Glossary · Chemistry
Hydrogen sulphide
Also written H2S. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
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.
- Subject
- Chemistry
- Also written
- H2S
In more detail
Caught early, hydrogen sulphide can be removed by aeration during racking or by copper contact. Left alone in the presence of ethanol and oxygen it reacts to form mercaptans and disulphides, which smell of drains, rubber and cooked cabbage and are much harder to remove.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Chemistry
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.
Fault
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.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
YAN
The nitrogen a yeast can actually use — free amino nitrogen plus ammonium — expressed in milligrams of nitrogen per litre. Apple juice is frequently short of it.
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.
Faults
Mercaptan
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.
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.
Fermentation
Yeast nutrient
Nitrogen and micronutrients added to a must that cannot supply the yeast’s needs. Diammonium phosphate provides inorganic nitrogen; complex nutrients also supply amino acids, vitamins and sterols.
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.
Cornell Cider Research and Extension programme
Cornell University, School of Integrative Plant Science · university · passage verified 2026-08-24
Runs cultivar trials in New York State and publishes juice chemistry for European cider varieties grown in a North American climate — the single most useful counterweight to treating English figures as universal.