Does this smell like Rotorua to you?

Its mid vintage, you’re the scribe for your daily ferment round with the winemaker, and they say, “Temp 14.5, brix 15, h2s/reductive”. You note this down and carry on.  But what does this really mean? To become a competent cellar hand, we need to understand what’s going on behind the scenes, and how we can best treat it. 

There are two kinds of Volatile Sulphur Compounds or VSC’s: Varietal Sulphur compounds which are good, and Fermentation Sulphur compounds which are not so good.  So, if you are smelling rotten eggs, Rotorua or that ‘reductive character’ in your ferment, you have the not so good kind of VSC’s and action mut be taken! We want to catch and clean up these Volatile Sulphur Compounds in our ferment before they become evident in our wines as Mercaptans, Di-Sulphides or Dimethyl Sulphides which are harder to deal with (we will cover this tricky situation in another blog).  

This rotten egg smell is Hydrogen Sulphide, and it’s your ferments way of telling you that its stressed out. All ferments produce H2S, but it is when yeast can no longer convert the H2S back to amino acids via the Sulphate Reduction Sequence (SRS) that the excess spills out into the wine and we smell it (at levels between 0.9 – 1.5 ppb). This excess of H2S is commonly caused by lack of available Nitrogen in the ferment, elemental Sulphur from the vineyard coming in on your fruit or a low YAN in your vineyard. Either way, if detected and dealt with earlier enough, H2S is easy to treat.  

First, we would try feeding the ferment some complex Yeast Nutrients such as goferm which is the same as giving a child with a low GI a banana-slow-release energy. We could also try DAP, which is more like giving a child a $2 mixture- fast and furious. If this did not work, we can try the risky business of adding small amounts of oxygen to the ferment, thus liberating the H2S and restoring balance (be careful not to overflow the tank!), or by adding copper sulphate at the absolute lowest rate. For those incredibly stubborn stinky ferments, you can also try racking off the stinky lees onto some nice clean silky ones (which is what we had to do last vintage and it worked a treat).   

So now when the winemaker says, ‘Temp 14.5, brix 15, H2S/reductive’ you can reply with, ‘Would you like me to add nutrients when we finish this round, try sparing with oxygen or set up a copper trial?’ Showing your knowledge and willingness to help will be sure to impress the boss. 

Better cellar handing everyone.  

Check out the next blog for a more in-depth look at Mercptans, Di-Sulphides and Dimethyl-Sulphides.  

References:  

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

ETS Labs (24/07/2020)  https://www.etslabs.com/library/31

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