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.
Also called Yeast nutritional requirement, Nitrogen status.
- 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
Yeast nutrition is the requirement side of the fermentation ledger: what the yeast must obtain from the juice in order to build a population large and healthy enough to consume the sugar. Sugar supplies carbon and energy, but a yeast cell is mostly protein and membrane, and both need materials the juice may or may not carry. The principal requirement is assimilable nitrogen, followed by thiamine and other vitamins, minerals, and — under fermentative conditions — preformed sterols and unsaturated fatty acids that the cell cannot make for itself once oxygen is gone. Apple and pear juice are frequently poorer in all of these than grape must.
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.
Why it is used
- Understanding the requirement is what allows a maker to diagnose a slowing ferment correctly rather than treating every stall as a temperature problem.
- Nitrogen status determines the final population size, and therefore whether the yeast is numerous enough to finish the sugar before ethanol stress becomes limiting.
- The most common aroma fault in an otherwise well-run inoculated cider — hydrogen sulphide — arises directly from nitrogen starvation, so the requirement explains the fault.
- Knowing that sterols and unsaturated fatty acids are only made when oxygen is present explains why the timing of aeration, not merely its presence, governs whether a ferment finishes cleanly.
How it works
- Yeast assimilable nitrogen comprises ammonium ions and the alpha-amino nitrogen of most free amino acids; the yeast takes these up through specific permeases and uses them to build the proteins and enzymes of a growing population.
- Proline, which is often a large share of the total amino nitrogen in fruit juice, requires oxygen for its catabolism and is therefore effectively unassimilable under fermentative conditions — a juice can appear nitrogen-rich on a total amino acid figure and still starve the yeast.
- Thiamine is a cofactor for pyruvate decarboxylase, the enzyme that carries pyruvate into the alcoholic pathway; a thiamine shortage slows the ferment and increases the accumulation of pyruvate-derived compounds including acetaldehyde.
- Sterols and unsaturated fatty acids maintain membrane fluidity and ethanol tolerance, and their synthesis is oxygen-dependent — a population that receives no oxygen in its growth phase inherits a fixed lipid ration diluted at every subsequent division, which is why nutrient-adequate ferments can still stall late.
- When assimilable nitrogen is exhausted while sugar remains, the yeast turns to the sulphate reduction pathway for sulphur but has no nitrogen skeleton onto which to fix the resulting sulphide, so hydrogen sulphide is released into the cider and may go on to form mercaptans.
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
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.
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.
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.
Thiamine
A vitamin yeast cannot ferment without, frequently limiting in cider juice, and destroyed by the sulphite added to protect that juice.
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.
Mercaptans
What hydrogen sulphide becomes if it is left alone: onion, garlic and rubber notes that are far harder to remove than the gas they came from.
Dissolved oxygen
Essential to a healthy yeast population at the start of fermentation and the principal enemy of a cider from the moment fermentation ends.
Acetaldehyde
The compound sitting one step short of ethanol, which smells of bruised apple and sherry, binds most of the sulphite added to a cider, and is the chemical signature of oxidation.
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.
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.
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.
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.
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.
Mercaptan taint
Onion, garlic, burnt rubber and cooked-cabbage aromas from thiols and disulphides formed when hydrogen sulphide is left in cider long enough to react onwards.
Reduction
A closed, stale, slightly sulphurous character in cider held under strongly oxygen-free conditions, sometimes clearing with air and sometimes not.
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.
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.
Recorded figures
Shown with the place, period and method each was taken under, and never averaged: the same step run in another cellar genuinely gives a different number.
Assimilable nitrogen59.0 mg/L (9.0–249.0)
Virginia, United States, 2014–2015 · 108 samples · Free amino nitrogen plus ammonium ion, summed, across two seasons · Boudreau, Peck, O’Keefe and Stewart, Food Science & Nutrition 6(1):119–123
The reason this page exists. Ninety-four per cent of these juices sat below 140 mg N/L, the figure wine practice treats as the minimum for a complete ferment, and the spread between the poorest and richest sample was twenty-sevenfold. Apple juice being short of nitrogen is the normal condition rather than a problem year.
The nitrogen yeast can actually use. Cider juice is often short of it, which is why ferments stall. Measured in milligrams per litre.
Ammonium nitrogen8.0–9.0 mg/L
Virginia, United States, 2014–2015 · 108 samples · Enzymatic ammonia assay; the paper’s abstract and results section round the mean differently · Boudreau, Peck, O’Keefe and Stewart, Food Science & Nutrition 6(1):119–123
The fraction yeast consumes first, and the fraction apple juice has almost none of — about a seventh of the assimilable nitrogen, against a much larger share in grape juice. This is the specific deficiency that diammonium phosphate addition is aimed at, and it is why grape-derived nutrient advice does not map cleanly onto cider.
