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The ''[[Saccharomyces]]'' dominated stage of fermentation is followed by prolonged and gradual acid and flavor development accompanied by the final points of attenuation, which lasts anywhere from 2 to 10 months <ref name="Roos_2018" />. This stage is dominated by lactic acid bacteria (LAB), primarily ''[[Pediococcus]]'' and sometimes ''[[Lactobacillus]]''. Several organic acids are produced during this stage with the majority of them being lactic acid and acetic acid, resulting in the pH of the beer dropping to below 3.5 <ref name="Van Oevelen et al., 1977" /><ref name="Bongaerts_2021" /><ref name="Roosa_2024"/>. Other sources describe the acidification and maturation phases as one extended maturation phase with acidification from ''Pediococcus'' and ''Brettanomyces'' growth occurring simultaneously <ref name="Spitaels et al., 2015" /><ref name="Bokulic et al., 2012" /><ref name="Spitaels et al., 2014" />. When the wort is pre-acidified, the acidification and maturation phases overlap <ref name="Roos_2018" />. Other yeasts such as ''Candida'', ''Cryptococcus'', and ''Torulopsis'' species have also been isolated from mature lambic, although their impact other than possibly being involved in the formation of a pellicle is unknown <ref>[https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.1977.tb03825.x MICROBIOLOGICAL ASPECTS OF SPONTANEOUS WORT FERMENTATION IN THE PRODUCTION OF LAMBIC AND GUEUZE. D. Van Oevelen M. Spaepen P. Timmermans H. Verachtert. 1977. DOI: https://doi.org/10.1002/j.2050-0416.1977.tb03825.x.]</ref>. As many of the flavor and aroma characteristics that we associate with spontaneously fermented beer are produced during this slow maturation/acidification phase, allowing sufficient aging time is important when producing spontaneously fermented beers <ref name="Van Oevelen et al., 1976" /><ref name="Spaepen et al., 1978" />. Specifically, the ratio of lactic acid to acetic acid greatly impacts the flavor profile of the beer. Lactic acid can range from 1.5 to 10 g/l, where as acetic acid is hopefully limited to 1.5 g/l due to he more harsh acidic flavor of acetic acid <ref name="Bongaerts_2021" />. Homebrewer Caleb Buck reported data on several batches of homebrewed spontaneously fermented beer and observed a slower drop in gravity for some batches than others over about a 7 month period (see [http://www.archaicpursuit.com/2018/08/2017-coolship-experiment-hopping-rate.html?m=1 this graph for details]). De Roos et al. (2018) reported a gradual increase in glucose, maltose, and maltotriose from week 7 to month 6 due to the degradation of maltooligosaccharides (higher chain sugars) <ref name="Roos_2018_2" />.
The acidification phase is also accompanied by the growth of acetic acid bacteria (AAB), which can be undesirable if this growth is excessive since it leads to greater [[Acetic Acid|acetic acid]] production (in high quantities, acetic acid smells and tastes like vinegar and is very harsh on the palate and throat) as well as acetoin. These microbes include species from the genera of ''Acetobacter'' and ''Gluconobacter''. The species diversity of these genre is lower than during the primary stage due to acid and ethanol selecting for species that are more tolerant to these harsher conditions. For example, De Roos et al. (2018) reported high numbers of ''Acetobacter pasteurianus'', which contains extra genes that code for acid and ethanol tolerance more so than other species of ''Acetobacter'', in lambic from month 3 to month 6, with it disappearing around month 9-13 as ''Pediococcus damnosus'' took its place. ''Acetobacter lambici'' is another species found in lambic during this stage and is well adapted to the lambic environment due to its ability to break down maltooligosaccharides (dextrins) via maltooligosyl trehalose synthase. These microbes are dependent on oxygen in order to metabolize ethanol into acetic acid (with acetaldehyde produced as an intermediary step) and acetoin from lactic acid and are found on the surface of the wort where oxygen is available. The beer/air interface (or surface of the beer that interfaces with the air above it) is also where higher concentrations of acetic acid, ethyl acetate, and acetoin are found due to the AAB being present there rather than deeper within the beer (this is similar to [[Flanders Red Ale]]) <ref name="Roos_2018_2" /><ref name="Roos_2018" /><ref name="Bongaerts_2021" />. With the flavor threshold of acetic acid in beer being 90 ppm <ref>[https://www.aroxa.com/beer/beer-flavour-standard/acetic-acid Aroxa website. "Acetic Acid". Retrieved 11/19/2018.]</ref>, and the levels of acetic acid in Belgian gueuze/lambic being reported in the range of 727-2240 ppm, acetic acid levels in this range is an important flavor compound in spontaneously fermented beers <ref>[http://beachwoodbbq.com/pdf/BBAIBLTBLENDERY.pdf Ryan Fields. "Brewing Beer in America Inspired By the Belgian Lambic Tradition". 2018.]</ref><ref name="Spitaels et al., 2015" />. During a second phase of growth of acetic acid bacteria starting at week 7 in lambic casks, significantly more acetoin (moldy/must flavor when above 50 ppm <ref>[https://www.morebeer.com/articles/Fatty_Flavors_Diacetyl "Fatty Flavors and Diacetyl - Should Your Beer Be Fat-Free?". MoreBeer website. Scott Bickham. Retrieved 04/04/2023.]</ref>) was found in the top portion of the lambic casks above flavor threshold. Acetic acid bacteria has been shown to reduce lactic acid into acetoin, and in another study by De Roose et. al (2023) the researchers found that ''Acetobacter lambici'' present in lambic samples that were tested had the genetic capability to utilize lactic acid as a food source <ref name="Roosa_2024"/>. Acetoin was gradually reduced (presumably metabolized by ''Brettanomyces'') below flavor thresholds at month 9 until it reached near 0 ppm around month 18 <ref name="De_roos_AAB_2018" />.
====Fourth Stage: Maturation====