Sensory descriptor
Flinty
A sub-threshold reductive note read as struck flint rather than as sulphur, the mildest state of reduction.
Also called Struck flint, Reductive.
- Aroma family
- Sulphur
- Does it indicate a fault?
- Usually, yes
What causes it
At very low concentrations, benzenemethanethiol and related thiols produce a struck-flint or gunflint character rather than anything recognisably sulphurous, and they arise in closed, oxygen-deprived fermentations where yeast is under mild reductive stress. The state is genuinely different from a sulphide fault in degree rather than in kind: it is the same pathway operating at a level where the products read as mineral. It is characteristic of ciders fermented and stored under strict oxygen exclusion, and it is favoured by high sulphite and low temperature.
This is the state below rotten egg and above nothing at all. Traditional practice in some categories tolerates or even seeks it, and a small aeration will remove it. Left in a reductive environment it does not stay where it is.
The compounds responsible
Where the molecule is established. Some descriptors in this lexicon name a compound the compound register does not yet hold a record for; those are named in the cause above instead.
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.
Sulphur dioxide
The antimicrobial and antioxidant on which most modern cidermaking depends, and whose effectiveness collapses as pH rises — which makes every sulphiting decision a pH decision first.
Dissolved oxygen
Essential to a healthy yeast population at the start of fermentation and the principal enemy of a cider from the moment fermentation ends.
What raises it
Processes that increase this character, or preserve it where it would otherwise be lost.
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.
Sulphiting
Adding sulphur dioxide to juice to suppress spoilage organisms and oxidative browning, at a level that only means anything once the juice pH is known.
Cold maturation
Holding cider at low temperature so that it settles bright, sheds colloidal material and stops changing microbially, without any additive or treatment being applied.
Sterile filtration
Membrane filtration at a rating that retains viable yeast and bacteria, which is what allows a sweet cider to be shelf-stable without heat or preservative — provided the filling that follows is aseptic.
The fault it points at
Meeting this descriptor above a certain intensity is how the fault below is usually noticed first.
Fault
A closed, stale, slightly sulphurous character in cider held under strongly oxygen-free conditions, sometimes clearing with air and sometimes not.
Where it is usually met
Fruit and styles this character is recorded in. A record here is where the descriptor is commonly found, not a claim that every example shows it.
About Flinty
Flinty is included because reduction has a mild state that panels genuinely encounter and that the fault vocabulary does not cover: calling it hydrogen sulphide overstates it and calling it wet stone hides its cause.
It responds immediately to oxygen. A cider showing it that opens up over ten minutes in the glass has confirmed the diagnosis.
Others in the sulphur family
Rotten egg, struck match, drains, rubber, cooked cabbage.
Rotten egg
Hydrogen sulphide from yeast under nitrogen stress — the commonest sulphur fault and the most correctable.
Drains
A heavy, stagnant, sewer-like note from mercaptans formed when hydrogen sulphide is left to react.
Struck match
The sharp, mineral, matchhead note of free sulphur dioxide above its perception threshold.
Cooked cabbage
A vegetal sulphur note from dimethyl sulphide, rising slowly during storage rather than during fermentation.
Lightstruck
A skunky, sulphidic note produced when light degrades riboflavin in clear or green glass.
Rubber
A burnt-rubber note from disulphides and higher mercaptans, generally a late stage of reductive fault.
Garlic
A pungent allium note from specific thiols and disulphides, occasionally from orchard spray residues.
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.
- Does cider contain sulphites — Most does. Sulphur dioxide is added to protect juice and finished cider, and fermentation itself produces a small amount even when none is added. European labels must declare it above 10 mg per litre.
- Why is sulphite added to cider — Sulphur dioxide suppresses spoilage bacteria and wild yeast before fermentation and protects the finished cider from oxidation. How much of it is active depends strongly on pH, which is why low-acid juice is harder to protect.
- Does cider need oxygen to ferment — Yeast needs a little oxygen early on to build the sterols it uses to stay healthy, but the ferment itself is anaerobic. After the first days oxygen is a liability, because it feeds acetic acid bacteria and oxidises the cider.
- Does carbonation change how cider tastes — Yes. Dissolved carbon dioxide forms carbonic acid, so a sparkling cider tastes sharper than the same cider still, and the bubbles carry aroma upwards and scrub fat from the palate.
- Why does my dry cider still taste sweet — Alcohol, glycerol and ripe fruit aroma all read as sweetness even when no sugar remains. High alcohol in particular is perceived as sweet before it is perceived as strong.
- Pasteurising or sterile filtering — what is the difference
Where to go next
- The sensory model — The thirteen structural dimensions and the twenty aroma families, in full.
- Compounds — The chemistry behind the words.
- Troubleshooting — When a perception turns out to be a problem, this is where it is diagnosed.
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.
Washington State University cider research programme
Washington State University Northwestern Washington Research and Extension Center · university · passage verified 2026-08-24
The single most useful open cider dataset CiderHQ has found. The programme’s cultivar performance database gives juice chemistry, orchard behaviour, bloom and harvest timing and cider-maker tasting notes for 73 cultivars grown at one maritime site over fifteen years, and — unusually — publishes its classification thresholds alongside its figures, so the classification can be checked rather than taken on trust. It also states outright that its results diverge from the English ones. Read in full on 2026-08-24 and transcribed into `data/trials/wsu-mount-vernon.ts`; four published figures were found to be impossible and are withheld there with their reasons.
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.