Fermentation
Yeast rehydration
Reviving active dried yeast in warm water before pitching, so the cells restore their membranes without being ruptured by the sugar concentration of juice.
Also called Rehydrating dried yeast, Yeast starter preparation.
- 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
Active dried yeast is supplied as dormant cells with most of their water removed. Rehydration is the short preparatory step in which those cells are stirred into clean water at a controlled temperature, left to take up water and repair their cell membranes, and then brought gently to the temperature of the juice before being pitched. It takes minutes and is skipped often, usually with no visible consequence and a quietly reduced viable population. Where a producer is pitching into cold, low-nitrogen juice, or asking the yeast to referment a cider that already contains alcohol, the difference between a properly rehydrated pitch and a sprinkled one becomes operationally significant.
Why it is used
- A dried cell dropped straight into juice meets a high sugar concentration before its membrane has reformed, and a substantial fraction of the population is killed outright by osmotic shock.
- A larger viable population reaches the exponential phase sooner, which shortens the lag phase during which spoilage organisms have an uncontested supply of sugar.
- Where the yeast is being asked to work in difficult conditions — cold juice, residual sulphite, or an existing alcohol level — the surviving fraction of a badly pitched population may not be enough to establish at all.
How it works
- A dried yeast cell has a disordered, leaky membrane; given plain water it rebuilds an intact lipid bilayer within minutes, after which it can control what crosses it.
- If the first liquid the cell meets is sugar-rich juice rather than water, water is drawn out of the cell across the still-permeable membrane and cytoplasmic contents leak out with it — the cell dies before it can begin metabolising.
- Rehydration water that is too cold leaves the membrane repair incomplete, and water that is too hot kills the cells directly, so the supplier’s stated range for that specific strain governs; there is no single correct figure across products.
- The temperature gap between the rehydrated slurry and the juice matters as much as the rehydration temperature itself, because a large sudden drop causes cold shock and a further loss of viability — the usual practice is to temper the slurry by adding juice to it in stages before pitching.
- Rehydration nutrients — proprietary preparations containing sterols, unsaturated fatty acids and micronutrients rather than simple nitrogen salts — are added to the rehydration water to give the reviving cells membrane precursors they cannot synthesise without oxygen.
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.
Thiamine
A vitamin yeast cannot ferment without, frequently limiting in cider juice, and destroyed by the sulphite added to protect that juice.
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.
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 it is done with
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.
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.
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.
More on yeast rehydration
Rehydration is one of the few steps in cider making where the mechanism is fully understood, the intervention is trivial, and it is nonetheless routinely omitted. The reason it is omitted is that the failure is invisible: a sprinkled pitch usually still ferments, because a surviving minority of a large population is still a large number of cells. What is lost is margin. The ferment starts later, the initial population is smaller and therefore has to divide more times to reach working numbers, and each of those divisions draws on the juice’s limited nitrogen. In a healthy, warm, nutrient-adequate juice none of this shows. In cold juice, in a heavily sulphited juice, or in a restart, it is the difference between a ferment and a failure.
The practical sequence is unglamorous. Weigh the water, bring it to the temperature the supplier specifies for that strain, stir in any rehydration nutrient, then add the yeast and leave it undisturbed briefly before stirring to a smooth suspension. Do not leave it standing far beyond the stated period: once rehydrated, the cells begin consuming their own reserves and lose viability in water. Then close the temperature gap by adding juice to the slurry in two or three increments, waiting between them, until the slurry is close to the temperature of the tank. Only then pitch. The whole operation is a matter of minutes, and the numbers on the sachet — not a remembered figure from another product — are what govern.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
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
Yeast selection
Choosing which cultured strain to pitch, on the basis of the temperature, nitrogen, alcohol and aroma behaviour that separates one commercial yeast from another.
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.
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.
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.
- What yeast should i use for cider — CiderHQ does not recommend brands. The choice is between a neutral, reliable strain that lets the fruit show, an aromatic wine strain that adds its own esters, and no addition at all. Alcohol tolerance, cold tolerance and nitrogen demand are the properties worth comparing.
- 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.
- 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.
- 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.
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.
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.