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'''Wort Souring''' (formally "sour worting") is a process in which ''[[Lactobacillus]]'' is given a "head start", pitched before the yeast so that it will be able to produce significant amounts of lactic acid before the ''Saccharomyces'' completes the main fermentation. There are several variations on souring wort, including souring in the primary fermenter, souring in a secondary vessel, or even souring in the boil kettle itself. There are also various methods of inoculating the wort with ''Lactobacillus''. Finally, the brewer has the option of pasteurizing the wort by heating it to kill the ''Lactobacillus'' before adding the yeast for the main fermentation. Some creative brewers have applied wort souring techniques to longer aged [[Mixed Fermentation]] beers and barrel aged beers. Many brewers prefer this process over [[Sour Mashing]] because it can be easier to control, and when implemented properly it produces a clean sour beer in a short amount of time. The possibility of pasteurizing the soured wort also makes this a good method for making sour beers with a lot of residual malt sweetness (e.g. sour barley wines), and should also make it attractive to brewers who are concerned about infection issues in their cold side equipment (equipment that is used post-boil) <ref>[http://sourbeerblog.com/fast-souring-lactobacillus/ Miller, Matt. Dec 20, 2014. "Fast Souring with ''Lactobacillus'' – Best Practices, Sensory, & Science". Sour Beer Blog.]</ref>. When sour wortingsouring wort, some brewers first [[Sour_WortingWort_Souring#How_to_Pre-Acidify|lower the pH of the wort to 4.0-4.3]] before pitching ''Lactobacillus''. This has sometimes been found to help the head retention of the beer, and helps to protect the wort from contaminating microorganisms. For more information, see the [[Lactobacillus#Foam_Degradation|''Lactobacillus'' page section on Foam Degradation]].
Generally, ''[[Pediococcus]]'' is not used with this method (''Pediococcus'' is generally used in long aged [[Mixed Fermentation]] sours with ''[[Brettanomyces]]''), however [[Pediococcus#Commercial_Pediococcus_Cultures|Bootleg Biology]] has released a blend of ''Pediococcus'' strains that are reportedly good for sour wortingsouring wort.
Important note regarding aluminum pots: souring in an aluminum vessel may strip the aluminum of the protective oxide layer. The oxide layer is only stable at a pH of 4.5 - 8.5. Therefore, kettle souring in an aluminum pot is generally not recommended <ref>[http://www.pfonline.com/articles/aluminum-surface-finishing-corrosion-causes-and-troubleshooting Aluminum Surface Finishing Corrosion Causes and Troubleshooting. W. John Fullen, Boeing Research and Technology & Jennifer Deheck, Boeing, Seattle, Washington, USA. 10/17/2014.]</ref>.
The brewing process is the same for any all grain batch up until the first wort and sparge runnings are collected into the boil kettle. The temperatures that a typical mash out/sparge reach should be enough to pasteurize the wort <ref name="pasteurization">[http://science.howstuffworks.com/life/cellular-microscopic/pasteurization4.htm Heat pasteurization]</ref>, however we advise heating the wort for a short (1-2 minutes) boil in order to kill a greater degree (2-3 logs more) of thermotolerant microbes <ref name="Heit_boiling">[https://www.facebook.com/groups/MilkTheFunk/permalink/1180630378631841/?comment_id=1180634488631430&reply_comment_id=1180677581960454&comment_tracking=%7B%22tn%22%3A%22R2%22%7D Conversation with Bryan of Sui Generis Blog regarding boiling versus lower temperature pasteurization. 11/18/2015.]</ref><ref>[http://sourbeerblog.com/lactobacillus-2-0-advanced-techniques-for-fast-souring-beer/ ''Lactobacillus'' 2.0 – Advanced Techniques for Fast Souring Beer. 11/18/2015. Retrieved 11/19/2015.]</ref>. Once all of the wort is collected in the boil kettle (and preferably brought to a boil), the wort is chilled to around 80-115°F (37-46°C), depending on the ''[[Lactobacillus]]'' culture that is being used. Once chilled to the temperature that is appropriate, the wort in the kettle is inoculated with a culture of ''Lactobacillus''. Hops should not be added at any point before inoculating the wort with a culture of ''Lactobacillus''; most species of ''Lactobacillus'' will be inhibited by the presence of even very small amounts hops (1-2 IBU or even just hop material from dry hopping). When using a pure culture of ''Lactobacillus'', it is generally a good idea to [[Lactobacillus#Starters_and_Pitching_Rate|create a 500 mL starter]] for ~5-6 gallons of wort.
