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→First Stage: Enterobacteria
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. This difference is attributed to different microbes being present in 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 mostly consume sucrose in the wort), 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), as well as a significant population of ''Komagataeibacter'', a genera normally found in kombucha, after 135 day <ref name="Curtin_2021">[https://www.tandfonline.com/doi/abs/10.1080/03610470.2020.1795607?journalCode=ujbc20 Avi Shayevitz, Keisha Harrison & Chris D. Curtin (2021) Barrel-Induced Variation in the Microbiome and Mycobiome of Aged Sour Ale and Imperial Porter Beer, Journal of the American Society of Brewing Chemists, 79:1, 33-40, DOI: 10.1080/03610470.2020.1795607.]</ref>.
Acetic acid bacteria are also present during the first stage of fermentation before alcoholic fermentation begins. These consist of a large diversity of species from ''Acetobacter'' and ''Gluconobacter''. Acetic acid bacteria are able to grow for the first few weeks because oxygen is available from filling the casks. Once alcoholic fermentation begins, oxygen becomes limited, and the acetic acid bacteria population greatly decreases. Acetic acid bacteria appear again after the alcoholic fermentation phase <ref name="Bongaerts_2021" />. For example, Curtin et al. (2018) reported that acetic acid bacteria came and went at various random points within a 0-4.5 month period of fermentation <ref name="curtain_asbc_2018" />(~26 minutes in).