Compound
Yeast-assimilable nitrogen
The nitrogen a yeast can actually use, which apple juice is chronically short of — the shortage behind both stuck fermentations and rotten-egg aromas, and the shortage keeving deliberately makes worse.
Also called YAN, Assimilable nitrogen, FAN.
- Class
- Nitrogen compounds
- Formula
- Not a single molecule — see below
- How often it matters
- Present in every cider
What it does in cider
- Supplies the nitrogen for every protein and every sugar transporter the yeast builds, so it sets how large a population the ferment can support and how fast it can work.
- Comprises free amino acids other than proline, plus ammonium: proline is present in apple juice but yeast cannot use it without oxygen, so it is excluded from the figure.
- Governs the completion of fermentation, because a population built on inadequate nitrogen loses sugar transport capacity late and leaves fructose behind.
- Determines whether a ferment produces hydrogen sulphide, since sulphide with no nitrogen partner to bond to leaves the cell as gas.
- Is removed on purpose in keeving, where the floating pectin gel carries nitrogen out of the juice so that the ferment must stop while sugar remains.
Measured figures
Shown as they were measured, with the context each was taken in. They are not averaged: a concentration recorded in one country's fruit in one decade is not a constant.
3 separate analyses of assimilable nitrogen. They are shown as they were measured, in their own contexts, and are not averaged — the same fruit grown somewhere else can genuinely give a different number.
Assimilable nitrogen30.0–150.0 mg/L
context not recorded · Assimilable nitrogen, excluding proline · Cornell University, School of Integrative Plant Science
The band cider apple juices commonly fall in, and the point of quoting it is the comparison: winemaking practice generally treats around 150 mg/L as the minimum for a clean, complete ferment, and a great many cider juices arrive below that before anything has been added. Orchard nitrogen status, cultivar, rootstock, crop load and season all move the figure, so it must be measured rather than assumed.
Assimilable nitrogen59.0 mg/L (9.0–249.0)
Virginia, United States, 2014–2015 · 108 samples · Enzymatic assay of free amino nitrogen plus ammonium ion, summed · Boudreau, Peck, O’Keefe and Stewart, Food Science & Nutrition 6(1):119–123
A twenty-sevenfold spread across a hundred and eight samples, which is the finding. Ninety-four per cent of them fell below 140 mg N/L, the concentration wine practice treats as the minimum for a complete ferment — so apple juice being nitrogen-poor is the ordinary case rather than the exception. The same cultivar on the same trees moved between the two seasons, so a figure measured once does not carry forward.
Assimilable nitrogen63.7 mg/L
Ithaca, New York, United States, 2016 · Rack-and-cloth pressed juice from a mixed culinary and European cider apple blend · Song, Gibney, Cheng, Liu and Peck, Frontiers in Microbiology 11:1264
One juice rather than a survey, recorded because the rest of that study’s findings only mean anything against it: this is the starting nitrogen from which three phosphate treatments were built. It sits almost exactly on the Virginia survey mean and well below the wine minimum.
The nitrogen yeast can actually use. Cider juice is often short of it, which is why ferments stall. Measured in milligrams per litre.
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Nutrient addition
Supplementing a characteristically nitrogen-poor juice so that yeast can complete fermentation without producing sulphide or stalling.
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.
Orchard nutrition
Deciding what the trees are fed and how much, a balance in which too little nitrogen shows up as a difficult fermentation and too much shows up as canker and soft growth.
Lees ageing
Deliberately holding a cider or perry on its fine yeast deposit so that autolysing cells release material that changes texture and foam behaviour.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Keeving
Forming a floating pectin gel that lifts nutrients and solids out of the juice, so that the ferment starves before it finishes and leaves natural sweetness behind.
Wild fermentation
Letting the microflora already present on the fruit and in the press house carry the ferment, in a succession rather than as a single organism.
Inoculated fermentation
Starting a ferment by pitching a chosen yeast culture so that one known strain, rather than the fruit’s resident population, does the work.
Juice settling
Letting freshly pressed juice stand cold and undisturbed so that gross solids fall, then racking the cleaner juice off the deposit before pitching.
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.
Faults it is implicated in
Being implicated is not the same as being a fault. Several of the compounds on this site are ordinary constituents of a sound cider and define a named fault only above a concentration.
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.
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.
Reduction
A closed, stale, slightly sulphurous character in cider held under strongly oxygen-free conditions, sometimes clearing with air and sometimes not.
Yeast haze
Cloudiness from yeast cells that have not settled out, usually because the strain flocculates poorly or the cider has not been left alone long enough.
Atypical ageing
A cider that loses its fruit unusually early and develops a flat, faintly acrid or naphthalene-like character — a syndrome described in white wine and less firmly established in cider.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
Saccharomyces bayanus
A name applied both to a hybrid *Saccharomyces* lineage and, loosely, to a whole class of commercial high-alcohol yeasts, and one of the least stable names in fermentation microbiology.
Hanseniaspora uvarum
The apiculate yeast most often reported from grapes and widely present on apples too, whose anamorph name *Kloeckera apiculata* still appears throughout older cider literature.
Metschnikowia pulcherrima
An early-succession yeast that suppresses competitors by locking up iron, and is used commercially as a controlled non-Saccharomyces partner rather than as a fermenter.
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.
