Equipment
The refractometer, and why it lies during fermentation
Can I use a refractometer to track cider fermentation?
In short
A refractometer shines light through a thin film of sample and reads the angle at which it is refracted, which depends on what is dissolved in the liquid. Calibrated against sucrose solutions, it gives a Brix reading.
For juice this is excellent: two drops, an instant reading, no sample lost. It is the standard field instrument for checking fruit and juice sugar at the mill and the press.
Once alcohol is present the reading is wrong, because ethanol has a much higher refractive index per unit mass than sugar and pushes the reading up while the actual sugar is falling. A refractometer cannot be used to follow a ferment without a correction, and corrections for cider are approximate.
What it is good at
The instrument’s great advantage is sample size. A hand refractometer needs a drop or two, which means a single apple can be tested, a press run can be checked every few minutes without losing volume, and a hundred lots can be screened in an hour. Nothing else in practical cider measurement is that quick.
For orchard and press work this is the right tool. Assessing ripeness across a block, checking whether an early press run differs from a late one, deciding which lots to keep separate, monitoring a juice blend — all of these are Brix questions on sugar-only liquid, which is exactly what a refractometer answers.
Digital and optical instruments both work on the same principle. The optical ones are read by eye against a scale through an eyepiece and need a light source; the digital ones read a sensor and are easier to use in poor light or with tired eyes. Both need automatic temperature compensation, or a temperature correction applied, because refractive index shifts with temperature.
Why it fails once yeast is working
A refractometer measures a bulk optical property of the solution, not sugar specifically. Every dissolved substance contributes. In unfermented juice the dissolved substances are overwhelmingly sugars, plus acids and minerals whose contribution is small and roughly constant, so Brix tracks sugar well. In fermenting cider a new substance appears in quantity — ethanol — and its effect on refractive index is disproportionately large relative to its mass.
The practical result is that the reading falls far more slowly than the sugar does, and a cider that has fermented to dryness still gives a substantial apparent Brix. A maker who reads that as residual sugar concludes the ferment is stuck when it is finished, or adds nutrient and warmth to a cider that needs neither.
Correction formulae exist that take the original Brix and the current Brix and estimate the true current gravity and alcohol. They are empirical fits derived largely from beer and wine work, they assume a particular fermentation stoichiometry, and their accuracy on cider is limited. They are usable for a rough indication and are not a substitute for a hydrometer reading at the points that matter.
Using both instruments together
The arrangement most working makers arrive at is to use the refractometer for everything before fermentation and the hydrometer for everything during and after it. The starting gravity is taken with a hydrometer even if a Brix figure is already in hand, because that is the number the finishing reading will be compared against and the two must be on the same scale and the same instrument.
During fermentation the refractometer can still be useful as a quick relative check — a reading that has stopped moving at all is worth investigating even if its absolute value is meaningless — but the decisions are made on gravity.
Calibration matters for both. A refractometer is zeroed on distilled water at its reference temperature before a session, using the adjusting screw or the digital zero function. An instrument that has not been zeroed since last autumn should not be trusted with a decision about when to press.
Brix is not a sugar assay
Also answered on this page
Questions this page covers, so you can tell at a glance whether it is the one you want.
- Why does my refractometer still read high in finished cider?
- Brix or specific gravity for cider?
- How do you calibrate a refractometer?
Related
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.
- How do i use a hydrometer to measure cider — A hydrometer floats at a depth determined by the density of the liquid it is in. Sugary juice is denser than water, so the hydrometer floats high; as fermentation converts sugar to alcohol the density falls and it sinks.
- 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.
- 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.
- When are cider apples harvested — In the northern hemisphere the cider harvest runs from about September to November, with late bittersweets such as Dabinett and Vilberie coming in last. Cider fruit is generally picked riper than dessert fruit, and often gathered from the ground.
- Which measurements can i do myself and which need a laboratory
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.
Hochschule Geisenheim University — beverage technology
Hochschule Geisenheim · university · retrieved 2026-08-24
German beverage-technology research covering apple wine and fruit juice processing, including the enzymology of clarification.
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.
Where to go next
- Cider making equipment — The rest of the equipment material, grouped by what each piece is for.
- How cider is made — The methods this equipment exists to carry out.
- Troubleshooting — The faults that are traced back to equipment and to cleaning.