Organism
Zymomonas mobilis
The bacterium behind cider sickness — maladie de la framboise — a fermentative spoilage of low-acid sweet cider that produces gas, haze and a raspberry-like odour.
Also called Zymomonas anaerobia.
- Kind
- Bacteria
- Binomial
- Zymomonas mobilis
- Role
- Spoilage
What it does
- Ferments glucose and fructose to ethanol and carbon dioxide through the Entner–Doudoroff pathway — unusually for a bacterium, an almost yeast-like fermentation, and a vigorous one.
- Produces acetaldehyde in quantity, which together with hydrogen sulphide and other volatiles gives the characteristic raspberry-like or framboise aroma the fault is named for.
- Generates copious carbon dioxide in sweet cider, causing gushing, over-carbonation, permanent haze and a milky, unstable appearance.
- Requires a relatively high pH to grow, so it is a fault of low-acid ciders specifically and is absent from sharp ones.
- Attacks cider with residual sugar, so it is a hazard of sweet and keeved styles rather than of dry cider.
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 across roughly 20 °C to 35 °C; warm storage of sweet cider is the classic trigger. |
| pH | The critical limit: growth requires roughly pH 3.7 and above, and adequate acidity is the primary defence. |
| Oxygen | Anaerobic to microaerophilic; it ferments happily in sealed containers. |
| Alcohol tolerance | Tolerates cider strength; ethanol is its own product. |
| Sulphite tolerance | Sensitive to free sulphur dioxide, which is the second line of defence after acidity. |
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.
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.
Faults it causes
Faults this organism is implicated in. Several are faults only against a particular expectation — the same activity is a signature elsewhere.
Framboise
A raspberry-and-rotten-fruit character with sulphurous overtones, produced by *Zymomonas mobilis* in sweet ciders that still contain sugar.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
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.
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.
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.
Acetaldehyde excess
A bruised-apple, green-nut or sherry aroma from acetaldehyde, produced by oxidation, by film yeast, or left behind by a ferment that was interrupted.
Where in the process it appears
Acid adjustment
Correcting the acidity of low-acid juice before fermentation, usually with malic acid, and the difference between the pH question and the titratable acidity question.
Sulphiting
Adding sulphur dioxide to juice to suppress spoilage organisms and oxidative browning, at a level that only means anything once the juice pH is known.
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.
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.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
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.
Acid balancing
Bringing a cider to the sharpness it needs, which requires separating perceived sharpness from titratable acidity from pH — three related things that do not move together.
About Zymomonas mobilis
Cider sickness is a fault with a long documented history in England and France, and it is one of the few in which the organism responsible was identified from cider itself rather than borrowed from wine or beer microbiology. Zymomonas mobilis is also known from palm wine and pulque, where it does the same thing on purpose.
What makes it unusual is that it is a bacterium fermenting like a yeast. Through the Entner–Doudoroff pathway it converts glucose and fructose to ethanol and carbon dioxide rapidly and in quantity, and the acetaldehyde it produces along the way gives a sweet, penetrating, raspberry-like aroma that is unmistakable once met. The cider becomes gassy, turns milky, throws a permanent haze, and in bottle can build dangerous pressure.
The defence is acidity, and this is one of the clearest cases in cider where pH is not an abstraction. The organism does not grow below roughly pH 3.7. Traditional low-acid bittersweet blends sit uncomfortably close to that line, which is exactly why blending in sharp fruit, or adjusting acid where the tradition permits it, is a microbiological measure and not only a matter of taste. Sulphite, adequate acidity and clean sweet-cider handling together make the fault rare in modern production; it was much commoner when none of the three was routine.
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 balance acid and tannin in a blend — Acid and tannin do different jobs and cannot substitute for each other: acid gives freshness and microbiological safety, tannin gives structure and length. A blend short of acid tastes flabby however tannic it is.
- 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.
- 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.
- 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.
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.
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.
Long Ashton Research Station cider fruit analyses
National Fruit and Cider Institute / University of Bristol · research institute · registered as competent for this subject · covers 1903–2003
The foundational body of cider-fruit science in English. Long Ashton produced the acid-and-tannin classification that divides cider apples into sweet, sharp, bittersweet and bittersharp, and analysed hundreds of cultivars grown at its Somerset site. Its figures are historic measurements of specific fruit at a specific place, not universal constants — a distinction CiderHQ preserves in every measurement record that cites it.