Organism
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.
Also called Leuconostoc oeni.
- Kind
- Bacteria
- Binomial
- Oenococcus oeni
- Role
- Malolactic conversion
What it does
- Decarboxylates L-malic acid directly to L-lactic acid using the malolactic enzyme, without free intermediates — replacing a diprotic acid with a monoprotic one, which lowers titratable acidity and raises pH.
- Softens the palate as a consequence of that substitution: lactic acid is perceived as less sharp than malic, so the cider tastes rounder even though little else has changed.
- Metabolises citric acid to diacetyl and acetoin, which give buttery aromas at low concentration and an intrusive butterscotch character above threshold — a separate reaction from the malolactic conversion itself.
- Grows heterofermentatively on residual sugars, producing lactic acid, acetic acid, ethanol and carbon dioxide, so a malolactic ferment in a cider with sugar left in it also raises volatile acidity.
- Removes malic acid permanently, which means a cider that has undergone malolactic conversion cannot be refermented on it in bottle — the standard reason the process is encouraged before packaging rather than left to chance.
- Proliferates past roughly a million cells per millilitre once it establishes, at which point it becomes the only bacterial species that can be cultured from the cider at all — the succession does not end in a community, it ends in a monoculture.
- Exists as cider-adapted strains rather than as wine strains that happen to be present. Genome comparison across fifty strains puts most cider isolates in one genetic group and the strain most basal to the whole species — also from cider — outside both major groups, which the authors read as separate domestication to the two drinks.
Conditions it works in
What the organism tolerates and what suppresses it. These are the levers a maker actually has: temperature, acidity, air, alcohol and sulphite.
| Condition | What is recorded |
|---|---|
| Temperature | Grows from roughly 15 °C to 25 °C, with malolactic conversion slowing markedly below about 12 °C — the mechanism behind the traditional practice of a cool winter and a spring malolactic. |
| pH | Notably acid-tolerant for a lactic acid bacterium, active down to around pH 3.0 and increasingly rapid above pH 3.5. |
| Oxygen | Anaerobic to microaerophilic; it does not need air and is not inhibited by its absence. |
| Alcohol tolerance | Tolerates cider strength readily, generally to around 12–14% ABV. |
| Sulphite tolerance | Sensitive. Free sulphur dioxide inhibits it strongly, which is why sulphiting a cider before malolactic conversion is the standard way to prevent it. |
What it produces
Compounds this organism makes. Which organism made a compound usually decides whether it reads as a feature or as a symptom.
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.
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.
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.
Carbon dioxide
The other product of fermentation, which protects a cider from air while it is being made and, dissolved in the finished drink, changes both its texture and its perceived acidity.
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.
Faults it causes
Faults this organism is implicated in. Several are faults only against a particular expectation — the same activity is a signature elsewhere.
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
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.
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.
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.
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.
Microbial haze
Cloudiness caused by a growing population of spoilage organisms, and therefore a symptom of something worse rather than a clarity problem in itself.
Excessive acidity
Acidity high enough to dominate everything else, leaving a cider that is thin, hard and sour rather than fresh.
Geranium taint
A sharp, unmistakable crushed-geranium-leaf smell produced when lactic acid bacteria metabolise sorbic acid added as a preservative.
Ropiness
Cider that pours thick and pulls into an oily thread, caused by lactic acid bacteria secreting long-chain glucan polymers into the drink.
Where in the process it appears
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.
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.
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.
Sulphiting
Adding sulphur dioxide to juice to suppress spoilage organisms and oxidative browning, at a level that only means anything once the juice pH is known.
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.
Fermentation monitoring
Watching a running ferment through gravity, temperature, smell and surface behaviour, and reading the rate of change rather than any single figure.
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.
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.
Bottle conditioning
Carbonating cider by letting a second fermentation finish inside the sealed bottle, so the gas is generated where it is going to stay.
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.
Microbial succession
The ordered handover of a spontaneous ferment from apiculate yeasts to *Saccharomyces* to lactic acid bacteria, and the spoilage organisms waiting at the end of it.
Fermentation vessels
The container a cider ferments in — wood, stainless, plastic, glass or concrete — and how its permeability, thermal mass and resident microflora shape the result.
Secondary fermentation
Any fermentative event that follows the primary ferment — residual sugar refermenting, a deliberate second alcoholic fermentation, or the malolactic conversion of the maturation phase.
Sorbate stabilisation
Adding potassium sorbate to prevent yeast from restarting a fermentation in the package — an inhibitor rather than a killer, and one that must never be used without sulphite.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Cold maturation
Holding cider at low temperature so that it settles bright, sheds colloidal material and stops changing microbially, without any additive or treatment being applied.
Measured figures
Growth limits, tolerances and population counts, shown with the context each was taken in rather than averaged into a constant.
Microbial population6.0 log CFU/mL
context not recorded · Reported as “above 10⁶ cells/ml”, the point at which it becomes the only detectable bacterial species · Campbell-Sills, El Khoury, Favier and others, Genome Biology and Evolution 7(6):1506–1518
A floor rather than a peak: the source states that the population exceeds this once the organism takes hold in wine or cider, not that it stops there. What the figure marks is the transition from an organism that is present to an organism that is running the process, and at that point nothing else bacterial is culturable alongside it.
Viable cells per millilitre, on the log scale. What separates an organism that is present from one that is running the ferment. Measured in log colony-forming units per millilitre.
Described in full
- The reaction
- Malic acid, which carries two acid groups, is decarboxylated to lactic acid, which carries one, releasing carbon dioxide. Nothing is fermented in the ordinary sense: no sugar is consumed and no alcohol is made.
