Microbiology and Biochemistry: Difference between revisions

[[File:LambicInfoDragon.png|right|375px]]

[[An_Overview_of_Lambic|← An Overview of Lambic]]

The spontaneous fermentation of lambic is a complex process involving a succession of bacteria and yeasts that progresses along with the chemical changes that occur during fermentation.<ref name=GeuzeKriek>Jef Van den Steen, [[Books#Geuze & Kriek: The Secret of Lambic Beer|Geuze & Kriek: The Secret of Lambic Beer]], 2012</ref> The spontaneous fermentation process has shown considerable variability even among different barrels of beer from the same brewery [2], though all spontaneiously fermented beers appear to follow a general sequence of microbes, which can be broken into four distinct stages [1-3]<ref name=AWAs>Nicholas A. Bokulich, Charles W. Bamforth, David A. Mills. [http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0035507|Brewhouse-Resident Microbiota Are Responsible for Multi-Stage Fermentation of American Coolship Ale], PLoS One, 7(4), 2012</ref> <ref name=Oevelen77 >D. Van Oevelen, M. Spaepen, P. Timmermans and H. Verachtert, [http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1977.tb03825.x/abstract|MICROBIOLOGICAL ASPECTS OF SPONTANEOUS WORT FERMENTATION IN THE PRODUCTION OF LAMBIC AND GUEUZE], 1977</ref>:

The microbes present in lambic may come from a variety of sources in the brewhouse and the surrounding environment, including the surrounding air, the walls and ceilings of the brewhouse, and the wooden barrels the beer is kept in [2]. The organisms in lambic are present in different parts of the environment and it is likely that the organisms responsible for lambic come from a number of different places.

IMAGE FROM PAPER

Lambic wort arrives in the coolship at approximately 5% sugar per weight of water, along with an assortment of proteins and fatty acids and other compounds.<ref name=Erbe >T. Erbe and H. Brückner, [http://www.sciencedirect.com/science/article/pii/S0021967300002557|Chromatographic determination of amino acid enantiomers in beers and raw materials used for their manufacture], 2000</ref> Negligible ethanol is present prior to fermentation, nor is there much of the organic acids that will give lambic its characteristic tartness; the wort has a pH around 5, which is similar to the wort of other beers.<ref name=Oevelen77 >D. Van Oevelen, M. Spaepen, P. Timmermans and H. Verachtert, [http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1977.tb03825.x/abstract| MICROBIOLOGICAL ASPECTS OF SPONTANEOUS WORT FERMENTATION IN THE PRODUCTION OF LAMBIC AND GUEUZE], 1977</ref> Enteric bacteria, including Enterobacter hormaechei, E. kobei, Klebsiella pneumoniae, and Escherichia coli, are the first to gain a foothold in this environment, with significant numbers found after three to four days. The enteric bacteria primarily consume glucose, which reduces the gravity of the wort from ~1.050 to ~1.040 after the first three weeks.

Low pH (below ~4.5) and an ethanol concentration higher than ~2% by volume is a hostile environment to the enterobacteria, and Saccromyces species are able to dominate in the wort once these conditions occur around 30-60 days into fermentation.

After the pH falls below ~4.5 and the alcohol content rises over ~2%, Saccromyces species take over as the dominant organisms in the wort, though Saccromyces is present in large numbers well prior to the dissapearance of the enterobacteria. Saccromyces will remain dominant until at least 8 months into fermentation, and usually maintain a considerable presence until ____, when Brettanomyces becomes dominant. Despite Saccromyces' importance to the fermentation, its concentrations remain below 10^7 cells per mL of wort, which is considerably lower than the 10^8 cells/mL found in commercial beers [6].

-- [[Lactic acid]]

Interestingly, lactic acid bacteria have been implicated in racemizing amino acids in beer, causing Lambic (and other beers which make use of lactic acid bacteria such as Berliner Wiesse) to have a high percentage of right-handed amino acid stereoisomers relative to both their starting materials and other beers.<ref name=Erbe >T. Erbe and H. Brückner, [http://www.sciencedirect.com/science/article/pii/S0021967300002557|Chromatographic determination of amino acid enantiomers in beers and raw materials used for their manufacture], 2000</ref>

Brettanomyces inherits the role of most prominant yeast genera from Saccromyces around 8 months, and continues consuming sugar 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 saccromyces alone, as brettanomyces is able to metabolise sugars that saccromyces 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 and lactic acid, respectively. 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 Saccromyces and is a characteristic odor compound in most other beers.[ref]

Gueuze and other lambic bottled with either some residual sugar left unfermented at the time of bottling or added priming sugars will undergo significant fermentation in the bottle, though all unpasteurized lambic will continue to ferment to some degree there. For lambic bottled after about 8 months without additional sugar, the fermentation in the bottle progresses much as an extension of the Brettanomyces stage of fermentation and negligible carbon dioxide and ethanol production occurrs, leaving most of this lambic still. For lambic bottled younger and those with additional fermentable sugars added at bottling, considerable fermentation by saccromyces occurs in the bottle, causing marked increases in ethanol and the production of carbon dioxide, leading to a carbonated product [ref].

The reactivation of saccromyces fermentation at the addition of additional sugar indicates that the dominance of brettanomyces is brought about by brettanomyces ability to ferment sugars that saccromyces cannot. Once saccromyces has consumed most of the available ______ and ______ in the wort, it goes dormant, though does not die completely, and brettanomycs is free to assume the role of primary yeast in the wort. Upon the re-introduction of _____ and _____, the faster-growing saccromyces once again flourishes, until the again the fermentable sugar is consumed and brettanomyces and its other associated yeasts can once again resume the slow procress of the final fermentation.

Eventually even the slow-fermenting Brettanomyces runs out of fermentable sugars and fermentation draws to a slow close. The beer will continue to change and evolve over time, though minimal interaction with active yeast occurs. This stage is marked by oxidation and breakdown of the more complex parts of the yeast itself.<ref name=Dalgliesh >C. E. Dalgliesh, Flavour stability, [http://www.europeanbreweryconvention.org/EBCmain/organisation/publication.php|Proceedings of the European Brewery Convention Congress], 1977</ref> <ref name=Vanderhaegen >B. Vanderhaegen, H. Neven, H. Verachtert, and G. Derdelinckx, [http://www.sciencedirect.com/science/article/pii/S0308814605000865|The chemistry of beer aging – a critical review], 2006</ref>

The decomposition of the yeast leads to _____.

The microbes found in Lambic may come from a variety of sources, as nearly every surface and even the air found in the brewery are teeming with life. While the air above the coelship is often cited as the source of the microorganisms in Lambic, other sources are now known to play a significant role.

# The surfaces of the barrels, both interior and exterior, as contamination from the exterior may occur during transfer of the wort.

Discussion about geographical variation of microorganims goes here.

Variation in the flora in Lambic, and thus the chemistry, due to differences in the season at innoculation have also been found [ref].

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American Coolship Ale