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Once the level of acidity is reached (this can be tested with a reliable pH meter, or in the case of using a pure culture can safely be taste tested), the wort is brought to a boil. ''Caution: the wort gravity may also be tested to check for alcohol presence. Alcohol can be more easily boiled off since it boils at 173.1°F or 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 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]), as long as it is heat pasteurized <ref name="pasteurization" />, so if alcohol is present then the brewer can choose to heat pasteurize the wort at 170°F (76.6°C) for 15 minutes.
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>.
===Souring in the Primary Fermenter===
The process is very similar to the sour worting technique, except the wort is never pasteurized after it is soured. The all grain brewing process is the same for any all grain brewing process, except that after the boil the beer is only chilled to the recommended temperature for the ''Lactobacillus'' strain that the brewer is going to use. Using grain husks for souring with this method is not advised since the grain will stay in the fermenter during primary fermentation, and unwanted microbes on the grain husks would potentially have a longer exposure to the wort. Instead, the brewer should use a pure strain of ''[[Lactobacillus]]''. As a result of not using grains to sour the wort, there is little concern of developing [[Butyric Acid]] or [[Isovaleric Acid]] with this method. There is also the option of [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|using a sour yeast cake]] from another sour beer as the bacteria inoculation.
Once cooled to the desired temperature (usually around 90-120°F or 32.2-48.8°C), the wort is racked to the primary fermenting vessel. Note that the wort should contain a low amount of IBU's when using this process since IBU's can inhibit many (but not all) species of ''[[Lactobacillus]]''. Using no hops is a good approach to getting more acidity, but if hops are required then using less than 6 IBU's is a good guideline in general (consider mash hopping; mash hopping has been reported to reduce IBU's by ~70% <ref>[http://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars/ ''Putting Some Numbers on First Wort and Mash Hop Additions''. David Curtis NHC 2014 Presentation.]</ref>). It may or may not be advised to create a ''Lactobacillus'' starter (see the ''[[Lactobacillus]]'' page) before hand. Once the wort is racked to the primary fermenting vessel, the ''Lactobacillus'' culture is added directly to the fermenter. No other yeasts are added at this time. The ''Lactobacillus'' bacteria is allowed to incubate by itself in the wort for 2-5 days with the before mentioned target temperature maintained throughout the incubation period (some ''Lactobacillus'' species/strains may continue to produce acidity under lower temperatures). The exact time frame of incubation depends on the species/strain of ''Lactobacillus'', the manufacturer's recommendation, and the brewer's desired acidity level. Acidity can safely be measured with a reliable pH Meter throughout this time. If possible, it is advised that the brewer fill the head space of the fermenter with CO2. Some brewers have reported that this helps to reduce sulfur production, but if ''Brettanomyces '' is added to the beer later on in the process and allowed to age, this shouldn't be a concern.
After the desired acidity level is reached from the incubating ''Lactobacillus'' bacteria, the brewer can crash cool the fermenter down to the temperature that is desired for the primary fermenting yeast. Both ''[[Saccharomyces]] '' and ''[[Brettanomyces]]'', or a blend can be used as primary fermenting yeast. ''Brettanomyces '' is often chosen because of it's higher tolerance of a low pH environment (3.4- pH <ref name="low ph">[http://www.homebrewtalk.com/f127/no-hop-berliner-weisse-415067/index3.html#post5280971 Michael Tonsmeire on HBT]</ref>), although many ''Saccharomyces'' strains have been successfully used (see the ''Souring in the Boil Kettle'' section above). If the chosen yeast requires aeration and the brewer has the ability, the sour wort should be aerated before pitching yeast. Brewers have had good luck using Fermentis dry yeast products in non-aerated wort. Re-hydrating the dry yeast as per the manufacturer's instructions and with a yeast nutrient such as Go-Ferm is effective <ref>[http://www.scottlab.com/product-102.aspx Go-Ferm]</ref>. The wort is then fermented out as normal. The brewer can consider other [[Brewing Methods]] such as pitching ''Brettanomyces'', a mixed culture, or commercial sour beer dregs into secondary.
===Souring in Another Vessel Before Racking to the Primary Fermenter===