Compound
Tetrahydropyridines
The compounds behind mousiness, which cannot be smelled in the glass because they are not volatile at cider pH and appear only as an aftertaste once saliva has raised the pH in the mouth.
Also called Mousy taint compounds, ACTPY, 2-acetyltetrahydropyridine.
- Class
- Nitrogen compounds
- Formula
- Not a single molecule — see below
- How often it matters
- Occasional
What it does in cider
- Are formed by Dekkera and certain lactic acid bacteria from lysine and ethanol.
- Exist in a protonated, non-volatile form at cider pH, so a badly affected cider can smell entirely normal.
- Become volatile when saliva raises the pH in the mouth, producing a delayed retronasal aftertaste rather than an aroma.
- Persist indefinitely: no treatment available to a cidermaker removes them once formed.
How it is perceived
What the compound registers as, and at roughly what concentration. Perception is not a property of the molecule alone: sugar, tannin and carbonation all change where a threshold falls.
- mouse cage
- stale popcorn
- cracker
- corn chips
- delayed and lingering aftertaste
Detected at extremely low concentrations by those who can detect them at all, and a substantial proportion of tasters cannot — sensitivity varies enough between individuals that a panel will genuinely disagree about whether a cider is affected. Rubbing a little cider between the palms and smelling the residue raises the pH and makes the taint apparent.
Descriptors it is responsible for
Sensory records that name Tetrahydropyridines as a cause. Each states the perception and the mechanism behind it.
- Mousy — A retronasal-only taint of mouse cage and stale grain, from tetrahydropyridines produced by lactic bacteria and Brettanomyces.
The structure it moves
| Dimension | What it is |
|---|---|
| Fermentation character | Aromas made by the ferment rather than carried in from the fruit. |
| Phenolic character | Smoky, spicy, leathery or medicinal aromas that sit apart from fruit. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Barrel ageing
Holding cider in wood — which may mean an old neutral vat used simply as a vessel, or fresh or spirit-seasoned oak used as a source of flavour, and the two are not the same operation.
Tank maturation
Resting cider in stainless steel or a lined vessel, where the point of the container is that it contributes nothing and admits almost no oxygen.
Lees ageing
Deliberately holding a cider or perry on its fine yeast deposit so that autolysing cells release material that changes texture and foam behaviour.
Wild fermentation
Letting the microflora already present on the fruit and in the press house carry the ferment, in a succession rather than as a single organism.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
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.
Faults it is implicated in
Being implicated is not the same as being a fault. Several of the compounds on this site are ordinary constituents of a sound cider and define a named fault only above a concentration.
Mousiness
A retronasal taint of mouse cage, stale popcorn or crackers that appears only after swallowing — and that a substantial fraction of people cannot detect at all.
Brettanomyces character
Farmyard, horse-blanket, smoky and sticking-plaster aromas from *Brettanomyces* yeast converting hydroxycinnamic acids into volatile phenols.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
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.
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.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
About Tetrahydropyridines
Mousiness is the strangest fault in cider because of a quirk of chemistry. The compounds responsible — 2-acetyltetrahydropyridine and its relatives, together with 2-acetylpyrroline — are nitrogen bases. At the pH of cider they are protonated, and a protonated base is not volatile, so it cannot reach the olfactory receptors through the nose. A cider carrying a severe mousy taint smells clean.
Take a mouthful and saliva, which is close to neutral, deprotonates them. They volatilise in the mouth and reach the olfactory epithelium from behind, which is why the taint arrives several seconds after swallowing, why it is described as an aftertaste rather than an aroma, and why it lingers so unpleasantly. The standard bench test exploits the same effect: a little cider rubbed between the palms until the alcohol evaporates leaves a residue at skin pH that can be smelled directly.
Sensitivity varies more between individuals than for almost any other fault. A proportion of tasters cannot detect these compounds at all, which means a tasting panel can split genuinely rather than through inattention, and a producer who cannot detect them can ship affected cider in good faith.
The organisms are Dekkera and several lactic acid bacteria, working on lysine, and the conditions are the familiar ones: high pH, low free sulphur dioxide, long unattended maturation, wooden vessels. Nothing removes the compounds once they are formed. As with the ethylphenols, the answer is cellar hygiene, sulphite management appropriate to the pH, and not leaving a cider alone in a barrel for a year without tasting it.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Nitrogen compounds
Amino acids
The largest usable nitrogen fraction in apple juice, and the raw material from which yeast builds both its own protein and most of the aroma compounds a cider carries.
Phenolics
4-Ethylphenol
The horse-and-sticking-plaster compound, made by *Dekkera* from a phenolic acid the fruit supplied, and the clearest case in cider of one molecule being a tradition in one glass and a fault in another.
Nitrogen compounds
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.
Alcohols
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.
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 many calories are in cider — Roughly 40 to 60 kcal per 100 ml for most ciders, so a UK pint falls somewhere around 200 to 250 kcal. Alcohol contributes about 7 kcal per gram and residual sugar about 4, so both strength and sweetness matter.
- What is mousiness in cider — Mousiness is a fault caused by tetrahydropyridines produced by *Brettanomyces* and some lactic acid bacteria. It tastes of stale grain or a mouse cage and appears in the aftertaste rather than in the aroma.
- How does cider ferment — Yeast consumes the sugar in apple juice and produces roughly equal masses of ethanol and carbon dioxide, along with the esters and higher alcohols that give cider much of its aroma. In a wild ferment several yeast species take turns before *Saccharomyces* finishes the job.
- What is brettanomyces — *Brettanomyces*, correctly *Dekkera* in its spore-forming form, is a slow yeast that ferments sugars other strains leave behind and produces volatile phenols smelling of farmyard, leather or sticking plaster. In cider it is common and not always unwanted.
- 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.
- Is brett a fault in cider — It depends on the tradition and the level. A trace of *Brettanomyces* character is expected in much West Country and Spanish cider; at high concentration it flattens fruit and dominates everything else, and few would defend it then.
Where to go next
- All compounds — Grouped by what the fruit brings and what the ferment makes of it.
- Sensory — Every descriptor, with the compound or process that causes it.
- Microbiology — The organisms whose metabolism most of this chemistry belongs to.
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.