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.
Also called Specific gravity, Hydrometry, Taking a gravity reading.
- 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
Gravity measurement is the determination of a liquid’s density relative to water, either by floating a calibrated hydrometer in a sample or by reading the refractive index of a drop on a refractometer. In juice, dissolved sugar is by far the largest contributor to density, so the reading is a workable proxy for fermentable sugar and thus for potential alcohol. During and after fermentation the relationship breaks down, because ethanol is less dense than water while glycerol, acids, unfermentable sugars and dissolved solids are denser, and the single figure reported is the net effect of all of them rather than a measurement of sugar.
Why it is used
- The fall in gravity from the starting reading to the finishing reading is the basis of the arithmetic by which alcoholic strength is estimated without distillation or a laboratory.
- Tracked over time it gives the fermentation rate, which is the primary diagnostic signal available to a cellar without instruments.
- A starting gravity taken before fermentation tells the maker whether the juice will reach the strength intended, and therefore whether chaptalisation or blending is being considered.
- A gravity that has been stable across repeated readings over a period is a necessary — though on its own not a sufficient — indication that a cider is safe to bottle without stabilisation.
How it works
- A hydrometer floats at the depth at which it displaces its own mass of liquid, so a denser liquid supports it higher; the calibrated stem reads density directly and is unaffected by the presence of ethanol as such.
- A refractometer measures how much a liquid bends light, which in a sugar solution scales with sugar concentration — but ethanol has a markedly different refractive behaviour from sugar, so any refractometer reading taken after fermentation has begun is invalid without a correction that is empirical, instrument-specific and unreliable in cider.
- Hydrometers are calibrated at one stated temperature, and the density of both the sample and the glass instrument change with temperature, so a reading taken at a different temperature must be corrected using the scale supplied with that hydrometer.
- Suspended solids, carbon dioxide bubbles clinging to the stem and surface tension at the meniscus all shift the reading, which is why a sample is settled and degassed and read at the same point on the meniscus each time.
- The conversion from gravity drop to alcohol is a convention rather than a physical identity; the factor used should be one stated by a single reference and applied consistently, since figures from different sources are not interchangeable.
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
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.
Sucrose
The disaccharide of apple juice and the sugar most often added to it, split into glucose and fructose by the yeast’s own invertase before any of it is fermented.
Sorbitol
The unfermentable sugar alcohol that pears carry in quantity and apples carry only in trace, and the single reason a fully fermented perry keeps a sweetness a fully fermented cider cannot.
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.
Glycerol
A syrupy three-carbon alcohol yeast produces as a side reaction of fermentation, which adds weight to a dry cider and is the raw material for one of its more obscure faults.
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.
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.
Laboratory measurement versus practical measurement
Gravity, temperature, pH and total acidity are all within reach of a careful person with modest equipment; alcohol by volume, sulphur dioxide for a declaration, patulin, methanol and microbiological counts are not, and pretending otherwise is how numbers on labels become wrong.
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.
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 gravity measurement
Two instrument facts do most of the practical work. The first is that a refractometer is excellent before fermentation and useless during it. It needs a drop rather than a jar, which makes it ideal for checking incoming juice, individual pressings or a blend on the press floor. Once ethanol is present its response no longer maps onto sugar concentration, because alcohol refracts light quite differently from sugar; the correction formulae in circulation were fitted to particular products and do not transfer reliably. The second is that a hydrometer needs a settled, degassed sample at a known temperature. Carbon dioxide bubbles adhering to the stem lift it and give a falsely high reading, which is exactly the error most likely to occur in a vigorous ferment where the reading matters most.
The alcohol arithmetic deserves stating plainly: converting a gravity drop into a percentage of alcohol by volume is a convention, and different handbooks use different factors that are not interchangeable. `jolicoeur-handbook` sets out one such convention, and the workable practice is to adopt a single stated convention and apply it consistently rather than to mix figures from several sources and imagine the result is more accurate. The estimate is good enough for cellar decisions and for a producer’s own record. It is not good enough where a legal declaration is at stake, which is why labelled alcoholic strength is determined by distillation or by an instrumental method rather than from a hydrometer log.
Perry is the case where reading gravity as sugar goes most badly wrong. Pears carry a substantial proportion of their carbohydrate as sorbitol, a sugar alcohol that Saccharomyces largely cannot ferment, so a perry that has fermented completely still contains that sorbitol and finishes at a gravity well above the point a dry cider would reach. A maker who reads that figure as residual sugar concludes that the perry is stuck or sweet when it is neither, and a maker who back-corrects for it by assuming a fixed offset will be wrong in a different direction, since the sorbitol content varies with the pear. Where the answer must be right — before bottling, above all — an enzymatic assay for glucose and fructose settles what a hydrometer cannot.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
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.
Juice treatment
Sugar addition
Adding sugar or apple juice concentrate to raise potential alcohol before fermentation, and the regulatory and compositional consequences of doing so.
Carbonation
Priming
Adding a measured, calculable quantity of fermentable sugar at bottling so that the fermentation which follows generates a predictable volume of carbon dioxide.
Carbonation
Bottle conditioning
Carbonating cider by letting a second fermentation finish inside the sealed bottle, so the gas is generated where it is going to stay.
Juice treatment
Juice blending
Combining juices from different cultivars before fermentation so that the blend ferments as a single batch, as against fermenting separately and blending later.
Fermentation
Arrested fermentation
Deliberately halting a ferment while sugar remains, to obtain natural sweetness from the fruit rather than from an addition — and accepting the instability that follows.
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 long does cider take to ferment — A warm ferment with cultured yeast can finish in one to two weeks; a cool wild ferment in a cellar may take three months or more. Slow is not the same as stuck — the test is whether gravity is still falling.
- Do you add sugar when making cider — You do not have to: apple juice already contains enough sugar to ferment to roughly five or six per cent. Sugar is added when a maker wants more alcohol than the fruit will give, and in some jurisdictions how much may be added is restricted.
- How do you stop cider from fermenting — By removing the yeast, by chilling, by filtering it out, by pasteurising, or by a combination — and in practice a home maker cannot reliably stop a ferment mid-way with chemicals alone. Sorbate prevents yeast multiplying but will not stop an active ferment.
- How is sparkling cider made
- What is priming sugar — Priming sugar is a measured dose of sugar added at bottling so that the remaining yeast produces carbon dioxide inside the sealed bottle. Too much of it is the usual cause of burst bottles.
- 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.
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.
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.
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.