Compound
Hexanal
The green-apple aldehyde generated within seconds of breaking fruit tissue, which gives fresh juice its cut-grass smell and is largely converted away by the yeast.
- Class
- Aldehydes
- Formula
- C6H12O
- How often it matters
- Occasional
What it does in cider
- Is produced by the lipoxygenase pathway when cell disruption brings enzymes and membrane fatty acids together, so it appears at milling rather than in the intact fruit.
- Contributes the green, grassy, cut-apple aroma of fresh juice.
- Is reduced by yeast to hexanol during fermentation, so most of it does not survive into the finished cider.
- Serves as the precursor of hexyl acetate, transferring some of the fresh-fruit character into a form that does survive.
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.
- cut grass
- green apple
- leafy
- green
A potent green-note contributor in juice; in finished cider it is normally present well below the level at which it would be identified, having been reduced during fermentation.
Descriptors it is responsible for
Sensory records that name Hexanal as a cause. Each states the perception and the mechanism behind it.
- Fresh pear — Clean, watery, faintly green pear aroma, the base fruit note of perry and of pear-fruited cider.
- Fresh-cut apple — The smell of a cut apple surface, produced within seconds of the fruit tissue being broken by six-carbon aldehydes released from membrane fats.
- Cooked apple — The soft, sweet, aldehyde-stripped apple character of heated fruit or heated juice, the base note of the whole cooked family.
- Pressed apple juice — The full, sweet, unfermented smell of fresh juice, met in cider where residual sugar and surviving fruit esters combine.
- Apple pomace — The wet, sweet-sour smell of freshly milled and pressed pomace: cut-apple aldehydes over the first traces of wild fermentation and browning.
- Cut grass — A sharp green note from cis-3-hexenol and cis-3-hexenal, the same green-leaf volatiles released by mown grass.
- Elderflower — A green-tinged floral note from terpene alcohols and cis-rose oxide, or from direct botanical addition.
- Stewed apple — Soft, wet, low-definition cooked fruit without browning, from prolonged gentle heat or long warm storage.
- Melon — A soft, watery, faintly green tropical note from hexyl esters and residual nine-carbon aldehydes.
The structure it moves
| Dimension | What it is |
|---|---|
| Fruit character | How strongly the drink smells of apple or pear, and of fruit generally. |
| 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.
Milling
Reducing whole fruit to a pulp so that the press has cell walls it can drain, rather than intact apples it can only bruise.
Maceration
Holding milled pomace before pressing so that phenolics, aroma precursors and pectin have time to move out of the solid tissue and into the juice.
Pressing yield
The proportion of a fruit charge recovered as juice, and why a single headline figure for it is not a meaningful number.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Wild fermentation
Letting the microflora already present on the fruit and in the press house carry the ferment, in a succession rather than as a single organism.
Inoculated fermentation
Starting a ferment by pitching a chosen yeast culture so that one known strain, rather than the fruit’s resident population, does the work.
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.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
About Hexanal
Cut an apple and it smells green within a second or two. That smell is not released from the fruit; it is manufactured on the spot. Breaking cells brings lipoxygenase and hydroperoxide lyase into contact with linoleic acid from the membranes, and the enzymes cleave the fatty acid into six-carbon fragments, hexanal among them. The whole sequence takes seconds, which is why a mill produces an aroma that a whole apple does not have.
How much is made depends on how the fruit is handled. Fine milling and long maceration produce more; a gentle press of coarse pomace produces less. Since the same conditions govern phenolic extraction and press yield, the green-note intensity of a fresh juice is partly a record of how hard the fruit was worked.
Little of it survives fermentation. Yeast reduces aldehydes to the corresponding alcohols, so hexanal becomes hexanol, which is far less aromatic. Some of the hexanol is then esterified to hexyl acetate, and it is that ester rather than the aldehyde which carries fresh apple-skin character into the finished cider.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Aldehydes
trans-2-Hexenal
The sharper, more penetrating green aldehyde of the same pathway as hexanal, made from linolenic rather than linoleic acid, and a marker of underripe fruit.
Esters
Hexyl acetate
The apple-skin and pear ester that is genuinely carried in from the fruit rather than made by the yeast, and one of the few fresh-fruit aromas that survives fermentation.
Alcohols
Higher alcohols
The group of larger alcohols yeast makes from amino acids, welcome as background complexity in trace and harsh and solvent-like in quantity.
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 does cider ferment — Yeast consumes the sugar in apple juice and produces roughly equal masses of ethanol and carbon dioxide, along with the esters and higher alcohols that give cider much of its aroma. In a wild ferment several yeast species take turns before *Saccharomyces* finishes the job.
- Can you make cider from shop bought apple juice — Yes, provided the juice contains no preservative — check for potassium sorbate or benzoate on the label. Pasteurised juice ferments perfectly well once yeast is added, because pasteurisation removes the organisms but not the sugar.
- How are apples milled for cider
- What is pressing yield — Pressing yield is the proportion of fruit weight recovered as juice. It varies from under half on a poorly loaded basket press to around seventy per cent on a well-run rack-and-cloth or belt press.
- What are fusel alcohols in cider — They are the higher alcohols yeast makes from amino acids rather than from sugar, mostly amyl alcohols. They carry weight and warmth of aroma, and in Asturian cider they run to a couple of hundred milligrams per litre.
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.
Institut national de recherche pour l’agriculture, l’alimentation et l’environnement (INRAE)
INRAE · research institute · retrieved 2026-08-24
French national agricultural research. Its Angers programme produced much of the published work on apple procyanidin chain length and on the relationship between polymer size, bitterness and astringency.
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.