Fault
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.
Also called sauerkraut character, lactic spoilage.
- Severity
- Quality — the drink is worse for it
- Where it starts
- Maturation, Blending
- Can it be fixed?
- Partly — it can be reduced, not removed
- When you notice it
- Fermentation, Maturation
- Signs
- 6 recorded
What it is
Lactic acid bacteria are a normal and often welcome part of cidermaking: a controlled malolactic fermentation converts sharp malic acid to softer lactic acid and is deliberate in much traditional practice. Lactic off-flavours are what happens when the same family of organisms goes beyond that — fermenting residual sugars, glycerol, citric acid or tartrate, and producing a set of aromas that read as sauerkraut, sour milk, silage, cheese or a general sourness with no fruit behind it.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the nose
- A sauerkraut or pickled-cabbage smell
- Sour milk, yoghurt or soft cheese notes
- A silage or farmyard-slurry impression distinct from the animal character of Brettanomyces
On the palate
- Acidity that has changed character — softer in shape but sourer in impression
To look at
- A slow, fine haze and a gentle prickle appearing in a cider believed finished
How the batch behaves
- The cider tasting flatter and less fruity than it did a few months earlier
The words for it: Cheesy, Soured milk, Yoghurt. Each links to what produces it.
When it appears. During or after a malolactic fermentation, which in traditional cider frequently happens without anyone having decided it should. A cider that has softened over winter has usually had one.
The organisms responsible are the same ones that carry out the wanted conversion, so this fault cannot be prevented by excluding them without also giving up the malolactic. The control is which organisms and at what pH, not whether.
Campbell-Sills, El Khoury, Favier and others, Genome Biology and Evolution 7(6):1506–1518, Sánchez, Coton, Coton, Herrero, García and Díaz, Food Microbiology 32(1):32–37
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.
- Diacetyl excess — Diacetyl is specifically butter or butterscotch; the broader lactic range is yoghurt, sour milk and cheese. Diacetyl is one product of the same organisms rather than a separate event.
- Mousiness — Same organisms, different compounds, and mousiness is not detectable in the aroma. A cider with lactic character is worth checking for mousiness in the finish.
- Successful malolactic softening — Whether anything has been added or only removed. A good malolactic lowers perceived acidity and leaves the fruit; a bad one leaves dairy notes on top of it.
Malolactic fermentation is deliberate in many traditional ciders and its milder products are part of the expected character. The line falls where the dairy notes stop being a background and start being the description.
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.
Malolactic fermentation proper is a decarboxylation: the malolactic enzyme converts L-malic acid to L-lactic acid and carbon dioxide, with no energy yield in the classical sense and essentially no off-flavour. Oenococcus oeni is the organism generally wanted for it, and a clean conversion softens acidity and lowers titratable acid without much else changing.
Off-flavours arise when the bacteria have substrates other than malic acid. Heterofermentative species — Lactobacillus brevis, Lactobacillus collinoides, Leuconostoc mesenteroides — ferment residual glucose and fructose to lactic acid, acetic acid, ethanol and carbon dioxide. The acetic acid contributes to volatile acidity, and the mixture of products gives the sour, vegetal impression.
Citric acid metabolism produces diacetyl and acetoin, adding buttery notes; ethyl lactate from lactic acid and ethanol adds a milky, slightly waxy character; and amino acid decarboxylation produces biogenic amines, which are not strongly flavoured but are the reason unmanaged lactic activity is not simply a sensory question.
The whole family is restrained by low pH and by free sulphur dioxide, and favoured by warmth, residual sugar and time. Cider from low-acid bittersweet fruit sits at a pH where they are comfortable, which is why lactic activity in cider is both more common and less controllable than in most wine.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Residual sugar left in a cider with an active lactic population | Maturation | common |
| High pH from low-acid fruit, offering little natural restraint | Blending | common |
| Spontaneous malolactic fermentation by whatever species the cellar holds | Maturation | common |
| No free sulphur dioxide during warm maturation | Maturation | common |
| Extended lees contact providing amino acids and nutrients to the bacteria | Maturation | occasional |
| Wooden vessels or equipment carrying resident strains | Maturation | occasional |
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 |
|---|---|---|
| Stop the activity where it is | Partial | Sulphiting once conversion is complete, or filtering, prevents further development. This preserves a lightly affected cider rather than repairing it. |
| Blend into a larger clean volume | Partial | Reasonable for mild sauerkraut or milky character; ineffective once the cider is obviously sour and vegetal. |
| Wait for the character to integrate with age | Unlikely to work | Sometimes claimed. In practice a cider with established lactic off-flavours does not grow out of them, and leaving it exposed invites the rest of what these organisms do. |
| Remove the off-flavours | Not possible | They are dissolved fermentation products and cannot be selectively taken out. |
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
- Decide whether malolactic fermentation is wanted, and if it is, run it with a known culture at a controlled point rather than leaving it to chance.
