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====Fourth Stage: Maturation====
The fourth and last phase of spontaneous fermentation, also known as the extended maturation phase, is dominated by ''[[Brettanomyces]]'' yeast, which is a genus of yeasts that are highly tolerant of low pH, high alcohol, and can survive in low-nutrient conditions, as well as lactic acid bacteria from the genera ''[[Pediococcus]]'' and to a lesser extent ''[[Lactobacillus]]'' and ''Pichia'' yeast species. ''Pichia membranifaciens'', ''Debaryomyces hansenii'', and ''Wickerhamomyces anomalus'' are examples of other yeast species that have been found in lambic during the maturation phase. This phase generally begins somewhere around month four to eight, with these microbes completely dominating at around 9-13 months <ref name="Roos_2018_2" /><ref name="curtain_asbc_2018" />(~26 minutes in). Additional attenuation occurs very slowly for another 7-18 months. De Roos et al. (2018) reported a gradual drop from 4 Plato to 0.5 °Plato during the maturation phase. <ref name="Roos_2018_2" />. During this extended maturation phase, ''Brettanomyces'' continues to ferment the residual sugars leftover in the beer using intra- and extracellular alpha-glucosidase, and produces most of the final aromatic and flavor compounds in the form of esters, phenols, and fatty acids found in finished Belgian lambic and other spontaneously fermented beers (see [[Brettanomyces#Brettanomyces_Metabolism|''Brettanomyces'' metabolism]]). The most abundant species of ''Brettanomyces'' found in spontaneously fermented beer are strains of ''B. bruxellensis'' (''B. lambicus'' is often found, but has been reclassified as a strain of ''B. bruxellensis''). ''B. anomalus'' and ''B. custersianus'' have also been found, but to a lesser extent than ''B. bruxellensis''. During the maturation phase, a [[pellicle]] is formed from the ''Brettanomyces'', as well as oxidative yeasts from the genera ''Pichia'', ''Candida'', ''Cryptococcus'', and ''Torulspsis'' <ref name="Van Oevelen et al., 1977" /><ref name="Roos_2018" /><ref name="Roos_2018_2" /><ref name="Bongaerts_2021" />. It is thought that the pellicle and the presence of these oxidative yeasts might reduce oxygen influx, and thus assist in inhibiting the growth of acetic acid bacteria <ref>[https://pdfs.semanticscholar.org/8c12/9985b9f1264179fe2e2f779bae1ff3e51a54.pdf Jacques De Keersmaecker. "The Mystery of Lambic Beer". Scientific American, Inc. 1996.]</ref>, however, this has not been proven in a scientific manner that we know of.
During the extended maturation phase, a beer may become "sick" or "ropey", though not all producers get this <ref name="Spontaneous Sour Hour" /> (~1:10 min in) <ref name="Vinnie sour beer talk">[http://www.thebrewingnetwork.com/post1863/ Recording of Vinnie's talk at NHC]</ref> (~1:44 in) <ref name="Vinnie on the Session Jan 2010"/> (~3:44 in). This is the result of exopolysaccharides, which some ''Pediococcus'' strains are known to produce. These exopolysaccharides can be broken down by other microbes present in the beer relieving the beer of its "sickness" (this exopolysaccharide breakdown is generally attributed to ''Brettanomyces''). Beer may also become "sick" in the bottle during bottle conditioning. This is likely due to enhanced ''Pediococcus'' activity from additional fermentable sugar, in the form of simple sugars or beer which has not completely attenuated yet <ref name="Vinnie sour beer talk">[http://www.thebrewingnetwork.com/post1863/ Recording of Vinnie's talk at NHC]</ref> (~1:47 in). A beer that is sick in the bottle will generally clear through the same process as a younger aging beer when given appropriate time. See the [[Pediococcus#.22Ropy.22_or_.22Sick.22_Beer|Pediococcus]] page for more information.