🍷 Reduction (wine)
Also known as: reduction, reductive aromas, reduced wine, sulfur smell, hydrogen sulfide in wine
Reduction is a wine fault in which volatile sulfur compounds give smells of rotten egg, onion or cooked cabbage.
Reduction is a wine fault in which the wine smells of rotten egg, onion, cooked cabbage or burnt rubber. The culprits are volatile sulfur compounds: hydrogen sulfide and the mercaptans, also called thiols. Human noses pick them up at tiny doses.
The Australian Wine Research Institute (AWRI) lists the thresholds. Hydrogen sulfide smells of rotten eggs from about 1 to 2 micrograms per litre. Ethanethiol, which reads as onion and rubber, shows from 1.1. Methanethiol, rotten egg and cabbage, from 0.02 to 2. Dimethyl disulfide, cooked cabbage, needs 29. Oddly, hydrogen sulfide just under its threshold may even add a little complexity.
Yeast makes the first sulfur compounds. The strain matters a lot, and so does a shortage of the nitrogen yeast can use, especially early in fermentation. So winemakers deal with it during the ferment, most often by adding copper sulfate, which takes away the hydrogen sulfide smell. Red ferments can be aerated instead, but once mercaptans are present, air turns them into disulfides, and the fix gets harder.
AWRI also warns against copper as a last-minute habit. If a wine shows no reductive character, there is no reason to add copper before bottling. Copper added at bottling was linked to more hydrogen sulfide and methanethiol appearing during bottle ageing.
Reduction is close to the opposite of oxidation: not too much oxygen, but sulfur compounds that never cleared. And unlike volatile acidity, which smells of vinegar, it smells of sulphur. All three sit under wine faults.
Facts
| Taste | Hydrogen sulfide: rotten egg (1 to 2 ug/L); ethanethiol: onion, rubber (1.1 ug/L); methanethiol: egg, cabbage 1 |
|---|---|
| Main use | Copper sulfate added during fermentation removes hydrogen sulfide odour 2 |
| Status | Hydrogen sulfide just below its threshold may add complexity; above it, a fault 1 |
How it connects to Wine
🍷 Reduction (wine) → then → 🍷 Racking → gives → 🍷 Wine
Links
- Next step
- 🍷 Racking
- Part of
- 🍷 Wine faults
Frequently asked questions
What is reduction in wine?
A fault where volatile sulfur compounds make the wine smell of rotten egg, onion, cooked cabbage or burnt rubber.
What causes the rotten egg smell in wine?
Hydrogen sulfide, detectable from about 1 to 2 micrograms per litre. Yeast makes it, and strain and low available nitrogen are key factors.
Why do winemakers add copper sulfate?
To remove the hydrogen sulfide smell during fermentation. AWRI discourages adding copper just before bottling to wines with no reductive character.
Is it true that a trace of sulfur smell is always a defect?
No. AWRI notes that hydrogen sulfide just below its threshold may add complexity.
Is reduction the same as oxidation?
No. Oxidation comes from too much oxygen and gives honey, nut and cider notes. Reduction is about sulfur compounds and gives egg, onion and cabbage.
Similar
🍷 Volatile acidity · 🍷 Acetic acid bacteria · 🍷 Cork taint · 🍷 Oxidation of wine · 🍷 Brettanomyces
Sources
- Australian Wine Research Institute. Recognition of wine faults and taints
- Australian Wine Research Institute. Copper and reductive sulfur compounds in wine