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