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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). These microbes include species from the genera of ''Acetobacter'' and ''Gluconobacter''. These microbes are dependent on oxygen in order to metabolize ethanol into acetic acid (with acetaldehyde produced as an intermediate step) and acetoin from lactic acid and are found on the surface of the wort where oxygen is available. The wort/air interface is also where higher concentrations of acetic acid are found due to the AAB being present there rather than deeper within the beer (this is similar to [[Flanders Red Ale]]). It has been shown that the species of AAB found in lambic and American spontaneous ales have adapted to high concentrations of ethanol and acetic acid <ref name="Roos_2018" />. 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" />.
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. This phase generally occurs somewhere around month eight. During this extended maturation phase, ''Brettanomyces'' continues to ferment the residual sugars left over in the beerusing 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''). 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" />. 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''). A 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 which 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.