H2S, Mercaptans, and Di-sulphide
Before discussing these volatile sulphur compounds we must first become familiar with the term ‘reduction’. The chemical explanation of reduction refers to the gain of electrons, and oxidation is the loss of electrons.
You have probably heard the word ‘reduction’ as a negative aromatic quality or described as a wine fault. When a wine is described as being ‘reduced’ it usually means it has above-threshold levels of volatile sulfur compounds. But not all reduction is bad, and reductive wine-making is not the same as reduction….
Positive reduction aromas
As described in my last blog-post, the production of volatile sulphur compounds (or ‘thiols’) such as 4MMP, 3MH, and 3MHA have a desirable effect of the aroma of a wine. Other volatile sulphur compounds that produce ‘flinty’ aromas can also be desirable at low levels.
Reductive vs oxidative wine-making
Reductive wine-making simply refers to wine-making in the absence of oxygen. For example, Sauvignon blanc is typically fermented in steel tanks, protected with CO2 and kept in cool temperatures.
Oxidative wine-making refers to wine-making in the presence of oxygen. For example, Chardonnay typically undergoes secondary fermentation in barrel, allowing micro-oxygenation.
H2S- Hydrogen Sulfide
H2S is formed during fermentation, and is often described as having the distinct smell of rotten eggs.
The formation H2S is mainly due to the following:
- Alcoholic fermentation– yeast produce H2S when they use one of the sulfur-containing amino acids as a nitrogen source. If a ferment is stressed there is more likelihood of H2S formation.
- Elemental sulphur– applied to the vineyard to control powdery mildew, if there are residues of elemental sulphur in the ferment it can cause yeast to convert S (elemental sulphur) into H2S.
During fermentation H2S can be removed by liberating the H2S through sparging with oxygen. Application of nutrients, such as DAP, can reduce H2S formation. H2S has a low sensory threshold, meaning that if H2S is not controlled during fermentation it will mask other aromas in the wine.
Mercaptans & Disulfides
As talked about in my previous blog-post, H2S plays a role in the formation of other volatile sulphur compounds.
A reaction between H2S and other compounds in the wine can result in the formation of mercaptans. Mercaptans cause aromas such as ‘cooked cabbage’, ‘burnt rubber’ and ‘onion’.
Mercaptans are also undesirable as they can rapidly oxidise into disulfides. These disulfides result in more undesirable aromas in wine (see image).
Mercaptans can be removed from wine through the application of copper sulphate. A process in which copper sulphate reacts to form copper sulphide, which is highly insoluble and odour less.
Disulfides however, do not respond to copper sulphate. The removal of disulfides requires first reducing it to it’s mercaptan form through time-consuming methods such as applications of ascorbic acid and SO2, once reduced to mercaptan form it can then be treated with copper sulphate.
Due to the difficulties of removing these volatile sulphur compounds at their mercaptan and disulfide forms, it becomes clear how important it is to either prevent or treat H2S during fermentation.
References:
Goode, J. (2007). Mercaptans and other volatile sulfur compounds in wine. Wineanorak. https://www.wineanorak.com/mercaptansinwine.htm
Volatile Sulphides: Detection and Prevention. (2015). ETS Labs. https://www.etslabs.com/library/31#:~:text=Volatile%20sulfur%20compounds%20can%20contribute,effective%20treatment%20and%20prevention%20plan.