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, which is a combination of alcohol and acid. Since fermentation (the process of yeast converting sugar into alcohol) produces isoamyl acetate, it is a common off-flavor in all beers. Although the content of this compound varies greatly in beer, its flavor threshold ranges from 0.6 to 1.2 parts per million.
So How Is Isoamyl Acetate Produced?
The esters in beer are formed through the reaction of organic acids and alcohol produced during fermentation. This reaction leads to the esterification of alcohols, including the most abundant one, ethanol, as well as other higher alcohols known as “fusel” alcohols. The type of ester formed depends on the specific alcohol involved in the esterification process. Most concentrated esters are “acetates,” which means they use acetate (acetyl-CoA) as part of the esterification process. However, some esters use other catalysts instead of acetyl-CoA.
Acyl-CoA has several 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 through the condensation of acetyl-CoA and isoamyl alcohol during fermentation. Most esters are formed in beer due to the reduction of carboxylic acid and ethanol. This reaction occurs at higher temperatures, making it common in top-fermenting yeast. That’s why ales tend to have more ester and fruity flavors compared to lagers.
Isoamyl acetate is commonly found in German beers like Hefeweizen and has become a symbol of German beer styles. Due to the demand for this ester, some brewers intentionally create conditions to enhance its production. If you want a high concentration of isovaleric acid during brewing, the first step is to select a yeast strain that is highly suited for producing its precursors and catalysts. Isoamyl acetate is a volatile ester and can be detected at levels as low as 2 parts per million. Unfortunately, when this ester appears in beer that shouldn’t contain it, it is considered an off-flavor. However, controlling or reducing the presence of this ester in beer is usually not difficult since specific elements are not easily produced by the selected yeast during fermentation.
How To Increase The Production Of Esters In Beer?
In German dark beer and Belgian beer, higher lipid concentrations are expected. If you want to increase or decrease lipid content in beer, it starts with selecting the yeast.
Each yeast strain has its own characteristics when it comes to ester production. Some strains produce more esters than others, and some produce different types of esters. The esterification of alcohol is controlled by an enzyme called alcohol acetate transferase (AAT). Therefore, the first step to increasing the fruity taste and aroma of the beer is selecting a yeast strain that produces a higher concentration of AAT enzyme. Enzymes are the catalysts for everything!
Another common method to increase ester production during fermentation is to slightly under-pitch the yeast. This puts some stress on the yeast and stimulates yeast growth, resulting in higher ester production. Additionally, temperature affects ester production. A slightly warmer fermentation temperature can promote lipid content in the yeast, which may lead to increased ester production. However, this approach carries some risks for most brewers.
Lastly, some brewers choose to promote ester production by limiting aeration. I don’t recommend this because it often leads to the production of additional aldehydes. Surprisingly, even the shape of the fermenter can affect ester production. That’s why some professional breweries have different shaped fermentation tanks.
How To Reduce The Production Of Esters In Beer?
If you want to reduce the occurrence of esters in beer, in other words, if you’re aiming for a “clean” beer flavor, consider these key points:
- Fully ventilate.
- Select yeast strains with low-fat formation potential.
- Pitch a large number of viable/vital yeast cells.
- Pitch and ferment at the ideal temperature.