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→Aroma and Flavor Production
Other intermediate flavor and aroma compounds are creating during the fermentation process. For example, 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. 2,3-Butanediol and 2,3-butanedione 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" />.
===Biogenic Amines===
Biogenic amines in spontaneously fermented beers are produced mostly by enterobacteria, but lactic acid bacteria and yeasts can also produce them. Wort that is pre-acidified greatly reduces the production of biogenic amines, but small levels can still be found. These levels are below the levels found in cheese and fermented sausage and are well below the levels that are acceptable for health (less than 400 mg/kg). Roos et al. (2018) measured biogenic amines over the fermentation lifespan of Belgian lambic beers that were pre-acidified to a pH of 4.5 before being cooled in a coolship and spontaneously fermented. They found that the initial wort had low concentrations of some biogenic amines, such as agmatine (9 mg/l), putrescine (8 mg/l), and cadaverine (3 mg/l). In one cask, the agmatine remained stable while in the second cask the agmatine declined to zero during the maturation phase, and then slightly increased to less than 5 mg/l. Cadaverine was produced during the first three weeks of fermentation and remained steady throughout the fermentation process at about 30 mg/l. Histamine was produced during the acidification phase by ''Pediococcus damnosus'' between 3 and 9 months and ended up at around 15 mg/l. Tyramine had final concentrations of around 30-40 mg/l and was formed either during the acidification phase (6 months) or the late maturation phase (18-24 months), potentially by ''P. damnosus'' or some other LAB that was at too low of a population to detect, or maybe as a result of autolysis of dead yeast cells. 2-Phenylethylamine and tryptamine were never found in the lambic beers <ref name="Roos_2018_2" />.
===Notes on the Source of Microbes===