Four common wine faults – that can be avoided with a touch of sulphur | Britt on Forbes

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Wine defects. No-one wants them. But what is it really? Numerous things can spoil a wine. No doubt many wines were spoilt back in the old days when wine producers had little knowledge of wine chemistry and microbiology. Today, things are different. Today, almost all wines leave their wineries faultless. But not quite all. The dangers of bacteria, bad yeast etc., are still there, lurking, but most producers know how to counteract. But sometimes mistakes can happen. Here are some of the most frequent wine faults, what effect they have on the wine and what they winery can do to avoid them. One of the most important keys is a judicious use of sulphur (sulphites).

Even if it today is a rare occurrence, sometimes the consumer does come across a faulty wine. Sometimes it is obvious, sometimes not. It might not be perceived as faulty. Instead the consumer just thinks “this is not a wine that I like”. You simply write off that wine as something you will not buy again. A bad outcome both for the producer and the consumer. Better to understand that there’s a problem with the wine and give it another chance.

To make things even more complex, certain “faults” are not always experienced as such. Or are not necessarily fault.

This is a longer version of an article published on

Or to put it in a different way: if something is a fault or if it is instead something that contributes to the complexity and character of the wine is often a question of degrees. All wines have a certain level of volatile acidity (more on that below). Many would be bland without it. But if it is too much it becomes aggressive and eventually undrinkable, and certainly defective.

We all have our own preferences when it comes to taste. A wine that is considered faulty by one person may be well-liked by another. Some like barn-yardy aromas, others detest them.

And we all have different sensitivity to the aromatic and volatile compounds in wine so perceive their presence and intensity differently.

A tasting recently showed this complexity. The tasting was organised by VinNatur, an Italian association for natural wine producers based in Veneto. The aim was to show some of the most commonly found faults in wine, faults that can appear if the winemaker has been careless or just unlucky.

Bottles of clean wines and doctored wines for a "wine faults" tasting with VinNatur
Bottles of clean wines and doctored wines for a "wine faults" tasting with VinNatur, copyright BKWine Photography

Emma Bentley from VinNatur, who organized this fascinating tasting, started by saying, “it’s easy to make wine without adding sulphites. It’s not easy to make consistently good wine without adding sulphites.”

That is a humble statement from a natural wine organisation, considering that many natural wine producers would prefer not to use any sulphites at all. However, the people running VinNatur are realistic. And they don’t tolerate faulty wines.

Alessandro Maule, son of VinNatur’s founder Angiolino Maule, put it bluntly in an article: “We’ve reached a stage where there’s a trend for ‘funky wines’ where the stinkier the wine is, the better. Having an imperfection or defect makes it cool and original! We assure you; this is not where we wanted to get…”

No, Alessandro and Angiolino want the around 200 VinNatur members to make wines as natural as possible. But without defects.

Bacteria that destroy the wine are still a significant problem in winemaking, especially for natural wine producers. “There is an easy solution to this problem,” says Emma, “namely to add large doses of sulphur dioxide, filter or manipulate the tartaric acid. But it is not allowed for natural wine producers” (“manipulate” meaning adding some extra in this case).

To illustrate what happens when a wine is faulty, Emma arranged a blind tasting where we tasters were exposed to wines that had been manipulated to show some common faults.

All wines were taste blind. The taster were not told which of the wines had been “doctored” with some additions to simulate a wine fault and which wine was the reference sample.

The initial analysis of the wines, to select an appropriate base wine, as well as the preparation of the samples with “flaws” was done by Yuri and Damiano at Food Micro Team. It is a Florence-based laboratory that works not only with wine but also with other beverages and foodstuff. They have a long-standing collaboration with VinNatur for quality assurance and research.

The white wines

We started with three white wines. All were from the local garganega grape, vintage 2019 and kept in a steel tank. One of the wines was a reference, i.e., unmanipulated. In the other two wines, Emma had added acetic acid in one and both acetic acid and lactic acid in the other. We recognised the unmanipulated wine fairly easily. In the other two wines, we felt that something was wrong.

Bottles of white wines for a "wine faults" tasting with VinNatur
Bottles of white wines for a "wine faults" tasting with VinNatur, copyright BKWine Photography

Acetic acid

There is always some acetic acid in a wine. The oxygen will permit acetobacteracetic acid bacteria – to form acetic acid during the vinification. This acid is volatile (we describe it as volatile acidity) and can, in reasonable quantities, add complexity to the wine. Too much, and the wine will be considered faulty. We are drinking wine, after all, not vinegar. In the unmanipulated wine, there was 0.49 gram/litre; in the other two wines, 1.4 g/litre.

The balance can be tricky. I remember tasting a delicious sauternes in the cellar of one of the very top estates in the appellation. Luscious, sweet, intense, fresh, all you would want. However, the winemaker commented “if you made a chemical analysis of this wine you’d find that the volatile acidity is above the permitted level”. It was certainly not a defective wine.

Acetic acid bacteria in combination with ethanol can also form ethyl acetate in the wine. This was precisely what had happened in the wine with added acetic acid. The wine had 200 mg/l of ethyl acetate. Low levels (30–60 mg/l) give fruity aromas to the wine. High levels, though (150-200 mg/l), give a rather unpleasant smell of nail polish remover or glue.

Acetobacter needs oxygen to work and to produce acetic acid. They are sensitive to sulphur, so they are relatively easy to keep in check by protecting the must/wine from too much oxygen and adding sulphur.

