Fault
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.
Also called spoilage haze, bacterial cloud.
- Severity
- Serious — the batch may not be salvageable
- Where it starts
- Stabilisation, Packaging, Blending, Storage
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Maturation, Packaging, Storage
- Signs
- 5 recorded
What it is
Microbial haze is turbidity produced by living organisms multiplying in the cider — spoilage yeasts, lactic acid bacteria or acetic acid bacteria. Unlike the physical hazes it is not a cosmetic matter: the cloudiness is evidence of an active population that is also producing flavour compounds, acid or gas. A cider that becomes hazy after having been clear, particularly one already in bottle, should be treated as a microbiological question first and an appearance question second.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
How the batch behaves
- A cider that was bright becoming cloudy over weeks — Direction matters. Clearing is normal; rehazing is not.
To look at
- A silky or slightly swirling appearance when the bottle is tilted — Fine bacterial haze often shows as delicate swirls rather than an even cloud.
- New sediment forming in bottles after packaging
On the nose
- Haze accompanied by a change in smell — sour, vinegary, buttery or vegetal
In the mouth
- A prickle of gas in a cider that should be still
The words for it: Lees. Each links to what produces it.
When it appears. Develops in a cider that was previously clear, and continues to develop. Increasing turbidity in a bright cider is the single most useful pointer to a microbial cause.
It is the only haze on this list that is a warning rather than a cosmetic finding, which is why the first diagnostic move for any haze is to establish whether it is changing.
Various journals, Peter Mitchell / Cider and Perry Academy
What it is mistaken for, and how to tell
One observation per rival, chosen because it separates them rather than because it describes either. Several of these are not faults at all — a style feature, a normal outcome, or the fruit behaving as it does.
- Pectin haze — Direction of travel. A pectin haze was always there; a microbial haze is arriving.
- Yeast haze — Whether it settles and whether it smells. Primary yeast settles and smells of bread; a spoilage population does neither.
- Film yeast growth — Look at the surface as well as the body. Film yeast grows as a mat on top; a microbial haze is distributed through the liquid.
- Unwanted refermentation — Check for pressure. A refermenting bottle hazes and gains gas; a spoilage haze usually does not.
What is actually happening
The chemistry or microbiology behind it. Understanding the mechanism is what makes the prevention make sense rather than being a list of rules.
Suspended cells scatter light in proportion to their number and size, so a population growing from an invisible level to a turbid one represents a very large increase in cell count. Bacteria are smaller than yeast and produce a finer, more silken haze; spoilage yeasts produce something closer in appearance to ordinary yeast haze.
The organisms involved differ in what they need. Zygosaccharomyces bailii is notorious for tolerating preservatives, high sugar and low pH, and can grow from a tiny inoculum in a sweetened cider. Saccharomycodes ludwigii tolerates sulphite well and ferments slowly. Dekkera bruxellensis works on residual sugars over months. Lactic acid bacteria grow on sugar, malic acid, citrate or glycerol depending on species.
Because these organisms are metabolising as they multiply, the haze arrives with company: acetic acid, lactic acid, diacetyl, volatile phenols, tetrahydropyridines or gas, according to what is growing. The haze is generally the first thing noticed and the least important thing happening.
The gas is the reason this matters beyond flavour. Any organism fermenting sugar in a sealed bottle produces carbon dioxide, and a hazy bottle of sweet cider is describing an active fermentation in a pressure vessel.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Residual sugar in a cider that was not stabilised | Stabilisation | common |
| No effective free sulphur dioxide, particularly at a high pH | Stabilisation | common |
| Contamination from unclean hoses, taps, bottles or a filler | Packaging | common |
| Back-sweetening followed by bottling without filtration or pasteurisation | Blending | common |
| Preservative-tolerant spoilage yeasts surviving the treatment used | Stabilisation | occasional |
| Warm storage allowing a small surviving population to grow | Storage | occasional |
Can it be put right?
Most cider faults cannot be reversed. Where that is the answer it is given plainly — an honest 'this batch is what it is' is more useful than a procedure that will not work.
| Option | Effectiveness | What it involves |
|---|---|---|
| Identify what is growing before treating anything | Reliable | The haze itself is not the problem. Smell and taste the cider, check the gravity, and check whether bottles are gaining pressure; the answer determines everything else. |
| Treat hazy bottles as potentially pressurised | Reliable | Where the haze is in bottle and any sugar was present, chill the batch and handle it as described under unwanted refermentation before opening anything. |
| Filter or pasteurise the batch to remove the population | Partial | Stops further development where the cider is still in bulk and the flavour damage is limited. Does not undo anything already produced. |
| Sulphite to arrest the population | Partial | Works against some organisms and poorly against others, and depends heavily on pH. Take the rate from supplier guidance for the measured pH and stay within the applicable limit. |
| Fine or filter to clear the appearance and leave it at that | Unlikely to work | Treats the symptom. If the population is still viable the haze returns, and the flavour damage continues meanwhile. |
CiderHQ gives no additive dosage figures. An addition is a food-safety decision that depends on legal limits, on the juice in front of you and on what you are protecting against, and the right figure comes from your supplier’s or regulator’s own guidance rather than from a general reference.
