Compound
Acrolein
A sharp aldehyde made by lactic bacteria from glycerol, which reacts with tannin to produce an intense, lingering bitterness in cider that was sound when it was bottled.
Also called Prop-2-enal.
- Class
- Aldehydes
- Formula
- C3H4O
- How often it matters
- Occasional
What it does in cider
- Is formed when Lactobacillus collinoides and related bacteria dehydrate glycerol during long, warm storage.
- Combines with procyanidins to give products far more bitter than either compound alone.
- Produces a bitterness quite unlike tannin bitterness — intense, delayed and persistent — which is why the fault is not recognised as excessive tannin.
- Develops slowly and after packaging, so it appears in cider that passed every check at bottling.
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.
- intense bitterness
- acrid
- peppery
- lingering
The perceived fault comes from the reaction products with tannin rather than from acrolein itself, so a phenolic cider shows it far more severely than a low-tannin one at the same acrolein concentration.
Descriptors it is responsible for
Sensory records that name Acrolein as a cause. Each states the perception and the mechanism behind it.
- Peppery bitterness — A harsh, burning bitterness from acrolein formed when lactic bacteria degrade glycerol.
The structure it moves
| Dimension | What it is |
|---|---|
| Bitterness | A taste sensed at the back of the tongue, distinct from the drying grip of astringency. |
| Astringency | The drying, rough sensation left after swallowing. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Tank maturation
Resting cider in stainless steel or a lined vessel, where the point of the container is that it contributes nothing and admits almost no oxygen.
Bottle maturation
What happens to a cider after it is sealed in glass — which for most ciders is slow decline rather than improvement, and saying so is more useful than implying otherwise.
Cellar storage
Holding packaged cider in conditions that let it change slowly and predictably, where stability of temperature matters more than the temperature itself.
Lees ageing
Deliberately holding a cider or perry on its fine yeast deposit so that autolysing cells release material that changes texture and foam behaviour.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
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.
Pasteurisation
Heating cider enough to inactivate the organisms that would spoil it, either in the sealed package or in-line before filling, at a measurable cost in fresh aroma.
Racking
Moving cider off the sediment it has thrown, which both cleans the liquid and — by taking yeast and nitrogen away with the deposit — slows what is left of the ferment.
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.
Acrolein bitterness
An intense, lingering bitterness quite unlike tannin, produced when lactic acid bacteria convert glycerol to acrolein and the acrolein reacts with the cider’s phenolics.
Excessive bitterness
Bitterness that dominates the palate rather than supporting it, usually from a blend weighted too heavily towards high-tannin fruit or extracted too hard.
Lactic off-flavours
Sauerkraut, sour milk, silage or cheesy notes from lactic acid bacteria working on sugars and other substrates rather than on malic acid alone.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
Lactobacillus brevis
An obligately heterofermentative lactic acid bacterium, renamed *Levilactobacillus* in 2020, associated with volatile acidity, biogenic amines and mousiness rather than with clean malolactic conversion.
Lactiplantibacillus plantarum
A versatile lactic acid bacterium, renamed out of *Lactobacillus* in 2020, capable of malolactic conversion and of a range of faults depending on conditions.
About Acrolein
The bitterness fault that French cellar literature calls amertume has a specific chemistry behind it. Lactobacillus collinoides, a species particularly associated with cider, can dehydrate glycerol to 3-hydroxypropionaldehyde and onward to acrolein. Acrolein is small and highly reactive, and it does not stay as acrolein: it adds to the phenolic rings of procyanidins, and the resulting adducts are far more bitter than the tannin was on its own.
This explains the fault’s two puzzling features. It appears late, often a year or more after bottling, because the bacterial conversion and the subsequent reaction are both slow. And it hits tannic ciders hardest, because the bitterness comes from the reaction with procyanidins — a traditional bittersweet cider has plenty to react with, and a light low-tannin cider has little.
Since the fault develops after packaging, everything about managing it happens before. A cider that goes into bottle with a viable lactic population, glycerol to work on, tannin to react with, low free sulphur dioxide and a warm cellar to sit in has all the ingredients. Sulphite at a level appropriate to the pH, prompt racking, and filtration or pasteurisation where the product allows it, are the controls; there is no treatment once the bitterness has arrived.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Alcohols
Glycerol
A syrupy three-carbon alcohol yeast produces as a side reaction of fermentation, which adds weight to a dry cider and is the raw material for one of its more obscure faults.
Phenolics
Procyanidins
The condensed tannins of cider fruit, whose chain length — not their quantity — decides whether the mouth registers bitterness or astringency.
Acids
Lactic acid
The softer acid that replaces malic when malolactic fermentation runs, halving the acid a cider carries and changing its texture as much as its sharpness.
Phenolics
Total phenolics
The single number used to summarise everything phenolic in a juice, useful for comparing fruit and misleading whenever it is used to predict how a cider will taste.
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.
- What are procyanidins in cider — They are the condensed tannins of apples: chains of catechin-type units whose length decides how much of the phenolic load reads as bitterness and how much as astringency. Two ciders with identical total tannin can taste nothing alike.
- Why does cider contain lactic acid — Because malolactic bacteria converted the malic acid into it. Malic acid has two acid groups and lactic acid has one, so roughly half the titratable acidity disappears and the cider tastes softer.
- Does cider get better with age — Some does. Tannic, dry, bottle-conditioned ciders and ice ciders can gain for several years. Light, fruit-driven and commercially filtered ciders lose their aroma and do not gain anything in exchange.
- How should i store cider — Cool, dark and at a steady temperature. Light causes light-strike in clear glass, warmth accelerates oxidation, and temperature swings push cider past the closure of a bottle that is not tightly sealed.
- What is racking in cider making — Racking is siphoning cider off the sediment it has thrown into a clean vessel, leaving the lees behind. It clarifies the cider and, in traditional practice, is also used to slow a ferment by removing yeast with the lees.
- Does cider contain antioxidants — Cider carries apple polyphenols, and tannic bittersweet ciders carry considerably more than pale ones made from dessert fruit. What that means for health is a clinical question this site does not adjudicate.
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.
Peer-reviewed literature on cider fermentation microbiology
Various journals · peer-reviewed literature · registered as competent for this subject
Covers the microbial succession of spontaneous cider fermentation, the role of non-Saccharomyces yeasts in the early stages, malolactic conversion by Oenococcus and Lactobacillus species, and the organisms behind the principal spoilage faults.
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 Français des Productions Cidricoles (IFPC)
IFPC · research institute · retrieved 2026-08-24
The French technical institute for cider production. The authority for the French cultivar classification families, for keeving as an industrial process, and for the pectin and nitrogen chemistry that keeving depends on.