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Sunday, December 21, 2014

Wet Dog or Wet Wool Smell in Wine

Well, it is not so easy to track down exactly what this reaction involves although Tom Stevenson's description in The Sotheby's Wine Encyclopedia 5th Edition of the wet dog or wet wool smell is often quoted on the Internet:
          These aromas are heat-generated volatile sulphur faults involving the           retro-Michael reaction of methional, which is thermally unstable and           evolves rapidly into acrolein and methanethiol, which are responsible           for the so-called wet-dog and wet-wool aromas and a stronger cooked-           cauliflower smell.
And many people reference the following URL for almost the same explanation:
http://www.wine-pages.com/guests/tom/taste.htm
I had no idea that this topic was rampant on the Internet, it is also a reaction involved in heated wort in beer making. I'm interested in seeing the chemistry and not only the description of the chemicals so my search continues, or I could just break out my Organic Chemistry books. But, hubby to the rescue. He looked up from his computer long enough to focus on my problem. To clarify what a retro-Michael reaction is, here is what the chemistry looks like according to Tom Stevenson's explanation:
A retro-Michael reaction is the opposite of a Michael reaction, so the products formed in the retro-Michael reaction can be the starting materials in the Michael reaction, which in this case are acrolein and methanethiol.
Retro-Michael reactions can be catalyzed in either a basic or an acidic medium and since we are now talking about wine, this particular retro-Michael is being catalyzed in an acidic medium, typical wine acidity being between pH 3.3-3.9. The barrier to the retro-Michael reaction occurring is lowered when the temperature is raised, which explains the thermally unstable nature of methional.
I learned a lot of interesting factoids while surfing the Internet. For example, methional is the notable flavor in potato based products like potato chips. Acrolein is a colourless liquid with a piercing, disagreeable, acrid smell and methanethiol is a colorless gas with a distinctive putrid smell.
Reference:
1. All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.

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