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→Aroma and Flavor Production
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 never occurred. The decline of acetoin during the maturation phase was attributed to ''Brettanomyces'', which is known to occur when oxygen is limited. 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 membranifaciens'' 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" />.
Below is a collection of data on spontaneously fermented beer; see the data sources for standard deviations and more information, methodology, etc.:
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Reported volatile compounds in mg/L:
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! Brand !! Isovaleric Acid !! Ethyl Octanoate !! 4-Ethylphenol !! 4-Ethylguaiacol !! Ethyl Caprylate Octanol !! octanol !! ethyl undecanoate Ethyl Undecanoate !! Ethyl Acetate !! Data Source
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| Hanssens Artisan Gueuze (vintage not reported) || Not measured || 2.74 || 0.57 || 1.36 || 0.084 || Not measured || 66.9 || Witrick et al. (2017) <ref name="witrick_2017" />
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| 3 Fonteinen Gueuze (vintage not reported) || Not measured || 1.36 || 0.44 || 0.99 || 0.034 || Not measured || 22.1 || Witrick et al. (2017) <ref name="witrick_2017" />
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| Oude Gueuze Boon (vintage not reported) || 2.3 || 1.66 || 0.28 || 0.52 || 0.02 || Not measured || 17 || Witrick et al. (2017) <ref name="witrick_2017" />
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| Gueuze Boon (vintage not reported) || Not measured || 1.62 || 0.32 || 0.97 || 0.02 || Not measured || 11.8 || Witrick et al. (2017) <ref name="witrick_2017" />
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| Cantillon Gueuze (vintage not reported) || 2.15 || 5.17 || 1.08 || 2.44 || 0.052 || 28.8 || 28.4 || Witrick et al. (2017) <ref name="witrick_2017" />
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| Cantillon Bio (vintage not reported) || 2.94 || 4.52 || 0.96 || 2.1 || 0.025 || 8.6 || 46.9 || Witrick et al. (2017) <ref name="witrick_2017" />
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| Oude Gueuze Vielle (vintage not reported) || 1.92 || 2.68 || 0.57 || 1.06 || 0.031 || 16.3 || 22.3 || Witrick et al. (2017) <ref name="witrick_2017" />
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| Girardin Gueuze (vintage not reported) || 2.95 || 2.22 || 0.28 || 0.52 || 0.02 || Not measured || 21.4 || Witrick et al. (2017) <ref name="witrick_2017" />
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| Lindemans Cuvee Renée (vintage not reported) || 3.01 || 5.67 || 1.13 || 5.77 || 0.041 || 46 || Not detected || Witrick et al. (2017) <ref name="witrick_2017" />
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