Beer Odor Guide: Isoamyl Acetate

The most important ester found in beer is isoamyl acetate, which has a flavor like rubber or pear blossom. Isoamyl acetate is considered an ester, or a mixture of alcohol and acid. Because fermentation (yeast converting sugar into alcohol) produces isoamyl acetate, isoamyl acetate is a very common off-flavors in all beer. Although the content of this compound in beer does vary greatly, its flavor threshold is between 0.6 and 1.2 parts per million.

So How Is Isoamyl Acetate Produced?

The esters in beer are formed by the reaction of organic acids and alcohol produced during the fermentation process. This reaction causes the esterification of alcohols, including the most abundantly created, ethanol as well as other higher, or “fusel” alcohols. The type of ester formed depends on the specific type of alcohol involved in esterification. Most concentrated esters are “acetates”. This means that they use acetate (known as acetyl-CoA) as part of the esterification process. Others do not use the same chemical acetyl-CoA as a catalyst but use others.

Acyl-CoA has several different sources, including the activation of wort fatty acids, the oxidative decarboxylation of keto acids, lipid catabolism, and fatty acid biosynthesis. The main acyl-CoA molecule is acetyl-CoA, which combines with ethanol and isoamyl alcohol to form ethyl acetate and isoamyl acetate, respectively.

Isoamyl acetate is formed by the condensation of acetyl-CoA and isoamyl alcohol during the fermentation process. Because of the reduction of carboxylic acid and ethanol, most esters are formed in beer. This reaction occurs at higher temperatures, so it is common in top-fermenting yeast. This is why ale has more ester and fruity flavors than lager.

Isoamyl acetate is usually found in many German beers such as Hefeweizen and has become a symbol of the German beer style. Because of the demand for this ester, some brewers will deliberately create conditions to expand production. If you want to have a high concentration of isovaleric acid in the brewing process, then the first step is to choose a yeast strain that is very suitable for the production of its precursors and catalysts. Isoamyl acetate is a corrosive ester and can be detected as low as 2 ppm on the tray. Unfortunately, when this ester appears in beer that shouldn’t contain it, it is considered an off-flavor. In beer, this ester is usually not difficult to control or reduce, because the selected yeast is not easy to produce specific elements during the fermentation process.

How To Increase The Production Of Esters In Beer?

In German dark beer and Belgian beer, it is expected that the beer contains a higher concentration of lipids. If you want to have higher or lower lipids in beer, you must first start choosing yeast.

All yeasts have individual ester production characteristics. Some yeasts produce more esters than other yeasts, and some produce different esters from other yeasts. The esterification of alcohol is controlled by an enzyme called alcohol acetate transferase (AAT). Therefore, the first way to increase the fruity taste and aroma of the beer is to select a yeast strain that produces a higher concentration of AAT enzyme. Enzymes are the catalyst for everything!

Another common way to increase ester production during fermentation is to slightly reduce the under-pitch of the yeast. This will put some pressure on the yeast and stimulate the growth of the yeast, resulting in higher ester production. In addition, temperature also affects ester production. A slightly warmer fermentation temperature can promote the lipids in the yeast, which may produce lipids. But this approach is relatively risky for most people.

Finally, some winemakers choose to promote the production of esters through insufficient aeration. I don’t recommend this because it usually also generates additional aldehydes. Believe it or not, even the shape of the fermenter can affect the production of esters. This is why some professional breweries also have different shapes of fermentation tanks.

How To Reduce The Production Of Esters In Beer?

If you want to reduce the incidence of esters in beer, in other words, if you are looking for a “clean” beer flavor, then there are several key points to consider:

• Fully ventilate

• Select yeast strains with low-fat formation potential

• Pitch many viable/vital yeast cells

• Pitching and fermentation at the ideal temperature