Compound
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.
Also called 2-hydroxypropanoic acid.
- Class
- Acids
- Formula
- C3H6O3
- How often it matters
- Central to how cider behaves
What it does in cider
- Replaces malic acid one molecule for one during malolactic fermentation, but carries a single acid group where malic carried two, so titratable acidity falls by roughly half.
- Raises pH as it does so, typically by one to three tenths — a change that makes the cider more vulnerable to further microbial activity, not less.
- Tastes rounder and less penetrating than malic acid at the same acidity, which is the sensory point of the conversion.
- Appears in small quantity from yeast metabolism even where no malolactic fermentation has occurred.
- Partly esterifies to ethyl lactate over time, adding a soft, milky note to long-matured cider.
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.
- soft acidity
- yoghurt
- milky
- rounded sourness
Described consistently as milder and less persistent than malic acid at equal titratable acidity; the perceived change during malolactic fermentation is one of quality of acid as well as quantity.
Descriptors it is responsible for
Sensory records that name Lactic acid as a cause. Each states the perception and the mechanism behind it.
- Butter — The buttery note of diacetyl, produced by lactic acid bacteria during malolactic conversion.
- Mousy — A retronasal-only taint of mouse cage and stale grain, from tetrahydropyridines produced by lactic bacteria and Brettanomyces.
- Cheesy — A rancid, sweaty note from short-chain fatty acids, chiefly butyric and isovaleric, produced by bacteria.
- Cream — A soft, rounded dairy impression from acetoin and lactic acid without the sharpness of free diacetyl.
- Geranium — The crushed-geranium-leaf note produced when lactic bacteria metabolise added sorbate — a fault with one cause and no remedy.
- Peppery bitterness — A harsh, burning bitterness from acrolein formed when lactic bacteria degrade glycerol.
- Ropy — An oily, viscous, pouring-in-a-thread texture from bacterial exopolysaccharide, with little aroma of its own.
- Soured milk — A sharp, unclean dairy note from uncontrolled lactic activity, recorded as a fault in every style.
- Yoghurt — A tangy fermented-dairy note from lactic acid with acetaldehyde, indicating vigorous bacterial activity.
The structure it moves
| Dimension | What it is |
|---|---|
| Acidity | The sharp, mouth-watering quality that makes a cider taste fresh rather than flat. |
| Body | How much weight and viscosity the drink has in the mouth. |
| Fermentation character | Aromas made by the ferment rather than carried in from the fruit. |
Measured figures
Shown as they were measured, with the context each was taken in. They are not averaged: a concentration recorded in one country's fruit in one decade is not a constant.
Lactic acid4.0–4.7 g/L
Villaviciosa, Asturias, Spain, 2001–2002 · 4 samples · HPLC, four ciders — two pressing technologies across two harvests · Suárez Valles, Pando Bedriñana, Fernández Tascón, Querol Simón and Rodríguez Madrera, Food Microbiology 24(1):25–31
Asturian sidra natural that has been through malolactic fermentation on its indigenous flora, so this is close to the upper end of what a cider carries: in the same ciders malic acid had fallen below 0.5 g/L. A cider protected from malolactic fermentation carries a fraction of this, and the figure is not a target.
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.
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
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.
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.
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.
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.
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.
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.
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.
Film yeast growth
A pale, powdery or wrinkled film of aerobic yeast growing on the surface of a standing cider, consuming alcohol and acid and producing acetaldehyde.
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.
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 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.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
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.
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 Lactic acid
Malolactic fermentation is not a fermentation in the sugar sense. A lactic acid bacterium takes malic acid and removes one carboxyl group as carbon dioxide, leaving lactic acid. Because the substrate had two acid groups and the product has one, the arithmetic of the change is simple and dramatic: roughly half the titratable acidity vanishes, and the pH climbs. In a cider that started sharp this is a gain in balance. In a cider that started at pH 3.8 it is a step towards a liquid that almost nothing will protect.
What the palate notices is not only that there is less acid but that the acid is different. Lactic acid reads as softer, broader and shorter-lived than malic, and the cider gains a rounder mid-palate. Alongside the acid change come the organism’s other products: diacetyl in the buttery range, acetoin, sometimes small amounts of acetic acid, and over the following months ethyl lactate, whose milky softness is a marker of long-matured traditional cider.
Whether this is wanted is a style question rather than a technical one, and traditional practice splits both ways. West Country farmhouse cider, most Normandy cider and much Asturian sidra go through it as a matter of course, and the resulting softness and lactic depth are part of what those styles are. A modern cider built on fresh fruit aroma and a clean line of acid loses something it cannot get back. The practical controls are the same in either direction: pH, free sulphur dioxide, temperature and how promptly the cider is racked off its lees.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Acids
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.
Ketones
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.
Ketones
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.
Esters
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.
Acids
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.
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 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 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.
- Which bacterium carries out malolactic fermentation in cider — Oenococcus oeni, in cider as in wine. Genome studies find that the cider strains are genetically distinguishable from the wine ones and that the strain most basal to the whole species was isolated from 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.
- Why does my cider pour thick and slimy — That is ropiness. Beating the cider vigorously to break the glucan chains and then sulphiting will usually restore the texture, though the bacteria remain a problem for the vessel.
- Why does my cider taste of butter — Diacetyl, produced by lactic acid bacteria and by yeast, smells of butter or butterscotch. A trace can add richness; above threshold it reads as artificial and stale.
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.
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.