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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 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>.
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" />.
Both primary and secondary metabolites play a large role in the flavor and aroma profile of wort fermented with ''Lactobacillus''. Secondary metabolites are compounds that are not directly related to the growth of an organism, but often assist with survival <ref>[http://www.ncbi.nlm.nih.gov/pubmed/11036689 The natural functions of secondary metabolites. Demain AL, Fang A. 2000.]</ref>. These secondary metabolites are produced by the pathways mentioned above, and different strains probably regulate the enzymes involved in various pathways differently and produce different secondary metabolites <ref>Private correspondence with Richard Preiss from Dan Pixley. 12/29/2015.</ref>. Thus, different species and strains can produce a wide variety of flavors and aromas (compare this to food grade lactic acid in which none of these secondary metabolites exist). These secondary metabolite are the result of carbohydrate fermentation and amino acid metabolism <ref name="peyer_review"></ref>. Different species can have varying ranges of flavor intensities, especially citrus flavor, which one study that soured wort and then fermented with US-05 two days later (not kettle soured) reported was higher for ''L. plantarum'', ''L. alimentarius'', ''L. brevis'', and ''L. buchneri'' versus ''L. rhamnosus''. Vinuous, malty, and sour flavors were also reported to vary in intensity based on species. ''L. buchneri'' was reported in the study to be the most vinuous, followed by (in descending order of intensity) ''L. brevis'', ''L. plantarum'', and ''L. alimentarius''. The sour flavor was reported highest for ''L. plantarum'' and ''L. alimentarius'', followed by (in descending order of intensity) ''L. brevis'', ''L. buchneri'', and finally ''L. rhamnosus''. ''L. rhamnosus'' had the most raspberry flavor, while the other species had a similar level of moderate raspberry flavor. All of the species tested had similar levels of apple flavor (moderate), acetic flavor (low), butyric (very low to absent, even though the fermenting vessels were never purged of oxygen), bitter aftertaste (low), and yeasty (low) <ref name="Tenhovirta_masters" />.
Another study showed that ''L. plantarum'' produced significantly more diacetyl, acetoin (yogurt-like flavor), and acetaldehyde than ''L. reuteri'' and ''L. brevis''. These three compounds were associated with dairy-related notes of "buttery", "lactic", and "yogurt" flavors identified during sensory testing <ref name="Peyer"></ref>. Some LAB can release these compounds through the catabolism of citric acid, which is found in wort. Ester production is generally insignificant, although significant ester formation has been found during malolactic fermentation in red wines, and ethyl acetate has been found to be produced in malt based beverages <ref name="peyer_review">[http://www.sciencedirect.com/science/article/pii/S0924224415300625 Lactic Acid Bacteria as Sensory Biomodulators for Fermented Cereal-Based Beverages. Lorenzo C. Peyer , Emanuele Zannini , Elke K. Arendt. 2016.]</ref><ref name="Tenhovirta_masters" />. Acetaldehyde produced from ''L. plantarum'' helps to produce pyranoanthocyanins that stabilize wine's red color <ref>[https://www.sciencedirect.com/science/article/pii/S0963996918302084 Acetaldehyde released by Lactobacillus plantarum enhances accumulation of pyranoanthocyanins in wine during malolactic fermentation. Shaoyang Wanga, Siyu Lic, Hongfei Zhaoa, Pan Gua, Yuqi Chena, Bolin Zhanga, Baoqing Zhu. 2018. https://doi.org/10.1016/j.foodres.2018.03.032]</ref>. Some strains may also produce fusel alcohols and other off-flavors. For example the referenced study found an accumulation of the fusel alcohol n-Porponal in the sample of ''L. reuteri'', and a small decrease of isovaleric acid coupled with a small increase of [https://en.wikipedia.org/wiki/Hexanoic_acid hexanoic acid] by ''L. brevis'', ''L. plantarum'', and ''L. reuteri'' (only 0.25-0.32 mg/L was found, and the flavor threshold of hexanoic acid is 5.4 mg/L <ref>[http://www.leffingwell.com/odorthre.htm Leffingwell & Associates website. Odor Thresholds. Retrieved 12/30/2015.]</ref>) <ref name="Peyer"></ref>. Heterofermentative species can also produce [[Tetrahydropyridine|tetrahydropyridines (THP)]], which is the cause of "mousy" off-flavors <ref name="Costello">[http://pubs.acs.org/doi/abs/10.1021/jf020341r Mousy Off-Flavor of Wine: Precursors and Biosynthesis of the Causative N-Heterocycles 2-Ethyltetrahydropyridine, 2-Acetyltetrahydropyridine, and 2-Acetyl-1-pyrroline by Lactobacillus hilgardii DSM 20176. Peter J. Costello and Paul A. Henschke. 2002.]</ref>. Aldehydes (2-methyl-1-propanal, 2-methyl-1-butanal, 3-methyl-1-butanal) and their associated non-fusel alcohols (2-methyl-1-propanol, 2-methyl-1-butanol, and 3-methyl-1-butanol) can be produced from amino acids such as leucine, isoleucine, and valine to form fruity flavors <ref name="peyer_review"></ref>. A few species, especially most strains of ''L. fermentum'', and some strains of ''L. delbrueckii subsp. bulgaricus'', can produce ropiness in the form of exopolysaccharides, similar to [[Pediococcus]] <ref name="peyer_review"></ref>.
