Mercaptans
Mercaptans are a more severe, less reversible cousin of reduction. Where mild struck-match aromas can often be blown off with a swirl or a decant, mercaptans are pungent sulphur compounds that linger and dominate the nose regardless of aeration.
What does it smell like?
Rotten egg, burnt rubber, garlic, or a skunky, pungent smell that dominates everything else in the glass.
Unlike mild reduction, mercaptans rarely blow off with swirling and are considered a genuine, persistent flaw.
Aroma Classification
Mercaptans sit outside the primary/secondary/tertiary framework, they form when hydrogen sulphide produced during fermentation reacts further under prolonged low-oxygen conditions.
Nature's Influence
Climate
Not climate-driven. Mercaptans form through nutrient-deficient fermentations and extended time on lees without protective racking or aeration.
Soil & Site
Not applicable.
Vintage
Not vintage-dependent.
The Winemaker's Hand
Harvest Timing
Not applicable.
Fermentation
Nitrogen-deficient musts are the leading cause, stressed yeast produce hydrogen sulphide, which can convert to mercaptans if left unaddressed.
Extraction
Not directly linked.
Maturation
Left unresolved after fermentation, hydrogen sulphide reacts with other wine compounds to form mercaptans, which are far harder to remove than the original sulphur smell.
Sealed tanks left unracked for long periods increase the risk of hydrogen sulphide converting to mercaptans.
Prolonged, unstirred lees contact without adequate oxygen is a common trigger.
Once formed, mercaptans are chemically stable and do not resolve with bottle age.
Prompt racking and copper fining shortly after fermentation are the standard preventative measures.
Blending
Not typically a blending consideration once mercaptans have formed.
Where You'll Find It
Typical Wine Styles
Can occur in any style where fermentation nutrition or post-ferment handling was inadequate.
Typical Regions
What Does It Tell You?
- A nutrient-deficient fermentation
- Inadequate racking after primary fermentation
- A persistent fault unlikely to resolve with air
This is one of the reasons winemakers monitor yeast nutrition closely during fermentation, well before the wine is finished.
Food Pairing
Similar Aromas
If I swirl or decant the wine, will the smell go away like reduction does?
Usually not. Mild reduction (struck match, flint) often clears with aeration, but true mercaptans are chemically more stable and typically persist regardless of how long the wine is exposed to air.
Learn to Recognise It
- Is there a rotten egg or garlic smell rather than a flinty, struck-match note?
- Does the smell persist after vigorous swirling or decanting?
- Does it get worse rather than better with air exposure?
- Is the aroma pungent enough to dominate the entire glass?
Persistence after aeration is the key test that separates mercaptans from ordinary, resolvable reduction.
Mercaptans show that some conditions, once left unattended, harden into outcomes that can no longer be undone. It is a reminder that timing matters as much as intention, the same issue caught early is easily corrected, but left too long becomes permanent.
The human nose can detect some mercaptan compounds at concentrations of parts per billion, they are among the most potent-smelling substances known, which is also why they are added to natural gas to make leaks detectable.