Fault
Mercaptan taint
Onion, garlic, burnt rubber and cooked-cabbage aromas from thiols and disulphides formed when hydrogen sulphide is left in cider long enough to react onwards.
Also called thiol taint, onion and rubber character.
- Severity
- Serious — the batch may not be salvageable
- Where it starts
- Fermentation, Maturation, Storage
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Fermentation, Maturation
- Signs
- 5 recorded
What it is
Mercaptans, more properly thiols, are sulphur compounds that form from hydrogen sulphide during and after fermentation. Where hydrogen sulphide smells of rotten eggs and is volatile enough to blow off, mercaptans smell of onion, garlic, drains, burnt rubber and cooked cabbage, and they are stable. They are the reason the advice about hydrogen sulphide is always to deal with it immediately: this is what it becomes if it is left.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the nose
- A raw onion or crushed garlic smell in an otherwise clean cider
- Burnt rubber or a struck tyre note
- Boiled or rotting cabbage, particularly on a cider that has been in bottle some time
- A stale, sewage-like note that does not disperse when the glass is swirled — Persistence on swirling is the practical way to separate this from simple hydrogen sulphide, which lifts away.
How the batch behaves
- A cider that seemed only faintly sulphurous in tank and is markedly worse months later in bottle
The words for it: Drains, Rubber, Garlic. Each links to what produces it.
When it appears. Weeks after the hydrogen sulphide that produced it, usually during maturation on lees. By the time it is recognisable, the reaction that formed it has already gone past the point where aeration helps.
Detection thresholds for the thiols involved are extremely low, and the smell persists in the glass rather than blowing off — which is itself the practical test that separates it from a reductive character that will lift.
What it is mistaken for, and how to tell
One observation per rival, chosen because it separates them rather than because it describes either. Several of these are not faults at all — a style feature, a normal outcome, or the fruit behaving as it does.
- Hydrogen sulphide — Rotten egg is sulphide and is treatable by racking with air; drains, rubber, garlic and onion are mercaptans and are not. This is the single most consequential distinction in the sulphur family, because it decides whether the batch is recoverable.
- Reduction — Reduction lifts when the cider is poured hard or left in the glass; mercaptan character does not.
- Plastic and packaging taint — A rubber note from a mercaptan sits with other sulphur character; a rubber note from a hose or seal arrives alone and can be traced to a component.
What is actually happening
The chemistry or microbiology behind it. Understanding the mechanism is what makes the prevention make sense rather than being a list of rules.
Hydrogen sulphide left in cider reacts with ethanol and with other alcohols and carbonyl compounds to give simple thiols, chiefly ethanethiol and methanethiol. These have very low odour thresholds and are far less volatile than hydrogen sulphide, so they cannot be removed by the racking that would have dealt with the parent compound.
Thiols oxidise readily to disulphides — diethyl disulphide and dimethyl disulphide — which smell of cooked vegetables and rubber. The reaction is reversible: under the reducing conditions inside a sealed bottle, disulphides can be reduced back to the more strongly smelling thiols. This is why a cider can pass inspection at bottling and be unmistakably faulty a year later, and why the problem sometimes appears to come and go.
A third group, the sulphides proper, includes dimethyl sulphide, which arises from precursors such as S-methylmethionine carried in from the fruit rather than from the sulphate reduction pathway, and which smells of cooked sweetcorn or tinned vegetables. It behaves differently and is not removed by the same measures.
Yeast autolysis contributes. Cider held on a heavy yeast bed for months releases sulphur-containing amino acids and peptides as cells break down, providing further substrate for thiol formation in a low-oxygen environment.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Hydrogen sulphide produced during fermentation and not dealt with at the time | Fermentation | common |
| A nitrogen-deficient ferment that ran under stress from the start | Fermentation | common |
| Extended ageing on heavy lees without racking or stirring | Maturation | occasional |
| Strongly reducing conditions in a sealed vessel or bottle over long storage | Storage | occasional |
| Light exposure of bottled cider, which generates thiols by a separate routeThis is light-strike, and it produces similar-smelling compounds from riboflavin-mediated chemistry rather than from sulphate reduction. | Storage | occasional |
Can it be put right?
Most cider faults cannot be reversed. Where that is the answer it is given plainly — an honest 'this batch is what it is' is more useful than a procedure that will not work.
| Option | Effectiveness | What it involves |
|---|---|---|
| Accept the batch as lost and correct the nitrogen management next season | Reliable | The honest answer for most home producers. A cider with an established onion-and-rubber character is not going to be recovered in a domestic setting, and the value of the diagnosis lies entirely in the next harvest. |
| Copper fining | Unlikely to work | It is the recognised commercial treatment and it is not a home procedure. Copper additions are legally limited, residual copper is itself a taint and a powerful oxidation catalyst, and disulphides are not removed by copper at all unless first reduced. CiderHQ publishes no dose; a producer considering it should be working from a supplier specification and a laboratory result. |
| Blend into a much larger clean volume | Unlikely to work | Thresholds are so low that the dilution needed usually spoils more cider than it saves. |
| Blow the compounds off by racking or splashing | Not possible | They are not volatile enough. This works for hydrogen sulphide and does not work here; the attempt simply oxidises the cider. |
CiderHQ gives no additive dosage figures. An addition is a food-safety decision that depends on legal limits, on the juice in front of you and on what you are protecting against, and the right figure comes from your supplier’s or regulator’s own guidance rather than from a general reference.
Preventing it
- Treat hydrogen sulphide during fermentation, not after it; the whole prevention strategy for mercaptans is upstream.
