Microbiology and Biochemistry: Difference between revisions
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==Continued aging== | ==Continued aging== | ||
The ongoing process of ageing of Lambic after its maturation is a very complex process and has both purely chemical as well as biochemical aspects. | The ongoing process of ageing of Lambic after its maturation is a very complex process and has both purely chemical as well as biochemical aspects. Much of the present literature is general to all beer. | ||
Eventually even the slow-fermenting Brettanomyces runs out of fermentable sugars and fermentation draws to a prolonged 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> | Eventually even the slow-fermenting Brettanomyces runs out of fermentable sugars and fermentation draws to a prolonged 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 progresses primarily through the action of their own enzymes in a process called autolysis. Autolysis releases a large number of other enzymes, which has the secondary effect of breaking down many other components of the beer.<ref name=Vanderhaegen1> B. Vanderhaegen, H. Neven, H. Verachtert, G. Derdelinckx [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCAQFjAA&url=http%3A%2F%2Fwww.researchgate.net%2Fprofile%2FGuy_Derdelinckx%2Fpublication%2F222839054_The_chemistry_of_beer_aging__a_critical_review%2Flinks%2F0c960523339c4b25a6000000.pdf&ei=Tq3IVKmfFcGyogSs_YLQCA&usg=AFQjCNFaBrvqDGjqEV2I9uQ73dYh_ParXg&sig2=Z8dY4iDHozbT1eb9JeAdrw&bvm=bv.84607526,d.cGU| The chemistry of beer aging – a critical review], 2006</ref> The release of esterase leads to the increased destruction of iso-amyl acetate and other esters, causing a loss of its light, fruity odor. Not all esters are broken down by this process, and a number of esters are formed during ageing. These include ethyl 3-methyl-butyrate and ethyl 2-methyl-butyrate which contribute a light floral or even tropical fruit odor.<ref name=Bohmann2> J. J. Bohmann [http://www.brewingscience.de/index.php?tpl=table_of_contents&year=1985&edition=0004&article=56947| Zum Alterungsverhalten des Bieres. 4 Teil, Kombinierte Alterungsversuche durch Begasung mit Kohlendioxid, Stickstoff, Luft und Sauerstoff], 1985</ref><ref name=Bohmann1> J. J. Bohmann [http://www.brewingscience.de/index.php?tpl=table_of_contents&year=1985&edition=0003&article=56946| Zum Alterungsverhalten des Bieres. 3. Teil, Der Einfluß der Strahlungsbelastung, dargestellt am Beispiel 2-Methyl-2-buten und Isopren], 1985</ref> The release of proteases by autolysis causes a breakdown of long protein chains, with its concordant thinning of mouthfeel and reduction in head. While autolysis is often seen as undesirable, it can contribute to a pleasant mouthfeel as well as some of the characteristic flavors found in very old Lambic.<ref name="Oxford pg 54">J. Robinson (ed) ''"The Oxford Companion to Wine"'' Third Edition pg 54 Oxford University Press 2006 ISBN 0-19-860990-6</ref> | |||
While oxidation can occur rapidly due to a break in the fidelity of the seal at the cork, oxidation can still occur without the passage of oxygen through the cork or significant oxygen gas in the headspace due to the transfer of oxygen from an oxygen-containing compound like ____ or ____ in the wort to others. The act of losing an oxygen, or more generally, of losing electrons, is called reduction. Reduction of ____ by ____ to (E)-2-nonenal (as well as other linear aldehydes) has been implicated as being of primary importance to the long-term oxidation of flavor compounds in beer,<ref name=Jamieson > A. M. Jamieson, E. C. Chen, and J. E. A. Van Gheluwe, A study of the cardboard flavour in beer by gas chromatography, [http://www.asbcnet.org/publications/journal/Pages/default.aspx | Proceedings of the American Society of Brewing Chemists], 1969</ref> which leads to a characteristic flavor of oxidized beer, commonly described as being of "wet cardboard". However, many other processes and compounds are important to the ageing of beer as well. | |||
The Mailard reaction, also responsible for the browning of toast and steak among many other things, | |||
=Sources of the microbes in Lambic= | =Sources of the microbes in Lambic= | ||