Changes

''Lactobacillus'' can have an impact on ''S. cerevisiae'' metabolism and the flavor-contributing metabolites that it produces. A study by [https://onlinelibrary.wiley.com/doi/10.1002/jib.569 Dysvik et al. (2019)] compared beer pre-soured with ''L buchneri'' versus pre-soured with lactic acid versus lactic acid added post yeast fermentation and found that lactic acid by itself did not significantly change the amount of volatile flavor compounds produced by the yeast strain that was tested (Fermentis US-05). However, the beers soured with ''L. buchneri'' had significantly different volatile alcohols compared to the beers with just lactic acid added to them. Specifically, the beer that was pre-soured with ''L. buchneri'' ended up with less 2-methyl-1-butanol (alcohol, malty notes), 2-methyl-1-propanol (fruity/winey) and phenylethyl alcohol (rose/honey). Acetic acid was also much higher in the beers soured with ''L. buchneri'' versus the beers with just lactic acid added, but the acetic acid was still below flavor threshold. Formic acid was around twice as much in the wort soured with ''L. buchneri'' versus wort with lactic acid or no lactic acid/bacteria, but the formic acid disappeared completely in all of the beers tested at bottling time and after maturation in the bottle. Although this study used a neutral ale yeast (US-05) and alcohols/esters across all samples were below flavor threshold (although combinations of different alcohols/esters under threshold can have a synergistic flavor impact), this supports anecdotal reports from brewers that adding lactic acid to beer to make a sour beer does not produce the same beer than when the souring is done with ''Lactobacillus'', as well as the anecdotes from brewers that say that pre-souring wort with ''Lactobacillus'' results in less yeast character in the final beer. It is interesting to note that both the beers with ''L buchneri'' and the beers with just lactic acid added had similarly significantly lower levels of pyruvic acid compared to the beer fermented with just yeast, as well as less haze, indicating that lactic acid alone inhibits the amount of pyruvic acid produced by the yeast as well as haze (perhaps because a lower pH reduces protein-polyphenol haze formation, or maybe the lower pH increased yeast flocculation) <ref name="Dysvik_2019">[https://onlinelibrary.wiley.com/doi/10.1002/jib.569 Pre‐fermentation with lactic acid bacteria in sour beer production. Anna Dysvik, Kristian Hovde Liland, Kristine S. Myhrer, Bjørge Westereng, Elling‐Olav Rukke, Gert de Rouck, Trude Wicklund. 2019. DOI: https://doi.org/10.1002/jib.569.]</ref>.

The presence of ''Lactobacillus'' can stall or slow yeast fermentation. This is likely a combination of low pH. The presence of lactic acid might change the way yeast ferments by allowing them to consume multiple types of sugars regardless of whether or not glucose is present, although it has been demonstrated that this alone is not the cause for stuck fermentations (see [[Lactic Acid]] for more information). Peyer et al. (2017) observed that growth of US-05 was 82% at a pH of 3.51, and 53% at a pH of 3.17. Fermentation was delayed by 2-4 days (the lower the pH, the longer the start of fermentation was delayed). In a co-fermentation of ''Lactobacillus amylovorus'' and US-05, the initial growth of the ''L. amylovorus'' continued for 3 days while the US-05 was delayed. On day 7, the US-05 recovered and continued growth, and the growth of the ''Lactobacillus'' was slowed starting on day 5. This was due to the increase in ethanol from fermentation, lower pH, and the depletion of nutrients for the ''Lactobacillus''. It is also possible that the yeast benefited from the autolysis of the ''Lactobacillus'', which is speculated to have released nutrients that were made available to the yeast <ref name="Peyer_2017" />. Santeri Tenhovirta's master thesis agreed with this. Tenhovirta pitched several species of ''Lactobacillus'' for 48 hours, and then pitched Fermentis US-05. The control US-05 fermentation without any ''Lactobacillus'' started to ferment as expected after 20 hours, while the samples that were pre-acidified with ''Lactobacillus'' took around 2 days to begin yeast fermentation <ref name="Tenhovirta_masters">[https://helda.helsinki.fi/handle/10138/303018 The Effects of Lactic Acid Bacteria Species on Properties of Sour Beer. Santeri Tenhovirta; master thesis in Food Science from the University of Helsinki. 2019.]</ref>.