Glossary · Fermentation
Fermentation
Definition
The conversion of sugar to ethanol and carbon dioxide by yeast, together with the many secondary reactions that occur alongside it and which supply most of a cider’s aroma.
- Subject
- Fermentation
In more detail
The primary equation is simple — roughly 51 per cent of the sugar by weight becomes ethanol and 49 per cent carbon dioxide — but the sensory result is decided by the minority pathways: esters formed from acids and alcohols, higher alcohols from amino acid metabolism, glycerol from the yeast’s redox balance, and sulphur compounds when nitrogen runs short.
Temperature governs the balance. A cool ferment at 12 to 15 degrees Celsius retains fruit esters and runs slowly; a warm one at 20 degrees or above is faster, throws off more volatiles and tends towards higher alcohols. Traditional cellar fermentation in an unheated building is cool by default, which is one reason farmhouse cider ferments for months.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Production
Fermentation temperature control
Managing the temperature at which a ferment runs, which sets not only how fast it goes but which aromatics survive it and what the finished cider tastes of.
Production
Fermentation monitoring
Watching a running ferment through gravity, temperature, smell and surface behaviour, and reading the rate of change rather than any single figure.
Microbiology
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Fermentation
Primary fermentation
The vigorous first phase in which the bulk of the sugar is fermented, usually over one to several weeks, marked by visible gas evolution and a rapidly falling gravity.
Fermentation
Secondary fermentation
Any fermentation after the primary: the slow finishing of residual sugar, a deliberate refermentation in bottle or tank for carbonation, or a malolactic conversion by bacteria.
Fermentation
Attenuation
The proportion of the original sugar that fermentation has consumed, calculated from the fall in gravity from the starting to the current reading and expressed as a percentage.
Microbiology
Yeast
A single-celled fungus that ferments sugar to ethanol and carbon dioxide. Cider is fermented by a succession of yeast species, of which *Saccharomyces cerevisiae* usually finishes the job.
Chemistry
Specific gravity
The density of a liquid relative to water at the same temperature. In cider it is the routine measure of dissolved sugar before fermentation and of progress during it.
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.
The New Cider Maker’s Handbook: A Comprehensive Guide for Craft Producers
Claude Jolicoeur, Chelsea Green Publishing, 2013. ISBN 9781603584739 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against the Open Library union catalogue: Chelsea Green Publishing, 2013, ISBN 9781603584739, one edition recorded. That establishes the citation points at a real book in a stated edition, which is what a citation needs and is all it establishes. No copy was opened and nothing is quoted from it. The book itself is in print and not digitised in any open collection; where CiderHQ needs a figure from this territory it uses an accessible research source instead and says so.