Fermentation
Fermentation monitoring
Watching a running ferment through gravity, temperature, smell and surface behaviour, and reading the rate of change rather than any single figure.
Also called Ferment tracking, Following the ferment.
- Stage
- Fermentation
- Traditional in
- No single tradition — used wherever it suits
- What it most changes
- Not recorded as moving a sensory dimension
- Safety
- None recorded
What it is
Monitoring is the routine of taking and recording a small set of observations on a fermenting batch at intervals: gravity, temperature, the smell at the vessel, the state of the surface, and whatever laboratory numbers the operation can obtain. Its purpose is diagnostic rather than descriptive. A single gravity reading tells a maker almost nothing on its own; a series of them tells whether the ferment is accelerating, holding, decelerating normally as sugar depletes, or decelerating abnormally while sugar remains. Nearly every intervention available during fermentation — nutrient, warmth, aeration, racking — has to be made before the ferment has finished, which means it has to be made on a trend.
Why it is used
- It converts a stuck ferment from a discovery made weeks later into a deviation spotted while the yeast population is still viable and can be helped.
- It establishes when the ferment is genuinely finished, which is the precondition for any decision about racking, stabilising or — critically — bottling.
- A record across batches and seasons is what lets a producer attribute a difference in the finished cider to something specific rather than to the vintage in general.
- Smell at the vessel catches nitrogen problems and early spoilage days before they become visible in any number.
How it works
- Gravity falls as sugar is converted to ethanol and carbon dioxide, so the slope of the gravity curve is a direct proxy for fermentation rate; the shape of that curve, not any point on it, is the diagnostic signal.
- Temperature taken at the same time separates causes — a decelerating ferment in a cooling vessel is a temperature problem, the same deceleration at steady temperature points to nitrogen or ethanol stress.
- Hydrogen sulphide is detectable by smell at concentrations far below anything a farm-scale operation could measure, so the nose is the most sensitive instrument most cellars possess.
- The cap of pomace fragments and yeast on an open ferment, or the head of foam in a closed one, indicates gas evolution rate directly, and its collapse marks the end of vigorous fermentation earlier than gravity does.
- A hydrometer reading in a briskly fermenting cider is disturbed by adhering carbon dioxide bubbles, so the sample is usually allowed to degas before the reading is taken.
The chemistry and the organisms
What is actually being changed, and by what. Each entry says what that compound or organism does in cider generally; this page is one place it does it.
Compounds involved
Carbon dioxide
The other product of fermentation, which protects a cider from air while it is being made and, dissolved in the finished drink, changes both its texture and its perceived acidity.
Ethanol
The alcohol yeast makes from fruit sugar, which converts a perishable juice into a keepable drink and carries most of its aroma to the nose.
Hydrogen sulphide
The rotten-egg gas a nitrogen-starved yeast produces, detectable at concentrations too small to measure easily, and removable only if it is caught before it becomes something worse.
Acetic acid
The vinegar acid, made by bacteria oxidising ethanol whenever air reaches a cider, and the one fault in cider that no later processing can undo.
Glucose
The sugar yeast takes first and the smallest of the three in apple juice, which is why the last sugar left in a slow ferment is almost never glucose.
Fructose
The dominant sugar of apple juice and the sweetest of the common sugars, which is why apple juice tastes sweeter than its total sugar figure implies and why a stuck ferment leaves sweetness behind.
Organisms involved
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
Acetobacter aceti
An acetic acid bacterium that oxidises ethanol to acetic acid wherever cider meets air, and the organism behind most volatile acidity in cider.
Film yeasts
A functional grouping rather than a taxon: the oxidative yeasts that form a skin on cider exposed to air and consume its alcohol and acid.
What it is done with
The hydrometer and the trial jar
A weighted glass float that sinks to a depth set by the density of the liquid, read against a scale on its stem; with a starting and a finishing reading it gives the sugar consumed and an estimate of alcohol produced.
The refractometer, and why it lies during fermentation
A refractometer measures how much a liquid bends light, which tracks dissolved sugar closely in juice and needs only a drop of sample — but ethanol bends light too, so once fermentation starts the reading is no longer a sugar measurement.
Thermometers and temperature measurement
Temperature drives fermentation rate, yeast stress and the aromatic compounds that result, and the only reading that means anything is one taken in the liquid — not on the outside of the vessel and not in the room.
Drawing a sample without spoiling the batch
Every sample is a small hole made in the protection around a batch: something goes in, air goes in with it, and cider comes out — so the technique is about drawing a representative sample while putting nothing back and letting in as little air as possible.
What can go wrong
Faults that begin at this step, or that this step makes more likely. Each one is set out with its causes and whether it can be put right.
Stuck fermentation
A fermentation that has stopped before the sugar is gone and will not restart, leaving a sweet, low-alcohol cider that is vulnerable to everything.
Sluggish fermentation
A fermentation that is still moving but far more slowly than it should, extending the period during which the cider is weak, sweet and exposed.
Hydrogen sulphide
A rotten-egg or drain smell from hydrogen sulphide produced by stressed yeast, usually the first visible consequence of a nitrogen-short juice.
Volatile acidity
The measurable fraction of acidity that can be steam-distilled off, dominated by acetic acid and read as vinegar sharpness in the nose and a hard, hot finish.
