Understanding wine: Are mousiness, volatile acidity, Brett, oxidation actual characteristics or faults?

Someone smelling wine

Oxidation, volatile acidity, Brettanomyces (more commonly known as Brett), gas and mousiness – are these characteristics or faults when it comes to some wines? We give you the keys to unlock these terms without entering into a pro/anti natural wine debate and its ‘deviations’.

Mousiness (as it sounds, a taste similar to that of a mouse), volatile acidity, Brett, and oxidation, once universally recognised as wine faults, have greatly developed in recent years alongside the natural wine trend. Indeed, crafting this type of wine carries inherent risks and requires a great deal of precision and the adherence to strict standards, particularly in terms of hygiene and ensuring a perfectly healthy harvest to prevent wine deviations. That being said, some wine lovers have grown accustomed to these deviations and even go as far as to appreciate and actively seek them out. This isn’t our stance at iDealwine, as we still consider them as faults. But this doesn’t stop us from loving natural wines as plenty are absolutely faultless!

But what exactly are oxidation, Brett and volatile acidity? And how can you identify these wine faults when you smell and taste a wine?

Are natural wines more prone to wine faults?

“I knew it was a natural wine, the nose was like an unkempt stable and the palate was all gas and vinegar…” Have you ever heard this kind of talk from wine lovers? Are natural wines more delicate than others, or are they simply more susceptible to deviations that sometimes lead them directly down the drain?

Before we look into these potential deviations, let’s quickly review the definition of natural wine and how it relates to the aforementioned faults. For many wine lovers, it’s essentially a sulphur-free wine. As a matter of fact, it’s much more intricate, given that there isn’t a definitive official definition of what constitutes a natural wine. Unlike with an AOC wine, there are no specifications recognised by the authorities. According to the Syndicat de Défense des Vins Nature’L association, which is arguably the most representative source of this loosely organised movement, for a wine to be considered natural, it must be produced from certified organic (or biodynamic) grapes, harvested by hand, fermented with native yeasts, vinified with no added inputs, no recourse to harsh and restrictive physical techniques (reverse osmosis, flash-pasteurisation, tangential filtration, thermovinification…). Sulphur is prohibited during the vinification process but is authorised during bottling provided it is kept below 30 mg/l in total. For further information on natural wines, read our article Natural wine: A passing trend or lasting phenomenon?.

Fundamentally, there is no reason why a natural wine should have more faults or deviations than a conventional wine. All wines can be susceptible to issues or faults for a wide variety of reasons. Regardless of whether winemakers use conventional, organic, or natural techniques, if they harvest unripe grapes or, conversely, grapes with rot that isn’t noble at all, they won’t be able to produce a ‘pristine’ wine. And regardless of a producer’s ethos, a wine may reveal too much oak, be overly alcoholic, show signs of over-extraction, and other such characteristics.

Natural wines, on account of the way they are produced – which involves greater risk, fewer safeguards and ‘no safety net’ – may indeed be prone to a number of faults or deviations (or at least be perceived as such) and may also develop them subsequently if they are stored in unsuitable conditions. Let’s take a closer look at the most common faults you might come across in wine.

Wine oxidation and how to recognise it

Oxidation is one of the main phenomena that affect the quality of wine and it impacts all sensory characteristics. In a nutshell, oxidation is a chemical reaction triggered by contact with oxygen in the air causing a portion of ethanol – the alcohol present in wine – to turn into ethanal and then acetic acid (the main component of vinegar). This chemical reaction also alters the wine’s colour with white wines taking on an unpleasant yellowish colour, and reds developing a brownish tinge. This natural and irreversible process may be coveted in so-called oxidative wines – when winemakers deliberately expose the wine to oxygen – but certainly not in other types of wine.

When the white wine in your glass gives off aromas of apples or chard pears, cider, walnuts and curry, which you can detect on both the nose and the mouth, it doesn’t bode well. This tells you that oxidation has struck, and this is not going to get any better with time… In the event you’ve poured a glass of red, oxidation will emerge in pronounced acetate aromas (nail varnish) and a more or less vinegary palate along with animal notes.

Oxidation is especially common if sulphur, an antioxidant, is not used in the winemaking process, if the wine comes into excessive contact with air during vinification, or if a bottle is left open too long.

For further information on this topic, read our article “Sniffing out wine faults“. 

Volatile acidity in wine and how to recognise it

This is rather difficult to distinguish from the acetate aromas mentioned above, as it is typical of wine that has turned vinegary. Ethyl acetate is the culprit here, causing the infamous aromas of nail varnish or glue in wine, which is then said to be ‘acescent’ (sour). Acetic acid may also come into play, as this is what turns wine into vinegar (at 37 g/L or more, according to the norm). Wine may contain one or both of these volatile acids.

