Glossary · Chemistry
Pectin methylesterase
Also written PME, pectinesterase. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
The native fruit enzyme that removes methyl groups from pectin, exposing the carboxyl groups that then bind calcium. It is the enzyme on which keeving depends.
- Subject
- Chemistry
- Also written
- PME, pectinesterase
In more detail
PME is active in apple tissue and continues to work in the juice at cellar temperatures. Its rate is why keeving is slow and why it fails when it is too cold to proceed or too warm for fermentation to hold back long enough. Commercial preparations are added where native activity is insufficient.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Chemistry
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.
Production
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.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Fermentation
Keeving
A pre-fermentation clarification in which pectin in the juice is gelled into a floating brown cap, taking nutrients and yeast with it, so that the clear juice drawn from beneath ferments slowly and stops with sugar remaining.
Chemistry
Pectin
A structural polysaccharide of fruit cell walls, released into juice during milling and pressing. It thickens juice, holds haze in suspension and is the raw material of the keeving cap.
Chemistry
Calcium pectate
The insoluble gel formed when calcium ions cross-link de-methylated pectin. It is the substance of the keeving cap, and the reason calcium salts are sometimes added to encourage one.
Fermentation
Chapeau brun
The brown gelatinous cap of pectate, solids and yeast that floats to the surface during keeving. Its formation is the visible test that keeving has worked.
Chemistry
Pectic enzyme
A commercial enzyme preparation, usually fungal, that depolymerises pectin so that juice clarifies, presses more freely and yields more.
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.
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.