Of mice and wine, Mousiness in (Natural) Wine

If, like me, you spend time in natural wine bars or at natural wine fairs, you’ve probably experienced one of these two faults often linked to the natural wine: Brett and Mousiness. The latter is the one many frequent natural wine drinkers will recognize easily. Mousiness is a mystery; the hunt to understand it often reads like a detective story.

Like any good mystery, the crime isn’t obvious: mousiness is notoriously hard to spot, at least by sniffing the bottle; it comes later.

You open a bottle, pour the glasses, and everything seems fine: the nose is engaging, full of aroma, and the first sips are fresh and pleasant. Nothing suggests a problem.

As time passes, something odd happens: the wine still smells the same, but the taste has changed. At first, it’s a faint impression; soon, the taste can become unpleasant, sweet earthy, with popcorn like or rodent like notes, and savory. Soon, you can not enjoy the wine anymore.

For people drinking the same wine, perception varies: Clarisse might find the wine merely odd but acceptable, while George detects nothing at all.

That is mousiness. The English term derives from the French goût de souris, literally “taste of mice.”

The cause

Mousiness was once very rare, nearly extinct in the 20th century, but it has become more visible with some natural wine practices. Reliable statistics are hard to find, but at recent wine fairs I attended, roughly 6-8% of bottles showed clear mousiness (personal estimation, not a scientific result).

The taste is caused by specific molecules produced by microbes. These compounds are weakly volatile in the bottle; you often can’t smell them after opening. But when the wine is in the mouth, however, saliva raises the local pH (less acidifying) and the compounds become volatile; you perceive them retro nasally after swallowing.

There is a large difference between individuals regarding the capacity of the mouth to reduce the acidity of a liquid, and sensitivity can be altered by recent food, drink, or even the time of day.

Sensitivity to the responsible molecules varies widely: some people never notice mousiness, while others detect it almost immediately.

Without lab analysis, it can be hard to confirm mousiness in a wine, but a simple trick sometimes helps: place a few drops on your hand and wait a few seconds; the compounds may become volatile and can be easily smelled. Some tasters also add a pinch of baking soda to a glass to raise the pH and reveal the fault on the nose.

The fault nearly disappeared during the 20th century; it became so uncommon that many sommeliers never encountered it. The last historical references to mousiness come from papers from the 19th century.

Scientific literature is limited but growing. Key contributions come from the University of Bordeaux and the Australian Wine Research Institute. Until recently, there were only a dozen or so papers; now the topic is gaining traction among oenology researchers and PhD students. What did these papers say about mousiness?

Research points to Pyridine molecules, exactly the three N heterocyclic compounds: 2 acetyl 1 pyrroline (APY), 2 acetyltetrahydropyridine (ATHP), and 2 ethyltetrahydropyridine (ETHP). Typically, at least two are present when mousiness is detected; ETHP appears to be the least influential.

Only certain strains of Brettanomyces bruxellensis, Lentilactobacillus hilgardii, and Oenococcus oeni (lactic acid bacteria) can produce one or more of these N heterocycles. Saccharomyces cerevisiae usually does not produce them directly but can create conditions or precursors that favor their formation. In short, strain specificity matters; there is a need for specific strains of yeast and bacteria to produce a mousy wine.

Prediction is difficult. Low pH may help to prevent mousiness, and SO₂ during fermentation can reduce the risk, but there are no foolproof predictors. The fault can develop over several weeks, long enough for a wine to be bottled and shipped before anyone notices.

To complicate matters, some researchers propose that oxidation or Maillard type reactions between amino acids and reducing sugars could contribute in certain conditions. These chemical pathways are plausible but less well supported than the microbial explanation.

How can a winemaker prevent mousiness? Science hasn’t given definitive answers, but empirical patterns emerge. Mousiness is rare in wet vintages and more common in warm years; some old vine sites and certain vineyards or cellars seem more prone than others. These observations point to environmental and cellar management factors.

There are PCR tests (just like the COVID test we know) that detect the microbes associated with mousiness, giving winemakers an early warning. Increasing SO₂ can suppress the organisms, but eradication, especially in barrels, is challenging.

If a cuvée becomes mousy, options are limited. Some producers report that extended aging can reduce the perception of mousiness, but this often comes at the cost of fresh fruit character.

I made this observation with a young red wine that was just put into bottles. The wine was fresh, fruity, and light but with a little fizziness (not a good sign). A few months later, when the wine started to be distributed around the world, it was faulty. Not only did it have mousiness, but also Brett. A year later, the wine was more acceptable, but the fruit was gone, replaced by forest floor and cooked fruit notes. This suggests that mousiness can be cured with age.

But if the mousiness is in the bottle, you just open, and if you can stand it, there is no other solution than the sink (don’t try to cook with it), or find someone who doesn’t care (or can’t taste) about mousiness.