[http://www.sciencedirect.com/science/article/pii/S0308814617302911#t0005 Dongmo et al. (2017)] found 56 volatile flavor compounds, including various esters, alcohols, ketones, aldehydes, acids, ethers compounds, sulfur compounds, heterocyclic compounds, phenols (including guaiacol and 4-vinylguaiacol), terpenes, lactones, and several unidentified compounds. Key compounds produced by ''Lactobacillus'' include acetaldehyde (thought to be a major flavor contributor to kettle soured beers <ref name="Peyer_2017" />), β-Damascenone, furaneol, phenylacetic acid, 2-phenylethanol, 4-vinylguaiacol, sotolon, methional, vanillin, acetic acid, nor-furaneol, guaiacol and ethyl 2-methylbutanoate. Acetaldehyde was the most impactful aroma compound found followed by propan-1-ol and γ-dodecalactone. Acetaldehyde was generally produced in much higher amounts (~23-64 µg/L) by the select strains of ''L. plantarum'', while ''L. amylolyticus'' and ''L. brevis'' produced only 1.5-3 µg/L. In fact, the levels of all of these compounds differed significantly based on the species and strain. The selected strains of ''L. brevis'' were associated as having worse aromas that were dominated by methional (cooked potatoes), acetic acid (vinegar), and nor-furaneol (caramel-like). The ''L. plantarum'' strains selected were identified as producing more positive aromas from compounds such as β-damascenone (apple/fruit juice), furaneol (strawberry), 2-phenylethanol (rose/caramel) and ethyl 2-methylbutanoate (citrus). Small but significant amounts of linalool and geraniol were also found, which are normally terpenes found in [[Hops|hops]]. Vanillan is formed from ferulic acid by some ''Lactobacillus'' species as well as ''Oenococcus oeni'' <ref name="Dongmo">[http://www.sciencedirect.com/science/article/pii/S0308814617302911 Key volatile aroma compounds of lactic acid fermented malt based beverages – impact of lactic acid bacteria strains. Sorelle Nsogning Dongmo, Bertram Sacher, Hubert Kollmannsberger, Thomas Becker. 2017. doi:http://dx.doi.org/10.1016/j.foodchem.2017.02.091.]</ref>. Some strains of ''Lactobacillus'' can break down chlorogenic acids, which are esters found in plants, into their phenolic derivatives. For example, some strains of ''L. fermentum'', ''L. helveticus'', and ''L. reuteri'' were found to hydrolize chlorogenic acids into caffeic acid, which can then be converted into other phenols such as 4-vinylcatechol and 4-ethylcatechol by ''[[Brettanomyces#Phenol_Production|Brettanomyces]]'' <ref>[https://www.sciencedirect.com/science/article/pii/S0963996918303363 Site-specific hydrolysis of chlorogenic acids by selected Lactobacillus species. Elsa Anaheim Aguirre Santos, Andreas Schieber, Fabia nWeber. 2018. DOI: https://doi.org/10.1016/j.foodres.2018.04.052.]</ref>.