Hydrogen Sulphide that Rotten Egg Smell
A rotten egg smell is due to the presence of hydrogen sulphide (H2S) gas. Although the production of hydrogen sulphide is a natural by-product of yeast metabolism, albeit in very small quantities, perceptible levels should not be detectable in the finished product.
Causes of elevated levels of Hydrogen Sulphide include stressed yeast from low available nitrogen during fermentation and/or lack of oxygen, grapes that have been over-treated with sulphur-based products, leaving the wine (or beer) on the lees (sediment) too long, or yeast choice.
Yeast under nutritional stress during primary fermentation will produce perceptible levels of H2S. Yeast requires nitrogen and oxygen to conduct a healthy fermentation. Higher gravity musts or worts create more challenging fermentation conditions for the yeast. Beers with an original gravity above 1.070 require additional yeast nutrient and oxygen as well as a higher yeast cell count when inoculated. This is also true for a must with a starting gravity of 1.100 or higher. The addition of diammonium phosphate in the form of Yeast Nutrient, Yeast Energizer or Fermaid K to all wine, mead and cider must is recommended as well as high gravity worts. This supplies the yeast with ammonia which gives the yeast a nitrogen source. Do not add diammonium phosphate after the gravity reaches 1.015-1.020 as the yeast can no longer take it up due to alcohol toxicity.
Although all yeast produces hydrogen sulphide during fermentation, some strains like Montrachet and lager yeast are predisposed to produce higher levels. Others strains like Lalvin 71B 1122 are low sulphur producers.
It is important to make sure there is enough dissolved oxygen in wine, mead, cider and beer. With insufficient oxygen yeast will become stressed producing off flavors and odors and possibly not completing fermentation before flocculating.
During primary fermentation, the top half of wine, mead and cider should be stirred daily to volatize the sulphur. Wine and cider should be racked from the primary off the sediment (lees) when the gravity reaches 1.010-1.020 not allowing the lees to form a deep compact layer because free sulphur becomes trapped in the lees of new wines and is readily reduced to hydrogen sulphide and a host of other sulphur based compounds. Lees lose their ability to reduce sulfur over time, so during the next month or two, weekly stirring is recommended.
Should you detect hydrogen sulphide in a very young wine that is still on the primary fermentation lees, rack off the lees vigorously splashing. If you’ve already done the initial racking and notice the hydrogen sulphide smell, rack from carboy to carboy a couple time aerating the must. The introduction of oxygen at this time is contrary to most practices, but the formation of hydrogen sulphide is a reductive process. Introduction of oxygen at this point shifts the reaction away from the formation of reductive compounds.
Catching the production of H2S early is fairly easy to reduce. Unfortunately, once the wine is more than a few weeks old, it is much more difficult to get rid of.