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===Aroma and Flavor Production===
Spontaneously fermented beers are usually characterized by the compounds produced by the complex and highly variable fermentation profile explained above. This generally includes the production of [[Lactic Acid|lactic acid]] by lactic acid bacteria and [[Acetic Acid|acetic acid]] by acetic acid bacteria and ''Brettanomyces'', which lend a sour and fruity flavor to the beer. ''Brettanomyces'' is responsible for many of the volatile aroma compounds, including [[Tetrahydropyridine|tetrahydropyridine]] (also produced by lactic acid bacteria), phenols such as 4-ethylguaiacol (smokey, spicy, clove) and 4-ethylphenol (barnyard, horsey, spicy, smokey, medicinal, Band-Aid), and esters such as ethyl acetate (pineapple in low concentrations, nail polish in high concentrations), ethyl lactate (fruity, creamy, rum-like), ethyl caproate (fruity/aniseed), ethyl caprylate (fruity with creamy mushroom and cognac notes), and phenylethyl acetate (sweet honey and rose-like). ''Brettanomyces'' can also produce volatile fatty acids such as [[Isovaleric Acid|isovaleric acid]] <ref name="Roos_2018" /><ref name="Roos_2018_2" />. Some strains of ''Brettanomyces'' are also known to release the enzyme beta-glucosidase to break down glycosides, which can result in the release of flavor compounds. This enzyme is also responsible for allow ''Brettanomyces'' to consume cellobiose (wood from barrels). However, Daenen et al. (2007) found that none of the ''B. bruxellensis'' strains isolated from lambic could utilize cellobiose, but strains of ''B. anomalus'' and ''B. custersianus'' isolated from lambic could utilize cellobiose, indicating that not all ''Brettanomyces'' strains (especially ''B. bruxellensis'') can break down glycosides or use cellobiose in wood as a food source (see [[Brettanomyces#Glycosides_and_Beta-Glucosidase_Activity|beta-glucosidase activity]] for more information). See the [[Brettanomyces#Secondary_Metabolites|''Brettanomyces'' secondary metabolites]] page for a complete list of flavor compounds that ''Brettanomyces'' can produce.
Other intermediate flavor and aroma compounds are creating during the fermentation process. For example, De Roos et al. (2018) reported the production of acetoin from week 3 to month 6 in lambic beers, and then a gradual decline after that. This corresponded with the growth of acetic acid bacteria (AAB), and the production of acetoin was attributed to the AAB oxidizing lactic acid. 2,3-Butanediol and 2,3-butanedione (diacetyl) were not found during the entire age of the lambics studied, so the full conversion of acetoin to these compounds. Malic acid was depleted as the lactic acid bacteria started to grow from month 6-9, and lactic acid increased (both D-lactic acid and L-lactic acid in nearly equal amounts at around 2.0 g/l), indicating malolactic fermentation occurred during this time. After the acidification phase and during the maturation phase where ''Brettanomyces'' and ''Pichia'' were dominant, small amounts of malic acid were produced, indicating that these yeasts are capable of producing small amounts of malic acid (less than 20 mg/l of malic acid was in the lambic beer at 24 months). Gluconic acid and citric acid, which were presumably introduced from the brewing process, were present at fairly stable levels during the entire fermentation process, with gluconic acid seeing a slight spike during the growth of acetic acid bacteria (~50 mg/l and ~220 mg/l in the final beers) <ref name="Roos_2018_2" />.