- Ferment sugars out before allowing any lactic activity, so the bacteria have malic acid and nothing else worth having.
- Keep pH low enough to restrain the species you do not want; blending for acid is the primary tool.
- Maintain free sulphur dioxide once malolactic conversion is complete, at a rate taken from supplier guidance for the measured pH.
- Rack off gross lees rather than leaving cider on a nutrient bed for many months.
The awkwardness of this fault is that the organisms responsible are the same ones that make traditional cider taste the way it does. Malolactic fermentation is not an infection; in much of the West Country and in Normandy it is expected, and a cider that has not been through it can taste harsh by comparison. The fault is the uncontrolled version of a wanted process.
What distinguishes the two is substrate. Bacteria given only malic acid convert it and stop. Bacteria given sugar, citrate, glycerol and lees nutrients do a great deal more, and produce the sour, vegetal, buttery, milky spectrum that this entry covers. The practical control is therefore to finish the alcoholic fermentation properly before the bacteria get their turn.
The second control is pH, and it is the one that cider makes hardest. A blend built on bittersweet fruit sits at a pH where lactic acid bacteria are comfortable and where sulphur dioxide is much less effective than the same addition would be in a sharper cider. The traditional inclusion of sharp fruit in a blend is a microbiological decision as much as a sensory one.
Where cider is being made deliberately without added sulphite, unmanaged lactic activity is the most likely thing to go wrong, and the honest way to make that choice is with a cider whose pH is low enough to give it a chance.
The compounds involved
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.
Malic acid
The acid of apples, which supplies almost all the sharpness a cider has and, through pH, decides how vulnerable that cider is to everything that could spoil it.
Acetic acid
The vinegar acid, made by bacteria oxidising ethanol whenever air reaches a cider, and the one fault in cider that no later processing can undo.
Diacetyl
The butter compound, made mainly by lactic acid bacteria and perceptible at very low concentration, which is a defining part of some traditional ciders and an obvious fault in others.
Acetoin
The intermediate between diacetyl and odourless 2,3-butanediol, far less aromatic than the compound it comes from and a useful marker of how far lactic and acetic activity has run.
Ethyl lactate
The slow-forming ester of lactic acid and ethanol, which accumulates after malolactic fermentation and contributes the soft, milky roundness of a long-matured traditional cider.
Biogenic amines
Amines produced when bacteria decarboxylate amino acids, associated with long lees contact at high pH and low sulphite, and a reason bacterial activity is managed rather than simply tolerated.
Citric acid
A minor acid in apples and a much more significant one in pears, whose metabolism by lactic bacteria is the reason perry gains more butter and more vinegar from malolactic fermentation than cider does.
The organisms involved
Oenococcus oeni
The acid-tolerant lactic acid bacterium that carries out most deliberate malolactic fermentation, converting malic acid to lactic acid after the yeast has finished.
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.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
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.
Leuconostoc mesenteroides
A heterofermentative lactic acid bacterium common on fruit and early in fermentation, and a classic producer of the dextran that causes ropiness.
Pediococcus species
Homofermentative lactic acid bacteria that grow in tetrads, associated in cider with ropiness, diacetyl and slow spoilage during maturation.
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.
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.
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 confused with
These present similarly. What separates them is set out on each page.
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Ropiness
Cider that pours thick and pulls into an oily thread, caused by lactic acid bacteria secreting long-chain glucan polymers into the drink.
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.
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.
Volatile acidity
The measurable fraction of acidity that can be steam-distilled off, dominated by acetic acid and read as vinegar sharpness in the nose and a hard, hot finish.
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.
- Can you fix a cider that has gone vinegary — Not really. Volatile acidity cannot be removed at small scale, and blending it away only dilutes it. The realistic options are to accept it, to blend a lightly affected batch into a much larger sound one, or to let it finish becoming vinegar.
- 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.
- What is malic acid and why does it matter in cider — Malic acid is the acid of apples and supplies almost all the sharpness a cider has. It also sets pH, which decides how well sulphite works and how exposed the cider is to spoilage, so it does far more work than its taste suggests.
- 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.
- 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 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.
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.
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.