Brettanomyces species, particularly B. limbicus and B. bruxellensis also become active during this period, from approximately 8 months onward (figure to follow). These yeasts are capable of fermenting dextrins (found in relatively high levels in lambic wort), maltotriose and maltotetraose, producing small amounts of acetic acid and ethanol. However, it is the production of unique flavour and aroma compounds that characterises the activity of Brettanomyces. Brettanomyces exhibits a high esterase activity, primarily resulting in the synthesis of large amounts of ethyl acetate and ethyl lactate. This esterase also cleaves iso- amyl acetate esters, thus accounting for its low concentration in lambic beer.
Tetrahydropyridines are also produced from ethanol and the amino acid lysine, which impart a ‘mousy’ or ‘horsy’ aroma to the beer. Volatile phenolics, with medicinal, ‘barnyard’ or ‘animal’ type odours are also from from 4-ethyl phenol and 4-ethyl guaiacol compounds.
Brettanomyces activity also leads to a significant increase in caprylic (C8) and caprix (C10) acids (small amounts are formed during earlier Saccharomyces acitivity) along with their associated esters, ethyl caprylate and ethyl caprate. These compounds produce a characteristic ‘goaty’ aroma/flavour to the final product. Capric and caprylic acids are short chain fatty acids and are thought to be by-products of yeast metabolism, produced during lipid synthesis by a yeast cell and released into the medium via leakage through membranes damaged by ethanol, or as a consequence of an autolytic mechanism. Higher temperatures, aeration and agitation during fermentation reduce the amounts of fatty acids in beer. Reduced aeration/agitation produced higher concentrations. The combined threshold for C6-C12 acids is 10ppm, beyond which ‘goaty’ aromas arise. C8 acid levels in gueuze as 12.4 – 21.85 ppm and C10 acids as 2.3 – 3.9p ppm.
Brettanomyces is active until the end of the fermnetaion period. Acetic acid bacteria of the genus Acetomonas and a number of oxidative yeasts (Pichia, ) are also found during lambic fermentations, however their influence is often minimal.Tags:beer, fermenation, ferment lambic beer