Is it Mercaptans?

You think you fixed that smelly ferment, only to smell the return of the reductive aromas in the now dry wine. But not to worry, we can fix it!

In the last blog we talked about VSC’s and H@S in ferments. In this blog, we will tackle a tricker problem: Mercaptans and Di-Sulfides in wine.  

To understand how they got there, we need to look at the chain reaction that brought us to this point. H2S, a form of Volatile Sulphur compounds developing in early stages of fermentation (see previous Blog), have reacted with components in the wine, thought to be ethanol or methanol, and created Mercaptans. If these Mercaptans oxidise we are going to see Di-Sulfides, either Dimethyl Disulfide (DMDS) or Diethyl Disulfide (DEDS). These last two are harder to get rid of and if your very unlucky, won’t show themselves until after bottling, having developed during storage. Much like H2S, at low thresholds, we cannot smell these reductive Sulphur compounds, infact at low levels they may even enhance certain aromas in the wine. But for the sake of good wine making, lets clear them up as soon as possible and not take the risk of off aromas developing in our bottled wines.  

First, we need to distinguish what we are dealing with: H2S, mercaptans, or di-sulphides. This means we can treat the wine correctly and not unknowingly make things worse. Conducting a copper cadmium test, commonly known as a Cu/Cd test is the best way to go. This is a quick and simple aroma diagnostic test that we can use to identify the reductive compounds causing the off aroma.  

For the sake of keeping this brief, how to conduct a Cu/Cd test can be found here AWRI Cu/Cd Test

Test done; reductive compound identified. Now let’s get rid of it!  

Mercaptans: Easily treated with copper sulphate. Remember, when using copper ALWAYS conduct a copper trial before any additions are made to wine (see the next blog). 

Di-Sulfides: Dimethyl Disulfide (DMDS), often smelling like cooked cabbage and onions, and Diethyl Disulfide (DEDS), often smelling like burnt rubber and garlic, are a little tricker to treat. We cannot simply hit them with copper because they don’t react with copper. So, we add ascorbic acid and SO2, turning them into mercaptans, which we can then treat with copper.  

There you have it folks. The ins and outs of volatile Sulphur compounds in winemaking. Hopefully, after reading this blog, you get an appreciation of how important it is to nip any H2S in the bud as soon as possible to save yourself trouble later down the line.  

Better cellar handing everyone.  

References: 

AWRI, Australian Wine Research Institute, Diagnostic test for ‘reductive’ wine characters, https://www.awri.com.au/industry_support/winemaking_resources/sensory_assessment/diagnostic_test/

Ruis A.  (n.d) NMIT, CCO410 Wine Finishing and Analysis, S2-20                    ‘Volatile Sulphur Compounds’  PowerPoint slide show 

Images taken from NMIT, CCO410 Wine Finishing and Analysis, S2-20              ‘Volatile Sulphur Compounds’  PowerPoint slide show 

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