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Blinded by Science: Flavor and Aroma Constituents of Yeast!

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August 11, 2015 by fhsteinbart


Here we see a diagram showing how acids are decarboxylated into esters.

Here we see a diagram showing how acids are dehydrated into esters.

The next time you have a well made, fresh, craft brewed beer, take a deep nasal inhalation of the aromatics that the yeast provide before swilling down that first gulp of delicious amber concoction we call beer. Brewers’ yeast creates relatively large quantities of carbon dioxide and alcohol during fermentation. About twelve pounds per barrel of carbon dioxide is produced during a normal standard fermentation. That is why beer is fizzy and gives you a mild feeling of euphoria. Ethyl alcohol is by far the most abundant alcohol produced by yeast. They do make some other alcohols, too, which are referred to as fusel alcohols. All of these alcohols can be changed into esters inside the yeast cells. The chemical reaction responsible for this conversion is called esterification. The specific alcohol that is esterified determines which ester is produced. If one starts with ethyl alcohol, one ends up with ethyl acetate after the esterification reaction. Similarly, isoamyl alcohol is esterified to the banana-like isoamyl acetate. Easy, right? Increasing the rate of yeast growth will increase the production of fusel alcohols, which are important ester precursors. Pitching too little yeast can do this, as can high temperatures early in the fermentation. Under pitching yeast can cause other problems, however, and is not a recommended method of increasing esters. The brewer’s safest method is to pitch the yeast into relatively warm wort (68-72 °F) to encourage rapid yeast growth. Like under-pitching, restricting oxygen is likely to cause more problems than the brewer wants to deal with, so it is not a recommended method of ester control for the home or small commercial brewery. In the end, the safest way to increase esters in a beer is to choose an appropriate yeast strain and increase the rate of yeast growth by pitching and fermenting at higher temperatures. To decrease esters, as is desirable in many beer styles, choose an appropriately clean tasting yeast strain and pitch at low temperature. Always give your yeast plenty of oxygen!

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