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'''Brettanomyces''', also known as ''Brett'' or ''Bretta'', is a yeast that was originally thought of as a spoilage yeast. The genus name ''Dekkera'' is used interchangeably with Brettanomyces, as it describes the teleomorph or spore forming form of the yeast <ref>[http://en.wikipedia.org/wiki/Brettanomyces Wikipedia. Brettanomyces. Retrieved 2/24/2015.]</ref>. Known for it's barnyard, fecal, horsey, metallic or Band-Aid flavors, brett ''Brett'' was unwelcome in most breweries. However, in some styles like Saison and Lambic these flavors add a layer of complexity to the beer. ''Brett '' can form a [[pellicle]]. See [[Lactobacillus]], [[Pediococcus]], [[Saccharomyces]], and [[Mixed Cultures]] charts for other commercially available cultures.

''Brettanomyces '' is capable of forming several ethyl esters (derived from ethanol and fatty acids). Among these are ethyl acetate, ethyl lactate and phenethyl acetate, along with the hydrolysis of isoamyl acetate. During non-mixed fermentations where lactic acid and acetic acid are minimal to none, these esters are produced in smaller quantities <ref>[http://www.brettanomycesproject.com/dissertation/introduction/ Yakobson, Chad]. Pure Culture Fermentation Characteristics of Brettanomyces Yeast Species and Their Use in the Brewing Industry. Production of Secondary Metabolites. 2011.</ref>.

In the presence of oxygen, ''Brettanomyces'' strains are capable of producing acetic acid. Depending on the brewer's palate and the degree of acetic production, this can be a desirable or undesirable trait. The degree of acetic acid production varies among different brett ''Brett'' strains. Acetic acid produced by ''Brettanomyces'' may also be used in the synthesis of [[Secondary metabolites|acetate esters]] such as ethyl acetate. ''Brett '' has been shown to produce enough fatty acids in anaerobic fermentation to drop the pH to 4.0, which can also be esterified (see the ester table above) <ref name="yakobson1"></ref>. Many of these acids can have an unpleasant rancid odor and/or taste, which may be noticeable in young ''Brettanomyces '' beers before these acids are esterified.

''Brettanomyces '' can be pitched into a beer at many points in the beer's fermentation life cycle. If used as the primary fermenter, the beer that is produced is often fruit forward and not very ''funky''. A large cell count will be needed (somewhere between an ale and lager pitching rate). See the [[100% Brettanomyces Fermentation]] page for more information. If pitched into a beer that has already been fermented by [[Saccharomyces]], a wider range of flavors including the ''funkier'' flavors can be produced (see the [[Brettanomyces#Brettanomyces_Metabolism|Brettanomyces Metabolism]] section above). A small cell count of ''Brettanomyces '' is plenty for creating these flavors, and normally a starter is not necessary. See the [[Mixed Fermentation]] and [[Funky Mixed Fermentations]] pages for more information on using Brett in secondary.

When pitching just ''Brettanomyces '' from a commercial pure or blended culture and no other microbes, it is recommended to make a starter for the culture. If the ''Brettanomyces '' is being pitched into secondary, no starter is necessary unless the brewer suspects that the ''Brettanomyces '' has lost a lot of viability due to age, heat exposure, etc.

There are two concepts unique to Brettanomyces compared to Saccharomyces when considering doing a starter. First, ''Brettanomyces '' performs best in a "semi-aerobic" environment. This means that the oxygen levels are low, but also not non-existent. An initial blast of oxygen into the wort, and foil left over the top to allow the air to slowly diffuse into the starter should be enough oxygen for the starter. With too much oxygen introduction, ''Brettanomyces '' can produce a lot of acetic acid (this is species dependent). Since ''Brett '' does not flocculate very well, and cold crashing a Brett starter can lead to difficulties, limiting the acetic acid production during the starter fermentation by refraining from too much oxygen exposure is advised <ref>[http://www.ncbi.nlm.nih.gov/pubmed/12655458 Brettanomyces bruxellensis: effect of oxygen on growth and acetic acid production. Aguilar Uscanga, Délia1, and Strehaiano. 2003.]</ref>.

The second aspect of ''Brettanomyces '' cell growth is that it typically takes about 7-8 days to reach it's maximum growth <ref>[http://www.brettanomycesproject.com/dissertation/propagation-and-batch-culture-growth/propagation-results/ Yakobson, Chad. The Brettanomyces Project. Propagation and Batch Culture Results. Retrieved 2/17/2015]</ref>. Thus, each step of a starter for Brett should be 7-8 days.

A ''Brett '' starter strength and size can be accurately calculated using an online yeast calculator such as [http://www.mrmalty.com/calc/calc.html Mr. Malty] or [http://yeastcalculator.com/ YeastCalc] using the 'hybrid' yeast type setting. This will calculate a pitching rate somewhere between ale and lager pitching rates, which should be adequate for 100% Brettanomyces fermentations. Maintaining a temperature of 70°-80°F/20°-26°C should be adequate for most strains. After the 7-8 days of growth, the entire starter should be pitched into the fermenter Brettanomyces is known for not flocculating well, even if cold crashed. Cold crashing will also slow the metabolism of Brett. Therefore cold crashing is generally not advisable.

It is worth noting that a "Malt Yeast Peptone Glucose" growth substrate has been shown to be a better substrate for growing an initial pitch of ''Brettanomyces''. Specifically, when grown in wort, ''Brettanomyces '' will go through a 24 hour lag phase, a growth phase, another lag phase, and a second growth phase (all within 7-8 days). When grown in MYPG substrate, there is only a single growth phase and no lag phase, which has been reported by Yakobson to produce a larger cell count in the same amount of time <ref>[http://www.brettanomycesproject.com/2009/08/mypg-vs-wort-as-the-growth-substrate/ Yakobson, Chad. The Brettanomyces Project. MYPG Compared to Wort as a Growth Substrate. Retrieved 2/18/2015.]</ref>. Cells grown in MYPG also are better adapted to grow in wort <ref>[http://www.brettanomycesproject.com/dissertation/propagation-and-batch-culture-growth/propagation-discussion/ Yakobson, Chad. The Brettanomyces Project. Propagation and Batch Culture Discussion. Paragraph 5. Retrieved 2/18/2015.]</ref>. Practical instructions for making this substrate can be found on Jason Rodriguez's blog, "[http://sciencebrewer.com/2011/04/29/wild-yeast-project-mypg-culture-media/ Brew Science - Homebrew Blog]".

Store ''Brett '' at room temperature, and not refrigerated. 60-75°F/15.5-23.9°C should be acceptable (reference needed). Chad Yakobson noted that after storing ''Brett '' in a refrigerated environment, after 6 months the ''Brett '' would die. If ''Brett '' is stored cold, it will be very sluggish and slow to start fermentation. Making a starter is highly recommended if the ''Brett '' culture has been stored cold <ref>[http://youtu.be/AjVOzBtE27Y?t=43m Yakobson, Chad. Presentation at 2012 Music City Brew Off. At 43:00.]</ref>.