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added short section on the stressing effects of LA
==Effects on Yeast==
===Stress on Yeast===
The presence of lactic acid has been identified as a yeast stressor. However, yeast can be acclimated to lactic acid and low pH conditions by growing them in media that has a slowly increasing amount of lactic acid. See [[Saccharomyces#Fermentation_Under_Low_pH_Conditions|''Saccharomyces'' Fermentation Under Low pH]]. ''[[Brettanomyces]]'' tends to be more acid tolerant than ''S. cerevisiae'', and the presence of lactic acid will change the way ''Brettanomyces'' ferments wort and the esters that it will produce. See [[Brettanomyces#Ester_Production|''Brettanomyces'' ester production]] for more information.
===Genetic Manipulation===
It has recently been discovered that both L-lactic acid and D-lactic acid produced by lactic acid bacteria can manipulate a genetic trait in yeast that dictates how yeast ferments different types of sugars, including some strains (but not all) of ''Saccharomyces cerevisiae'' and ''Brettanomyces bruxellensis'' <ref name="Garcia_2016">[https://elifesciences.org/content/5/e17978 A common bacterial metabolite elicits prion-based bypass of glucose repression. David M Garcia, David Dietrich, Jon Clardy, Daniel F Jarosz. 2016.]</ref>. Normally ''Saccharomyces'' and many other types of yeast will preferentially metabolize glucose when glucose is present and they ignore other sugars such as maltose and maltotriose until the glucose is completely consumed. This is called "glucose repression". Recently it has been identified that lactic acid produced by lactic acid bacteria in the presence of some yeast strains turns off this "glucose repression" in yeast, allowing them to simultaneously ferment all types of sugars. This has a side effect of limiting attenuation in wine, and has been one of the identified causes of stuck wine fermentations (it has been observed as far back as Louis Pasteur that stuck wine fermentations often contain lactic acid bacteria) <ref name="cross-kingdom">[http://weitzlab.seas.harvard.edu/files/weitzlab/files/2014_cell_jarosz.pdf Cross-Kingdom Chemical Communication Drives a Heritable, Mutually Beneficial Prion-Based Transformation of Metabolism. 2014. Daniel F. Jarosz, Jessica C.S. Brown, Gordon A. Walker, Manoshi S. Datta, W. Lloyd Ung, Alex K. Lancaster, Assaf Rotem, Amelia Chang, Gregory A. Newby,David A. Weitz, Linda F. Bisson, and Susan Lindquist. Cell. 2014 Aug 28;158(5):1083-93.]</ref>.