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Why cider fermentations stall

Why has my cider stopped fermenting?

In short

Most often because the yeast has run out of something other than sugar. Assimilable nitrogen is the usual culprit in cider, because apple juice frequently carries less than the population needs to finish.

The other common causes are temperature — yeast activity falls sharply as a cellar cools, and a ferment can go dormant rather than finish — and an excess of sulphur dioxide, which suppresses the population at the outset.

A distinction worth making is between sluggish and stuck. A sluggish ferment is running slowly and will finish given time and warmth; a stuck one has stopped and will not restart without intervention. In cider the first is far more common than the second, and is frequently mistaken for it.

Nitrogen and sulphide are not a straight lineTwo yeast strains fermenting one apple juice at three nitrogen levels. The middle level produced the most hydrogen sulphide; one strain produced none at any level.0100200300123.750Low22.3 mg/L288.250Intermediate144.3 mg/L44.1250High369.3 mg/Lµg H₂S per 100 mLDiammonium phosphate addedStrain UCD522Strain UCD932 — none detectedBase juice 63.7 mg N/L, fermented at 20 °C
Two yeast strains fermenting one apple juice at three nitrogen levels. The middle level produced the most hydrogen sulphide; one strain produced none at any level.
Described in full
Shape
A grouped bar chart. Three groups along the bottom are the three diammonium phosphate treatments — low at 22.3, intermediate at 144.3 and high at 369.3 milligrams per litre, all added to one juice that already carried 63.7 milligrams of assimilable nitrogen per litre. Each group holds two bars, one per yeast strain, and each bar is labelled with its value.
Strain UCD522
Low 123.75, intermediate 288.25, high 44.125 micrograms of hydrogen sulphide per 100 millilitres. The intermediate treatment produced twice what the low one did and more than six times what the high one did.
Strain UCD932
Zero at all three treatments. No hydrogen sulphide was detected from this strain regardless of how much nitrogen it was given, so its three bars are drawn as a flat line at the axis and labelled “none detected”.
The threshold line
A dashed horizontal reference is not drawn, because the sensory threshold — about 0.41 micrograms per litre — is roughly a thousandth of the smallest bar here and would sit indistinguishably on the axis. That is the point worth taking away: every one of these fermentations was far above the concentration a nose can find.
What it corrects
The common rule is that low nitrogen causes sulphide and adding nutrient cures it. On these data, adding some nutrient was worse than adding none, and adding a lot was better than either — but only for the strain that made sulphide at all. Strain choice came before nitrogen.
Scope
One juice, one temperature — 20 degrees Celsius — one pitching rate, two strains, three treatments, in triplicate. The shape of the relationship is the finding; the numbers belong to this experiment.

The causes, and how to tell them apart

Why fermentations stop, and the evidence that points to each cause.
CauseWhat is happeningPointers
Nitrogen deficiencyThe population is too small and too poorly equipped to sustain sugar uptakeA slow ferment from the start; often sulphide aroma; low-input orchard fruit
Low temperatureYeast metabolism slows steeply with falling temperature and can effectively haltA ferment that stopped when the weather turned; restarts if warmed
Excess sulphur dioxideMolecular sulphur dioxide inhibits the yeast, particularly at low pH where the active fraction is higherA ferment that never started properly after sulphiting; a heavier dose than the pH warranted
Over-clarified juiceClarification removes nutrients and the solid particles yeast uses as nucleation and supportVery clear juice; a ferment that starts and then fades
Alcohol and pH togetherHigh alcohol and low pH stress the membrane simultaneously; either alone is tolerableA ferment that stops near the end, in a strong, high-acid juice
Poor early oxygenThe yeast could not synthesise sterols and unsaturated fatty acids for its membranesA ferment that starts vigorously and dies away early
Killer yeast or competitionOne yeast population suppresses anotherA spontaneous ferment that halts abruptly; uncommon and hard to confirm

Confirming that it has actually stopped

The first step is measurement rather than observation. Airlock activity is a poor indicator: a slow ferment can produce too little gas to be visible, and a vessel with a slight leak produces none at all. What settles the question is specific gravity, taken twice, several days apart, at a consistent temperature.

If gravity is falling, even very slowly, the ferment is running and the correct action in most cases is to wait. Cider ferments in cold conditions routinely take months, and a slow ferment is the traditional method rather than a malfunction.

If gravity is stable and above the expected end point, the ferment has stopped. The next question is whether the residual sugar matters: a cider that has stopped at a pleasant sweetness and is stable can simply be treated as a sweet cider, provided it is stabilised so that it does not restart in the bottle.

Restarting, and why it is difficult

Restarting is harder than starting, because a stuck ferment is a hostile environment: it already contains alcohol, may contain sulphite, and may contain compounds released by dying yeast that inhibit new cells. Adding fresh dry yeast directly to it usually fails.

The established approach is to build a starter and acclimatise it in stages — establishing a healthy population in a small volume of fresh juice, then adding portions of the stuck cider progressively so the population adapts to the alcohol before being asked to work in it. Warming the cider into the yeast’s active range, and correcting any nitrogen deficiency at the same time, are usually necessary alongside.

Prevention is materially easier. Adequate nitrogen, a stable temperature within range, sulphite dosed against the measured pH rather than by habit, a healthy inoculation or a properly built pied de cuve, and juice that is not clarified beyond what the ferment needs will together avoid most stuck fermentations.

A stopped ferment is not necessarily a finished oneCider that has stalled with sugar remaining can restart in a sealed bottle when the weather warms, producing unwanted carbonation and, at worst, burst glass. A cider bottled short of dryness must either be stabilised, kept cold, or bottled deliberately in pressure-rated glass.

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