- Address juice nitrogen before and early in fermentation rather than reacting later.
- Rack off gross lees on a sensible schedule instead of leaving cider on a deep yeast bed for many months.
- Give maturing cider a small, controlled amount of oxygen at racking where the style allows it, so conditions never become extremely reducing.
- Store bottles away from light, which prevents the parallel light-driven route.
Mercaptan taint is most usefully understood as hydrogen sulphide that was allowed to grow up. Everything about the fault — its persistence, its resistance to treatment, its habit of appearing months after bottling — follows from the fact that the volatile parent compound has been converted into stable derivatives.
The reversible relationship between thiols and disulphides explains an otherwise baffling pattern. A cider assessed at bottling may show little; in the reducing environment of a sealed bottle the disulphides are progressively reduced to thiols, and the fault emerges. A cider poured and left in the glass may then improve slightly as oxygen pushes the equilibrium the other way. Neither observation means the compounds have gone.
It matters that this is not an infection and not a hygiene failure. No spoilage organism is involved. The cause is nutritional stress in an ordinary fermentation, which is why the corrective work belongs at the juice stage of the following season and not in the cleaning cupboard.
For a home producer the realistic position is that this fault should be prevented rather than treated. The commercial treatment exists, but it involves adding a metal to a drink under laboratory control, and improvising it is a genuinely bad idea.
The compounds involved
Mercaptans
What hydrogen sulphide becomes if it is left alone: onion, garlic and rubber notes that are far harder to remove than the gas they came from.
Hydrogen sulphide
The rotten-egg gas a nitrogen-starved yeast produces, detectable at concentrations too small to measure easily, and removable only if it is caught before it becomes something worse.
Dimethyl sulphide
A sulphur compound that reads as cooked vegetable in quantity and adds an indefinable savoury depth in trace, and which builds slowly during storage rather than during fermentation.
Ethanol
The alcohol yeast makes from fruit sugar, which converts a perishable juice into a keepable drink and carries most of its aroma to the nose.
Amino acids
The largest usable nitrogen fraction in apple juice, and the raw material from which yeast builds both its own protein and most of the aroma compounds a cider carries.
The organisms involved
Where in the process it arises
Racking
Moving cider off the sediment it has thrown, which both cleans the liquid and — by taking yeast and nitrogen away with the deposit — slows what is left of the ferment.
Lees ageing
Deliberately holding a cider or perry on its fine yeast deposit so that autolysing cells release material that changes texture and foam behaviour.
Yeast nutrition
What a fermenting yeast population actually needs from apple juice — assimilable nitrogen, vitamins and membrane lipids — and what goes wrong when the juice cannot supply it.
Oxygen management in fermentation
Giving the yeast the oxygen it needs early to build viable membranes, then excluding it once fermentation is under way and especially once it slows.
Cellar storage
Holding packaged cider in conditions that let it change slowly and predictably, where stability of temperature matters more than the temperature itself.
Faults it is confused with
These present similarly. What separates them is set out on each page.
Hydrogen sulphide
A rotten-egg or drain smell from hydrogen sulphide produced by stressed yeast, usually the first visible consequence of a nitrogen-short juice.
Reduction
A closed, stale, slightly sulphurous character in cider held under strongly oxygen-free conditions, sometimes clearing with air and sometimes not.
Light strike
A skunky, cooked-cabbage or drain-like aroma produced when light acting on riboflavin generates sulphur compounds in a bottle.
Nitrogen deficiency character
The set of characters a nitrogen-starved fermentation produces together — sulphide, a stalled or dragging ferment, harsh higher alcohols and a thin, hard cider.
Where this comes up in a guide
The link lands on the step where the fault arises rather than at the top of the pathway, because that is where the decision that causes it is taken.
What people ask next
Questions readers ask about the things this page mentions. Each one goes to the section that answers it rather than to a page written to receive the question.
- How do i get rid of the sulphur smell in my cider — Racking with a little splashing usually blows off free hydrogen sulphide while it is still fresh. Once it has reacted into mercaptans the smell becomes rubbery and no longer responds to aeration.
- What does a nitrogen-starved cider taste like
- How many calories are in cider — Roughly 40 to 60 kcal per 100 ml for most ciders, so a UK pint falls somewhere around 200 to 250 kcal. Alcohol contributes about 7 kcal per gram and residual sugar about 4, so both strength and sweetness matter.
- Which yeast ferments cider — Saccharomyces cerevisiae finishes almost every cider fermentation, whether it was pitched or arrived from the press house. In a spontaneous ferment it is not the first organism present, only the one that survives the alcohol it makes.
- How should i store cider — Cool, dark and at a steady temperature. Light causes light-strike in clear glass, warmth accelerates oxidation, and temperature swings push cider past the closure of a bottle that is not tightly sealed.
- What is racking in cider making — Racking is siphoning cider off the sediment it has thrown into a clean vessel, leaving the lees behind. It clarifies the cider and, in traditional practice, is also used to slow a ferment by removing yeast with the lees.
Where to go next
- The fault finder — Describe what you can smell and see, and narrow it down from the signs.
- All faults — Grouped by where they come from and how serious they are.
- Cider science — The chemistry and microbiology these faults come out of.
Sources
What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.
The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Peer-reviewed literature on cider fermentation microbiology
Various journals · peer-reviewed literature · registered as competent for this subject
Covers the microbial succession of spontaneous cider fermentation, the role of non-Saccharomyces yeasts in the early stages, malolactic conversion by Oenococcus and Lactobacillus species, and the organisms behind the principal spoilage faults.
Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.