The Complete Guide to Mousiness

Everything we know about natural wine's bête noire

It’s a crutch for those of us who love natural wine, and particularly for those who fetishise no added sulphites. At some point, you will make the unwelcome acquaintance of our friend Mr. Mouse.

Mousiness, or mousy taint, is a technical term for a type of bacterial spoilage that can occur in any fermented beverage or foodstuff. In wine, it creates a foul aftertaste that has been variously described as stale popcorn, dog halitosis or - yes- mouse excrement. It’s a devious pest that is more or less undetectable on the nose, usually making its mark on your palate about 10 seconds after you ingested a sip.

I published a deep dive into mousiness in natural wine, back in 2017. It is the most-read post ever on my site. That tells you how much growers, natural wine lovers and the industry obsess about the problem. Since then, there continues to be a slew of articles, Instagram posts and rants on the topic. I still hear people talking about mousiness as if it is a complete mystery. It’s not. We know plenty about what causes it, both scientifically and anecdotally.

In this post, I set down the current state of play. Here’s everything we currently know, where the research stands and everything I can add from 12 years of unwillingly identifying as ‘mouse-aware’.

WARNING: This post is super technical and geeky. If it’s all too much, look out for next week’s article ‘How Drinking Orange Wine WILL Improve Your Sex Life’ - only available to premium subscribers.

What is Mousiness?

Also known as mousy taint, goût de souris by the French, Mäusel or Mäuseln by German speakers, mousiness is a bacterial infection in fermented beverages and foodstuff which creates an extremely unpleasant aftertaste after the beverage or foodstuff is ingested or expectorated.

Can you smell mousiness?

Not without first ingesting a sip, because the compounds that cause it (see below) are not volatile at the normal pH of wine (roughly between 2.8 and 4). They need to hang out with your saliva, which has a pH of between 5.76 and 7.96 before they can release their payload - which they do via your retronasal passage: a tube that connects the back of your mouth with your nose.

This is why it can take as long as 10-20 seconds to perceive the taint. In an environment where you’re tasting multiple wines in quick succession (such as a professional tasting or competition), this can make it onerous to identify which wine is the culprit.

Why do some people say they can smell when a wine is mousy?

They are most likely smelling off-flavours caused by Brettanomyces, which is one of several precursors to mousiness (see below). A wine that has mousy taint will often suffer from a host of other problems, which could include oxidation or excess volatile acidity. These are all perceived on the nose.

What causes mousiness?

Combinations of two or more of the following three compounds¹ are responsible for the taint:

2-ethyltetrahydropyridine (ETHP)

2-acetyltetrahydropyridine (ATHP)

2-acetylpyrroline (APY)

There is reasonable evidence that the presence of only ETHP and ATHP (or possibly other combinations of two compounds) is enough to cause mousy taint².

What causes those compounds to develop in wine?

The compounds are produced either by lactic acid bacteria (LAB) such as Oenococcus oeni or members of the Brettanomyces (aka Dekkera) family of spoilage yeasts, such as Brettanomyces bruxellensis³.

Research done in 2013 established a potential chemical pathway where the taint could also be produced as a result of hyperoxidation and a Maillard reaction involving amino acids and reducing sugars. However, the research is not conclusive⁴.

In layman’s terms, what are the risk factors that could lead to the development of mousy taint?

Here’s where we get to stuff that is anecdotally well known, but not 100% proven by cold hard science. Wine scientist Geoff Taylor of Campden BRI told me in 2017 that low levels of free SO₂, high pH (usually equivalent to low acidity), poor hygiene and the presence of oxygen provide the best conditions for mousy taint to develop.

To put that even more simply, the biggest risks are

• ripe or over-ripe grapes with low acidity (and thus high pH)

• working with no or very little added sulphites

• winemaking in the presence of oxygen, for example the use of open-top fermenters and/or barrels or tanks that are not completely topped up.

• uncontrolled fermentations where the risk of LAB or Brettanomyces joining the party is higher

If there are LAB or Brettanomyces in a wine, what can prompt mousy taint to develop in the bottle?

