Anxious to check out the grape variety of your new bottle of wine, you pour, swirl and raise the glass to your nose expecting a fruity, spicy or perhaps a floral aroma. What you get though is an obnoxious smell, a disgusting odor similar to rotten eggs. Unfortunately, your wine has one of the undesired defects of reduction; you’re getting a stinky odor because it contains specific volatile compounds.

This unwanted aroma shows a lack of oxygen in the winemaking style: it is the opposite defect of oxidation when the wine has too much oxygen.

A scent of rotten eggs in wine can destroy the nose of a possible fine wine, but luckily winemakers can avoid and treat reduction. Rotten egg aroma should not be confused with the common problem of cork taint which is a more mustier smell and is caused by a fungus in the cork.

What causes rotten egg smell in wine?

The reduction flaw comes about because of two of the many sulfur contents in the wine, sulfur dioxide, (SO2) and hydrogen sulfide (H2S). What your nose has picked up is the colorless hydrogen sulfide gas (a mixture of sulfide and hydrogen) All wines contain the sulfide compound; it is a natural byproduct of fermentation in winemaking.

Winemakers have been using sulfur for centuries. Today, they spray vines with sulfur to protect the grapes from powdery mildew; they use sulfides in the form of sulfur dioxide to get rid of unwanted harmful microbes and bacteria during the fermentation stage. Important too is the preservation properties of sulfur dioxide, which allow the wine to keep its fresh flavor and helps prevent oxidation. The reduction problem of rotten egg smell in wine and in the must show the wine has been reduced; it has not been exposed sufficiently to oxygen.

Although it’s the winemaker’s job to make sure sulfur reduction does not have detrimental effects on the wine, bottled wine can also suffer from a lack of oxygen and air. Screw caps seem to be more affected by reduction as natural cork supposedly allows more oxygen to seep into the wine.

Removing the Rotten Egg Smell From Wine

Once detected during winemaking, it is essential to remove the hydrogen sulfide as soon as possible, if not, the gas will react with the ethanol in the wine to form mercaptans. These, in turn, will give rise to the second stage of reduction and yet another unwanted odor.

One solution employed by professional winemakers to remove the odor is the use of carefully measured small quantities of copper sulfate. The wine is stirred and left for a few days and checked again.

You can probably fix the problem at home by first exposing the wine to air. Mixing it with a spoon should resolve the issue; if not just drop a clean copper coin into the container. Copper will react with H2S to form copper sulfide.

Avoiding the Rotten Egg Smell in Wine

In winemaking, It’s almost impossible to do away with sulfur altogether. To limit the problems like rotten eggs, winemakers today employ the best practices such as:

  • Cultivating healthy untreated soil, which, in turn, will yield healthy organic grapes
  • Spraying their vines with sulfur a couple of weeks before harvest time or refrain from over treating them
  • Abstaining from excessive use of sulfite in winemaking.

The maximum sulfur doses stipulated by the European Union are the following:

  • § Red: 160 mg / l
  • § White / rosé: 210 mg / l
  • § Sweet : 400mg/l

Granted, sulfur compounds can have an adverse effect on the aroma of wine, but it’s important to consider the protective and conservative properties of sulfur.

Over the years winemakers have been reducing the sulfur content in wine not only in harvesting but also in the pressing, fermenting and bottling stages. To avoid reduction and the disgusting odor of rotten eggs depends a lot on the skills of the winemaker, his ability to balance oxidation and reduction