Changes

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 (or until 40-50% of glucose is consumed in the case of wort and in anaerobic conditions <ref>Stewart, G. G. (2006). Studies on the uptake and metabolism of wort sugars during brewing fermentations. Tech. Q. Master Brew. Assoc. Am. 43:265-269. </ref>). 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. It has been proposed by some researchers to explain stuck fermentations in wine (it has been observed as far back as Louis Pasteur that stuck wine fermentations often contain lactic acid bacteria), but this claim has been disputed by Ramakrishnan et al. (2016) who showed that yeast that have been effected this way can still ferment wine adequately and is not directly the cause of stuck wine fermentations, partly because in natural conditions (juice fermentations versus lab growth media) only 50% of the total yeast population becomes <nowiki>[</nowiki>GAR⁺<nowiki>]</nowiki>changes in this way, leaving plenty of yeast that still demonstrate normal glucose repression. They (and others) were also able to show that the presence of acetic acid might also play a role in triggering this state in yeast <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><ref name="Ramakrishnan_2016">[https://www.frontiersin.org/articles/10.3389/fevo.2016.00137/full Inter-Kingdom Modification of Metabolic Behavior: [GAR+] Prion Induction in Saccharomyces cerevisiae Mediated by Wine Ecosystem Bacteria. Vidhya Ramakrishnan, Gordon A. Walker, Qingwen Fan, Minami Ogawa, Yan Luo, Peter Luong, C. M. Lucy Joseph and Linda F. Bisson. 2016. DOI: https://doi.org/10.3389/fevo.2016.00137.]</ref>.