The inorganic fraction of assimilable nitrogen, taken up first and exhausted first. Measured in milligrams per litre.
More on yeast nutrition
Apple juice is a difficult substrate for yeast in a way that is easy to miss because it ferments anyway. Its assimilable nitrogen is typically well below what a wine yeast would meet in grape must, and the position is worse than a total nitrogen figure suggests because much of the amino nitrogen in fruit is proline, which yeast cannot use without oxygen. The juice is also low in the vitamins and in the lipid precursors a growing population needs. Fruit that has been kept in store, juice that has been heavily settled or clarified, and juice from trees on poor or heavily grassed orchard floors are all poorer again, because settling and racking remove the suspended solids that carry much of the lipid and micronutrient load.
The consequence is that nitrogen becomes the limiting resource in most inoculated cider ferments, and the timing of its exhaustion determines what happens next. Nitrogen consumed early builds a large population that then runs out of food and turns to sulphur metabolism; the ferment continues but the cider stinks. Nitrogen exhausted before the population is large enough gives a sluggish ferment that fades and stops. The rate of change in gravity is the practical warning: a ferment slowing steadily is normal, one that decelerates sharply while sugar remains is telling the maker something about nitrogen. A whiff of hydrogen sulphide at the airlock partway through is the same message arriving through a different sense.
The mirror image of this is the keeved ferment, in which nitrogen starvation is the entire objective. Keeving strips the juice of nitrogen and suspended solids by pectin gelation, and the resulting ferment is slow and weak by design, stopping with sugar unfermented. That is the clearest demonstration that yeast nutrition is not simply a thing to be maximised: it is a variable a maker sets according to the drink they want. The additions themselves — diammonium phosphate, complex organic preparations, thiamine and their staging — belong to `nutrient-addition`. What this record establishes is why those additions exist, why an organic nutrient behaves differently from an ammonium salt, and why adding all of it at pitching is worse than adding it in stages.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Juice treatment
Nutrient addition
Supplementing a characteristically nitrogen-poor juice so that yeast can complete fermentation without producing sulphide or stalling.
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.
Fermentation
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.
Fermentation
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.
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.
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.
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.
- Can you make cider from shop bought apple juice — Yes, provided the juice contains no preservative — check for potassium sorbate or benzoate on the label. Pasteurised juice ferments perfectly well once yeast is added, because pasteurisation removes the organisms but not the sugar.
- 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 keeving — Keeving is a technique for starving a ferment of nitrogen so that it stops before all the sugar is gone, leaving a naturally sweet cider. Pectin is made to gel and float as a brown cap, carrying nutrients and yeast out of the juice with it.
- 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.
- 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.
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 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.
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.
Peer-reviewed literature on cider fermentation microbiology
Various journals · peer-reviewed literature · registered as competent for this subject
Covers the microbial succession of spontaneous cider fermentation, the role of non-Saccharomyces yeasts in the early stages, malolactic conversion by Oenococcus and Lactobacillus species, and the organisms behind the principal spoilage faults.
Boudreau, Peck, O’Keefe and Stewart, Food Science & Nutrition 6(1):119–123 · peer-reviewed literature · passage verified 2026-08-24 · covers 2014–2015
Open access; read in full on 2026-08-24. The paper that gives cider its own nitrogen numbers instead of borrowing wine’s. A hundred and eight apple samples from Virginia over two seasons, with the two fractions of assimilable nitrogen measured separately — which matters, because the finding is that apple juice is short of ammonium specifically, and that the wine practice of measuring both fractions may be unnecessary for apples while the wine practice of assuming there is enough nitrogen is badly wrong.
Song, Gibney, Cheng, Liu and Peck, Frontiers in Microbiology 11:1264 · peer-reviewed literature · passage verified 2026-08-24 · covers 2016
Open access; read in full on 2026-08-24. This is the paper CiderHQ cites against the folk rule that low nitrogen causes sulphide and adding nutrient cures it. Two Saccharomyces strains fermented the same Cornell juice at three nitrogen levels: one strain produced no detectable hydrogen sulphide at any level, and in the strain that did, the middle nitrogen treatment produced the most — twice the low treatment and six times the high. A rule that is true on average is being applied to individual batches where it can be exactly backwards, and the correction is worth a page of its own.