In winemaking, reduction is a term used to describe a wine fault characterised by the presence of unpleasant sulphurous aromas, such as rotten eggs or burnt rubber. A wine is “reduced” when it displays above-threshold levels of volatile sulphur compounds (VSCs) such as hydrogen sulphide, mercaptans, and disulfides. There are also some important and positive wine aroma compounds that are VSCs such as disulfides (such as dimethyl sulphide in red wines) and thiols.
Regarding reduction faults, the most famous of these is hydrogen sulphide (H2S). It is made by yeasts during fermentation when there is a lack of oxygen or nutrients for the yeast.
Excessive use of DAP (diammonium phosphate) has shown that it can cause the production of hydrogen sulphide (H2S) and other sulphurous compounds, which can cause wine to get reductive notes. To prevent wine reduction from excessive use of DAP, winemakers should measure the YAN (Yeast Assimilable Nitrogen) level before considering adding DAP and adjust the dosage of DAP accordingly if needed. In general, a YAN level of 140–220 mg/L is considered suitable for a healthy and complete fermentation without the need to add DAP.
Supplementation with organic nutrients like Natuferm products should be considered when DAP is not needed and you want to reach another level of yeast metabolism support, including the aroma quality provided by the yeast.
For the growth of yeast, adding oxygen is mandatory. When smelling reduction, racking with air or doing macro-oxygenation, which can be done for a longer or shorter amount of time depending on how strong the reduced taste is, can break down molecules like H2S, which smells like rotten eggs.
However, the action is ineffective on more complex compounds such as ethanethiol (the smell of onions) or methionol (the smell of cooked cabbage).
Worse, some foul-smelling monomer compounds become more stable as they oxidise. On the other hand, contact with oxygen can lead to a loss of positive varietal thiols or even lead to oxidation tastes. There is therefore a risk of obtaining a wine combining the two defects, oxidised and reduced, which are not incompatible.
Using yeast strains that are known to produce lower levels of H2S can help reduce the risk of wine reduction when you know you have already faced this issue and know your vineyard well.
It is an empirical observation that the addition of fresh yeast lees can have a positive effect on the wine in the event of reduction. Although the mechanisms are still unclear, the researchers observed that such a practice contributes to the elimination of heavy compounds (in particular methanethiol and ethanethiol). We have noticed that winemakers also use Extraferm D’tox as a fresh lees, especially when they want to avoid any cross-contamination of odours or spoilage. This is a fresh start with less instead of recycling, which can be risky.
Addition of copper sulphate or copper ions: these compounds can bind with the sulphurous compounds and reduce their concentration. However, it’s important to note that adding copper can also affect the flavour and colour of the wine, so it should be used with caution and only as a last resort.
With a highly qualified team of experts in many fields, Oenobrands strives to offer winemakers’ novel and scientifically sound solutions, as well as to highlight the positive synergies between its products.
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