Diacetyl smells like butter or cream, acetaldehyde smells like cut green apples, and both of them show up in beer because of a normal part of yeast metabolism that a brewer has to help finish properly. Neither compound is a contaminant and neither comes from bad beer, and the good news is that both are usually fixable while the beer is still in the fermenter. The bad news is that they behave differently, and the two get confused often enough to be worth keeping straight: diacetyl clears itself as the yeast finishes and is reduced, while acetaldehyde tends to signal a problem, and a packaged beer will not clean it up on its own.
What Diacetyl Is And Why Yeast Makes It
Diacetyl is a real compound with a buttery smell, and yeast produces it as a normal step in making two of the amino acids it needs, valine and isoleucine. While the yeast is actively growing and dividing, it makes more diacetyl than it needs and simply excretes the excess into the wort. Late in fermentation, when growth slows and the yeast stops needing those amino acids, it does something different: it takes the diacetyl back out of the beer and reduces it, first to acetol and then to a compound that is essentially flavourless. The whole process is a handover, and butter only becomes a fault when the second half of the handover never happens.
That reduction needs the yeast to be healthy, plentiful, still active and supplied with zinc, which is a cofactor the enzyme responsible depends on. A stressed or underpitched yeast, a fermentation that stalled early, a wort that ran out of fermentable sugar before the yeast got to the end of its work, and a shortage of available zinc all push in the same direction, which is more diacetyl in the finished beer. There is also a second, sneakier route. The acetol formed during fermentation is not stable, and once the yeast drops out of suspension it can convert back into diacetyl on its own, which is why some beers seem to throw a late buttery note as they finish. A brewer who tastes a clean sample one week and a buttery one the next has met this.
What Acetaldehyde Is Doing There
Acetaldehyde is a different smell and a different origin. It carries a hard, tart, solvent note that people describe as green apple, cut apple, raw apple, or occasionally sulphur or nail-polish remover, and it is the compound your nose picks up in almost every case of oxidized beer. Yeast makes acetaldehyde as a step on the way from pyruvate to ethanol, and it makes it in quantity when the yeast is stressed or when the wort is deficient in the nutrients the yeast needs to finish, so like diacetyl it points at how the fermentation went. But acetaldehyde also has a second, more troublesome source: oxygen. Ethanol in the beer reacts with dissolved oxygen and becomes acetaldehyde, which is why a bottle that has been open, warm or shaken in transit can pick up a green apple note without the yeast ever being involved. If you are chasing this in a young beer with no oxygen exposure, think fermentation. If you are chasing it in a bottle that has travelled, think oxygen. The two are linked in our piece on oxidation in the bottle.
The Fix Depends On Which Compound You Have
The two faults respond to different treatment, so the first job is telling them apart in the glass before you do anything else.
- If it smells buttery or like cream or butterscotch, especially at warm temperature, you have diacetyl, and the fix is to let the yeast finish the job: leave the beer on its yeast, keep it a little warmer rather than colder, make sure the yeast is healthy and has zinc, and only package once attenuation has finished and the buttery note has dropped.
- If it smells of cut or green apple, solvent or sulphur, treat it first as a fermentation problem, which means checking the yeast health, the nutrient levels and the temperature at which it finished, and second as an oxygen problem, which means reducing the oxygen the beer sees in the fermenter, in the transfer and in the packaging.
- If it smells both buttery and green apple together, you likely have both compounds, which is common, because a stressed fermentation leaves leftover acetaldehyde and a stalled one leaves diacetyl, and the remedy is to finish cleanly and then limit oxygen.
- If the beer is young and the note is green apple and nothing else, give it a little more time on a healthy yeast, since young beer is allowed to be a little sharp before the yeast tidies it up.
The temperature detail is worth dwelling on, because it catches people. Diacetyl is more noticeable warm than cold, so a cold sample of a beer with a trace of diacetyl can taste clean while the same beer, warmed in the hand or served at cellar temperature, tastes obviously buttery. This is why a beer should be tasted at more than one temperature before you pass judgement on it, and why a buttery note that appears only when the bottle has been sitting in a warm room is a warning rather than a curiosity.
The Zinc Piece Nobody Thinks To Check
Zinc is the mineral a homebrewer most often runs short of without noticing, and it is exactly the one the diacetyl-reduction enzyme needs. Brewer's yeast will limp through a fermentation with very little available zinc and still attenuate the wort, so a beer can drop in gravity right on schedule and still come out buttery, which is why the fault is so often blamed on the yeast strain when the real issue is what the yeast did not have to work with. If your water is very soft, which a lot of British Columbia water is, there may be little natural zinc in it, and that is a case where a small, labelled zinc addition made with a product sold for exactly this purpose can clear a persistent diacetyl problem. More is not the answer here, because an overdose of any mineral is its own fault.
Bottle Conditioning Makes The Problem Temporary
There is one case where a buttery or green apple note in a young beer is a reason for patience rather than alarm. A bottle-conditioned beer finishes its yeast cleanup in the bottle, over weeks and months, so a diacetyl note that would be a fault in a filtered, force-carbonated package often fades on its own. That is the useful contrast with oxidation, which is stable and does not improve, while diacetyl is a temporary state that a living yeast will resolve. The same reasoning does not apply to a canned or kegged beer that has been dealt with at the brewery, because there is no yeast left in it to do the cleanup, which is why a commercial beer should not have a buttery note in the first place.
Frequently Asked Questions
Is a little diacetyl in a pale ale a fault?
Diacetyl is not a traditional character of any classic ale style. Where the yeast strain itself throws a subtle buttery note as part of its profile, a brewer may choose to keep it deliberately, but that is a decision, and a trace that a brewer did not intend is still a fault. If you can taste butter clearly in a pale ale, it is more butter than intended, and you are entitled to prefer it without it.
Does dry hopping or hop oil cause diacetyl?
A dry hop does not cause this fault. Dry hopping adds hop material to the finished beer for aroma and a little bitterness, but it does not produce diacetyl, and the timing of a dry hop charge has nothing to do with it. If a beer develops a buttery note after a dry hop, the underlying cause is the fermentation, not the hops, and a common trigger is hopping into a beer that was not fully attenuated, because an early hop addition only distracts from the real problem, which is an unfinished ferment.
Can I add yeast to a beer to clean it up?
Often, yes, and it is the standard rescue for a beer that still has both fermentable sugar and a healthy yeast. Re-pitching a healthy, active culture into a beer that is not yet at its final gravity gives the yeast the chance it needed to reduce the diacetyl, particularly if you keep it a little warm. Two things will not work. A beer that has already reached its final gravity has no fermentable sugar left to sustain new yeast, so a re-pitch will do very little. And an old or heat-stressed culture will simply add its own problems, so a fresh, healthy pitch at a moderate temperature is the move, not a dusty packet from the back of the cupboard.
Two Compounds, One Lesson
The reason these two come as a pair is that both are the normal work of a yeast cell mid-fermentation, left behind when a fermentation is hurried, stressed or cut short. Butter means the cleanup did not finish, and green apple means something went wrong in the yeast's environment or oxygen got in behind it. Learn to name which one you are smelling, and the fix stops being guesswork. A British Columbia homebrewer working with soft water and a quick five-day ferment has the two conditions for both faults sitting right there in the kettle, and adjusting for them, rather than masking the result, is what a clean-tasting beer actually comes down to.
