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===Fermentation Under Low pH Conditions===
''Saccharomyces'' species and individual strains have a wide range of tolerance to low pH and lactic/acetic acid concentrations, which have been identified as stressors for yeast fermentation. For ideal fermentation conditions for ''S. cerevisiae'', lactic acid should not exceed 0.8%, acetic acid should not exceed 0.5%, and wort should not fall below 4.0 pH. Since pH is on a log-based scale, even small differences in pH (especially below 3.5) can make a large impact on whether or not a given yeast strain is able to ferment. This obviously presents a challenge to brewers when [[Sour WortingWort Souring]] or [[Packaging#Re-yeasting|naturally carbonating with yeast]] for sour beers <ref name="rogers2016">[http://www.sciencedirect.com/science/article/pii/S0740002016301605 Terminal acidic shock inhibits sour beer bottle conditioning by Saccharomyces cerevisiae. Cody M. Rogers, Devon Veatch, Adam Covey, Caleb Staton, Matthew L. Bochman. 2016.]</ref>.
Yeast that fails to bottle condition sour beer may not be due to death of the cells. Rogers et al. <ref name="rogers2016"></ref> published a study that found that yeast used to bottle condition a sour beer at [http://uplandbeer.com/ Upland Brewing Co.] was still ~80% viable after two weeks, but the surviving cells were small and unbudded, indicating that they ceased growing and entered the stationary phase. This effect has been referred to as "terminal acid shock" <ref name="rogers2016"></ref>.