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===Microbial Succession During Fermentation===
The number of different species found in lambic and spontaneously fermented beers is very large and diverse from brewery to brewery and batch to batch, however, scientific research in Belgium and the US has shown a regular general pattern to the microbial succession of spontaneous fermentation beer at the genus level, with only minor genera differences between Belgian lambic beers and American spontaneous ale being . This difference is attributed to different microbes being present in the American brewery that was studied different breweries <ref name="Van Oevelen et al., 1977">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1977.tb03825.x/abstract/ MICROBIOLOGICAL ASPECTS OF SPONTANEOUS WORT FERMENTATION IN THE PRODUCTION OF LAMBIC AND GUEUZE. Van Oevelen et al., 1977.]</ref><ref name="Bokulic et al., 2012" /><ref name="Spitaels et al., 2014">[http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0095384#pone-0095384-g004/ The Microbial Diversity of Traditional Spontaneously Fermented Lambic Beer. Spitaels et al., 2014. DOI: https://doi.org/10.1371/journal.pone.0095384.]</ref><ref name="Spitaels et al., 2015" /><ref name="Roos_2018_2" /><ref>[http://www2.parc.com/emdl/members/apte/slides_nchf.pdf Raj Apte Concepts of sour Beer, 2004]</ref>. The first stage, which lasts for approximately 1 month <ref name="Van Oevelen et al., 1977" /><ref name="Martens et al., 1992">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1992.tb01126.x/abstract/ Martens et al., 1992]</ref>, is dominated by [https://en.wikipedia.org/wiki/Enterobacteriaceae enterobacteria] and [http://laboratoryresearch.blogspot.com/2008/07/yeasts-and-yeastlike-fungi.html?m=1 oxidative yeasts] that produce large amounts of DMS which can be smelled during the early stages of fermentation (see [[Dimethyl Sulfide]] for more details). Although spontaneous ales have a common pattern of fermentation by groups of genera of microbes, the diversity in specific species is large across different lambic producers and American spontaneous ale producers (although data for American spontaneous ale producers is limited). In American spontaneous ale producers, ''Klebsiella'' spp., ''Enterobacter'' spp.,'' Pectobacterium carotovorum'', and ''Serratia ureilytica'' have been found. In Belgian lambic producers, ''Enterobacter'' spp., such as ''Enterobacter aerogenes'', ''Enterobacter cloacae'', ''Enterobacter hormaechei'' and ''Enterobacter kobei'', ''Klebsiella aerogenes'', ''Klebsiella oxytoca'', ''Klebsiella varicola'', ''Escherichia coli'', ''Hafnia alvei'', ''Hafnia paralvei'', and ''Citrobacter freundii'', have been found in lambic, with ''E. cloacae'' and ''K. aerogenes'' as the most frequently found ones. Although these enterobacteria contribute little in terms of gravity drop over the first month of fermentation, they do contribute aroma and flavor compounds and precursors during the initial stages of spontaneous fermentation, particularly acetoin, 2,3 butanediol, acetic acid, lactic acid, succinic acid, DMS, acetaldehyde, long-chain fatty acids (these play a role in both flavor impact and providing nutrients for yeast later in the fermentation process), and small amounts of glycerol, ethyl acetate, and higher alcohols which might form esters in the later stages of fermentation. Enterobacteria can also contribute to the production of [https://en.wikipedia.org/wiki/Biogenic_amine biogenic amines] in fermented foods and beverages, including spontaneously fermented beers. Enterobacteria usually disappear after 30-40 days of fermentation due to the increase in ethanol, decrease in pH, and a decrease in food availability <ref name="Martens et al., 1992" /><ref name="Roos_2018">[https://www.ncbi.nlm.nih.gov/pubmed/30246252?dopt=Abstract Microbial acidification, alcoholization, and aroma production during spontaneous lambic beer production. Jonas De Roos and Luc De Vuyst. 2018. DOI: 10.1002/jsfa.9291.]</ref>, although one study by Curtin et al. reported finding at least small populations of enterobacteria as late as up to 4.5 months <ref name="curtain_asbc_2018">[https://www.asbcnet.org/lab/webinars/webinars/Pages/funkyFermentationsWebinar.aspx Chris Curtin. ASBC webinar: "Funky Fermentations". 