Fault
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.
Also called amertume, bitterness fault, acrolein taint.
- Severity
- Serious — the batch may not be salvageable
- Where it starts
- Maturation, Blending, Fermentation
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Maturation, Storage
- Signs
- 5 recorded
What it is
Acrolein bitterness — the fault French cidermakers call amertume — is a bacterial spoilage in which glycerol produced during fermentation is metabolised by certain lactic acid bacteria into acrolein. Acrolein itself is pungent and irritating; more importantly it reacts with the phenolic compounds already present in cider to form intensely bitter adducts. The result is a bitterness that is harsher, more persistent and quite differently shaped from ordinary tannin bitterness, and it appears in a cider that was previously balanced.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the palate
- A harsh, scraping bitterness that arrives late and stays for a long time
- Bitterness quite unlike the fruit tannin the cider had before — Tannin bitterness comes with astringency and a drying grip; this arrives without them.
- Bitterness developing alongside a rise in lactic character or a loss of clarity
On the nose
- A pungent, slightly acrid note in the nose in a strongly affected sample
How the batch behaves
- A cider that has become steadily more bitter over months in store
The words for it: Peppery bitterness. Each links to what produces it.
When it appears. Develops in a finished cider containing glycerol, over months, and is usually attributed to age or to tannin before it is correctly identified.
A bitterness that was not present when a cider was young and has appeared since is the single most useful pointer to this fault, because tannin-derived bitterness moves in the other direction with age.
Various journals, Andrew Lea
What it is mistaken for, and how to tell
One observation per rival, chosen because it separates them rather than because it describes either. Several of these are not faults at all — a style feature, a normal outcome, or the fruit behaving as it does.
- Excessive bitterness — Bitterness from tannin arrives with astringency and a drying texture; acrolein bitterness is peppery and sharp and comes without the drying.
- Excessive astringency — Ask whether the tongue is being dried or the throat is being caught. Acrolein catches.
- A tannic cider from bittersweet fruit — Whether the bitterness was there when the cider was young. Acrolein bitterness appears later, in a cider that had none.
What is actually happening
The chemistry or microbiology behind it. Understanding the mechanism is what makes the prevention make sense rather than being a list of rules.
Glycerol is a normal and substantial by-product of alcoholic fermentation, contributing to a cider’s body. Certain lactic acid bacteria, above all Lactobacillus collinoides in cider and some Lactobacillus brevis strains, possess glycerol dehydratase, which converts glycerol to 3-hydroxypropionaldehyde. Under acidic conditions and over time, that intermediate dehydrates to acrolein.
Acrolein is an alpha,beta-unsaturated aldehyde and is highly reactive. In cider it undergoes nucleophilic addition with the phenolic compounds present, principally the flavan-3-ols epicatechin and catechin and the procyanidins built from them. The resulting adducts are intensely bitter and are not present in unaffected cider at all.
This is why the fault is worse in traditional bittersweet cider than in a low-tannin one. The bitterness produced is proportional to the phenolic material available to react, so exactly the fruit that makes a fine West Country cider provides the substrate that makes the fault severe. A low-phenolic cider suffering the same bacterial activity is far less affected.
The reaction continues in store, since the intermediate persists and converts slowly. A cider can therefore leave the cellar tasting acceptable and become markedly bitter months later, which is one of the harder things about the fault.
How it happens
| Cause | Stage | How often |
|---|---|---|
| High pH from a blend built entirely on low-acid bittersweet fruit | Blending | common |
| Lactobacillus collinoides active in a cider with little or no free sulphur dioxide | Maturation | occasional |
| Extended maturation in wood or in tank at cellar temperature without protection | Maturation | occasional |
| A vessel or cellar carrying a resident population between seasons | Maturation | occasional |
| A ferment that produced a high glycerol level, supplying more substrate | Fermentation | rare |
Can it be put right?
Most cider faults cannot be reversed. Where that is the answer it is given plainly — an honest 'this batch is what it is' is more useful than a procedure that will not work.
| Option | Effectiveness | What it involves |
|---|---|---|
| Treat the batch as lost and correct the blend pH next season | Reliable | The honest answer once the bitterness is established. The fault is a consequence of a cider that was too low in acid to defend itself, and that is fixed at blending, not in treatment. |
| Stabilise to stop the reaction progressing | Partial | Halting the bacteria stops further acrolein being formed. It does not remove the intermediate already present, so some further bittering can still occur. |
| Fine to reduce bitterness | Unlikely to work | Phenolic-binding finings lower general bitterness slightly, taking body and structure with them. Bench trial will usually show the cost exceeds the benefit. |
| Blend into a much larger volume of sound cider | Unlikely to work | Bitterness of this kind is perceptually persistent and does not dilute as cleanly as an aroma fault. |
| Remove the bitter adducts from finished cider | Not possible | They are stable phenolic derivatives. Nothing available removes them selectively. |
CiderHQ gives no additive dosage figures. An addition is a food-safety decision that depends on legal limits, on the juice in front of you and on what you are protecting against, and the right figure comes from your supplier’s or regulator’s own guidance rather than from a general reference.
