Organism
Zygosaccharomyces bailii
A preservative-resistant spoilage yeast that refements sweetened cider and juice, and one of very few organisms able to grow through sorbate and benzoate at cider strength.
- Kind
- Yeast
- Binomial
- Zygosaccharomyces bailii
- Role
- Spoilage, Secondary fermentation
What it does
- Actively expels weak-acid preservatives from the cell and metabolises them, so sorbic and benzoic acid at legal use levels inhibit it far less than they inhibit other yeasts — the defining property of the species.
- Ferments glucose and fructose to ethanol and carbon dioxide in sweetened product, causing gas, haze, sediment and in sealed containers a pressure hazard.
- Grows at high sugar concentration and low water activity, so back-sweetened cider, concentrate and syrup are all within its range.
- Establishes from extremely low cell numbers, meaning that a small contamination surviving a filtration or a fill is enough to spoil a batch given time.
Conditions it works in
What the organism tolerates and what suppresses it. These are the levers a maker actually has: temperature, acidity, air, alcohol and sulphite.
| Condition | What is recorded |
|---|---|
| Temperature | Grows from roughly 5 °C to 37 °C; refrigeration slows it without stopping it. |
| pH | Tolerates low pH well and remains active through the whole cider range and below. |
| Oxygen | Facultative; ferments readily in sealed packaging. |
| Alcohol tolerance | High for a spoilage yeast, generally reported to around 12–15% ABV. |
| Sulphite tolerance | Notably resistant, and free SO2 at normal cider levels should not be relied on to control it. |
What it produces
Compounds this organism makes. Which organism made a compound usually decides whether it reads as a feature or as a symptom.
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.
Carbon dioxide
The other product of fermentation, which protects a cider from air while it is being made and, dissolved in the finished drink, changes both its texture and its perceived acidity.
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.
Sorbic acid
A preservative that stops yeast but not bacteria, and whose effectiveness — like that of sulphur dioxide — depends on pH, because only the undissociated molecule works.
Potassium sorbate
The soluble salt in which sorbic acid is actually added, used to hold sweetness in a bottled cider, and dangerous to use without sulphite because of what lactic bacteria do to it.
Benzoic acid
A weak-acid preservative added as sodium benzoate, more useful the lower the pH, and encountered in cider mainly in low-alcohol and soft products rather than in traditional ones.
Faults it causes
Faults this organism is implicated in. Several are faults only against a particular expectation — the same activity is a signature elsewhere.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
Bottle over-carbonation
More dissolved carbon dioxide in the bottle than intended or than the glass is rated for — a presentation problem at the mild end and a physical hazard at the severe one.
Gushing
Cider that erupts from the bottle on opening, either because it is over-pressurised or because something in it is nucleating the dissolved gas violently.
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.
Excessive sediment
More deposit in the bottle or vessel than the presentation intends, ranging from a normal conditioning yeast layer to a loose sludge that clouds every pour.
Sorbate off-flavour
A faint plastic, candle-wax or celery-like note from sorbic acid used as a preservative, distinct from the geranium taint that bacteria produce from it.
Geranium taint
A sharp, unmistakable crushed-geranium-leaf smell produced when lactic acid bacteria metabolise sorbic acid added as a preservative.
Microbial haze
Cloudiness caused by a growing population of spoilage organisms, and therefore a symptom of something worse rather than a clarity problem in itself.
Where in the process it appears
Sterile filtration
Membrane filtration at a rating that retains viable yeast and bacteria, which is what allows a sweet cider to be shelf-stable without heat or preservative — provided the filling that follows is aseptic.
Filtration
Passing cider through a porous medium to reach a stated visual brightness, at a real cost in colloidal material, body and aroma.
Sorbate stabilisation
Adding potassium sorbate to prevent yeast from restarting a fermentation in the package — an inhibitor rather than a killer, and one that must never be used without sulphite.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Pasteurisation
Heating cider enough to inactivate the organisms that would spoil it, either in the sealed package or in-line before filling, at a measurable cost in fresh aroma.
