Esters in home brew beer can be both a blessing and a curse. Ester (a fruity flavor) can be highly desirable in many English ales or Bavarian Weizen, but can also be a curse in other styles like lager. This week we take a look at esters in beer and what you can do about it.
What are Esters?
Esters are a fruity flavor produced during fermentation that can vary in taste and aroma between pears, roses, bananas or other light fruits. In very high concentration it can create a solvent-like flavor.
Esters are formed in beer by the “esterification” of ethanol which is the primary alcohol in beer. Ethanol combines with fatty acids and a molecule called acetyl coenzyme (ACOA) forming ethyl acetate. Ethyl acetate’s flavor varies from a light pear-like character to solvent-like in high concentrations.
Other alcohols present in the beer may also combine to produce additional esters. For example isoamyl alcohol will combine and produce isoamyl acetate which tastes like bananas in low concentration. This ester is the distinct banana flavor that is the defining characteristic of Bavarian Hefeweizen.
Are Esters Bad?
Different styles require different levels of esters. For example, esters are highly undesirable for most lagers, so you would want to minimized ester production when brewing a lager. Esters are a feature of many English ales and as noted above in Weizen/Hefeweizen.
Choosing a Yeast Strain
The production of esters is primarily driven by the yeast used. Yeasts have an enzyme called acetate transferase (AAT) which drives the production of esters. A low AAT yeast will produce far fewer esters, so choosing a yeast that is appropriate to the style you are brewing is the #1 way to control ester production and make sure it is appropriate for your beer.
Fermentation Temperature
A second way to control ester production is by controlling the fermentation temperature. Higher temperatures in fermentation result in rapid yeast growth, more AAT and more ester production. This is why, in general, ale yeast produces more ester than lager yeast. So if you are brewing an estery English ale you might want to target the high end of the yeast’s fermentation temperature.
If you are brewing a style such as lager where you want few esters, be sure to ferment at appropriate lager temperatures. One common beginner mistake is to attempt fermentation of a lager at room temperature which will result in a fruity lager – not what you were shooting for at all.
Yeast Pitch Rate and Esters
A third way to control ester production is by varying your pitch rate. If you under-pitch yeast (i.e. don’t pitch enough yeast for your volume of wort) the yeast will reproduce rapidly during the short lag phase. Rapidly reproducing yeast enhances AAT production and subsequently produces more esters in the finished beer. This is another common problem for beginners who often brew with no starter and get fruity esters in their lagers. Pitching enough yeast (or even overpitching) will result in less ester production.
Oxygenation of Wort and Esters
Finally you can reduce esters by properly oxiginating your wort. During the growth phase, the yeast will actually consume ACOA (above) which is a precursor of ester production to reproduce. However this only continues until the yeast run out of oxygen. So if you properly oxygenate your wort it will reduce overall ester production. Conversely if you under-oxygenate your wort it will actually enhance ester production in the finished beer.
I hope you enjoyed this week’s article on esters and homebrewing. Please subscribe to my newsletter for a weekly article on homebrewing.
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“to minimized ester production” -> minimize
Yup, under-pitching can definitely throw off esters. Check out a recent experiment I did:
http://sciencebrewer.com/2012/03/02/pitching-rate-experiment-part-deux-results/
J
Great information. I found the part about isoamyl acetate in Bavarian heffs to be interesting. I never knew that. I got a true fresh Bavarian Heff once while in the airport in Berlin. Good stuff and now I know why it’s so unique. Thanks for sharing.
I have a beer that is high in esters and I’m curious if this quality will increase over time in the bottle or become more subtle. It’s not a trait I am particularly fond of in my ESB, so I’m wondering if I should drink them all quickly before it gets worse or let the fruity taste age out.
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Evan: I realize this reply is coming way too late, but from my limited experience (one batch with too much esters), they will usually get less prominent over time after the beer’s been bottled. My beer got noticeably better after about a month, but that was the last bottle I had so I don’t know if it would have gotten even better had I waited some more.
I believe AAT is alcohol acyl-transferases, not acetate transferase as stated in your article, correct me if I am wrong.
Otherwise great article, thanks!
You write that, “Ethanol combines with fatty acids and a molecule called acetyl coenzyme (ACOA) forming ethyl acetate.” but as I understand it, ethyl acetate is a simple ethanol + acetate (“acetic acid” in water) joined by an ester bond. Is this incorrect? Is the route to becoming ethyl acetate in beer somehow more complicated?
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I watched yesterday a very good you tube video made by White Labs (Neva Parker)
They showed a nice slideon what will influence among others the ester production – they showed that actually overpitching yeast will result in higher ester production. The slide shows: Higher Pitching rate leads to Lower Acetaldehyde, Lower Fusels, but higher Esters.
Rationale: Acetyl Coa is responsible for yeast metabolism and also ester formation. If there is high yeast growth rate, this compound is directed to growth, but since you are overpitching, there is already enough yeast, so the compound is free and starts ester production.
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Didn’t mention anything about brewing under pressure …
This is obviously all about plastic bucket brewing..
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Esters are indeed the key to flavors. It takes a great formulate to get consistent flavors.