Stabilisation
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.
Also called Membrane filtration, Cold sterile filtration, Microbiological stabilisation by filtration.
- Stage
- Stabilisation
- Traditional in
- No single tradition — used wherever it suits
- What it most changes
- Body down
- Safety
- None recorded
What it is
Sterile filtration uses a membrane whose pore structure is fine enough and uniform enough to hold back viable microorganisms, so that the cider on the downstream side carries no organism able to grow in the package. Unlike a depth medium, a membrane has a defined and testable structure: it can be integrity-tested in place before and after a run, so the producer knows the barrier was intact rather than assuming it. It is always the last stage of a train, protected by coarser filtration upstream, and it is meaningless unless the filler, hoses, bottles and closures downstream of it are themselves sterile.
Why it is used
- A cider carrying residual or added fermentable sugar will referment in a sealed package if a single viable yeast cell is present, and removing the cells is the only intervention that addresses the cause rather than suppressing the symptom.
- It achieves microbiological stability at cellar temperature, so the fresh esters and fruit aroma that heat drives off are retained in a way pasteurisation cannot match.
- It allows a producer to present a sweet or medium cider without a preservative on the label, which matters commercially in markets where additive declarations are read closely.
- It gives a testable, documentable control point — the integrity test on the membrane — which a quality system can be built around.
How it works
- The membrane retains organisms mainly by size exclusion at a rating chosen for the target organisms, and its structure is regular enough that a physical integrity test — measuring the pressure at which gas breaks through the wetted membrane, or the rate of diffusion across it — reports whether the barrier held.
- Bacteria are smaller and more deformable than yeast, so a rating that reliably retains Saccharomyces cerevisiae is not automatically adequate against lactic acid bacteria; the choice of rating follows from which organisms actually threaten the product.
- A membrane has very little dirt-holding capacity of its own. Prefiltration through progressively finer depth or cartridge stages is what keeps it from blinding within a fraction of a batch.
- The filter defines the upstream edge of a sterile zone. Everything after it — the buffer tank, the filler bowl, the filling valves, the closures — must be sanitised and held sterile, because a single point of recontamination downstream makes the whole exercise worthless.
What it changes
The direction this step pushes the finished drink in, dimension by dimension. A direction, not a measurement: how far it moves depends on the juice, the temperature and how the step is carried out.
| Dimension | Direction | Why |
|---|---|---|
| Sweetness | Preserves | Residual and added sugars survive intact in the package because the organisms that would otherwise consume them have been physically taken out of the liquid. |
| Freshness | Preserves | The operation runs cold, so the volatile esters that a thermal process would strip stay in solution. |
| Body | Lowers | The prefiltration train needed to protect the membrane is itself adsorptive, so colloidal material is removed on the way to the membrane rather than by it. |
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.
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.
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.
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.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
Saccharomycodes ludwigii
A large, sulphite-resistant yeast that refements sweet cider in bottle and is one of the classic causes of unwanted secondary fermentation.
What it is done with
Filters: plate, sheet, cartridge and crossflow
Filtration ranges from coarse polishing that only brightens a cider to sterile membrane filtration that removes yeast and bacteria entirely; the equipment differs in whether it traps particles in a depth of material or on a surface, and in what it takes out of the cider along with the haze.
Sanitation: clean, sanitised, and why it decides the batch
Cider is an unsterile liquid at a pH and alcohol level that many spoilage organisms tolerate, so what survives on the equipment goes into the batch — which makes hygiene the largest single determinant of whether a small-scale ferment turns out drinkable.
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.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
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.
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.
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.
Brettanomyces character
Farmyard, horse-blanket, smoky and sticking-plaster aromas from Brettanomyces yeast converting hydroxycinnamic acids into volatile phenols.
Cooked character
A stewed-apple, caramel or jam-like character from heat applied to juice or cider, most often through pasteurisation or hot storage.
Flat and lifeless
A cider with nothing obviously wrong and nothing to say: aroma faded, acidity dulled, finish short — usually the cumulative result of over-processing or slow oxidation.
Geranium taint
A sharp, unmistakable crushed-geranium-leaf smell produced when lactic acid bacteria metabolise sorbic acid added as a preservative.
Colour loss
A cider left noticeably paler than it should be, usually because fining, filtration or sulphite has removed the phenolic material that gave it colour.
