“Corked wine” isn’t the lone culprit that can mar a wine’s flavour. There’s an increasing discussion around a “new” defect, typically addressed as “mouse” or “mousiness”. This flaw was hardly recognized or even existent a few decades prior, but recently, its occurrence has escalated, courtesy of a rising trend of utilizing minimal to nil sulphur in wine-crafting and producers plucking mature grapes with lower acidity. Here’s your need-to-know on this.

Pinpointing this flaw with precision can be challenging. The commonly perceived manifestations can be likened to corn, popcorn, basmati rice, tortillas, and, predictably, a mouse though admittedly, the actual smell of a mouse remains ambiguous).

A person sniffing a wine in a wine glass, copyright BKWine Photography

The chemical compounds that give rise to mousiness don’t evaporate easily in the acidic habitat (low pH) of wine. But they do when you take a sip of the wine, as the acidity level alters.

Interestingly, the pH level in humans’ mouths is noticeably higher, so when the wine is introduced to your mouth, these compounds become more volatile, allowing you to experience the “flavour”, or more aptly, the smell.

Unusually, mouse is a flaw that can’t be detected by simply smelling the wine. Its presence only becomes apparent when you actually taste the wine.

Typically, a sommelier will present the wine for you to taste at a restaurant before service. Often, a sniff is all that’s required to identify flaws like TCA or “corked” wine, but with mouse, you actually have to taste it to confirm its presence. Adding to the challenge, not everyone has the sensitivity to detect mouse (this is true for other wine faults as well).

On multiple occasions, I’ve attended tastings where wines were intentionally tainted with the mouse flaw (alongside others presenting different types of faults). Even then, it was still difficult to definitively say if a wine was affected or exactly what the fault truly was. Sometimes it’s just a vague feeling that something about the wine is slightly off. Maybe if mice were more prevalent in our area, the identification would come easier…

Scientists believe that mousiness is caused by a group of compounds called pyridine. This compound consists of a basic molecular formula of C5H5N, and is even used industrially as an additive to render spirits non-potable.

A woman in a lab coat in a laboratory in a winery, copyright BKWine Photography

So, what brings about mousiness in wine?

Based on a recent research conducted at the University of Bordeaux, three main factors are found guilty: Brettanomyces, also known as the “wild” yeast that may lead to its own wine fault, “brett”, the so-called “lactobacteria” that execute the malolactic “fermentation” of malic acid to lactic acid in wine, and some inorganic chemical activities, majorly, the Maillard reaction.

The research implies that the risk of having a mousy wine elevates with very low sulphur levels. Consequently, the ongoing trend of low-sulphur wines and even no-sulphur wines have significantly amplified the risk of experiencing mousiness. Different researches provide different statistics. Some tagging 20% of low or no sulphur wines as impacted while others mention numbers up to 50%.

Potassium metabisulphite is a form that SO2, sulphur, can be added to wine, copyright BKWine … [+]. Photography

Another impactful factor is the prevalent acidity in the wine. With the shift in weather patterns and changing consumer preferences, grapes are being harvested at a later, more mature stage, leading to diminished acidity levels. This, in turn, increases the risk of yielding a mousy wine, an issue perplexed by the effects of climate change.

From my perspective, there’s no easy solution for the winemakers besides being cautious and continuously monitoring the acidity levels, perhaps tweaking them periodically, and ensuring appropriate levels of sulphur are used. I am not aware of any quick fixes that can eradicate the issue if it occurs.

If you ever get a bottle with it, unfortunately, there’s not much to be done. If it is a mere trace, maybe it won’t make much difference.

The good news are that in most cases, it is not a very stinky fault, it is relatively rare, and some people don’t even notice it.

—Per Karlsson