Glossary · Faults
Mercaptan
Also written thiol. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
A sulphur compound formed when hydrogen sulphide reacts with ethanol and other constituents, smelling of onion, rubber, garlic or drains. Far harder to remove than the hydrogen sulphide it comes from.
- Subject
- Faults
- Also written
- thiol
In more detail
The practical significance is timing: hydrogen sulphide caught during fermentation can be aerated or copper-treated away, whereas once it has reacted to mercaptans and then oxidised further to disulphides, ordinary treatment fails and the cider is usually blended off or lost.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
Hydrogen sulphide
A volatile sulphur compound produced by stressed yeast, smelling of rotten eggs at very low concentrations. In cider it is usually a symptom of nitrogen deficiency during fermentation.
Faults
Reduction
The accumulation of volatile sulphur compounds — hydrogen sulphide, mercaptans and disulphides — in an oxygen-starved ferment or maturing cider, giving rotten egg, drain and rubber aromas.
Sensory
Reductive
Showing sulphurous, struck-match, rubbery or drain-like character arising from a lack of oxygen during fermentation or lees ageing. The opposite condition to oxidative.
Fermentation
Racking
Transferring cider off its sediment into a clean vessel, leaving the lees behind. It is the basic maintenance operation of the cellar and the traditional method of slowing a fermentation.
Where to go next
- Cider glossary — The whole vocabulary, by subject and A–Z.
- Comparisons — Terms that are only really clear when set against the thing they are not.
- Search — Finds terms by their aliases and by their spellings in other languages.
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.
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.