Most esters in beer are formed by esterification of ethanol with fatty acids and a molecule called acetyl coenzyme A (or aCoA for short). Ethanol is by far the most abundant alcohol produced by brewers' yeast. There are other types of alcohols which are produced and these are called fusel alcohols.
All alcohols can undergo the chemical changes inside the yeast cell to form these compounds, but those produced from fusel alcohols tend have very low concentrations.
The specific compound produced depends on which alcohol gets converted. Since ethyl alcohol, or ethanol, is the most abundant alcohol in beer, the ester called ethyl acetate, which is formed by the esterification of ethanol with acetic acid, is the most abundant ester in beer. Ethyl acetate's flavor and aroma is described as being pear-like at small concentrations and solvent-like at higher concentrations (>30 ppm).
Wild yeast is sometimes blamed for the solvent-like flavors in beer but you can get them from commercial yeast strains as well.
Another of the prominent brewing esters produced by yeast is called isoamyl acetate. It is produced by the esterification of isoamyl alcohol. Isoamyl acetate is the ester which gives Hefeweizens their characteristic fruity banana-like aromas and flavors. It can be tasted in concentrations as low as 2 ppm.
For some beers, such as German hefeweizens, fruity esters define the style. But for other beers, like clean lagers, ester formation is not wanted and is considered a flaw.
You as the brewer have some control of the amount and type of esters in your beer.Ester formation is primarily dependent on the yeast strain.
Some yeast will produce more esters than others and some will produce different esters than others. Esterification of alcohol is controlled by an enzyme called alcohol acetate transferase (AAT). So, the first way you can increase your beer's fruity flavors and aromas is to select a yeast strain which produces more of the AAT enzyme.
A second method of increasing esters in your beer is to under-pitch yeast in your wort. The yeast will grow rapidly until a total yeast mass is established that can utilize all of the resources in the wort. More yeast growth means more cell wall production, which means more AAT production, since it is localized in the cell membranes of brewer's yeast.
According to Jamil and Dr. Chris White from White Labs on the podcast entitled "Homebrewing Saison" originally aired on 09-10-2007, the yeast must go through a growth phase so it can produce the compounds that are the precursors to fruity esters.
Pitching too much yeast, such as when you reuse the entire cake from a previous batch, will cause the yeast to have a very short lag phase with very minimal AAT production (probably the compound Jamil and Dr. White were talking about) and thus very little ester production.
For some beers, especially those from Great Britain, this makes an uninteresting example of the style. Some beers must have some fruity esters to fall within the BJCP guidelines for that style. If you find your beers need more esters, try pitching on the low end of the scale for that style so the yeast have a chance to grow and produce the fruity esters you expect.
A third way to increase the fruity esters in beer is to increase the fermentation temperature, which will encourage rapid yeast growth and accomplish the same end result, lots of AAT production.
Of the two methods, raising the fermentation temperature during the lag phase is probably the best choice because under pitching your yeast is not generally recommended due to possible problems associated with yeast stress and competing contaminants ie. wild yeast and bacteria. Some people do under-pitch their yeast to increase ester production, especially in beers such as hefeweizens, weizenbocks, and English Ales, but unless you understand all the factors involved, such as pitching rates and yeast stress variables and their consequences, it's probably best to pitch plenty of healthy yeast at a warmer temperature (68-72°F or 20-22°C) and leave the finessing to the experts.
Another method of increasing esters in beer is under oxygenating your wort. Yeast use lots of oxygen building their cell walls (4-14 ppm) and will continue reproducing until all the dissolved oxygen is gone.
During reproduction, yeast use oxygen to produce unsaturated fatty acids which also uses up aCoA, a precursor for ester production. Therefore by reducing the amount of oxygen available for the yeast, you will effectively be increasing the available ester precursor acetyl coenzyme A, making it available to produce more of the fruity esters you want.
But again, just because the method is available and some may be applying it doesn't make it the best method for most homebrewers. Without the advantages of sophisticated labs full of over-educated technicians, we as homebrewers should probably stick to what we can control. It seems the best method for us is to pick the correct yeast strain and to pitch and ferment at higher temperatures to increase yeast growth.
When brewing lagers or other styles with subdued esters, we can take the opposite approach. Choose a clean fermenting yeast strain and ferment at the lower end of its temperature tolerance.
Many ale yeast will ferment perfectly well at 58-62°F (14-17°C) and produce an ale that is almost lager-like in its flavor profile. As a general rule, always pitch plenty of healthy yeast and oxygenate those yeast well for the best results.
Information for this article on Esters in Beer was adapted in part from New Brewing Lager Beer by Gregory J. Noonan, the article Beer Flavor Primer: Fruity Flavors, AKA, Esters by George de Piro, Brewmaster, C.H. Evans Brewing Company, How To Brew by John J. Palmer and Evaluating Beer edited by Brewer's Publications.
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