Anecdotally, as observed by me and thousands of other natural wine fans on a very regular basis, two things:

• oxygen

• warmth

So guess what, when you pop the cork and pour a glass, the wine gets liberal amounts of both. This leads to the ‘ticking mouse-bomb’ phenomenon - a wine that may seem delicious on opening, but that then ‘goes mousy’ within a certain time window. This could be anywhere from 24 hours to as little as 30 minutes.

I’m personally OK with a wine that develops mousiness after being open for 24 hours. It can be annoying to find that last glass you saved for the next night isn’t drinkable, but hey. In contrast, a wine that dies 30 minutes after opening is absolutely not OK. Not for me, and especially not for the poor somm or restaurateur who decided to offer that wine by the glass.

Why is the absence of sulphites such a big deal?

Because, again according to Taylor, even 10mg/L of free (unbound) SO₂ in a finished wine can provide enough protection against mousy taint

Right, let’s have it out with this free/unbound versus bound SO₂ thing

This is complicated chemistry stuff but it’s really important when we talk about mousiness.

When natural wine acolytes, fanatics and winemakers talk about sulphites, they will usually quote the total SO₂ in the finished wine. Many voluntary natural wine certification schemes, fairs and associations set limits for this figure. Raw wine sets it at a maximum of 70mg/L, the Vin Méthode Nature certification scheme at 30mg/L.

Here’s the thing: total SO₂ means zilch in terms of how protected the wine might be from mousiness. It all depend on how much of that total is still free (molecularly available), and thus available to protect the wine against oxidation or bind with bacteria to neutralise them. There is no exact science as to what proportion of a wine’s total SO₂ remains free. It depends partly on how dirty/microbioly unstable the wine was to begin with. Here’s an article that explains this pretty concisely.

Therefore as a winemaker, you need to know how much SO₂ remains free. If that figure starts creeping under 10mg/L, you either accept a heightened risk of mousiness or consider topping it up with a sulphites addition at racking or bottling time.

Can a wine made with no added sulphites still have 10mg/L or more free SO₂?

It’s possible, but unlikely. A wild yeast fermentation always produces some sulphites naturally, but the resulting total SO₂ in this case is rarely much over 10mg/L - meaning free SO₂ is likely to be close to zero.

How long have we known about this problem?

There are references dating back to 1894 where mousy taint was observed and described as “a peculiarly disagreeable flavour in wine, which is closely resembling the smell of a residence of mice”⁵. The compounds were identified as least as far back as 1977, in the context of fermented beverages in general⁶.

Why was no-one talking about mousiness a few decades ago?

Because the problem had all but disappeared by the mid 20th century, due to the availability and usage of sulphites in dosable forms such as powders or tablets. Mousiness reared its ugly head again with the rise in popularity of low intervention winemaking, and specifically winemaking with no added sulphites.

Why doesn’t everyone notice mousiness in affected wines?

Drinkers who don’t typically drink natural or no added sulphites wines may simply never have experienced it. Unfortunately, mousiness is something that you start to recognise - and even obsess about - with repeated exposure.

I’m worried I can’t taste mousiness, or I don’t know how to recognise it. What can I do?

Try buying a naturally made, unfiltered kombucha, such as this one. I’ve had a very high incidence of mousiness in these drinks! It’s a cheaper and easier way to start to recognise the taint.

Is it common to find other alcoholic beverages with mousy taint?

Yes. Once you learn to recognise it (not that I advise this), you’ll start finding some craft beers (especially unfiltered kegs) and ciders with mousiness too. Not to mention kombucha and just about anything else that underwent a natural fermentation and very limited processing thereafter.

Nope, I still really can’t taste it.

A certain proportion of people genuinely can’t taste mousiness. This can be due to various factors. Some people have anosmia (smell blindness) to one or more of the mousy taint compounds, due to genetic factors/missing genes. Others may have saliva with a particularly low pH which isn’t enough to make the mousy taint volatile. The pH of your saliva also varies day by day. So you might find a wine mousy one day and not the next.

Most scary of all is that there are winemakers who can’t taste mousiness.

