Compound
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.
Also called Thiols, Ethanethiol, Methanethiol.
- Class
- Sulphur compounds
- Formula
- Not a single molecule — see below
- How often it matters
- Occasional
What it does in cider
- Forms when hydrogen sulphide reacts with ethanol and with other cider components, or directly from the degradation of sulphur-containing amino acids.
- Contributes onion, garlic, cooked cabbage and burnt rubber aromas at extremely low concentration.
- Can be partly removed by copper while still in the thiol form, at the cost of stripping desirable aroma alongside.
- Oxidises to disulphides that copper does not touch and that can revert to mercaptans later in the bottle, producing a fault that reappears after apparently being cured.
How it is perceived
What the compound registers as, and at roughly what concentration. Perception is not a property of the molecule alone: sugar, tannin and carbonation all change where a threshold falls.
- onion
- garlic
- cooked cabbage
- burnt rubber
- drains
Perceptible at concentrations comparable to hydrogen sulphide — the low micrograms per litre — and generally described as more objectionable and more persistent than the parent gas.
Descriptors it is responsible for
Sensory records that name Mercaptans as a cause. Each states the perception and the mechanism behind it.
- Drains — A heavy, stagnant, sewer-like note from mercaptans formed when hydrogen sulphide is left to react.
- Flinty — A sub-threshold reductive note read as struck flint rather than as sulphur, the mildest state of reduction.
- Grapefruit — A bitter-edged citrus note from volatile thiols released by yeast, distinct from acid-driven lemon character.
- Lightstruck — A skunky, sulphidic note produced when light degrades riboflavin in clear or green glass.
- Passion fruit — A penetrating tropical-sulphur note from volatile thiols released by yeast β-lyase activity.
- Rubber — A burnt-rubber note from disulphides and higher mercaptans, generally a late stage of reductive fault.
- Wet stone — A cool, damp, faintly mineral impression, in cider a product of low-level sulphur compounds and high acidity rather than of minerals.
- Blackcurrant — A dark, slightly green berry note from sulphur-containing thiols, or from direct fruit addition.
- Garlic — A pungent allium note from specific thiols and disulphides, occasionally from orchard spray residues.
- Nettle — A sharp green-vegetal note with a faint sulphurous edge, met in early-picked and reductively handled cider.
The structure it moves
| Dimension | What it is |
|---|---|
| Fermentation character | Aromas made by the ferment rather than carried in from the fruit. |
| Freshness | Whether the drink smells and tastes of live fruit or of time and air. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
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.
Tank maturation
Resting cider in stainless steel or a lined vessel, where the point of the container is that it contributes nothing and admits almost no oxygen.
Bottle maturation
What happens to a cider after it is sealed in glass — which for most ciders is slow decline rather than improvement, and saying so is more useful than implying otherwise.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Fining
Adding a reactive agent that binds a target colloid and carries it to the bottom, chosen according to whether the problem is tannin, protein or a phenolic taste fault.
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.
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.
Faults it is implicated in
Being implicated is not the same as being a fault. Several of the compounds on this site are ordinary constituents of a sound cider and define a named fault only above a concentration.
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.
Reduction
A closed, stale, slightly sulphurous character in cider held under strongly oxygen-free conditions, sometimes clearing with air and sometimes not.
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.
Atypical ageing
A cider that loses its fruit unusually early and develops a flat, faintly acrid or naphthalene-like character — a syndrome described in white wine and less firmly established in cider.
Light strike
A skunky, cooked-cabbage or drain-like aroma produced when light acting on riboflavin generates sulphur compounds in a bottle.
Plastic and packaging taint
Plastic, rubber, adhesive or chemical notes picked up from unsuitable containers, liners, hoses, gaskets or cleaning residues.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
About Mercaptans
Mercaptans are the second act of a reduction fault. Hydrogen sulphide left in contact with a cider does not simply dissipate; it reacts, forming ethanethiol and methanethiol among others, and these smell of onion, garlic and rubber rather than of eggs. The change usually takes days to weeks, and it is the reason cellar advice about sulphide is always about acting quickly.
Removal is possible but imperfect. Copper reacts with thiols to form insoluble sulphides that settle out, and copper fining is the standard treatment. It is not free: copper also strips desirable volatile thiols and can leave residual metal that promotes later oxidation, and jurisdictions regulate how much may be used and how much may remain.
The harder problem is the disulphides. Oxidation joins two thiol molecules into a disulphide, which copper does not remove and which is often less obviously smelly, so a cider can appear to have recovered. Under the reducing conditions of a sealed bottle the reaction can run backwards, and the fault returns months later in a package nobody can treat. This is why reduction is described as a fault to prevent rather than to manage, and why prevention means nitrogen management during fermentation rather than intervention afterwards.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Sulphur compounds
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.
Sulphur compounds
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.
Nitrogen compounds
Yeast-assimilable nitrogen
The nitrogen a yeast can actually use, which apple juice is chronically short of — the shortage behind both stuck fermentations and rotten-egg aromas, and the shortage keeving deliberately makes worse.
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.
- 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 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 is yan and why does cider juice run short of it — Yeast assimilable nitrogen is the nitrogen yeast can actually use. Apple juice is usually short of it — in one Virginia survey of 108 samples, 94 per cent fell below the level wine practice treats as a minimum — which is why cider ferments stall more readily than wine ferments.
- Is cider vegan — Cider itself is a plant product, but some producers clarify it with animal-derived finings such as gelatine, isinglass or chitosan. Vegan status therefore depends on the fining regime, which is why some ciders are certified and others are not.
- Does cider get better with age — Some does. Tannic, dry, bottle-conditioned ciders and ice ciders can gain for several years. Light, fruit-driven and commercially filtered ciders lose their aroma and do not gain anything in exchange.
- 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
- All compounds — Grouped by what the fruit brings and what the ferment makes of it.
- Sensory — Every descriptor, with the compound or process that causes it.
- Microbiology — The organisms whose metabolism most of this chemistry belongs to.
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.