Framboise
A raspberry-and-rotten-fruit character with sulphurous overtones, produced by Zymomonas mobilis in sweet ciders that still contain sugar.
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.
Styles it produces
Categories in which this step is characteristic or required. Some name it in their definition; for others it is simply how they have always been made.
Sweet cider
Cider in which sugar is the leading sensation, whether retained from an arrested fermentation, added after it, or produced by keeving.
Medium cider
Cider carrying enough residual or added sugar to be perceptible without dominating, the commonest sweetness level in British retail.
Modern mainstream cider
The industrially produced, consistent, carbonated cider that accounts for most of what is sold worldwide, generally made partly from concentrate and finished to a fixed specification.
Sparkling cider
Cider carrying enough dissolved carbon dioxide to produce a persistent bubble, by bottle fermentation, tank fermentation or injection.
Ice cider
Cider made from apple juice concentrated by freezing, fermented slowly and stopped while a large residual sugar remains, giving a dessert-strength drink of high acid and high sweetness.
Fruit cider
Cider with other fruit added as juice, purée or flavouring, a category that now accounts for a large share of the global market and spans a very wide range of practice.
Modern American cider
The dominant contemporary American category: cider from culinary and dessert apples, fermented clean, often carbonated and frequently flavoured, defined against the heritage sector rather than by any tradition.
Quebec ice cider
Ice cider made under Quebec’s reserved designation, which sets out how the juice may be concentrated, what may be added and what the finished product must contain.
Alcohol-free cider
A cider-derived drink at or near zero alcohol, distinct from apple juice in having been fermented, and facing the same problem of what replaces the ethanol.
Ice perry
Perry made from pear juice concentrated by freezing, in which the fruit’s unfermentable sorbitol adds to the residual sweetness the arrested ferment leaves behind.
Low-alcohol cider
Cider at a reduced strength, reached either by removing alcohol from a finished cider or by arranging for less to be produced, with different consequences for flavour.
More on sterile filtration
The problem sterile filtration solves is narrow and specific: a cider with fermentable sugar in it and a live yeast population is a fermentation waiting for a warm shelf. In a sealed bottle that fermentation raises pressure, throws sediment and can burst glass. There are only three ways out — remove the organisms, kill them, or inhibit them — and membrane filtration is the first of those. Because it works by physical exclusion rather than by chemistry or heat, it changes nothing about the liquid’s composition except what it takes out with the prefiltration, and it leaves no residue to declare. That combination is why it became the standard approach in commercial sweet cider wherever the capital could be justified.
It is also the least forgiving of the three. Pasteurisation in the closed package treats whatever is in the bottle, sealed, at the end of the line; a preservative travels with the liquid and keeps working. A membrane protects only the liquid that passed through it, at the moment it passed through, and hands responsibility for everything afterwards to the filling operation. A filler that has not been properly sterilised, a closure fed from an unsanitised hopper, a hairline fault in a hose seal — any of these puts organisms back in and the cider is no more stable than if the membrane had never been fitted. Small producers frequently underestimate this and end up with an expensive filtration followed by an ordinary bottling, which buys nothing.
The decisions in practice are the rating, the prefiltration train and the integrity-test regime. The rating follows from the risk: a dry cider going into keg for quick turnover is a different problem from a sweetened cider going into bottle for a long shelf life, and lactic acid bacteria change the answer from yeast alone. The prefiltration exists to make the membrane economic. The integrity test exists so that a failure is discovered before ten thousand bottles have been filled rather than afterwards, which is the difference between a scrapped filter cartridge and a recall. Done badly, the characteristic outcome is not immediate — it is a proportion of the pack refermenting weeks later, showing as haze, sediment, unexpected gas and, in the worst case, bottles failing in the trade.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Stabilisation
Filtration
Passing cider through a porous medium to reach a stated visual brightness, at a real cost in colloidal material, body and aroma.
Stabilisation
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.
Stabilisation
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.
Blending
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.
Packaging
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.
Juice treatment
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
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 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.
- 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 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.
- Why does everyone say cleaning matters more than anything else — Cleaning removes soil — pomace, lees, tartrate, protein and the biofilm growing in it. Sanitising reduces the number of viable organisms on a surface that is already clean. They are two separate operations and the second does not work without the first.
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.
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.