Fermentation
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.
Known as Fermentation malolactique where it originates. Also called MLF, Malolactic conversion.
- Stage
- Fermentation
- Traditional in
- Asturias, Basque Country, Normandy, Pays d’Auge and 3 more
- What it most changes
- Acidity down, freshness down
- Safety
- None recorded
What it is
Malolactic fermentation is not a fermentation in the yeast sense at all. It is a bacterial decarboxylation in which lactic acid bacteria convert L-malic acid, the dominant and sharpest acid in apple juice, into L-lactic acid and carbon dioxide. Because malic acid carries two carboxyl groups and lactic acid only one, the conversion removes roughly half the acid’s titratable contribution and raises pH. Sensorially the drink loses hardness and gains a rounder, softer, sometimes faintly dairy or nutty character. In cider it usually happens spontaneously in the spring following the harvest, as cellar temperatures rise and the bacteria that survived the primary ferment become active.
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.
Why it is used
- Cider from sharp and bittersharp fruit can be uncomfortably acidic when fully dry, and the conversion softens it without dilution or added sweetness.
- Raising pH and consuming malic acid removes a substrate that spoilage organisms could otherwise use later, so a cider that has completed the conversion is more microbially settled than one that has not.
- The conversion contributes its own aroma and texture, which several traditions treat as part of the style rather than as a side effect.
- Allowing it to complete before packaging prevents it happening later in the bottle, where the carbon dioxide it releases produces unwanted spritz and haze.
How it works
- The malolactic enzyme decarboxylates L-malate directly to L-lactate, using NAD+ and manganese as cofactors. It is a single step, not a full metabolic pathway, and it yields the bacterium energy indirectly by exporting protons.
- In cider the organism is most often Oenococcus oeni, which tolerates low pH and ethanol well, but Lactiplantibacillus plantarum, Lactobacillus collinoides, Leuconostoc mesenteroides and Pediococcus species all occur and behave differently.
- Citric acid is metabolised alongside malic acid by many of these organisms, and that pathway produces diacetyl and acetoin — the buttery note — as intermediates, along with a little acetic acid.
- Ethyl lactate forms slowly by esterification of the newly abundant lactic acid, contributing to the softer, rounder aroma associated with a completed conversion.
- The conversion is inhibited by free sulphur dioxide, by low temperature and by low pH, which is exactly how a maker who does not want it prevents it.
What it changes
The direction this step pushes the finished drink in, dimension by dimension. A direction, not a measurement: how far it moves depends on the juice, the temperature and how the step is carried out.
| Dimension | Direction | Why |
|---|---|---|
| Acidity | Lowers | Replacing a dicarboxylic acid with a monocarboxylic one removes about half the titratable acid contribution of every molecule converted, and raises pH with it. |
| Freshness | Lowers | Malic acid is the component the palate reads as crisp and apple-like, and lactic acid does not substitute for it. |
| Body | Raises | The loss of sharp acid and the arrival of lactic and ester character shift the perceived texture from angular to rounded, an effect of contrast rather than of added substance. |
| Fermentation character | Raises | Diacetyl, acetoin and ethyl lactate produced during and after the conversion add a lactic, buttery or nutty layer that was not present in the young cider. |
| Fruit character | Lowers | Fresh apple aroma is closely tied to malic acidity, and the same bacterial activity also metabolises some of the fruit-derived compounds that supported it. |
The chemistry and the organisms
What is actually being changed, and by what. Each entry says what that compound or organism does in cider generally; this page is one place it does it.
Compounds involved
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.
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.
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.
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.
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.
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.
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.
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.
Tetrahydropyridines
The compounds behind mousiness, which cannot be smelled in the glass because they are not volatile at cider pH and appear only as an aftertaste once saliva has raised the pH in the mouth.
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.
Sulphur dioxide
The antimicrobial and antioxidant on which most modern cidermaking depends, and whose effectiveness collapses as pH rises — which makes every sulphiting decision a pH decision first.
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.
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.
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.
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 it is traditional
The places this step belongs to as a matter of practice. It is not a claim of exclusivity — a method can be traditional in one region and perfectly ordinary in another.
