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[[File:Mark Horsley Foam.jpg|thumb|300px|[https://www.facebook.com/groups/MilkTheFunk/permalink/1145033525524860/ Photo provided by Mark Horsley. Left is a 5.30 pH wort and the right is a 4.75pH wort. Both were kettle soured with ''L. delbrueckii''. Both reached 3.30 pH post ferment with finishing gravity of 1.7 Plato.]]]
:''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 to inhibit contaminants 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]]. There have also been observations by Jeff Mello of [[Bootleg Biology]] and others that pre-acidification results in a 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 wort down to a pH of 4.0 - 4.4 before adding ''Lactobacillus''.