We have read about this from other home winemakers, but this is the first time it happened to us. Our Chardonnay which has finished primary fermentation and started MLF has acquired a rotten egg smell. It is actually H2S or Hydrogen sulfide. It comes from lees (sediment) hanging around too long after fermentation. Yeast cells without nutrition with nothing to do.
It is a common problem that fortunately has a solution. The first step is to “splash rack” the wine off the lees. Giving it some oxygen and taking it off the lees which are causing the problem. The second amendment is to add 25 ppm of Potassium Metabisulfite (K2S205) .. for you chemistry nerds out there. It is an antioxidant to protect the wine from damage from being in contact with air.
Just racking it twice took away a lot of the bad smell. The Potassium Metabisulfite should assist in ridding us of the rest.
Hopefully we won’t need to go to the next – – more drastic step. Copper Sulfate.