden, N., Vanderputten, D. and Derdelinckx, G. (2004). Effect of pitching yeast preparation on the refermentation of beer in bottles. Cerevisia 29:140-147.]</ref><ref>Van den Berg, S., Demeyere, K. and Van Landschoot, A. (2001). The use of dried yeast for the bottle refermentation of beer. Cerevisia 26:102-108.</ref>. 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#Acid_Shock_Starters|Acid Shock Starters]] 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. Dry yeast should be rehydrated in ten times its weight in 80-90°F (27-32°C) water for ~15-20 minutes <ref name="Zandycke_Bottling">[http://www.mbaa.com/publications/tq/tqPastIssues/2011/Abstracts/TQ-48-1-0225-01.htm S. M. Van Zandycke, T. Fischborn, D. Peterson, G. Oliver, and C. D. Powell (2011). The Use of Dry Yeast for Bottle Conditioning. MBAA Technical Quarterly. doi:10.1094/TQ-48-1-0225-01.]</ref>.
Many wine yeast strains are known to be "killer" yeast strains. In ''Saccharomyces'', killer strains produce toxins that kill sensitive strains. Neutral strains do not produce toxins, nor are they killed by them <ref>[https://books.google.com/books?hl=en&lr=&id=mvORN6OXHh4C&oi=fnd&pg=PA93&dq=Bussey,+H.+1981.+Physiology+of+killer+factor+in+yeast.+Adv.+Microb.+Physiol.+22:93-121&ots=jUY4T9NpgB&sig=aw-Y1um0KsDnGe6rRe5PTWIDYdI#v=onepage&q&f=false Advances in Microbial Physiology, Volume 22. Academic Press, Sep 15, 1981. Pg 94-95.]</ref>. Almost all ale and lager strains are sensitive to the toxins produced by killer strains <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1179271825434363/?comment_id=1179424538752425&offset=0&total_comments=5&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Bryan of Sui Generis Blog on MTF on Killer Factor for Saccharomyces. 11/16/2015.]</ref><ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1973.tb03515.x/pdf Strains of Yeast Lethal to Brewery Yeasts. A.P. Maule and P.D. Thomas. 1972.]</ref>. In ''Saccharomyces'', four toxins have been identified: K1, K2, K28, and Klus, the first three of which can only kill other strains/species of ''Saccharomyces''. The Klus toxin has been found to kill yeast from other genra, such as ''Hanseniaspora spp.'', ''Kluyveromyces lactis'', ''Candida albicans'', ''Candida dubliniensis'', ''Candida kefir'' and ''Candida tropicalis'', and the K1, K2 and K28 killer strains of ''S. cerevisiae'' <ref name="Rodriguez">[http://aem.asm.org/content/77/5/1822.long A New Wine Saccharomyces cerevisiae Killer Toxin (Klus), Encoded by a Double-Stranded RNA Virus, with Broad Antifungal Activity Is Evolutionarily Related to a Chromosomal Host Gene. Nieves Rodríguez-Cousiño, Matilde Maqueda, Jesús Ambrona, Emiliano Zamora, Rosa Esteban and Manuel Ramírez. 2011]</ref>. However, none of the toxins have been found to kill ''Brettanomyces'' <ref>[http://www.scielo.org.za/scielo.php?pid=S2224-79042015000100010&script=sci_arttext&tlng=pt Non-Saccharomyces killer toxins: Possible biocontrol agents against Brettanomyces in wine? S. Afr. J. Enol. Vitic. vol.36 n.1 Stellenbosch. 2015.]</ref>. The K1 toxin is most active between a pH of 4.6 and 4.8, while K2 and Klus are active around a pH of 4.0 to 4.3 <ref name="Rodriguez"></ref>. See [[Saccharomyces#Killer_Wine_Yeast|Killer Wine Yeast]] for more information on these toxins.
This tends to create some fears for brewers who believe that the ''Saccharomyces'' strains in their beer might still be alive. However, in a highly acidic sour beer, ''Saccharomyces'' wine strains tend not to live for extended periods of time <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1268830686478476/?comment_id=1269605776400967&reply_comment_id=1270016239693254&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Richard Preiss on MTF. 03/28/2016.]</ref>. Even if some cells do, their activity would be next to none due to no available sugars left in the beer (other than priming sugar), and thus their contribution to the flavor development of the beer would be minimal to none. Assuming good bottling practices are followed, fear of autolysis occurring due to killer strain activity is usually unfounded because most of the ''Saccharomyces'' cells are left behind with the trub at bottling. Additionally, ''Brettanomyces'' has the ability to metabolize the acids and proteins that are released during whatever autolysis that might occur in the bottle. Re-yeasting with wine yeast for priming has the additional advantage of not fermenting maltose or maltotriose, so unexpected attenuation from the wine yeast will generally not occur as long as the beer is already fully attenuated. Flavor impact by the wine yeast is also probably not significant due to the small amount of fermentables provided by the priming sugar. Thus, there is little argument against re-yeasting with wine yeast at packaging time, other than a desire to approach carbonation in a traditional sense such as is the case with [[lambic]]. (In progress) The flavor impact by the wine yeast in beers that contain living ''Brettanomyces'' is also probably minimal or not significant. This is due to the small amount of fermentables provided by the priming sugar, as well as the ability of ''Brettanomyces'' to metabolize many secondary metabolites that would be produced by a bottling yeast.
* See [[Saccharomyces#Killer_Wine_Yeast|Killer Wine Yeast]] for more information.