Brettanomyces claussenii (anamorph of Dekkera claussenii) is a wild yeast of the genus Brettanomyces which has a negative pasteur effect. It and Brettanomyces Anomalus share identical mtDNA. In the wild, it is found on the skins of fruit. It has been shown to be useful for wine and beer fermentation as well as ethanol production.
Brettanomyces claussenii was first isolated by N. Hjelte Claussen at the New Carlsberg Brewery in 1904 while he was investigating the causes of spoilage in English ale. He named the genus Brettanomyces, which is Greek for British Fungus. For the most part Bretanomyces is viewed as a contaminant, as it forms compounds that lead to 'off-flavors in both wine and beer.  However, several wines and beers use strains of Brettanomyces in low concentrations in order to achieve the desired taste.
Brettanomyces claussenii is used for brewing beer. Originally isolated from strong English stock beer, it is a key component in some Belgian ales and sour beers.  It is said that fermentation with Brettanomyces claussenii will help a beer achieve English character.
Brettanomyces claussenii is a yeast in the kingdom fungi, the phylum ascomycota and the subphylum saccharomycotina, the so called true yeasts. Yeasts in this subphylum reproduce asexually through budding. Baker’s yeast and Brewer’s yeast are in saccharomycotina as well.
The genus Dekkera can be used interchangeably with Brettanomyces when describing species; Dekkera are the telemorphic (spore-forming) versions of the Brettanomyces species. Dekkera claussenii differs from other Dekkera species in its lack of blastese and inability to ferment lactose. 
Examination of the Mitochondrial DNA of the genus Brettanomyces showed identical genomes in three pairs of species: Dekkera bruxellensis/Brettanomyces lambicus, Brettanomyces abstinens/Brettanomyces custerianus and Brettanomyces anomalus/Brettanomyces clausenii. It is suspected that the genus Dekkera has similar taxonomical redundancies. 
Brettanomyces claussenii is used for a variety of purposes. As a yeast, it is able to ferment cereals and fruits to produce beers and wines with unique flavor profiles. Brettanomyces claussenii can ferment a variety of other substances as well.
Ethanol fuel is becoming more prevalent as an alternative fuel source for automobiles. Using waste wood and agricultural residue is one way to produce ethanol without using crops that could potentially be food sources, such as corn. Combined with Pichia Stipitis R, Brettanomyces claussenii has been shown to produce ethanol from lignocellusic material. The material is prehydrolysed with sulfur dioxide and then simultaneously saccrified and fermented. This fermentation is rapid and efficient, producing between 360 and 370 Liters of Ethanol per ton of prehydrolysed lignocellsic material.  However, Brettanomyces Claussenii is unable to convert lactose to ethanol, like several other yeasts can. 
In clusters of Brettanomyces claussenii a negative pasteur effect has been demonstrated. The rate of alcohol fermentation is reduced under anaerobic conditions and stimulated by the presence atmospheric oxygen. The fermentation rate is also sensitive to alkali concentrations. A heavily potassium solution will increase the rate of fermentation while increasing sodium will slow the processes. 
- "Oxford Companion to Wine — Brettanomyces". Also Heresztyn, T (1986). "Formation of substituted tetrahydropyridines by species of Brettanomyces and Lactobacillus isolated from mousey wines". American Journal of Enology and Viticulture (37): 127–32.
- Burningham, Lucy. "Brewers Take Risks to Make Sour Beer - NYTimes.com." The New York Times - Breaking News, World News & Multimedia. 1 June 2010. Web. 20 Apr. 2011. <http://www.nytimes.com/2010/06/02/dining/02sour.html>.
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- Bothast, R.J., C.P. Kurtzman, M.D. Saltarelli, and P.J. Slininger. "Ethanol Production by 107 Strains of Yeast on 5, 10 and 20% Lactose." Biotechnology Letters 8.8 (1986): 593-96.
- Wilken, T. O., A.J.M. Verhaar, and W. A. Scheffers. "The Influence of Potassium and Sodium Ions on the Negative Pasteur Effect in Brettanomyees Claussenii Clusters." Archiv Für Mikrobiologie 42.2 (1962): 226-36.