Is acetaladehyde a leading cause of the harshness in spirits?

I was thinking about how we us specific process to improve our spirits and how they operate:

  1. Foreshots/heads cut - Removal of low boiling point/high volatility chemicals
  2. Airing out the heads - Removal of low boiling point/high volatility chemicals, Oxidation
  3. Slow proofing - Oxidation
  4. Barrel aging - Removal of low boiling point/high volatility chemicals, Oxidation

Then I started researching acetaldehydes (using Google Search and Google Bard):

Acetaldehyde appears as a clear colorless liquid with a pungent choking odor. Boiling point 69 °F.

Acetaldehyde smells and tastes like green apples. Sometimes it's described as “oxidized apples” or “acetic cider”.

Aldehydes are more reactive than alcohols. This is because the carbonyl group in aldehydes is more polarized than the hydroxyl group in alcohols. The carbonyl group is electron-deficient, while the hydroxyl group is electron-rich. This difference in electronegativity makes the carbonyl group more susceptible to attack by electrophiles.

Aldehydes are more prone to oxidation than alcohols. This is because the carbonyl group in aldehydes is more susceptible to attack by electrophiles. Electrophiles can oxidize aldehydes to carboxylic acids.

Alcohols can also be oxidized, but the oxidation process is usually slower than the oxidation of aldehydes. Alcohols can be oxidized to aldehydes, which can then be further oxidized to carboxylic acids.

Oxidation of aldehydes can be used to synthesize a variety of organic compounds, including carboxylic acids and ketones. Oxidation of alcohols can be used to synthesize a variety of organic compounds, including esters and amides.

There's probably many other chemicals that fit same bill, but we know for certain that there are aldehydes in spirits. I've had this suspicion for while so it probably is at least partially confirmation bias. Regardless it seems like acetaldehydes are a good candidate for being responsible for part of the "unsmoothness" of a spirit.

Acetaldehydes would be more present in the foreshots/early heads. It would evaporate or be oxidied if you were to air out the spirit. Slow proofing and aging allow oxidation and evaporation to occur. All of those processes would reduce acetaldehydes.

It is not definitive by any means but makes a good candidate for research.


  • edited March 2023

    The problem with this the oxidation 'solution' is that it's actually alcohols that are more easily oxidized into aldehydes than aldehydes are into acids. Fixing the acetaldehyde problem with oxidation will just result in more acetaldehyde than you started with.

    Keep in mind, in long term aging studies, you generally see acetaldehyde concentrations increase. This phenomenon is even seen to a limited extent when spirits are aged in sealed containers (likely via dissolved/residual oxygen).

    Fermentation is going to be the single largest source of acetaldehyde, since the biochemical pathways (enzymatic) are far, far more efficient in facilitating these reactions. There is no mechanism for enzymatic reaction in distilled spirits (obviously), but there is some speculation that there might be enough metal ion in distillate to serve as a metal catalyst.

  • edited March 2023
    800 x 485 - 74K
    800 x 361 - 78K
    800 x 195 - 40K
  • edited March 2023

    Keep in mind, that's total aldehydes, not JUST acetaldehyde.

    More recent studies that look at acetaldehyde independently see a similar trend:


    Alcarde, A. R., Souza, L. M., & Bortoletto, A. M. (2014). Formation of volatile and maturation-related congeners during the aging of sugarcane spirit in oak barrels. Journal of the Institute of Brewing

    800 x 445 - 26K
  • edited March 2023

    I guess, in short, acetaldehyde as a fault begins in fermentation, and if not corrected during distillation (through a very wide heads cut), this fault will simply not age out, but will progressively worsen over time.

    Raises the question, how much is too much if the increase seen during maturation isn’t at all problematic?

    Asian distilled spirits are notorious for having absurdly high acetaldehyde concentrations, high enough to raise concerns about being carcinogenic. You see similar around Brazilian sugar cane distillates, and Calvados as well.

    I’ve seen studies showing some Korean distillates being 5-10x the concentration of traditional European and American whiskies.

    “Tastes like burning” is probably accurate here, so clearly there is a level at which acetaldehyde is a prominent fault, there is a level under which, acetaldehyde is a normal and expected congener in mature spirits. Clearly cultural and other class/category expectation-based factors in play here as well

  • Maybe we're thinking about it wrong. What if it's not a lack of a thing that makes spirits smooth, but the presence of something else that reduces the harshness?

    An example from the bartending world: In a drink that has a lot of bitter or astringent flavors, they use sugar or cream/fats to balance (smooth) out the drink.

    Just making this up: What if it's the Alcohol oxidizing to Aldehyde oxidizing to Organic acid chain, where it is the Organic acid (or the ester from) that reduces the harshness of the spirit? There's a myriad of different breakdown chemicals that could be a candidate too, or a combination of several.

    We also know that many producers use sugar, even in small amounts, to increase the perceived quality of their spirit. Barrel aging would gain sugars from the wood.

  • Good aldehydes might mask the bad aldehydes - that’s a plausible theory.

    Vanillin is an aldehyde, probably the most desirable aldehyde that’s formed in maturation. Syringaldehyde the number 2 behind that, and a dominant contributor to flavor.

  • We do understand why spirits burn: Feel The Burn: Why Alcohol Burns In Your Mouth @ VinePair

    There are two parts to it: Oral and esophageal. Each one has different reasons for the burning/warming sensation.

    I have observed that people can differentiate between oral and esophageal sensation and can report them separately. I know a lot of folks that say a spirit is orally smooth but then describe a warming sensation esophageally. This could mean any masking is only effective orally. This makes some sense as the reason that the reason it burns orally is different than esophageally.

    There is a beginning of a hypothesis here. I need to do more research to be able to properly set up an experiment to see if we could prove anything.

  • I used to think that acetaldehyde just turned into ethyl acetate and was the cause for a light crystallization around the neck of my first batches of malt barley vodka, but I know it is much more complicated than that, and the solution of better ferments and narrower cuts fixes that AND many other problems ;-)

    But I love reading your guy's conversations....

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