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I am reading a lot about yeast cell production and ester production. I think I have a good handle on the yeast cell production, but what are esters? Is that part of what the yeast consume that give beer a off flavor? I am trying to get as much info as I can to produce the best possible brew.

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Esters in our context are the "fruity" aromas and flavors you get from beer. The type and amount of them are related to the type of yeast you use and the temperature you ferment at are the two components that mainly determine the esters. From the BJCP study guide......

"Esters may be controlled by the choice of yeast strain, wort gravity, wort aeration, and fermentation temperature. In general, ale yeast strains produce higher ester levels, although there are variances among different ale strains. Lager yeast strains can, if fermented too warm, also produce esters as is practiced in the making of French Bière de Garde styles. Wort gravity also is a factor; the hallmark esters of Belgian Trappist styles are not only due to the yeast strains used but also a result of their high gravity wort. Wort aeration also plays an important role, as the ester production pathway directly competes with the absorption of oxygen and metabolism into sterols (16). Lastly, fermentation temperature also plays an important role. A four-fold increase in ester production may be observed as a result of increasing the fermentation temperature from 60 to 68 °F, 16-20 °C (1)."

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