Wine Chemistry 101 on Sulfur

On Cory Cartwright’s blog, as a part of his “31 days of natural wine” series, the Bay area chemist Arjun Mendiratta has written an entertaining and dense post on sulfur.

Here are important points we want to remember from his post:

SO2 advantages

  • SO2 averts wine oxidation (which happens with the oxygen in the air)
  • SO2 does not kill saccharomyces cerevisiae (the important enzyme)

SO2 disadvantages

  • SO2 smells like burnt match;
  • SO2 can combine with certain flavor compounds present in wine and deprive them from their flavor;
  • SO2 reduces the microbial complexity of fermenting wine, which some believe that microbial complexity is directly related to flavor complexity;

Some Other Notes

  • even wines to which no SO2 has been added will nonetheless contain just a little SO2, since it is naturally produced during fermentation;
  • When SO2 is dissolved in water (and wine is 85% water), a small portion of it remains as SO2, but much of it reacts with water to form a mixture of sulfite, SO32-. and bisulfite HSO3-;
  • KHSO3, potassium bisulfite, a white powder, is easier to handle than the stinky SO2 gas;
  • H2S (hydrogen sulfide, aka rotten eggs) is a product of fermentation, and if it sticks around, it can turn into sulfides and mercaptans, which can be a problem
  • sulfides and mercaptans are heavier cousins of H2S. Sulfides and mercaptans are an intrinsic part of the flavor of many wines.  However, when present in excess, they can impart an overpowering odor akin to cooked cabbage;
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