If you have harvested rotten or damaged grapes, you need to be extra careful as these contain more acetic acid bacteria. You can often feel that if you sniff on a bunch of damage or slightly rotten grapes.

Household vinegar
Household vinegar, copyright BKWine Photography

Lactic acid

To the third wine, Emma had added both acetic acid and lactic acid. The lactic acid in itself is not harmful. During the malolactic fermentation, the malic acid present in the wine is converted to lactic acid. The malolactic fermentation happens in most red wines and also in many white wines.

However, lactic acid brings with it new flavours, which can be positive or negative for the wine.

There are three types of lactic acid bacteria. Oenococcus is the one that works best for malolactic fermentation. In addition, there are pediococcus and lactobacillus. All three can bring different types of flavours and even faults to the wine. It can form diacetyl which makes the wine will feel buttery (often positive, but not always) or mannitol, giving it a kind of sweetish sensation. Mannitol is used in the food industry and also has many medical applications.

Mannitol had formed in wine number 3. Mannitol is a sugar alcohol produced by oenococcus. It is formed especially if there are high levels of acetic acid and lactic acid in the wine.

The red wines

The four red wines were all sangiovese 2019, fermented and kept in steel tank.

Actually, it wasn’t totally obvious to find the unmanipulated wine. We did suspect brettanomyces in one of the wines.  

It turned out that number one was the unmanipulated wine. Number 2 had added 4-ethylphenol, and number 2 had added 4-ethylguaiacol as well as 4-Ethylphenol. Number 4 had added acetaldehyde.

Bottles of red wines for a "wine faults" tasting with VinNatur
Bottles of red wines for a "wine faults" tasting with VinNatur, copyright BKWine Photography


4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG) are volatile phenolic compounds formed by the infamous brettanomyces (or brett, as it is affectionately called). Brett is a type of yeast often found in and around a winery and produces these molecules with their metabolism, i.e. when it consumes the sugar. Many red wines “contain” a bit of brett (or, to be precise, contain the volatile compounds produced by brettanomyses), which isn’t a problem. If it is too much, though, most people wouldn’t like it. You can be more or less sensitive to brett.

4-EG, 4-ethylguaiacol, will give smoky and spicy notes, for instance, cloves, to the wine. It can be hard to tell if these aromas come from the grape variety or from the oak, or from brett. But with brett, the wine can also lose some of its fruitiness.

Oak barrels in a wine cellar in Bordeaux
Oak barrels in a wine cellar in Bordeaux, copyright BKWine Photography

4-EP, 4-ethylphenol, is more of a problem, though, as the smell tends to be “horse sweat”, barnyard, medicinal etc. Red wine number two had 300 µg/l of 4-EP added (the reference wine had zero 4-EP).

With 300 µg/l it is still not obvious that everyone agrees that a wine is faulty. Wine number three had 400 µg/l, and here it was more distinct but still open for discussion. At 600 µg/l, which can happen, opinions might still differ, but most people will dislike the wine.

Brett usually starts working after the alcoholic fermentation if there is any sugar left in the wine for the brettanomyces yeast to work with. Wines with high potential alcohol content may not have been able to ferment out completely. New oak barrels contain sugar, and the oak barrel ageing is a perfect breeding ground for brett because it works slowly, and during barrel ageing it has time. Ageing on the lees, sur lie, also encourages brett to spread, as well as not filtering the wine.

Brett is mainly a problem for red wines, as they have a higher content of polyphenols than white and higher pH, two things that encourage the development of brett.

A barn in a field in the Swedish country side early in the morning
A barn in a field in the Swedish country side early in the morning, copyright BKWine Photography


To the last red wine, Emma had added acetaldehyde.

Acetaldehyde can be produced either from the yeast during alcoholic fermentation or by acetic acid bacteria due to the oxidation of ethanol. It occurs naturally in e.g. coffee, bread and fruit but is toxic in higher concentrations.

Acetaldehyde gives aromas of overripe apples and nuts, which can be pleasant in a white wine. The acetaldehyde gives fino – dry sherry – its unique character. In other wines, however, the winemaker mostly wants to avoid the formation of acetaldehyde, not least in red wines. It can make the wine feel rather flat. Another problem is that acetaldehyde binds sulphur dioxide, which thereby it loses its protective effect.

This tasting showed us only a few examples, but they make you realise that the winemaker always needs to be vigilant.

It also highlighted the difficulties in identifying faulty wines. One person’s impression of a wine defect can be another person’s extra spice and character.

How to avoid wine faults

But, says Emma, it is really not that difficult to avoid these faults, not even for a natural wine producer.

She recommends:

  • Good cellar hygiene
  • To add a little bit of sulphur dioxide, at the right time
  • Store the wines at a cool temperature. 15 degrees C instead of 18 makes a difference
  • Do laboratory analyses; do not be afraid of science
  • Better awareness of the problem by winemakers, critics, writers and sommeliers

Good hygiene is a must in a wine cellar, but it is not always enough. To combine with a small dose of sulphur dioxide is sometimes the best choice. For a natural wine producer, adding a small amount of sulphur can make a huge difference. And protect the wine from faults as well as sparing the consumer from disappointments.

VinNatur has a scientific and serious approach to natural winemaking and viticulture. They know that no matter how natural you want to be, you can’t escape the chemistry of wine.

Read more about the VinNatur’s interesting research at

Read more on this subject on BKWine Magazine:

Adding a tablet of effervescent sulphur to a barrel of wine
Adding a tablet of effervescent sulphur to a barrel of wine, copyright BKWine Photography

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