Glossary · Fermentation
Malolactic fermentation
Also written MLF, malolactic conversion. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
The bacterial conversion of malic acid into lactic acid and carbon dioxide, which lowers total acidity, raises pH slightly and softens the drink. It is a bacterial process, not a yeast fermentation.
- Subject
- Fermentation
- Also written
- MLF, malolactic conversion
In more detail
Malic acid has two acid groups and lactic acid one, so the conversion removes roughly half the acidity of the malic fraction while replacing a sharp acid with a softer one. In high-acid ciders — Asturian, Basque, much of northern Europe — that transformation is essential to the style, and the accompanying diacetyl and lactic esters are part of its signature.
It is not always wanted. In a low-acid bittersweet cider, malolactic conversion can leave the drink flat and raise pH into a range where spoilage is easier. And the same family of bacteria, in the wrong conditions, produces ropiness, mousiness and acrolein bitterness rather than a clean conversion.
It can be encouraged by warmth and a low sulphite regime, suppressed by sulphite, cold and sterile filtration, or directed by inoculating a selected Oenococcus oeni culture.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Production
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.
Microbiology
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.
Chemistry
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.
Chemistry
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.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
Malic acid
The dominant acid of apples and pears, giving the sharp, clean acidity of fresh juice. Its name comes from *malum*, the Latin for apple.
Chemistry
Lactic acid
A softer, milder acid produced from malic acid by lactic acid bacteria, and in smaller quantities by yeast. Its arrival is what makes a malolactic cider taste rounder.
Microbiology
Lactic acid bacteria
The bacteria that convert malic acid to lactic acid and, in other conditions, produce the disorders of ropiness, mousiness and acrolein bitterness. *Oenococcus*, *Lactiplantibacillus* and *Lactobacillus* species dominate in cider.
Faults
Diacetyl
A compound produced by lactic acid bacteria from citrate and by yeast as a fermentation intermediate, smelling of butter, butterscotch or hot popcorn.
Cider
Sidra natural
Asturian and Basque cider fermented to complete dryness from pure juice, unfiltered, uncarbonated and bottled without added gas or sugar. It is poured from a height to raise a momentary foam.
Faults
Ropiness
A texture fault in which the cider becomes viscous and pours in an oily rope, caused by lactic acid bacteria producing extracellular glucan polysaccharide.
Where to go next
- Cider glossary — The whole vocabulary, by subject and A–Z.
- Comparisons — Terms that are only really clear when set against the thing they are not.
- Search — Finds terms by their aliases and by their spellings in other languages.
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.