Changes

The spontaneous fermentation of lambic is a complex the process in by which non-cultivated yeasts and bacteria present in the environment around the wort take up residence in, the beer and which are responsible for the its fermentation and many of, the beerits final characteristics. In contrast to the controlled fermentation found in most other modern beers, spontaneous fermentation involves many different organisms and proceeds through at least four distinct, but often overlapping, stages, each characterized by the dominance of a different set of microbes which are responsible for different aspects of the finished lambic. The rapid rate of evolution of these microorganisms makes them unique to their geographic, chemical, and even seasonal environment.

Brettanomyces inherits the role of most prominant prominent yeast genera from Saccharomyces around 8 months, and continues consuming sugar sugars in the wort. Final attenuation can reach over 80% in lambic through the continued action of Brettanomyces, which is often referred to as "overattenuation" or "superattenuation". This is greater than is usually possible with Saccharomyces alone, as Brettanomyces is able to metabolise sugars that Saccharomyces cannot, generally known as "dextrins".

Brettanomyces has been implicated in producing most of the aroma compounds in Lambic.<ref name="Guinard">Jean-Xavier Guinard, [[Books#Classic Beer Styles: Lambic|Classic Beer Styles: Lambic]], 1990</ref> Sensory-significant quantities of ethyl acetate and ethyl lactate form at this time from ethanol entering into an ester bond with [[Acetic acid|acetic]] and [[lactic acid]], respectively. In addition, ethylphenols formed from hydroxycinammic acid contribute an odor often described as "horse sweat", "barnyard", or "leather" <ref name=Crauwels1> S. Crawels et. al. [http://link.springer.com/article/10.1007%2Fs00253-015-6769-9| Comparative phenomics and targeted use of genomics reveals variation in carbon and nitrogen assimilation among different Brettanomyces bruxellensis strains], 2015</ref>. The esterization process is greatly helped by the enzyme esterase provided by Brettanomyces. However, the enzymatic esterization is highly reversible and esters found in high concentrations in the lambic prior to the presence of the esterase will achieve a lower equilibrium. This is the case with iso-amyl acetate, which is produced by Saccharomyces and is a characteristic odor compound in many other beers. Tetrahydropyridines (THPs) produced by Brettanomyces (as well as some Lactobacilli) have a wide variety of odors and give lambic much of its "mousey" aroma, as well as cider- and horse-like aromas, though the concentrations and thus smells of THPs are variable.<ref name=Heresztyn1> T. Heresztyn [http://ajevonline.org/content/37/2/127.short| Formation of Substituted Tetrahydropyridines by Species of Brettanomyces and Lactobacillus Isolated from Mousy Wines], 1986</ref> Other important odor and flavor compounds produced by Brettanomyces include 4-ethylphenol, 4-ethylguaiacol, and isovaleric acid. 4-ethylphenol produces barnyard and horsey flavors which can taste like Band-aids in higher concentrations. 4-ethylguaiacol lends spicier flavors of bacon and cloves and can be smoky, while isovaleric acid gives lambic its sweaty and cheesy flavors and odors.