What it is done with
Temperature control: glycol, coils and a cold room
Cooling can be applied to the vessel — a glycol jacket or a coil in the liquid — or to the air around it, and for most small producers a cold room, an insulated container or simply a cold building is the cheaper and more reliable answer.
Drawing a sample without spoiling the batch
Every sample is a small hole made in the protection around a batch: something goes in, air goes in with it, and cider comes out — so the technique is about drawing a representative sample while putting nothing back and letting in as little air as possible.
What can go wrong
Faults that begin at this step, or that this step makes more likely. Each one is set out with its causes and whether it can be put right.
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.
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.
Framboise
A raspberry-and-rotten-fruit character with sulphurous overtones, produced by Zymomonas mobilis in sweet ciders that still contain sugar.
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.
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.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
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.
Styles it produces
Categories in which this step is characteristic or required. Some name it in their definition; for others it is simply how they have always been made.
Sidra natural
The still, dry, unfiltered cider of Asturias and the Basque Country, wild-fermented from local high-acid fruit and poured from a height to raise a momentary sparkle.
Euskal Sagardoa
Basque cider under its own protected designation, made from Basque-grown apples in the sagardotegi tradition of barrel service and dry, high-acid, still cider.
West Country farmhouse cider
Cider made on the farm from tannic bittersweet fruit, wild-fermented in wood and sold still and unfiltered, in a tradition whose variability is one of its defining features.
Cidre de Normandie
Protected Normandy cider made from the region’s bitter and bittersweet fruit, characteristically low in alcohol, sweet-edged and lightly sparkling.
Traditional draught cider
Cider dispensed from cask, bag-in-box or gravity without added gas, in the form that British cider has historically been drunk in pubs and at the farm gate.
Perry
The fermented drink of pears, a tradition parallel to cider rather than derived from it, in which unfermentable sorbitol leaves a sweetness the maker never chose.
Apfelwein
The dry, sharp, still apple wine of Hesse, fermented out from culinary and local fruit and served in ridged glasses from a stoneware jug.
Galician cider
Cider from Galicia and the north-western Iberian fringe, made in a smaller and less codified tradition than its Asturian and Basque neighbours, from a distinct local fruit population.
Recorded figures
Shown with the place, period and method each was taken under, and never averaged: the same step run in another cellar genuinely gives a different number.
Microbial population6.0 log CFU/mL
context not recorded · Reported as the level Oenococcus oeni proliferates past in wine and cider after alcoholic fermentation · Campbell-Sills, El Khoury, Favier and others, Genome Biology and Evolution 7(6):1506–1518
A threshold for the process rather than a target for a maker. Below it the conversion is slow and uncertain; past it the organism is the only culturable bacterium present and the conversion runs to completion. Nothing about the figure says how long that takes, which depends on temperature, pH and free sulphur dioxide.
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.
Lactic acid4.0–4.7 g/L
Villaviciosa, Asturias, Spain, 2001–2002 · HPLC on finished sidra natural after spontaneous malolactic conversion by the indigenous flora · Suárez Valles, Pando Bedriñana, Fernández Tascón, Querol Simón and Rodríguez Madrera, Food Microbiology 24(1):25–31
What the conversion actually leaves behind, measured in the same ciders whose malic acid had fallen below 0.5 g/L. Reading the two together is the clearest statement of the process there is: the acid did not go away, it changed to a milder one, and lactic acid became the major acid in the cider.
The acid malolactic bacteria make out of malic acid — softer on the palate than the acid it replaces, and in a fully malolactic cider the dominant one. Measured in grams per litre.
More on malolactic fermentation
The reaction itself is disarmingly simple. A single enzyme removes one carboxyl group from L-malate and releases it as carbon dioxide, leaving L-lactate. There is no chain of intermediates and no net gain of carbon for the organism; what the bacterium gets is a proton gradient it can use, which is why the conversion proceeds vigorously in a medium that offers it little else. The consequence for the drink is arithmetical: malic acid is diprotic, lactic acid is monoprotic, so each molecule converted contributes about half as much to titratable acidity as it did, and pH rises accordingly. In a bittersharp cider that has fermented to dryness this is often the difference between a drink that is uncomfortably hard and one that is balanced.
