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Once the soured wort is boiled or heat pasteurized, it can be safely added to the primary fermenting vessel without worries of future infections. The wort is aerated as normal, and brewer's yeast, or ''[[Brettanomyces]]'' yeast, is then pitched into the wort as normal (usually brewer's yeast is used if infection of cold side equipment is a concern). At a pH of 3.4 or lower, the acidity of the wort can reportedly effect the fermentation of some strains of brewer's yeast <ref name="low ph">[http://www.homebrewtalk.com/f127/no-hop-berliner-weisse-415067/index3.html#post5280971 Michael Tonsmeire on HBT]</ref><ref>[http://www.themadfermentationist.com/2016/12/quick-sour-then-what-acid-tolerance-of.html "Quick Sour, then what? Acid Tolerance of Brewer’s Yeast." The Mad Fermentationist blog. Michael Tonsmeire. 12/13/2016. Retrieved 12/14/2016.]</ref>. For example, a published study showed that growth of US-05 was 82% at a pH of 3.51, and 53% at a pH of 3.17. Fermentation was delayed by 2-4 days (the lower the pH, the longer the start of fermentation was delayed) <ref name="peyer_2017">[http://www.asbcnet.org/publications/journal/vol/2017/Pages/ASBCJ-2017-3861-01.aspx Sour Brewing: Impact of Lactobacillus amylovorus FST2.11 on Technological and Quality Attributes of Acid Beers. Lorenzo C. Peyer, Martin Zarnkow, Fritz Jacob, David P. Schutter, Elke K. Arendt. 2017.]</ref>. It is recommended to pitch a healthy starter of yeast, possibly with a higher cell count than normal. When using dry yeast, re-hydrating as per the manufacturer's recommendations, using a yeast nutrient like Go-Ferm is recommended. Many yeast strains have been successfully used by MTF members to ferment pre-acidified wort: US05, S04, WY1098/WLP007/OYL-006, Belle Saison, Sacch Trois, Bret brux, B. clausenii, B. custersianus, Bret Drie (BSI), WY3711, and WY3726 to name a few (see reference) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1105185972842949/ Conversation on MTF about using specific yeast strains in acidic wort. 7/6/2015.]</ref>. [http://www.themadfermentationist.com/2016/12/quick-sour-then-what-acid-tolerance-of.html Michael Tonsmeire] has shown results that suggest that English yeast strains might attenuate slightly more and give better flavor results than other strains; Richard Preiss from Escarpment Labs expressed similar observations <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1509509862410556/?comment_id=1509520015742874&comment_tracking=%7B%22tn%22%3A%22R4%22%7D MTF conversation with Richard Preiss about English ale strains and pH tolerance. 12/13/2016.]</ref>. Brewers who are having difficulty fully fermenting pre-acidified wort can try growing their yeast in the soured wort (pasteurize the soured wort first if needed) with yeast nutrients (Fermaid K + DAP, for example). This assumes that the wort still has a lot of sugar left over after souring (if not, DME can be added). See [[Packaging#Acid_Shock_Starters|Acid Shock Starters]] for more information on how to acclimate yeast to a highly acidic environment to improve fermentation.
The usual kettle souring process involves fermenting with a normal ale strain of some sort, and lacks ''Brettanomyces''. Without ''Brettanomyces'', there is a lack of ''Brettanomyces'' character which is the result of the unique esters, phenols, and fatty acids that this genus of yeast produces (see [[Brettanomyces#Secondary_Metabolites|''Brettanomyces'' secondary metabolites]]). For example, traditional [[Berliner_Weissbier|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]). Kettle sours can also be racked into oak barrels and have ''Brettanomyces'' pitched at that time. This approach is taken by some commercial brewers who don't want to expose their fermentation vessels to living ''Lactobacillus'' or ''Brettanomyces''. It is also one method for controlling the level of sourness in the beer since the ''Lactobacillus'' is killed off.
See also: