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Souring wort with ''Lactobacillus'' can stall or slow a sequential yeast fermentation. This is likely due to 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>.
Ciosek et al. (2019) observed the opposite effect. A faster fermentation was achieved when pitching ''Lactobacillus brevis'' WLP672 (White Labs Inc, USA) for 1, 2, or 3 days before pitching Fermentis Safale US-05. However, the yeast reached a slightly higher final gravity after 7 days and the yeast growth was lower than when the yeast was pitched first or at the same time as the WLP672 ''L. brevis''. Interestingly, the lowest final gravity was achieved when the yeast and ''L. brevis'' were pitched at the same time. This indicates that some species of ''Lactobacillus'' can have a synergistic effect on yeast, while other species might be more antagonistic towards yeast, and that multiple stress factors such as a combination of both low pH and the presence of ethanol can be factors that prevent yeast from attenuating as well as it would have done if the pH wasn't lowered by the presence of lactic acid. Another surprising observation was made by Ciosek et al. (2019) for the samples that were fermented with US-05 first, and then after 1, 2, or 3 days the WLP672 ''L. brevis'' was added and allowed to ferment for 7 days total: these samples did not have a significant drop in pH from the ''L. brevis'', and remained at a pH of 4.0 or higher. This result indicates that this particular strain does not produce much lactic acid in the presence of ethanol or because there aren't enough simple sugars left after the yeast fermentation (at least in the short amount of time that they were tested). When the ''L. brevis'' was pitched first, it took 72 hours for the pH to get lower than 4, but this approach ended up with the lowest final pH after yeast fermentation was finished (~3.4 pH), and the co-pitch ended up at ~3.7 pH. Only The researchers declared that only when the ''L. brevis'' was allowed to ferment by itself for 2-3 days before the yeast was pitched did the finished beers have enough lactic acid content to be considered "sour beer" (~6 g/l of lactic acid for the samples fermented with ''L. brevis'' first versus 1.8-2.8 g/l for all of the other samples), which is based on a definition from "American Sour Beers" by Michael Tonsmeire that states that sour beers are defined as having 3-6 g/l of lactic acid. However, no sensory data was presented in this study, nor were titratable acidity measurements taken, and Methner's 1987 thesis on Berliner Weisse reported 1-3 g/l of lactic acid in a survey of these German sour beers, so it is conceivable that all of the sample beers in this study had an adequate sour flavor <ref name="Ciosek_2019">[https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.590 Sour beer production: impact of pitching sequence of yeast and lactic acid bacteria. Aneta Ciosek, Iga Rusiecka, Aleksander Poreda. 2019. DOI: https://doi.org/10.1002/jib.590.]</ref><ref>[http://herr-rausch.de/MethnerBerliner.pdf Methner's thesis on Berliner Weisse, 1987 (German). Retrieve 11/15/2019.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/3052332944794899/?comment_id=3053207211374139 Benedicth Koch. Milk The Funk Facebook group thread on the lactic acid content of Berliner Weisse. 11/15/2019.]</ref>.
Also found in the Peyer study was an increase in [https://en.wikipedia.org/wiki/Diacetyl diacetyl] and [https://en.wikipedia.org/wiki/Acetoin acetoin] in the beers that were co-fermented with ''L. amylovorus'' and US-05 versus the beers that were kettle soured or mash soured. Both of these compounds are responsible for the buttery taste in beer. Normally, after primary fermentation the yeast reduces diacetyl to acetoin, which is then converted to butanediol, however during a co-fermentation with ''Lactobacillus'', this conversion was inhibited in this study <ref name="Peyer_2017" />.