Preventing it
- Ferment to genuine dryness, or stabilise deliberately if the cider is to retain sugar.
- Clean and sanitise everything the cider passes through on its way to the container, particularly the filler and the bottles.
- Maintain free sulphur dioxide appropriate to the measured pH, from supplier guidance and within legal limits.
- Where sweetness is wanted in a packaged cider, use sterile filtration or pasteurisation rather than relying on a preservative alone.
- Store finished cider cool, which slows every organism involved.
The value of separating microbial haze from the physical hazes is that it changes what the observation means. A pectin haze is a fact about the fruit. A microbial haze is a fact about a population that is currently growing, and the useful response is investigative rather than corrective.
The direction of travel is the single most informative sign. Cider clears as it matures, so a bright cider going cloudy is running the process backwards, and something is adding to the suspended matter faster than gravity is taking it away.
It is also the haze that has a safety dimension, because several of the organisms responsible ferment sugar and produce gas. A hazy bottle of sweet cider is not primarily an appearance problem; it is a bottle that may be gaining pressure, and it should be handled accordingly.
Prevention here is almost entirely about the two things that spoilage organisms need: a substrate and an opportunity. Removing the residual sugar removes the substrate for most of them, and cleaning the filler and the bottles removes the commonest opportunity.
The compounds involved
Glucose
The sugar yeast takes first and the smallest of the three in apple juice, which is why the last sugar left in a slow ferment is almost never glucose.
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.
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.
Acetic acid
The vinegar acid, made by bacteria oxidising ethanol whenever air reaches a cider, and the one fault in cider that no later processing can undo.
Lactic acid
The softer acid that replaces malic when malolactic fermentation runs, halving the acid a cider carries and changing its texture as much as its sharpness.
Sulphur dioxide
The antimicrobial and antioxidant on which most modern cidermaking depends, and whose effectiveness collapses as pH rises — which makes every sulphiting decision a pH decision first.
The organisms involved
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.
Saccharomycodes ludwigii
A large, sulphite-resistant yeast that refements sweet cider in bottle and is one of the classic causes of unwanted secondary fermentation.
Dekkera bruxellensis
The yeast behind 4-ethylphenol, and the organism that a cider tradition may regard as its signature or its ruin depending on where and how it is made.
Brettanomyces anomalus
The second Brettanomyces species regularly recovered from cider and beer, less studied than *B. bruxellensis* but capable of the same phenolic chemistry.
Lactobacillus brevis
An obligately heterofermentative lactic acid bacterium, renamed *Levilactobacillus* in 2020, associated with volatile acidity, biogenic amines and mousiness rather than with clean malolactic conversion.
Pediococcus species
Homofermentative lactic acid bacteria that grow in tetrads, associated in cider with ropiness, diacetyl and slow spoilage during maturation.
Acetobacter aceti
An acetic acid bacterium that oxidises ethanol to acetic acid wherever cider meets air, and the organism behind most volatile acidity in cider.
Where in the process it arises
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.
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.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
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.
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.
Faults it is confused with
These present similarly. What separates them is set out on each page.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
Brettanomyces character
Farmyard, horse-blanket, smoky and sticking-plaster aromas from *Brettanomyces* yeast converting hydroxycinnamic acids into volatile phenols.
Lactic off-flavours
Sauerkraut, sour milk, silage or cheesy notes from lactic acid bacteria working on sugars and other substrates rather than on malic acid alone.
Volatile acidity
The measurable fraction of acidity that can be steam-distilled off, dominated by acetic acid and read as vinegar sharpness in the nose and a hard, hot finish.
Framboise
A raspberry-and-rotten-fruit character with sulphurous overtones, produced by *Zymomonas mobilis* in sweet ciders that still contain sugar.
Ropiness
Cider that pours thick and pulls into an oily thread, caused by lactic acid bacteria secreting long-chain glucan polymers into the drink.
Where this comes up in a guide
The link lands on the step where the fault arises rather than at the top of the pathway, because that is where the decision that causes it is taken.
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.
- Can you fix a cider that has gone vinegary — Not really. Volatile acidity cannot be removed at small scale, and blending it away only dilutes it. The realistic options are to accept it, to blend a lightly affected batch into a much larger sound one, or to let it finish becoming vinegar.
- 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.
- Does cider contain sulphites — Most does. Sulphur dioxide is added to protect juice and finished cider, and fermentation itself produces a small amount even when none is added. European labels must declare it above 10 mg per litre.
- 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 much acetic acid is normal in cider — A small amount is present in every cider and contributes lift. Sound Asturian sidra natural has been measured at 0.2 to 0.3 g/L, and a clean modern cider much above about 0.7 g/L is generally showing a fault rather than a style.
Where to go next
- The fault finder — Describe what you can smell and see, and narrow it down from the signs.
- All faults — Grouped by where they come from and how serious they are.
- Cider science — The chemistry and microbiology these faults come out of.
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.
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.