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When attempting to use pH for sensory parameters (how sour something tastes), the measurement falls short. Humans perceive acidity from tasting acids rather than the dissociated H+ ions which determine pH. With strong acids, where the H+ ions are fully dissociated from the remaining acid molecules, the amount of acid is always equal to the amount of H+ ions. However, lactic acid, the primary acid in sour beer, is a weak acid. Weak acids do not completely ionize/dissociate, leaving some portion of the H+ still bound to the acid. The degree to which a weak acid dissociates depends on the identity of the acid and the composition of the solution it is in.
Any acid not dissociated (in other words, an acid still holding onto its H+) does not affect pH. On the other hand, humans will perceive an increased "sourness" based upon the amount of acid in a beer, dissociated or not. The best way to test for total acidity, regardless of dissociation, is using a test called ''Titratable Acidity''. Because different acids have different characteristic flavors and some acids might taste more smooth or aggressive than others (think lactic acid verses acetic acid), samples with the same titratable acidity may have either different perceived acidity or acidity that seems more or less pleasant than other samples. However overall titratable acidity does show a stronger correlation to perceived acidity than measures such as pH.
Kara Taylor at White Labs has done a reasonable amount of professional sensory panels showing the correlation between ''TA'' and perceived sour taste, as well as showing no a more poor correlation between pH and sour taste. Her full presentation, where most of this information was sourced, is available to any attendee of the 2015 Craft Brewer's Conference.
==Testing for Titratable Acidity in Sour Beer==