12/12/2018. Retrieved 01/03/2019.]</ref>(~25 minutes in). Acidifying the wort to a pH below 4.5 before cooling and exposing to ambient microbes in a coolship can partially eliminate the enterobacteria phase of spontaneous fermentation and thus avoid or limit biogenic amine production, which is a practice for some lambic breweries <ref name="Spitaels et al., 2015" /><ref name="Roos_2018_2" />. While enterobacteria and oxidative yeasts are not considered to be a part of the core microbes in spontaneous fermentation, it has been shown that ''Saccharomyces cerevisiae'' is metabolically stimulated when co-fermented with some of these species, allowing the ''S. cerevisiae'' to consume more glucose and nitrogen and to more quickly replicate <ref name="Roos_2018" />. Roos et al (2018) reported significant populations of the enterobacteria species ''Klebseilla variicola'', ''Klebsiella oxytoca'', and the yeast species ''Hanseniasspora uvarum'', ''Saccharomyces cerevisiae'' during the first week or two of lambic fermentation that was pre-acidified (see [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6252343/figure/F3/?report=objectonly Figure 3]). '''Wort or beer fermenting during this stage should not be consumed due to the fact that some of these are pathogenic bacteria and pose potential health risks.''' Oxidative yeasts are also present during the first stage of fermentation, including species of ''Rhodotorula'', ''Candida'', ''Cryptococcus'', ''Hanseniaspora'', and ''Pichia'', some of which might survive pre-acidification <ref name="Bokulic et al., 2012" />. Zach Taggart found that this initial stage also corresponded with a pH drop from 5.0 to 4.5 in 6 days and the aroma went from sweet-smelling wort to phenolic and a light burnt rubber character during this time in one batch of spontaneous fermentation <ref>Zach Taggart (using his wife's Facebook account). Milk The Funk Facebook group post on analysis of spontaneous fermentation at 42 North Brewing Co. 11/09/2018.]</ref>.
The second stage of spontaneous fermentation is dominated by ''Saccharomyces'' species (predominantly ''S. cerevisiae'', ''S. bayanus'', and ''S. pastorianus'', the latter often being present towards the end of this phase). Most of the attenuation is accomplished during this stage, which lasts approximately 3-4 months. One study also found populations of ''Kazachsania'' yeast species and ''Cellulosimicrobium'' yeast species early on in the second stage <ref name="Roos_2018_2" />. In addition to the bulk of the overall ethanol production, this phase also sees the production of higher alcohols and the synthesis of esters, especially isoamyl acetate, as well as glycerol, caprylic acid, and capric acid <ref name="Van Oevelen et al., 1977" /><ref name="Roos_2018" />. It has been reported by some brewers that this stage might begin as early as 3-14 days and corresponds with a drop in pH below that of regular beer, indicating that the first stage for some spontaneous fermentations might be shorter and faster than reported in the other literature <ref>[http://www.spontanmanc.co.uk/?p=66 Zach Taylor of Chorlton Brewing Co. "The Lab Work Begins". Spontanmanc blog. 08/01/2018. Retrieved 08/29/2018.]</ref>. MTF members (both homebrewers and professionals) have observed yeast fermentation activity typically at 3-7 days <ref>[https://www.facebook.com/events/666424196868756/ Various MTF members. Milk the Funk - Collaboration Brew #3: Spontaneous. 05/01/2017. Retrieved 08/29/2018.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1571139996247542/?comment_id=1571597289535146 Raf Soef, James Howat, Levi Funk. Milk The Funk Facebook thread on how long it takes for yeast to start fermenting in a spontaneous fermentation. 2017.]</ref>. However, these reports are anecdotal based on visual fermentation and microbe analysis was not done in many cases. Roos et al. (2018) reported that for wort that is pre-acidified to a pH of 4.5, and after an initial drop in pH to 3.8 by enterobacterial and acetic acid bacteria, the pH rose to 4.0 during the secondary fermentation phase, indicating that the yeast consumed some of the organic acids that were produced during the initial enterobacteria phase <ref name="Roos_2018_2" />.