Reduction
Wine reduction is a common flaw that is often misjudged. It exists because of the presence of sulfur compounds. But is it always a fault or can it give some flavor and character to the wine?
Before answering this question, we must understand reduction. Reduction is caused by volatile sulfur compounds originating from various sources during the fermentation process. It can cause a real fault, the wine will smell of rotten eggs or backwater, but it can also produce certain desired flavors, such as gunflint or citrus zest… The subject is indeed controversial.
These volatile compounds are diverse and are the result of the fermentation process. The role of yeasts in the must is the transformation of sugar into alcohol, but, as it is a natural process, many other reactions occur, some are well known, and some are not well understood. They all produce various byproducts including volatile sulfur compounds. The vast majority of the volatile sulfur compounds are produced by yeast to build protein in their cells. But not all yeasts are equal, some are more prone to produce sulfur compounds than others.
This complex biologic process can be simplified like this, yeast needs to form proteins from amino acids in chemical precursors, but if there is a deficiency of precursors, which is frequent, hydrogen sulfide is released. It will combine with other molecules and create volatile compounds.
Also, yeast can create special types of volatile compounds by reacting with the must. The compound family is named thiols and plays an important role in the taste of Sauvignon Blanc and other wines.
So, what are the most common volatile sulfur compounds? Here are a few, with associated tastes.
| compound | taste | Consequence |
|---|---|---|
| Hydrogen Sulfide | Rotten Eggs | It is a real fault, at high concentrations there is nothing that can be done |
| Dimethyl sulfite | Asparagus, cabbage | Contribute to the fruity flavor in young red wine and truffle flavor in old wine |
| Mercaptan / Methanethiol 3MHA and 3MH | Grapefruit and passion fruit | Very important for Sauvignon Blanc wine (Sancerre, Pouilly Fumé, or Marlborough) |
| Furfurylthiol mercaptan | Roasted coffee | In sweet wine, Bordeaux but also in Champagnes wine |
| Benzylmercaptan | Gunflint, flint, mineral, and smoke | Responsible for the mineral character in wines |
Adding a layer of complexity, the concentration of sulfur compounds is a significant factor. “The dose makes the poison,” said Paracelsus. It could not be more true for wine. A small amount of Hydrogen Sulfide (less than 1 mg per litter) will not ruin the wine, nobody will notice, and it could even participate in the personality of the wine. But if there is more; you can pour the wine down the sink.
Reduction is an important aspect of the taste of wine. Sulfur compounds can have positive effects. The best example is minerality, the taste of stone, flint, and smoke comes from Benzylmercaptan molecules and it’s an important part of the taste of Chablis and other chardonnay. Other reduction compounds are responsible for the nuttiness effect of chardonnay. The brioche character in aged Champagne and white wine from Bourgogne is a type of reduction coming from the dead yeast cells. But reduction can have positive effects on red wine too, it can enhance its fruitiness.
The concept of wine reduction is intriguing, it is both a fault and not a fault. Like Schrodinger’s cat paradox, it’s alive and it’s not alive until we taste the wine to know for sure. It question of context and concentration.