I am aware that esters in finished beer are usually caused by high temperatures during fermentation. This fact is not confusing to me and needs no elaboration. What has me confused is this article that another user posted recently in the comments section of this question. The article had he and I very confused.
It basically says that an increase in fermentation temperature will actually decrease ethyl acetate production and lower temps actually encourage ethyl acetate production. It even sites a source (that I couldn't find) that greatest production of ethyl acetate was between 10C and 21C. Everything I've ever read and tried (I accidentally made "banana-beer" once) indicates that this is the exact opposite of the truth.
I've only been brewing a year but I've been reading about brewing for several years so I feel like even though I'm lacking in experience, I have a pretty decent base knowledge. Either this article is completely inaccurate or I have overlooked something.
3 Answers
Ethyl Acetate is the ester of ethanol and acetic acid and has the aroma of pear drops to solvent depending on the concentration. The taste threshold is roughly 120mg/L.
Isoamyl acetate also known as isopentyl acetate, is the ester which tastes like bananas in low concentrations. [1]
Depending on the yeast strain there will be differing levels of higher alcohols which will be oxidised to organic acids which with the react with Acetly-CoA to form different esters.
So it is true that the temp range commonly used for brewing may heighten production of these compounds but it does so thusly:
"These include fermentation temperature, where an increase from 50°F to 77°F (10°C–25°C) has been found to increase the concentration of ethyl acetate from 12.5 to 21.5 mg/L." [2]
As you can see this does not, under ideal conditions, push Ethyl Acetate levels beyond the 120mg/L taste threshold. There are a range of other factors that can push ester production up to higher levels that break this taste threshold.
These include:
- low pitching rates;
- low oxygen availability in the wort;
- high C to N ratio in higher gravity worts;
[1] https://en.wikipedia.org/wiki/Isoamyl_acetate
[2] https://beerandbrewing.com/dictionary/O1rjQz3DYu/ethyl-acetate/
Please feel free to ask more questions in the comments and I will expand the answer as requested. I have to run now.
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1Thank you for Your comment Mr_road. It seems though that your answer confirms what I already thought... that raising fermentation temperature increases the concentration of ethyl acetate as well as other esters. The article I'm asking about seems to claim that raising fermentation temps actually decreases these compounds. Even more confusing is that they claim 10C-21C, an ideal fermentation temp for most ales, yields the highest concentration of esters. Commented May 15, 2017 at 13:28
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1This may be different in beers to wines, unlikely I would guess, or it may be that maximal ester production just happens to be inextricably linked to maximal fermentation, 10-21 is a pretty happy range for most yeast. Also at higher temperature you may get less retention but still high production, or at higher temps you may get more higher alcohol and ester production. I will keep digging in to this as you have spiked my interest, and if I find more I will update the answer,– Mr_roadCommented May 15, 2017 at 21:56
Ethyl acetate is created by all brewery yeast. This is a volatile ester as are all the ethyl esters.
Generally these esters are created when the yeast is stressed but remain when yeast isn't given the chance to reconsume them or is incapable due to conditions.
Also these esters being volatile are subject to being blown off with cO2. Which may account for the article stating warmer temp reduced ethyl acetate, not that it reduced its production but rather was easily blown off with c02 in vigorous fermentation.
To reduce the ethyl esters aerate wort and make sure yeast is healthy and pitched in proper amount for your brew and fermentaion temp is according to makers recommendations.
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Okay, that makes a bit more sense. So the production is not reduced, rather the extra ethyl acetate is removed by the vigorous fermentation. Would I be correct in assuming that the rate of production and the rate of expulsion (due to volatility) are linear or at least mostly linear? And if so, can I assume that the slope of production is flatter than expulsion? I'm still trying to wrap my mind around a situation where esters in the beer would be lower at higher fermentation temperatures. Commented May 15, 2017 at 14:37
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@thekolnik usually what happens to result in ethyl esters in the finished beer, is often too warm of temperature and too much alcohol is produced before the cell walls can tolerate it. Reducing the effective ABV tolerance of the yeast. So the yeast dies out before it's blown off or reconsumed. Commented May 17, 2017 at 14:06
The above answers are all quite correct. One additional comment, though:
All things being equal, the yeast generally tends to produce esters during the earlier stages of fermentation, while the re-uptake of these esters tends to occur during the later stages.
Therefore your finished beer will have higher levels of esters if the fermentation starts at a higher temperature (more ester production) and completes at a lower temperature (less ester re-uptake) and lower levels of esters if the fermentation starts cool and finishes warm.
