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added warning for vaporizing ethanol as being flammable
Once the level of acidity is reached (this can be tested with a reliable [[PH_Meter|pH meter]], or in the case of using a pure culture can safely be taste tested), the wort is brought to a boil. The wort may be boiled normally in the case of any style of beer that requires a longer boiling process, or it it may be boiled for no more than a minute or two in the case of making a [[Berliner Weissbier]]. Technically speaking, the wort doesn't need to be boiled at all (this is called [http://www.garshol.priv.no/blog/331.html Raw Ale]). Heat pasteurization at 170°F (76.6°C) for 15 minutes should kill the ''Lactobacillus'' culture being used to sour the wort <ref name="pasteurization" />.
Deciding whether or not to boil the wort can also depend on whether or not there was a considerable amount of alcohol produced, which commonly happens when the wort is contaminated with yeast. Pure cultures of ''Lactobacillus'' do not show typical signs of fermentation that we are used to seeing with yeast fermentations, such as forming a krausen, producing a lot of CO2, or fermenting wort more than ~1.005 gravity points (see [[100% Lactobacillus Fermentation]]). If a yeast contamination produces a significant amount of alcohol during the souring process, then this presents a problem when it comes to boiling. Although 100% pure ethanol boils at 173.1°F/78.4°C <ref name="boiling_of_alcohol">[http://chemistry.about.com/od/moleculecompoundfacts/f/What-Is-The-Boiling-Point-Of-Alcohol.htm Boiling temperature of ethanol]</ref>, the lower the concentration of ethanol in the wort (technically beer at this point), the higher the temperature required for boiling off the ethanol. For example, at 5% ABV it takes approximately 197°F/92°C for the ethanol to boil <ref>[http://www.clawhammersupply.com/blogs/moonshine-still-blog/12243869-making-moonshine-still-temperature Making Moonshine: Still Temperature. Retrieved 01/11/2016.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1212116758816536/?comment_id=1212140998814112&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Russell Carpenter on MTF. 01/11/2016.]</ref>. Time is also required to boil off the ethanol, so this may not be as big of a concern as it first appears(''warning: vaporized ethanol is highly flammable)''. Another and perhaps more important consideration is that, unlike unfermented wort, boiling and high heat pasteurization temperatures can have a negative effect on the flavor of fermented beer. Beer already has anti-bacterial properties, such as low pH, presence of alcohol, and hops (although hops may not be present in wort being soured), so higher pasteurization temperatures aren't necessarily required for beer. For these reasons, the beer industry commonly heat pasteurizes beer at 140°F/60°C for 15 minutes, and this is also adequate for pasteurizing soured beer <ref>[http://www.sciencedirect.com/science/article/pii/S0023643806002854#bib16 A suitable model of microbial survival curves for beer pasteurization. Sencer Buzrul. 2006.]</ref>. In the case of an accidental yeast contamination during the souring process, another option is to simply dump the batch and start again with a pure culture of ''Lactobacillus''.
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>. It is recommended to pitch a healthy starter of yeast, possibly with a higher cell count than normal. In the case of using dry yeast, re-hydrating as per the manufacturer's recommendations and 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>. 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 [[Saccharomyces#Fermentation_Under_Low_pH_Conditions|Fermenting in low pH conditions]] for more information.