There are various ways of inoculating the wort. A reliable method is pitching a pure culture of ''Lactobacillus'', or a blend of ''Lactobacillus'' cultures. Alternatively, a handful of unmilled malted barley can be added to the kettle for inoculation instead of a pure culture, since the husks of grain carry many microorganisms. If unmilled grain is added, it is thought that filling the head space of the kettle with CO2 will help decrease off-flavors such as "footiness" from [[Isovaleric Acid]], which are produced by aerobic microbes that are naturally present on the grain <ref name="khris_johnson">Personal correspondence with Khristopher Johnson of Green Bench Brewing Co. and Dan Pixley. 05/24/2016.</ref>. Keeping the temperature between 109-115°F (42.8-46°C) will encourage the ''Lactobacillus'' resident on the grain and will discourage other bacteria. Temperature consistency is critical during this process <ref name="young_grains">[https://www.facebook.com/groups/MilkTheFunk/permalink/1356058381089039/?comment_id=1356464531048424&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Jeff Young from Blue Owl Brewing Co on souring from grains. 07/21/2016.]</ref>. Lowering the pH of the [[Sour_WortingWort_Souring#How_to_Pre-Acidify|wort to under 4.5 (ideally 4.0 - 4.3)]] will also discourage many other bacteria from thriving in the wort during the incubation period. This will also help with head retention <ref>[[Lactobacillus#Foam_Degradation]]</ref>. Souring with grains should occur within 1 or 2 days if done correctly <ref name="young_grains"></ref>. Consider [[Alternative Bacteria Sources]] for more reliable approaches to using "wild" ''Lactobacillus'', or ''Lactobacillus'' from sources other than yeast labs.
If a pure culture of ''Lactobacillus'' bacteria is used it is ideal but not necessary to fill the head space of the fermenter with CO2 gas (some brewers have reported that this will help reduce sulfur in the finished beer). Keeping positive pressure in the kettle with CO2 will help prevent contaminates that create [[Butyric Acid|butyric acid]] and other off-flavors from entering the kettle due to negative pressure, and is often the approach that commercial brewers take <ref>Personal correspondence with Steph Cope of CraftHaus Brewing Co. 02/06/2016.</ref>. The kettle should be held at the desired temperature for 24-72 hours (in some cases longer, but no longer than 5 days). Depending on the strain of ''Lactobacillus'', and the desired sour level, the time of incubation is ultimately a variable that is up to the brewer (see the ''[[Lactobacillus]]'' page for suggested temperatures and times for specific strains). The kettle lid should be firmly in place and optionally sealed with plastic wrap so that other microorganisms do not get in. Potential for formation of [[Butyric Acid]] and [[Isovaleric Acid]] when using only a pure culture is extremely slight to none assuming contamination does not occur.
Wort can be soured in the primary fermenter before adding other yeasts. This is generally a good approach for brewers who aren't concerned with pasteurization and infections of their cold side equipment. This has the advantage of possibly producing a more complex sour beer overall, or at least a sour beer that will evolve over time. It has the advantage over a more traditional [[Mixed Fermentation]] in that ''Lactobacillus'' is used to guarantee at least a certain level of sourness. This is also a good process to use for making a [[Berliner Weissbier]].
The process is very similar to the kettle souring 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 less concern of developing [[Butyric Acid]] or [[Isovaleric Acid]] with this method. Even still, lowering the pH of the [[Sour_WortingWort_Souring#How_to_Pre-Acidify|wort to under 4.5 (ideally 4.0 - 4.3)]] will also discourage contaminating bacteria from thriving in the wort during the incubation period. This will also help with head retention <ref>[[Lactobacillus#Foam_Degradation]]</ref>. 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 90-115°F or 32.2-46°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 is generally a good idea to create a 500 mL starter before hand for ~5-6 gallons of wort (see [[Lactobacillus#Starters_and_Pitching_Rate|''Lactobacillus'' starters]]). 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 [[Sour_WortingWort_Souring#Souring_in_the_Boil_Kettle|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===
This process is very similar to sour worting souring wort in the kettle. This method is ideal for those who wish to use grains to introduce ''Lactobacillus'' to the beer. If done properly, the formation of [[Butyric Acid]] and [[Isovaleric Acid]] should be minimal.
The wort is mashed and sparged as normal (and alternatively brought to a short boil), and is then lowered to somewhere between 109-115°F (42.8-46°C). This temperature favors ''Lactobacillus'', while discouraging Enterobacteriaceae. Optionally, the mash pH can be lowered to 4.4 with lactic acid or acidulated malt to further discourage Enterobacteriaceae activity. Once the desired temperature (and optionally pH) is reached, a handful of fresh malted unmilled grain is added to the mash and allowed a few minutes to inoculate the mash with the microbes found naturally on the grain husks. The wort is then transferred to a second vessel such as a glass carboy. The vessel should be filled to the very top, minimizing the oxygen levels inside the vessel. The vessel should be stored in a heated environment that maintains a temperature between 109-115°F (42.8-46°C) for 1 to 3 days depending on how much acidity the brewer wants (the faster the souring process the better; Jeff Young from Blue Owl achieves the desired acidity in about 18 hours usually) <ref name="young_grains"></ref><ref name="james_spencer">[http://beerandwinejournal.com/sour-wort-berliner/ Spencer, James. December 15, 2014. Beer and Wine Journal.] </ref>.
===Tips on Maintaining Heat for Homebrewers===
Keeping the temperature as steady as possible for a pure culture ''Lactobacillus'' fermentation is not that important. Just try to stay in the range of the temperature best suited for a given species (see the [[Lactobacillus#Culture_Charts|''Lactobacillus'' culture charts]]). Here are some tips from MTF members on maintaining warm temperatures for sour worting wort souring <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1279628395398705/ Tips from many MTF members on maintaining heat for sour worting. 04/07/2016.]</ref>:
:''Editor's note: please be cautious when using heat sources to heat plastic fermenters; PET bottles have been known to melt or warp when applying too much heat to them.''