Insights - March 7, 2022

Catching the Mouse, In Your Wine

A sneaky flaw is on the rise, invading wine and leaving a trail of off-flavors

Written by

Aleks Zecevic

Talking about wine flaws often becomes a controversial topic among wine lovers. Some have no tolerance. Others embrace the flaws and say that in small doses, they can add character. However, one thing that most people agree on as being unacceptable is the off-putting flavors that are a result of the so-called mousy taint, mousiness, or simply, mouse.

Of course, despite the name, the flaw will not make your cat pounce on your wine glass. Mouse got its name because it creates an aftertaste that resembles the smell of mouse droppings or mouse cage. Unlike most other wine taints, winemakers and professionals alike don’t fully understand what causes the phenomena. However, one clear thing is that it is only common in natural wine—which those who think that the category is a fad or an excuse for flawed wines will celebrate. Furthermore, mouse seems to especially affect wines that have no added widely used preservative, sulfur dioxide, or those who have minimal additions of it. Another supposition is that wines with higher pH and those with more exposure to oxygen are more susceptible to the flaw. For this reason, wines hailing from regions with warmer climate tend to have more problems than those from cooler climate regions.

What we do know is that the compounds responsible for the mouse are known as N-heterocyclic bases, and there are three of them: 2-acetyl tetrahydropyridine (ATHP), 2-ethyl tetrahydropyridine (ETHP), and 2-acetyl-1-pyrroline (APY). For the odd flavor to emerge, two of these compounds need to coincide. The problem is that it is not entirely clear how they end up in wine in the first place.

There is evidence that certain strains of lactic acid bacteria, which carry out malolactic fermentation, can produce these compounds. Considering that most natural wines undergo malolactic fermentation, this adds up. Jamie Goode, a British wine columnist with a Ph.D. in plant biology, explains it is the subset of lactic bacteria known as heterofermentative strains that are most responsible. The other offender is a yeast strain called Brettanomyces, most commonly referred to as Brett. This also might be the reason why mouse can be found in other raw fermented drinks, like kombucha and even certain beers.

Therefore, many factors can contribute to a mousey wine, but it is most often a combination of no added sulfur, exposure to oxygen, higher pH levels and bad strains of lactic bacteria and yeast. What makes this flaw even more enigmatic is that the mouse is perceived retronasally, sometimes taking a few moments to build up. This means that a wine that is sound on the nose and the initial sip, can be undrinkable when the aftertaste hits you. It literally sneaks on the back-end, leaving an aftertaste that most drinkers who have experienced it, describe as resemblant to caged mice odor, animal breath, sausage casing or even taco shell, popcorn and water biscuits. Interestingly, the aforementioned compounds are found in some of these goods.

Chemical compounds responsible for the off-flavors aren’t aromatic in the low pH environment, like in wine, and are responsible for the detection delay. The nose cannot detect the mouse. Rather, the aromas go through activation once our mouth’s pH and wine create a chemical reaction, which then turns the compounds in question into an off-flavor that repels most wine drinkers.

Considering that it is subjective to our own pH levels, things become even more convoluted. Since we are all built differently, some people simply cannot even detect the mouse. Instead, they have a physical disposition to taste it, which in this case, maybe even works in their favor. Some people can tolerate it more than others. Given that the pH levels in our mouths vary, so does the perception of this sly flaw. Eating certain foods may also accentuate the unpleasant flavor, otherwise hardly noticeable. Claus Preisinger, of his eponymous winery in Burgenland, Austria, tastes all of his wines during the maturation process while still in the barrel. “When I notice that there is a mouse around a corner, I add a bit of sulfur to a barrel,” Preisinger explains on his methods of prevention.

“However, I have noticed that the mouse can go away over time, once the wine is in the bottle or with longer maturation in the bottle, but once you pull the cork, it is over.” Indeed, many wine professionals would say that the modern market demands push winemakers to bottle wines a lot earlier than before. Uli Stein, of his eponymous winery in Mosel, Germany, says that wines without added sulfur may need even more time before their release, so they become more naturally stable.

Most of us who can taste mouse will agree that it ruins the wine, despite what we say about volatile acidity, Brett, etc. Even natural winemakers and lovers alike, who tend to tolerate other flaws or even embrace certain imperfections, dislike the mouse. “I have no problem replacing a bottle for any of my mousey wines,” says Sepp Muster from Styria, Austria. “But, since I don’t work with sulfur, sometimes this can happen.” I have not tasted mouse in any of Muster’s wines yet, but his sentiment shows the general intolerance of the pest.

Although documentation of the mouse phenomenon dates back to over 100 years ago, it seems like any mention, research, and explanation are few and far between over that time. However, due to the rise of craft wine and within that space, the natural wines without added sulfur, and the climate change that has consequently brought up the pH levels in wines, we are experiencing the sneaky little pest more often. And while some drinkers say that if you drink a bottle fast enough, not exposing it to too much oxygen, you can still enjoy a mousey wine. Others regret that they even opened the bottle. What has your experience been?

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