Preventing it
- Get the pH down through blending; low-acid bittersweet juice alone is the standing risk factor.
- Maintain free sulphur dioxide appropriate to the measured pH, from supplier guidance and within legal limits.
- Keep maturation cool and vessels clean, and retire wood that has produced the fault before.
- Where malolactic fermentation is wanted, manage it deliberately with a known culture rather than leaving it to whatever the cellar contains — the useful organisms and the harmful ones are close relatives.
- Taste through storage, because the fault develops over months and is much easier to act on early.
Acrolein bitterness is the fault that explains why traditional cidermaking has always cared about acid as well as about tannin. A blend of pure bittersweet juice is a drink with high pH, ample phenolic substrate and a supply of glycerol — which is to say, everything Lactobacillus collinoides requires and none of the defence that would restrain it.
It is also unusually cruel in its timing. Acrolein forms from an intermediate that persists in the cider, so the bitterness continues to develop after the bacteria have been dealt with and after the cider has been bottled. A batch judged sound at packaging can turn in the bottle over the following year.
The bitterness itself is worth learning to recognise, because it is not tannic. Tannin bitterness arrives with astringency, with a drying, gripping sensation on the gums, and it is part of a structure. This bitterness arrives without that structure, later in the palate, and it goes on far longer than it should. A cider that has become bitter without becoming more astringent is describing this fault.
French cider literature has treated amertume as a named condition for a long time, which is a reasonable indication of how seriously it is taken in a tradition built on low-acid fruit. Anyone making cider from bittersweets alone should treat pH as a control point rather than an outcome.
The compounds involved
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.
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.
Procyanidins
The condensed tannins of cider fruit, whose chain length — not their quantity — decides whether the mouth registers bitterness or astringency.
Epicatechin
The flavan-3-ol that apple procyanidins are almost entirely built from, and the most bitter of the phenolic monomers a cider contains.
Catechin
The minor flavan-3-ol of apple, present largely as the terminal unit of procyanidin chains, and consequently a useful analytical handle on chain length.
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.
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.
The organisms involved
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.
Where in the process it arises
Malolactic fermentation
A bacterial conversion of sharp malic acid into softer lactic acid, which lowers acidity, changes aroma, and in most traditional cider happens whether it was planned or not.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Acid balancing
Bringing a cider to the sharpness it needs, which requires separating perceived sharpness from titratable acidity from pH — three related things that do not move together.
Blending
Combining separate lots of cider or perry into one, which in cider is the historically normal way of making the drink rather than a remedy applied when single lots disappoint.
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.
Faults it is confused with
These present similarly. What separates them is set out on each page.
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.
Ropiness
Cider that pours thick and pulls into an oily thread, caused by lactic acid bacteria secreting long-chain glucan polymers into the drink.
Low acidity
A cider without enough acid to give it definition, tasting soft, heavy and dull — and sitting at a pH that leaves it exposed to spoilage organisms.
Mousiness
A retronasal taint of mouse cage, stale popcorn or crackers that appears only after swallowing — and that a substantial fraction of people cannot detect at all.
Where this comes up in a guide
The link lands on the step where the fault arises rather than at the top of the pathway, because that is where the decision that causes it is taken.
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 is mousiness in cider — Mousiness is a fault caused by tetrahydropyridines produced by *Brettanomyces* and some lactic acid bacteria. It tastes of stale grain or a mouse cage and appears in the aftertaste rather than in the aroma.
- How do i balance acid and tannin in a blend — Acid and tannin do different jobs and cannot substitute for each other: acid gives freshness and microbiological safety, tannin gives structure and length. A blend short of acid tastes flabby however tannic it is.
- How do you blend cider
- What is malolactic fermentation — Malolactic fermentation is a bacterial conversion of sharp malic acid into softer lactic acid, releasing carbon dioxide. It lowers total acidity and raises pH, and in cider it is often the source of a farmyard or buttery note as well.
- 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.
Where to go next
- The fault finder — Describe what you can smell and see, and narrow it down from the signs.
- All faults — Grouped by where they come from and how serious they are.
- Cider science — The chemistry and microbiology these faults come out of.
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.
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.
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.