Film yeast growth
A pale, powdery or wrinkled film of aerobic yeast growing on the surface of a standing cider, consuming alcohol and acid and producing acetaldehyde.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
Acetaldehyde excess
A bruised-apple, green-nut or sherry aroma from acetaldehyde, produced by oxidation, by film yeast, or left behind by a ferment that was interrupted.
Nitrogen deficiency character
The set of characters a nitrogen-starved fermentation produces together — sulphide, a stalled or dragging ferment, harsh higher alcohols and a thin, hard cider.
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Styles it produces
Categories in which this step is characteristic or required. Some name it in their definition; for others it is simply how they have always been made.
More on fermentation monitoring
The single most useful habit in monitoring is to plot rather than to list. A column of gravity figures in a notebook invites the wrong question — "what is it now?" — where a curve invites the right one, which is "is this still behaving like a healthy ferment?" A ferment slows as it proceeds because sugar is depleting and ethanol is accumulating, and that deceleration is entirely normal. What is not normal is a sharp break in the slope while a good deal of sugar remains. That break, dated and matched against the temperature log, is what makes the difference between a diagnosis of cold, of nitrogen exhaustion, or of a strain that has hit its ethanol limit. The reading alone cannot distinguish those; the shape of the curve and its context can.
What is measurable varies enormously with the scale of the operation, and it is worth being clear that the difference is one of resolution rather than of competence. A commercial plant with a laboratory can run an enzymatic assay for residual glucose and fructose, which is the only way to establish genuine dryness, since a hydrometer reading in finished cider reflects everything dissolved and not just sugar. It can measure yeast assimilable nitrogen in the juice and plan the nutrient regime against it, and it can track volatile acidity to catch acetic spoilage before it is obvious. A farm operation typically has a hydrometer, a thermometer, a notebook and a nose, and these are enough to run good cider, provided the readings are taken often enough and actually written down.
Perry deserves a specific caution here. Because sorbitol is largely unfermentable, a perry finishes at a gravity that would indicate substantial residual sugar in a cider, and a maker reading it as a cider gravity may conclude that the ferment has stuck when it has finished. Conversely a genuinely stuck perry can look normal. Where the distinction matters — and it matters absolutely before bottling — the answer is an assay for fermentable sugar rather than a gravity reading. The other thing to record is what was done and when: pitched, racked, nutrient added, moved, topped up. A gravity curve without a record of interventions explains nothing when a batch turns out unlike its neighbours.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Fermentation
Gravity measurement
Measuring the density of juice or fermenting cider to follow sugar depletion and estimate alcohol, with the instrument limits and the perry complication that make the number less simple than it looks.
Fermentation
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.
Fermentation
Yeast nutrition
What a fermenting yeast population actually needs from apple juice — assimilable nitrogen, vitamins and membrane lipids — and what goes wrong when the juice cannot supply it.
Fermentation
Restarting a stuck fermentation
Diagnosing why a ferment has stopped with sugar remaining, then building an acclimatised starter and stepping the cider into it rather than pitching yeast into the problem.
Maturation
Racking
Moving cider off the sediment it has thrown, which both cleans the liquid and — by taking yeast and nitrogen away with the deposit — slows what is left of the ferment.
Fermentation
Oxygen management in fermentation
Giving the yeast the oxygen it needs early to build viable membranes, then excluding it once fermentation is under way and especially once it slows.
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 know when my cider has finished fermenting — Take two hydrometer readings several days apart: if the gravity has not moved, the ferment is over. Airlock activity is not a reliable test, because a slow ferment can produce gas too slowly to see.
- What temperature should cider ferment at — Most cider is fermented cool, commonly between about 12 and 18 °C. Cooler ferments keep more fruit aroma and run slower; above the low twenties the cider tends towards hot, solvent-like higher alcohols.
- How do i restart a stuck cider fermentation — Warm the batch gently, then build an active starter and acclimatise it to the cider in stages rather than pitching dry yeast straight in. Yeast dropped into a cold, alcoholic, nutrient-poor liquid usually dies without restarting anything.
- What is racking in cider making — Racking is siphoning cider off the sediment it has thrown into a clean vessel, leaving the lees behind. It clarifies the cider and, in traditional practice, is also used to slow a ferment by removing yeast with the lees.
- How many calories are in cider — Roughly 40 to 60 kcal per 100 ml for most ciders, so a UK pint falls somewhere around 200 to 250 kcal. Alcohol contributes about 7 kcal per gram and residual sugar about 4, so both strength and sweetness matter.
- Which yeast ferments cider — Saccharomyces cerevisiae finishes almost every cider fermentation, whether it was pitched or arrived from the press house. In a spontaneous ferment it is not the first organism present, only the one that survives the alcohol it makes.
Where to go next
- How cider is made — The whole sequence, stage by stage, with the choices open at each one.
- Cider science — The chemistry and microbiology the methods on this page rest on.
- Troubleshooting — What goes wrong, how to recognise it, and whether it can be reversed.
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.
Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.
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.
Research on pear juice composition and sorbitol in perry
Various journals and institute reports · peer-reviewed literature · registered as competent for this subject
The evidence base for the single most important chemical difference between cider and perry: pears carry substantial sorbitol, which yeast does not ferment, so a fully fermented perry retains sweetness a fully fermented cider cannot.