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:''Note: pre-acidifying applies to the wort that will become the batch of sour beer, '''not''' Lactobacillus starters. Pre-acidifying a Lactobacillus starter will slow the growth of the Lactobacillus in a starter. The idea of the starter is to grow an effective amount of cells while pre-acidifying the main batch of wort aims to inhibit contaminating microbes and retain foam positive proteins. See [[Lactobacillus#Samuel_Aeschlimann.27s_Starter_Procedures|''Lactobacillus'' starters]] for more information.''
After the production of the wort, but before pitching the culture of ''Lactobacillus'', some brewers like to slightly lower the pH of the wort with food grade lactic acid (available at homebrew stores) or phosphoric acid before adding the ''Lactobacillus''. Pre-acidifying wort can be applied to any wort souring method in general, and while using any species/source of ''Lactobacillus'' including lab cultures, probiotics, yogurt, grains, sauerkraut, etc. Acidifying the wort before pitching ''Lactobacillus'' has several benefits, such as discouraging unwanted microbes that may have accidentally been introduced into the wort, and helping to prevent [[Lactobacillus#Foam_Degradation|''Lactobacillus'' from degrading foam proteins]](although both of these points have been debated on MTF; see [https://www.facebook.com/groups/MilkTheFunk/permalink/1925483400813198/ this MTF thread] as an example). There have also been observations by Jeff Mello of [[Bootleg Biology]] and others that pre-acidification results in a .1 to .3 pH points lower final pH than if pre-acidification is not applied (this pH difference is not explained by the addition of acid because pH is logarithmic; see [https://www.facebook.com/groups/MilkTheFunk/permalink/1925483400813198/ this MTF thread]). The idea is to get the Pre-acidified wort down also appears to reach a low pH of 4.0 - 4.4 before adding in less time after the ''Lactobacillus''is pitched for souring.
There currently is no formula for how much lactic acid to add to a volume of wort due to the different buffering capacities of wort <ref>[http://www.mbaa.com/districts/Northwest/Events/Documents/Study%20in%20the%20Practical%20Use%20of%20Lactic%20Acid%20Bacteria.pdf A Study in the Practical Use of Lactic Acid Bacteria. Greg Doss from Wyeast Laboratories Inc. 2014.]</ref>. Water chemistry spreadsheets and formulas geared towards mash pH adjustments may not be accurate for wort pH adjustments since wort does not contain grain material, however there has been reports on MTF and recommendations from Martin Brungard (author of Bru'n Water) that [https://sites.google.com/site/brunwater/ Bru'n Water] can accurately determine how much lactic acid is needed to lower a wort's pH, or at least provide a starting point <ref name="delange_brungard"></ref>. We encourage readers to experiment with water chemistry calculators to see if they can accurately predict wort pH adjustments or get a starting point for how much acid to add <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1244954842199394/?comment_id=1245053585522853&comment_tracking=%7B%22tn%22%3A%22R%22%7D MTF Thread with Landon Ortiz. 03/03/2016.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1290987234262821/?comment_id=1353870694641141&reply_comment_id=1354548117906732&comment_tracking=%7B%22tn%22%3A%22R9%22%7D Conversation with Adam Boura and Mark Trent on MTF regarding using Bru'n Water for adjusting wort pH. 07/18/2016.]</ref>.