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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 4 2 days to begin yeast fermentation <ref name="Tenhovirta_masters">[https://mediatum.ub.tum.de/doc/1444162/1444162.pdf Flavor design by advanced fermentation techniques for lactic acid fermented malt-based beverages. Santeri Tenhovirta; master thesis in Food Science from the University of Helsinki. 2019.]</ref>
Also found 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" />.