Sensory descriptor
Soured milk
A sharp, unclean dairy note from uncontrolled lactic activity, recorded as a fault in every style.
Also called Sour milk, Off milk.
- Aroma family
- Dairy and lactic
- Does it indicate a fault?
- It depends on the style it is met in
What causes it
Where lactic acid bacteria continue working after the malic acid is exhausted, they turn to other substrates — residual sugars, glycerol, quinic acid, amino acids — and the products are less predictable and less welcome than lactic acid and diacetyl. The combination of lactic acid, excess acetaldehyde, short-chain fatty acids and amine compounds reads as soured or spoiled dairy rather than as clean fermented dairy. It is a signature of low-sulphite, warm, long-standing cider with an established and unmanaged bacterial population.
Unlike butter, this has no level at which it belongs. It marks the point at which malolactic bacteria have stopped doing something useful and started doing something else.
The compounds responsible
Where the molecule is established. Some descriptors in this lexicon name a compound the compound register does not yet hold a record for; those are named in the cause above instead.
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.
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.
Biogenic amines
Amines produced when bacteria decarboxylate amino acids, associated with long lees contact at high pH and low sulphite, and a reason bacterial activity is managed rather than simply tolerated.
Glycerol
A syrupy three-carbon alcohol yeast produces as a side reaction of fermentation, which adds weight to a dry cider and is the raw material for one of its more obscure faults.
The organisms that make it
Where a specific organism is implicated rather than the fruit. Naming one answers a different question from naming a molecule: what is in the cellar, rather than what is in the glass.
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.
Lactiplantibacillus plantarum
A versatile lactic acid bacterium, renamed out of *Lactobacillus* in 2020, capable of malolactic conversion and of a range of faults depending on conditions.
What raises it
Processes that increase this character, or preserve it where it would otherwise be lost.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Racking
Moving cider off the sediment it has thrown, which both cleans the liquid and — by taking yeast and nitrogen away with the deposit — slows what is left of the ferment.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Filtration
Passing cider through a porous medium to reach a stated visual brightness, at a real cost in colloidal material, body and aroma.
The fault it points at
Meeting this descriptor above a certain intensity is how the fault below is usually noticed first.
Fault
Sauerkraut, sour milk, silage or cheesy notes from lactic acid bacteria working on sugars and other substrates rather than on malic acid alone.
About Soured milk
Soured milk is the descriptor for lactic activity that has outlived its purpose, and its appearance is a prompt to check whether the bacteria are still viable and whether glycerol is being consumed — the same population is capable of producing acrolein by that route.
It travels with other lactic faults rather than alone, and a cider showing it should be assessed for ropiness, mousiness and acrolein bitterness at the same time.
Others in the dairy and lactic family
Butter, butterscotch, yoghurt, cream.
Butter
The buttery note of diacetyl, produced by lactic acid bacteria during malolactic conversion.
Butterscotch
Diacetyl at higher concentration with a caramel edge, sitting at the boundary between character and fault.
Cheesy
A rancid, sweaty note from short-chain fatty acids, chiefly butyric and isovaleric, produced by bacteria.
Cream
A soft, rounded dairy impression from acetoin and lactic acid without the sharpness of free diacetyl.
Yoghurt
A tangy fermented-dairy note from lactic acid with acetaldehyde, indicating vigorous bacterial activity.
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 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 racking in cider making — Racking is siphoning cider off the sediment it has thrown into a clean vessel, leaving the lees behind. It clarifies the cider and, in traditional practice, is also used to slow a ferment by removing yeast with the lees.
- Why does cider contain lactic acid — Because malolactic bacteria converted the malic acid into it. Malic acid has two acid groups and lactic acid has one, so roughly half the titratable acidity disappears and the cider tastes softer.
- Do i need a filter to make cider
- What does glycerol do in cider — Glycerol is a by-product of fermentation that is faintly sweet and viscous, and it contributes to the impression of body and roundness in a dry cider.
- What is acetaldehyde
Where to go next
- The sensory model — The thirteen structural dimensions and the twenty aroma families, in full.
- Compounds — The chemistry behind the words.
- Troubleshooting — When a perception turns out to be a problem, this is where it is diagnosed.
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.