However, volatile acidity is naturally present in all wines; it’s the acidity that is volatile, meaning it easily evaporates at room temperature. It’s the excess of volatile acidity levels that can be problematic. Under legal rules, it must not exceed 0.98 g/l for reds and 0.88 g/l for whites and rosés. If it’s excessive, you might feel a slightly ‘sharp tingle’ on the nose and the finish can become rather fiery because it’s quite an aggressive acidity. On the other hand, high volatile acidity (within the rules) can prove beneficial for some southern wines, as it contributes to freshness in the mouth. Volatile acidity is sometimes excessive in natural wines, also encouraged by the absence of sulphur or the use of too little sulphur.

What causes volatile acidity?

Several scenarios can cause this to occur – natural yeasts can produce acetic acid or ethyl acetate in the early stages of fermentation; or lactic acid bacteria can produce acetic acid from sugars during the alcoholic fermentation process, a phenomenon known as ‘lactic sting’; or acetic acid bacteria can turn ethanol into acetic acid during ageing, the so-called acetic sting. High temperatures, low juice acidity and high sugar levels are all risk factors favouring the appearance of volatile acidity.

Carbon dioxide and how to recognise perlage

Many wine lovers recognise a natural wine because it sometimes leaves a fair amount of gas in the mouth. Perlage is a French word meaning ‘string of pearls’ and refers to bubbles that can form in the wine. Carbon dioxide is naturally generated during all winemaking processes.  There may be varying amounts of it at the time of bottling. Many winemakers degas their wines before bottling. Others don’t, because carbon dioxide naturally protects the wine from oxidation and allows sulphur levels to be lowered during bottling or dispensed with altogether. For many wine lovers, the slightly pearly character of a white wine or, worse still, a red wine, is perceived as a fault. True, but this is a fault that can easily be corrected or greatly reduced, for example through vigorous decanting of the wine in question. Some people find this slight effervescence, which does not alter the taste of the wine, quite pleasant.

Brett in wine and how to recognise it

If your wine (often a red, where this fault is more obvious) gives off powerful phenolic smells (ink, leather, barnyard, sweat), there’s a good chance that it’s contaminated with Brettanomyces, a yeast that overpowers others during the winemaking process. This deviation isn’t specific to natural wines, but is perhaps a little more common among them, when hygiene in the winery isn’t perfect and the absence of sulphur fails to inhibit the infamous Brett.  With other beverages such as lambics (spontaneously fermented beers) and geuzes (blends of lambics), it is precisely this typical Brett taste that is sought after.

Again, lower acidity, higher alcohol levels and little protection from SO2 present risk factors.

Mousiness and how to recognise it

Previously little talked about, this flaw has become ‘fashionable’ in recent years. Badmouths might say that we’re talking about it more because there are more and more natural wines. There probably is a correlation… That said, this fault has always existed, even if it seemed far less common a few years ago.

This defect is not immediately noticeable on the palate. It only emerges in the retronasal smell, once the wine has been swallowed, or spat out in the event of a tasting. It is characterised by a strong taste of sausage skin or stale cereal grain, some mention floor-cloth or peanuts and even – and this is where it gets its name – rodent urine. This fault can either be understated or, on the contrary, can overpower all the other aromas in the wine. In addition, it is highly influenced by the acidity of the taster’s saliva and may therefore not be perceived in the same way by two people tasting the same wine. And it’s more easily detected in whites than in reds. Finally, it’s important to point out that, unlike many other faults, mousiness usually disappears after a certain amount of time in the bottle (between one and two/three years). This is a temporary fault; if a bottle presents mousiness and you have other bottles of the same wine, cellar them for a few years and the fault should subside. On the other hand, once the bottle has been opened, its intensity tends to increase quite rapidly.

What causes mousiness?

While this phenomenon is not fully grasped, we know it is caused by the development of unwanted bacteria; some winegrowers believe the problem is on the rise as a result of global warming, which is lowering acid levels. It would seem that initial research results also point in this direction. Additionally, lower SO2 levels also reduce protection against this fault.

In concrete terms, this is a microbial fault caused by lactobacilli.

After this overview, which is probably not comprehensive as various deviations are possible in any wine, let’s conclude with a controversy that sometimes divides the wine world. It involves those who criticise natural wines because they always perceive them as riddled with faults and the camp of extreme ‘naturalists’ who argue that a wine without faults cannot truly be natural. As is so often the case, we believe that the truth lies somewhere between these two extremes. It is clear that producing a natural wine free of faults requires absolute precision at every stage: a perfectly healthy harvest, impeccable hygiene in the winery, gentle vinification, and minimal manipulation. A tough challenge indeed, but one successfully met by many talented winemakers.

Have you ever attempted to make wine yourself, entirely artisanally in your kitchen? Then, inevitably, you must have produced … vinegar. It’s also important to be honest and humble enough to recognise that if you find the above-mentioned faults in a natural wine, chances are it is simply produced by a winemaker who has not fully mastered the demanding process described above. Fortunately, though, there is now a plethora of winemakers producing natural wines without the slightest flaw, in almost every appellation. And the phenomenon is gaining momentum, as the virtuous circle of talented natural winemakers taking inspiration from each other continues to expand. A real treat for wine lovers. But how do you identify them? We urge you to follow the recommendations offered by iDealwine, of course, or by your friends and the press, so that you make the right choices and fill your cellar with the best natural, flawless wines on the market.