Compound
Pectin methylesterase
The enzyme that strips methyl groups from pectin, exposing the charged sites that calcium bridges — which is the reaction the whole of keeving is built on.
Also called PME, Pectinesterase.
- Class
- Enzymes
- Formula
- Not a single molecule — see below
- How often it matters
- Central to how cider behaves
What it does in cider
- Removes methyl ester groups from the pectin backbone, releasing methanol and leaving free carboxyl groups behind.
- Creates the negatively charged sites that calcium ions cross-link, converting soluble pectin into an insoluble calcium pectate gel.
- Occurs naturally in apple tissue and is also supplied deliberately in modern keeving, where the fruit’s own activity is unreliable.
- Works slowly at the low temperatures keeving requires, which is why the process takes days and cannot be hurried without losing the gel.
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Keeving
Forming a floating pectin gel that lifts nutrients and solids out of the juice, so that the ferment starves before it finishes and leaves natural sweetness behind.
Maceration
Holding milled pomace before pressing so that phenolics, aroma precursors and pectin have time to move out of the solid tissue and into the juice.
Pectinase treatment
Adding pectin-degrading enzyme preparations to juice so that haze-forming and viscosity-forming pectin is broken down before fermentation.
Enzyme addition to pomace
Dosing pectin-degrading enzyme into milled pomace rather than into juice, to raise press yield and change how the cake drains.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Juice pasteurisation
Heat treatment of juice before fermentation, which inactivates enzymes and microorganisms, and in doing so removes the wild flora that would otherwise ferment it.
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.
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.
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.
About Pectin methylesterase
Pectin methylesterase does one thing: it hydrolyses the methyl ester groups that cap many of the galacturonic acid units in a pectin chain. The methanol released is the trivial part of the reaction. The important part is what is left behind — a free carboxyl group, negatively charged at cider pH, at each position the enzyme has visited.
A pectin chain with enough of these free sites can be cross-linked. Calcium ions, each carrying two positive charges, bridge carboxyl groups on adjacent chains, and once enough bridges form the dissolved polymer becomes a coherent three-dimensional gel. This is the défécation of French practice and the keeve of English practice, and the whole apparatus of it — cold juice, calcium salt, slow incipient fermentation, days of patience — exists to give this one enzyme time to do its work at a temperature that stops yeast doing anything faster.
The difficulty in practice is that native activity varies enormously with cultivar, with maturity and with how the fruit was stored, so a traditional keeve either works or does not, and generations of makers have described it as unpredictable. Commercial keeving preparations supply the enzyme directly and make the outcome repeatable, at the cost of the thing being wholly the fruit’s doing.
The same enzyme is a nuisance in other contexts. In cloudy juice production it de-esterifies pectin gradually in the pack, and the calcium naturally present then forms a gel or a haze in a product that was stable at bottling — which is why juice intended to stay cloudy is usually pasteurised warm enough to kill the enzyme as well as the microorganisms.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Polysaccharides
Pectin
The structural polysaccharide of fruit cell walls, which decides how much juice a press releases, whether a cider ever clears, and whether keeving is possible at all.
Polysaccharides
Calcium pectate
The gel formed when calcium bridges de-esterified pectin chains, which floats to the surface as the brown cap of a keeve and carries the juice’s nutrients out with it.
Alcohols
Methanol
A small alcohol released not by fermentation but by enzymes stripping methyl groups off pectin, which is why juice handling rather than yeast choice governs how much a cider contains.
Acids
Galacturonic acid
The sugar acid that pectin chains are built from, released as they break down, and a significant part of why juice from rotten fruit binds so much of the sulphite added to it.
Enzymes
Pectin lyase
The chain-cutting activity in a commercial pectinase preparation, which raises press yield and clears a juice by destroying the colloid pectin creates — and destroys the possibility of keeving in the same stroke.
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 do i clear a cloudy cider
- What does pectin do in cider — Pectin is the structural polysaccharide that holds fruit cells together. In juice it holds haze in suspension, and it is the molecule keeving depends on: strip its methyl groups and it will gel with calcium and float the nutrients out of the juice.
- What is keeving — Keeving is a technique for starving a ferment of nitrogen so that it stops before all the sugar is gone, leaving a naturally sweet cider. Pectin is made to gel and float as a brown cap, carrying nutrients and yeast out of the juice with it.
- Why is sulphite added to cider — Sulphur dioxide suppresses spoilage bacteria and wild yeast before fermentation and protects the finished cider from oxidation. How much of it is active depends strongly on pH, which is why low-acid juice is harder to protect.
- Is there methanol in cider — Yes, in small quantities: methanol is released when pectin in the fruit is broken down, and it is present in all fruit juices and fruit-based drinks. The concentration in cider is far below any level of concern, though it matters when cider is distilled.
- How is cider pasteurised
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.
Institut Français des Productions Cidricoles (IFPC)
IFPC · research institute · retrieved 2026-08-24
The French technical institute for cider production. The authority for the French cultivar classification families, for keeving as an industrial process, and for the pectin and nitrogen chemistry that keeving depends on.
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.
Hochschule Geisenheim University — beverage technology
Hochschule Geisenheim · university · retrieved 2026-08-24
German beverage-technology research covering apple wine and fruit juice processing, including the enzymology of clarification.