About Yeast-assimilable nitrogen
Yeast is mostly protein, and protein is mostly nitrogen. To ferment a juice a yeast must first build a population, and the size of the population it can build is set by how much usable nitrogen the juice contains. Not all nitrogen counts: yeast can use ammonium and most free amino acids, but it cannot use proline anaerobically, and apple juice carries a substantial share of its nitrogen as exactly that. The figure that matters, assimilable nitrogen, is therefore smaller than a total nitrogen analysis suggests, and in cider it is often much smaller.
Why apple juice is short of it is a question about trees rather than about juice. An apple is a low-nitrogen fruit to begin with; the tree preferentially directs nitrogen to leaves and wood, and traditional cider orchards — grassed down, unfertilised, often with old standard trees carrying heavy crops — produce fruit lower in nitrogen still. Fruit stored for weeks before milling loses more. Where wine grapes routinely arrive at levels adequate for a clean ferment, cider apples routinely do not, and it is not unusual to find a juice at a third of what would be considered sufficient.
The consequences arrive in two forms, and both are commonly misdiagnosed. The first is a ferment that slows, sticks, or crawls for months. A yeast that could not build a proper population, and that then had to break down its own sugar transporters for nitrogen when the supply ran out, cannot finish, and the sugar it cannot finish is fructose. The second is hydrogen sulphide, because yeast reduces sulphate to sulphide as a matter of routine and needs a nitrogen skeleton to attach the sulphide to; if there is none, the sulphide leaves as gas. A cider that smells of rotten eggs is not usually dirty. It is hungry. Very low nitrogen also raises fusel alcohol production, so the ferment tastes hot and coarse as well.
The fix is to measure the juice and to feed the ferment — ammonium salts, complex organic nutrients, or both, added at pitching and again a third of the way through, since a single large addition at the start is used inefficiently and can encourage the wrong organisms. Thiamine is worth attending to alongside, because it is often limiting in cider juice as well, and a nitrogen addition alone will not fix a thiamine shortage.
And then there is keeving, which does the opposite on purpose. The whole point of the keeve is that the floating calcium pectate raft takes the juice’s nitrogen with it as it rises, leaving a clear juice too poor to support a complete fermentation. The ferment crawls, produces very little in the way of fusel and ester character, and finally stops with several per cent of sugar unfermented. A traditional French cidre doux is sweet because a maker deliberately starved the yeast. This is the clearest illustration in cider of a principle that runs through all of it: the same chemistry is a fault or a technique depending entirely on whether it was chosen.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Nitrogen compounds
Amino acids
The largest usable nitrogen fraction in apple juice, and the raw material from which yeast builds both its own protein and most of the aroma compounds a cider carries.
Nitrogen compounds
Ammonium nitrogen
The nitrogen form yeast takes up fastest and the one most nutrient additions supply, useful for rescuing a ferment and a poor substitute for a properly balanced juice.
Nitrogen compounds
Thiamine
A vitamin yeast cannot ferment without, frequently limiting in cider juice, and destroyed by the sulphite added to protect that juice.
Sulphur compounds
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.
Alcohols
Higher alcohols
The group of larger alcohols yeast makes from amino acids, welcome as background complexity in trace and harsh and solvent-like in quantity.
Sugars
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.
Polysaccharides
Pectin
The structural polysaccharide of fruit cell walls, which decides how much juice a press releases, whether a cider ever clears, and whether keeving is possible at all.
Polysaccharides
Calcium pectate
The gel formed when calcium bridges de-esterified pectin chains, which floats to the surface as the brown cap of a keeve and carries the juice’s nutrients out with it.
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.
- Why did my cider stop fermenting — The usual causes are a shortage of yeast-available nitrogen, a temperature that has dropped, too much sulphite at the start, or a yeast that has reached its alcohol limit. Check the gravity before assuming anything is wrong: many ciders simply finish.
- 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.
- How do i get rid of the sulphur smell in my cider — Racking with a little splashing usually blows off free hydrogen sulphide while it is still fresh. Once it has reacted into mercaptans the smell becomes rubbery and no longer responds to aeration.
- What does a nitrogen-starved cider taste like
- What does pectin do in cider — Pectin is the structural polysaccharide that holds fruit cells together. In juice it holds haze in suspension, and it is the molecule keeving depends on: strip its methyl groups and it will gel with calcium and float the nutrients out of the juice.
- Which sugar is most of the sugar in apple juice — Fructose, by a long way. Apple juice carries roughly twice as much fructose as glucose, with a smaller amount of sucrose, and fructose is the sweetest of the three on the tongue.
Where to go next
- All compounds — Grouped by what the fruit brings and what the ferment makes of it.
- Sensory — Every descriptor, with the compound or process that causes it.
- Microbiology — The organisms whose metabolism most of this chemistry belongs to.
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.
Cornell Cider Research and Extension programme
Cornell University, School of Integrative Plant Science · university · passage verified 2026-08-24
Runs cultivar trials in New York State and publishes juice chemistry for European cider varieties grown in a North American climate — the single most useful counterweight to treating English figures as universal.
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.
Institut Français des Productions Cidricoles (IFPC)
IFPC · research institute · retrieved 2026-08-24
The French technical institute for cider production. The authority for the French cultivar classification families, for keeving as an industrial process, and for the pectin and nitrogen chemistry that keeving depends on.
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.