Glossary · Chemistry
Polyphenol oxidase
Also written PPO, phenolase. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
The enzyme, released when fruit cells are broken, that oxidises phenolics to quinones in the presence of oxygen. It is the cause of the browning of cut apples and of milled pomace.
- Subject
- Chemistry
- Also written
- PPO, phenolase
In more detail
PPO activity is not simply a fault. Controlled oxidative browning in the pomace and in settling juice removes the most reactive phenolics as insoluble complexes, which lowers harsh astringency and, importantly, protects the finished cider against later oxidation — the phenolics that would have oxidised in the bottle have already gone.
It is inhibited by sulphite, by heat and by the removal of oxygen, so a producer’s choice about sulphiting at the mill is simultaneously a choice about colour, astringency and oxidative stability.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Chemistry
Polyphenol oxidase
The copper enzyme that turns cut apple brown within seconds, and the reason a cidermaker has to decide, at the press, whether to let the juice oxidise or to stop it.
Production
Juice oxidation
The enzymic browning that begins the moment apple juice meets air, which some traditions deliberately allow and others suppress.
Fault
Enzymatic browning
The rapid darkening of milled fruit and fresh juice as polyphenol oxidase converts phenolics to quinones, taking colour and some tannin structure with it.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
Polyphenol
The broad class of plant compounds carrying multiple phenol rings. In apples this includes procyanidins, hydroxycinnamic acids such as chlorogenic acid, flavonols, dihydrochalcones such as phloridzin, and anthocyanins in red-fleshed fruit.
Chemistry
Chlorogenic acid
The most abundant hydroxycinnamic acid in apple juice, and the principal substrate for enzymatic browning. It is also a precursor of the ethylphenols that *Brettanomyces* produces.
Fermentation
Sulphiting
Adding sulphur dioxide, usually as potassium or sodium metabisulphite, to juice or cider to suppress spoilage organisms, inhibit oxidation and enzymatic browning, and give a selected yeast a clear field.
Faults
Oxidation
The chemical damage done by oxygen to a finished cider: loss of fresh fruit aroma, browning, and the arrival of acetaldehyde, nutty and sherry-like notes.
Milling and pressing
Maceration
Holding milled pomace before pressing so that enzymes soften it and phenolic and aromatic material moves from the skins into the juice. Times range from an hour to a full day.
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.
Peer-reviewed literature on apple phenolics and cider sensory perception
Various journals · peer-reviewed literature · registered as competent for this subject
Registered as a class rather than as one paper, because the mechanisms CiderHQ describes — tannin chain length driving the split between bitterness and astringency, salivary protein precipitation, enzymatic browning — are established across many studies rather than resting on any single one. Individual papers are cited where a specific number is quoted.
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 national de recherche pour l’agriculture, l’alimentation et l’environnement (INRAE)
INRAE · research institute · retrieved 2026-08-24
French national agricultural research. Its Angers programme produced much of the published work on apple procyanidin chain length and on the relationship between polymer size, bitterness and astringency.