Wednesday, November 22, 2017

Happy Thanksgiving: Something to Gnaw On

Here is a pre-Thanksgiving blogpost. I am currently reading a review called Origins of Grape and Wine Aroma. Part 1. Chemical Components and Viticultural Impacts that appeared in 2014 in the American Journal of Enology and Viticulture. It is 24 pages long so, I've been plodding along and trying to digest the review in small bites.
This morning, I came upon a paragraph that is almost incomprehensible by a lay person. I reproduce it below:
          Methional concentrations increase in white wines exposed to           elevated temperatures and oxygen. Under these conditions, the           compound is produced via a Strecker degradation of methionine to           methional in the presence of a dicarbonyl compound or via direct           peroxidation of methionol.
Whaaaaat?
Fortunately, my husband who is a chemist was sitting across from me, drinking his morning coffee, and I asked him do you know what a "Strecker degredation" is? He said, "sure, is that an oxidation?" (Close but no cigar!) Back in the days of taking organic chemistry, there were certain "name reactions" that one could memorize. So, here is my take on what the above paragraph translates into in organic chemistry terms.
So, the diagram above can be captioned: In the presence of a compound with 2 carbonyl groups, the amino acid methionine can react with one of the carbonyl groups forming an unstable intermediate. With the incorporation of 2 molecules of water, the unstable intermediate results in the formation of ammonia, carbon dioxide and methional.
What is wrong with having methional in white wine? The smell for one thing is of "cooked vegetables". Methional is also thermally unstable and undergoes a retro-Michael reaction that leads to the wet dog or wet wool smell in wine that comes from methyl mercaptan and acrolein.
Here is hoping that your Thanksgiving is a wonderful one with wines innocent of Strecker degradation and retro-Michael products. Cheers!
References:
1. Wil Konings, ‎Otto Kuipers, ‎O.P. Kuipers, Lactic Acid Bacteria: Genetics, Metabolism and Applications, 1999, pg. 250.

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