Which organism does it matters more in cider than the textbooks written for wine suggest. Oenococcus oeni is the well-behaved case: acid- and ethanol-tolerant, and the species most commercial malolactic cultures are built from. But cider is fermented cooler than wine, often at higher pH, and frequently without sulphite, and that combination opens the door to lactobacilli that would not establish in a wine cellar. Lactobacillus collinoides is the cider-specific problem child. Given glycerol it can produce 3-hydroxypropionaldehyde, which dehydrates to acrolein and reacts with phenolics to give a searing, persistent bitterness that cannot be blended away. Related organisms produce exopolysaccharide and turn a cider ropy, or generate tetrahydropyridines and the mousy off-note that only appears retronasally after swallowing, or produce the raspberry-like framboise character from fructose. A malolactic conversion is a bacterial ferment, and bacterial ferments in a nutrient-poor, low-alcohol medium are not automatically benign.
Traditions treat it very differently, and none of them treats it the way modern winemaking does. In Asturias and the Basque Country the conversion is expected: sidra natural and sagardoa are fermented spontaneously in wood and go through malolactic as a matter of course, and the resulting lactic and slightly volatile profile is what the appellations and the drinking public recognise as correct. English farmhouse and Norman practice likewise let it happen in the spring, and much of what the West Country palate reads as farmhouse character is post-malolactic. Modern producers making a fresh, aromatic, acid-driven cider often want the opposite, and prevent it deliberately by keeping the cider cold, maintaining free sulphur dioxide, filtering out the bacteria, or all three. Producers making a sparkling cider by any bottle-fermented method have a more urgent reason to settle the question before packaging.
That is the practical decision: not whether malolactic conversion is good, but whether it will happen in the tank where it can be watched or in the bottle where it cannot. A cider bottled with malic acid still present and viable lactic acid bacteria in it will convert eventually, releasing carbon dioxide into a package that may not be built for it, dropping a haze, and shifting the drink’s balance months after it left the producer. The conversion is also close to irreversible: acid can be added back, but the aromatic and textural change cannot be undone, and a low-acid bittersweet cider pushed through a complete malolactic can end up flabby with nothing to give it definition. Makers who want it monitor for the disappearance of malic acid by paper chromatography or enzymatic assay rather than by taste, and those who do not want it act before the cellar warms rather than after.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Fermentation
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.
Fermentation
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.
Maturation
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.
Blending
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.
Stabilisation
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Maturation
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 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.
- 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 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 sulphites — Most does. Sulphur dioxide is added to protect juice and finished cider, and fermentation itself produces a small amount even when none is added. European labels must declare it above 10 mg per litre.
- 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.
- What is perry — Perry is an alcoholic drink made by fermenting the juice of pears, traditionally of specific perry pear cultivars rather than eating pears. It is to pears what cider is to apples, and has its own history in the three counties of England and in Normandy.
Where to go next
- How cider is made — The whole sequence, stage by stage, with the choices open at each one.
- Cider science — The chemistry and microbiology the methods on this page rest on.
- Troubleshooting — What goes wrong, how to recognise it, and whether it can be reversed.
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.
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.
Yeast species associated with the spontaneous fermentation of cider
Suárez Valles, Pando Bedriñana, Fernández Tascón, Querol Simón and Rodríguez Madrera, Food Microbiology 24(1):25–31 · peer-reviewed literature · passage verified 2026-08-24 · covers 2001–2002
SERIDA’s survey of what is actually growing in an Asturian cellar during a spontaneous ferment, across two harvests and two pressing technologies. Read in full from the author institution’s open repository (ria.asturias.es, handle 123456789/925) on 2026-08-24. Two things make it worth citing rather than summarising: it identifies its isolates molecularly rather than by colony appearance, and it publishes the analytical composition of the finished ciders alongside the microbiology, so a reader can see the organisms and the numbers they produced in the same paper. It also contradicts the textbook account of apiculate yeasts dying out early, which is why CiderHQ cites it on that point specifically.