- What performs it
- Lactic acid bacteria, chiefly Oenococcus oeni and Lactiplantibacillus plantarum. They arrive on the fruit and in the vessel, and in traditional cidermaking they are rarely inoculated.
- Acidity falls
- Losing one of two acid groups roughly halves the acid contribution of every malic molecule converted. Titratable acidity drops and the cider tastes rounder.
- pH rises
- The same change raises pH, which matters because a higher pH is a less protected cider. Malolactic makes a drink softer and simultaneously more vulnerable to spoilage.
- A buttery note may appear
- Diacetyl is a by-product. In small amounts it reads as butter or butterscotch, which is traditional in some styles and a fault in a fresh, fruit-driven one.
- Carbon dioxide is released
- A slow bead in a maturing vessel is often malolactic rather than residual fermentation. In a sealed bottle, an unfinished malolactic is a source of unplanned pressure.
- Not always wanted
- A cider made for freshness is usually kept below the pH and temperature where the bacteria establish, or filtered and sulphited to prevent it. The same conversion is a goal in one cellar and a fault in another.
About Oenococcus oeni
Oenococcus oeni is the third act of the cider succession. The yeasts have gone; the sugar is gone; what remains is malic acid, and this organism can live on it. Formerly classified as Leuconostoc oeni and moved to its own genus in the 1990s, it is distinguished among lactic acid bacteria by tolerating conditions the others cannot — low pH, ethanol, and the nutrient-poor state of a finished ferment — which is exactly why it dominates malolactic conversion in wine and cider while its relatives cause faults instead.
The chemistry is deceptively small and its effects are large. One decarboxylation replaces malic acid with lactic acid: acid quantity falls, pH rises by a few tenths, and a cider that was hard and sharp becomes rounder. In high-acid seasons and high-acid regions this is the principal tool for softening a cider without adding anything to it. In a low-acid West Country blend already sitting near pH 3.8 it may be the last thing wanted, since the pH rise moves the cider towards a range where other spoilage is easier.
Two side reactions matter. Citric acid metabolism gives diacetyl, welcome as a whisper of butter and unwelcome as butterscotch, with the balance depending on how much citrate is present and whether yeast lees remain to reduce diacetyl to acetoin. And this species, like other lactic acid bacteria, can produce biogenic amines and — under some conditions — the tetrahydropyridines responsible for mousiness.
The practical decision is one of timing and control. Left to itself, malolactic conversion in a cool cellar tends to happen in spring as temperatures rise, which is late enough to be a hazard if the cider is already in bottle. A maker who wants it encourages it deliberately and confirms it is finished before packaging; a maker who does not want it prevents it with sulphite, with cold, or by removing the organism through filtration.
The strains in cider are not the strains in wine, and that is a genomic finding rather than an inference from where they were found. Across fifty sequenced genomes the species divides into two major groups, with most wine isolates in one and a champagne-adapted cluster inside it; cider isolates fall in the other group, and the single strain that sits basal to the entire species — outside both — was also isolated from cider. The reading the authors offer is that an ancestral organism adapted to low-ethanol environments such as overripe fruit was domesticated separately into the two drinks, and that the wine strains travelled further from the ancestor than the cider ones did. The practical version of that is short: a malolactic culture sold for wine is a wine organism, and the environment it is being asked to work in — six per cent alcohol rather than thirteen, and often a higher pH — is not the one it was selected in.
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 long does cider take to ferment — A warm ferment with cultured yeast can finish in one to two weeks; a cool wild ferment in a cellar may take three months or more. Slow is not the same as stuck — the test is whether gravity is still falling.
- 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.
- 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 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 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
- All organisms — Grouped by what each does in the ferment, and by what kind of organism it is.
- Compounds — The chemistry this microbiology produces.
- Troubleshooting — Work from the symptom in the glass back to the organism.
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.
Servicio Regional de Investigación y Desarrollo Agroalimentario (SERIDA)
Principado de Asturias · research institute · retrieved 2026-08-24
The Asturian regional agri-food research service, and the technical authority behind the Asturian cultivar classification and the analysis of sidra natural. SERIDA’s own site has not been opened; what CiderHQ has read is SERIDA’s varietal characterisation as republished by the Consejo Regulador of DOP Sidra de Asturias, which credits it explicitly. The Asturian measurements therefore cite the Consejo rather than SERIDA — citing the body that did the work, at a document CiderHQ has not seen, would be exactly the kind of borrowed authority this register exists to prevent.
Campbell-Sills, El Khoury, Favier and others, Genome Biology and Evolution 7(6):1506–1518 · peer-reviewed literature · passage verified 2026-08-24
Open access; read in full on 2026-08-24. Fifty genomes of the bacterium that performs malolactic fermentation, and the finding that the cider strains are not simply wine strains that happened to land in cider. The strain basal to the whole species came out of cider, and the authors read the pattern as separate domestication of one ancestral low-alcohol fruit organism into two drinks. It is also the source CiderHQ uses for what a malolactic population actually reaches, which is the difference between an organism being present and an organism running the process.
Prevalent lactic acid bacteria in cider cellars and efficiency of Oenococcus oeni strains
Sánchez, Coton, Coton, Herrero, García and Díaz, Food Microbiology 32(1):32–37 · peer-reviewed literature · retrieved 2026-08-24
Which lactic acid bacteria are actually in working Asturian cider cellars, rather than which ones the wine literature would predict. Bibliographic record verified through Crossref on 2026-08-24 and the abstract read; the full text is paywalled on ScienceDirect. Cited for the identification of the prevalent species and for the finding that strains sort by producing area, not for any figure.