You may have read an oft-quoted statistic that as many as 30% of tasters/winemakers/human beings are unable to taste mousiness. I had always wondered where this figure came from. Thanks to this excellent article at Punch Drink, I discovered it was an estimate by Hermann Müller (who crossed Müller-Thürgau) and Adolf Osterwalder from 1913. In short, it’s little more than a back of an envelope figure.

Should mousy wines be put up for sale?

Opinion varies. They’re not dangerous to your health but it can be very frustrating to spend good money on a bottle, only to find that it is undrinkable - or has a lifespan of 30 minutes before it goes to the dark side.

My 0.02mg/L? I’d prefer if winemakers are transparent about wines that have developed mousiness during or after bottling, and ideally don’t sell them. Ideally, they’d sit on the batch until (if) the wine has stabilised and the mousiness has gone.

Wait, mousiness can disappear?

According to thousands of anecdotal reports, yes. This is the baffling part, and frustratingly there is still no research to explain it. But low/no sulphite wines seem at most risk of developing mousy taint during periods of stress such as bottling or transport. As oxygen and heat are two of the vectors that appear to help the taint develop, this isn’t surprising.

Mousy wines often ‘come round’ after a few months or a year or two in bottle. The challenge for importers and retailers is that they may end up with stock that is unsaleable until this has happened.

Winemakers have also shared countless experiences of “that barrel that went mousy”, but then magically cleared up by the time they were ready to bottle.

Perhaps the ever changing balance of free versus bound SO₂ plays a role?

So is it correct to consider mousiness as a fault?

In my opinion, 100% Even a small amount of mousy taint makes a wine not just distasteful but pretty much undrinkable for many wine lovers. Myself included.

I’m baffled by the apologists from the natural wine world, who claim that it is just part of the equation, or “a phase that the wine naturally goes through”.

Why isn’t there more research into all this?

Because mousy taint still only affects a tiny proportion of wines that generally sit in the natural wine niche. It’s been estimated that only around 1% of all wine produced worldwide qualifies as ‘natural’. An even smaller proportion is produced with no added sulphites.

Unsurprisingly, it’s hard for research institutes - even major ones such as AWRI - to get funding for studies that target such a tiny segment of wine. That said, low intervention winemaking keeps getting more popular, and as evidenced by the recent studied I referenced in this post, we are slowly seeing more research into mousiness.

What about you?

Are you, like me, an unwilling mouse-spotter? Or do you live your wine life in blissful ignorance of the problem? Let me know in the comments. I’d also love to hear from those of you of a scientific bent, particularly if there’s anything in this post that I could improve.

1. There are numerous sources for this, including the Australian Wine Research Institute’s advisory page here and this summary of recent research on Jamie Goode’s site.

2. Daiki Kiyomichi, Céline Franc, Pierre Moulis, Laurent Riquier, Patricia Ballestra, Stéphanie Marchand, Sophie Tempère, Gilles de Revel (2022). Investigation into mousy off-flavor in wine using gas chromatography-mass spectrometry with stir bar sorptive extraction. https://doi.org/10.1016/j.foodchem.2023.135454

3. Moulis, P., Miot-Sertier, C., Cordazzo, L., Claisse, O., Franc, C., Riquier, L., … Ballestra, P. (2023). Which microorganisms contribute to mousy off-flavour in our wines?. OENO One, 57(2), 177–187. https://doi.org/10.20870/oeno-one.2023.57.2.7481

4. Künzler, L. Nikfardjam, M. P. (2013). Investigations into the formation of 2-acetylpyridine and the mousy off-flavor in wine. Mitteilungen Klosterneuburg, Rebe und Wein, Obstbau und Früchteverwertung 63 (4): 187-198;

5. Erckmann, L. Einiges u¨ber wein- und essiguntersuchung. Chem. Unserer Zeit 1898, 22, 673 and Thudichum, J. L. W. A Treatise on Wines; George Bell & Sons: New York, 1894.

6. Tucknott, O. G. Mousiness: A taint in fermented beverages. Ph.D. Thesis, University of Bristol, Bristol, England, 1977.