Changes

Producers of spontaneously fermented beer typically do not oxygenate their wort <ref name="Beer Temple interview with De Garde"></ref> (~27 minutes in) and traditional producers conduct fermentation for a long time period (1-3+ years) in wooden vessels. The long fermentation process allows the different microbes present to carry out their slow metabolism of the complex carbohydrates present in the beer, developing the flavors and acidity associated with spontaneous beers <refname="Van Oevelen et al., 1976">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1975.tb06953.x/abstract Van Oevelen et al., 1976. Synthesis of aroma components during the spontaneous fermentation of lambic and gueuze]</ref> <refname="Spaepen et al., 1978">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1978.tb03888.x/abstract Spaepen et al., 1978. Fatty acids and esters produced during the spontaneous fermentation of lambic and gueuze]</ref>. The wooden fermentation vessels are frequently oak wine barrels in the 220-400 L (58-105 gal) range but other woods such as chestnut are used and the vessels may also be large tuns or foeders holding upwards of 45 HL (about 1200 gal, or about 34 bbl). These barrels provide two primary benefits for the fermentation - they allow a small amount of oxygen permeability and they provide a environment which houses some of the microbes active in the fermentation (notably Brettanomyces, which can penetrate into the wood and in some cases can metabolize compounds present in the wood such as cellobiose, which is produced from toasting of the wood) <refname="Vinnie on the Session Jan 2010">[http://www.thebrewingnetwork.com/post1940/ Vinnie Cilurzo of Russian River on the Brewing Network's Sunday Session, 17-January-2010]</ref>(~3:22 in). While a controlled micro-oxidation can be beneficial to the beer, too much oxygen exposure can lead to excessive acetic acid and/or ethyl acetate production (either from ''Brettanomyces'' or ''Acetobacter'') (----ref).

The ''Saccharomyces'' dominated stage of fermentation is followed by prolonged and gradual acid and flavor development accompanied by the final points of attenuation. In some descriptions this is split into an "acidification phase" which is dominated by lactic acid bacteria (LAB), primarily ''Pediococcus'', and a "maturation phase" driven by ''Brettanomyces'' <ref name="Van Oevelen et al., 1977" />. Other sources describe these 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" />. Note that many scientific publications use the terminology ''Dekkera'' rather than ''Brettanomyces''. 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" />.

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) (---also Vinnie on the sunday session and maybe lambic summit---). This is the result of exopolysaccarides, 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 exopolysaccaride 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. --sources for this--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 wen when given appropriate time.