Compound
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.
Also called (E)-2-hexenal, Leaf aldehyde.
- Class
- Aldehydes
- Formula
- C6H10O
- How often it matters
- Specialist — met in particular circumstances
What it does in cider
- Arises by the lipoxygenase pathway from linolenic acid when fruit tissue is broken.
- Contributes a sharper, more bitter green note than hexanal, closer to leaf and stem than to grass.
- Is higher in underripe fruit, whose membranes are richer in the precursor fatty acid, making it an indicator of early picking.
- Is reduced by yeast during fermentation, largely disappearing from the finished cider.
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.
- leaf
- green stem
- sharp green
- unripe
Perceived as harsher and more intrusive than hexanal at comparable concentration, and a recognised marker of green, underripe fruit character in fresh juice.
Descriptors it is responsible for
Sensory records that name trans-2-Hexenal as a cause. Each states the perception and the mechanism behind it.
- 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.
- Green apple — A harder, sharper apple note than fresh-cut apple, driven by (E)-2-hexenal and reinforced by high malic acid.
- Hay — A dry, sweet-grassy note from coumarin and degraded green-leaf volatiles, characteristic of traditional farmhouse cider.
- Crab apple — A small-fruit character combining very high acidity, hard green aldehydes and a marked phenolic grip.
- Cut grass — A sharp green note from cis-3-hexenol and cis-3-hexenal, the same green-leaf volatiles released by mown grass.
- Hedgerow — A mixed green-and-floral note combining leaf volatiles with faint blossom and a damp edge, common in wild-fermented cider.
- Unripe apple — A hard, starchy, low-aroma character from fruit milled before the climacteric, with acid high and esters not yet formed.
- Unripe pear — Hard, green, markedly astringent pear character from fruit pressed before it softened.
- Lime — A green, zesty citrus impression from very high malic acidity combined with the green aldehyde fraction.
- 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 |
|---|---|
| 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.
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.
Fruit storage before milling
Holding a gathered crop between harvest and the mill, and managing what respiration, water loss and spoilage do to it while it waits.
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.
Thin body
A cider with no weight or texture in the mouth, most often from over-watered pomace, low-gravity juice or a blend with nothing to give it substance.
Cooked character
A stewed-apple, caramel or jam-like character from heat applied to juice or cider, most often through pasteurisation or hot storage.
About trans-2-Hexenal
The lipoxygenase pathway acts on two membrane fatty acids and produces two different green notes. Linoleic acid gives hexanal, which reads as cut grass and fresh apple. Linolenic acid gives trans-2-hexenal, which reads as leaf and stem and is distinctly harsher — the smell of a torn leaf rather than a mown lawn.
Underripe fruit carries proportionally more linolenic acid, so juice pressed from fruit picked early is greener in this specific, unattractive way, and the difference is one of the sensory arguments for leaving cider fruit on the tree or in store until it is properly ready.
Like hexanal it is largely reduced by yeast during fermentation and is not usually a feature of finished cider. It is a juice compound, most relevant to anyone tasting through a pressing to judge the fruit.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Aldehydes
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.
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.
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.
- 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
- Why is my cider thin and watery — Thin body usually follows from dessert-fruit juice with little tannin, over-dilution, aggressive filtration, or a ferment that stripped everything out. Body in cider comes mostly from tannin, glycerol and residual sugar.
- How long can apples be kept before pressing
- What is maceration in cider making — Maceration is letting milled pomace stand before pressing, so that enzymes soften the tissue and phenolics and aroma move from the skins into the juice. It raises both colour and tannin, and raises the risk of oxidation with them.
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.
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.
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.