Back-sweetening
Adding sugar, juice or concentrate to a cider that has fermented dry — a straightforward adjustment that leaves the drink microbiologically unstable until something is done about it.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Bottling
Transferring finished cider into glass, where the dominant variable is how much oxygen the liquid picks up in the few seconds it takes to fill and close each bottle.
Ice concentration
Freezing pressed juice and drawing off the unfrozen fraction, so that water is removed as ice and everything else in the juice is left behind more concentrated.
Microbial succession
The ordered handover of a spontaneous ferment from apiculate yeasts to *Saccharomyces* to lactic acid bacteria, and the spoilage organisms waiting at the end of it.
Arrested fermentation
Deliberately halting a ferment while sugar remains, to obtain natural sweetness from the fruit rather than from an addition — and accepting the instability that follows.
Cold chain
Keeping an unpasteurised, unfiltered or back-sweetened cider refrigerated from the packaging hall to the point of sale, because refrigeration is the only thing holding it stable.
Secondary fermentation
Any fermentative event that follows the primary ferment — residual sugar refermenting, a deliberate second alcoholic fermentation, or the malolactic conversion of the maturation phase.
Juice storage
Holding unfermented juice sound between pressing and fermentation, by chilling, sulphiting, gas blanketing, freezing or aseptic filling.
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.
About Zygosaccharomyces bailii
Zygosaccharomyces bailii is the reason a back-sweetened cider cannot be stabilised by adding preservative and hoping. Most yeasts are inhibited by undissociated sorbic or benzoic acid, which crosses the membrane at low pH and acidifies the cytoplasm. Z. bailii runs energy-dependent efflux and degradative pathways that let it hold that off, and it can grow in the presence of these preservatives at the concentrations food law permits.
It compounds the problem by needing almost nothing to start. Spoilage from a handful of cells is documented for this species, which makes it a filtration and hygiene problem rather than a dosing problem: the intervention that works is removing it — sterile filtration, pasteurisation, scrupulous fill hygiene — not out-dosing it.
Where it does grow in sorbate-treated cider there is a second consequence. Lactic acid bacteria can reduce sorbic acid to a compound that rearranges and esterifies into 2-ethoxyhexa-3,5-diene, the intensely geranium-like taint that is one of the more unpleasant faults a cider can carry. The organisms differ but the trigger is the same: a preservative that failed to sterilise, in a product that still has live microbes in it.
For a cider maker the practical rule follows from the mechanism rather than from any figure. If a cider contains fermentable sugar and is to be sealed in a container, the sugar must be protected by removing the organisms or by killing them, and Z. bailii is the organism that decides whether the method chosen was adequate.
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 bottles explode — Because fermentable sugar was still present, or too much priming sugar was used, and the pressure exceeded what the bottle could hold. Never bottle a cider whose gravity is still falling, and never use bottles not designed for pressure.
- How do i make my cider sweeter
- How do i sterilise cider bottles — Wash them clean first, then sanitise with a no-rinse sanitiser or a sulphite solution, and fill while still wet with it. Bottles that look clean but have dried deposits inside are the usual source of bottle spoilage.
- How do i sweeten cider without it fermenting again — Either remove or kill the yeast — sterile filtration or pasteurisation — or use a non-fermentable sweetener. Sorbate alone is not enough: it prevents yeast multiplying but does not kill what is already there, and needs sulphite alongside it.
- Why did my sweetened cider start fermenting again — Live yeast met the sugar that was added back. Sweetening is only stable if the yeast has been removed by sterile filtration, killed by pasteurisation, or held in check by sorbate together with sufficient sulphite.
- Why does my cider gush out when i open it — Either the cider is over-carbonated, or it is warm, or nucleation sites such as yeast and haze are releasing the gas all at once. Chilling the bottle thoroughly before opening solves most cases that are not genuine over-carbonation.
Where to go next
- All organisms — Grouped by what each does in the ferment, and by what kind of organism it is.
- Compounds — The chemistry this microbiology produces.
- Troubleshooting — Work from the symptom in the glass back to the organism.
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.
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.
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.
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.