Is That Mold on My Wine Cork?

Meet Quercetin, Sangiovese’s Strange Little Crystal Problem

Spoiler: your bottle of Sangiovese may be doing something completely natural, and rather fascinating.

Recently, one of our guests opened a bottle of Sangiovese and noticed something strange on the cork.

There was a grey-brown, slightly crystalline-looking deposit stuck to the end of it.

Understandably, the first thought was:

“Is that mold?”

A photograph was sent. Questions were asked. Should they drink the wine? Had something gone wrong with the bottle?

And that sent me down one of those wine rabbit holes that led to this article.

Because what looked suspiciously like something growing on the cork was very likely something completely different.

Quercetin.

And if you drink Sangiovese, which around here we certainly do, quercetin is a rather interesting little molecule to know about.

First things first: what is quercetin?

Quercetin is not something a winemaker has added to the wine.

It is naturally present in grapes.

More specifically, quercetin belongs to a family of plant compounds called flavonols, which themselves belong to the enormous family of compounds we call polyphenols.

If you have been reading wine articles for a while, you have probably heard plenty about polyphenols. Tannins are polyphenols. Anthocyanins (the compounds responsible for much of the colour in red grapes and young red wines) are polyphenols too.

Quercetin is another one.

And it isn't remotely exclusive to grapes.

It occurs naturally in apples, onions, berries, capers and many other fruits and vegetables. In the vineyard, flavonols such as quercetin accumulate principally in the grape skins and play, among other things, a protective role against sunlight and UV radiation.

So far, so perfectly normal.

The interesting part begins when we turn those grapes into wine.

And Sangiovese seems to have a particular relationship with it

This is where our Tuscan protagonist enters the story.

Researchers studying quercetin deposits in wine compared Sangiovese with 11 other monovarietal wines. Sangiovese turned out to be particularly rich in both quercetin and its related compounds, the quercetin glycosides.

A major scientific review published in 2026 goes even further, describing Sangiovese as the grape variety most sensitive to the risk of quercetin deposits.

Which means this isn't some obscure theoretical problem.

It is particularly relevant to wines such as Chianti Classico, Brunello di Montalcino and Vino Nobile di Montepulciano, as well as the many other Tuscan wines based principally or entirely on Sangiovese.

And it can create something that wineries generally don't particularly want us to see:

crystals.

How does something dissolved in wine suddenly become a crystal?

Here comes the slightly geekier part. But it's actually quite simple.

In grapes, much of the quercetin exists attached to a sugar molecule. These forms are known as quercetin glycosides.

Think of the sugar as quercetin's helpful travelling companion.

In this form, it is relatively soluble.

During winemaking and particularly during ageing, however, some of those bonds can slowly break down. The sugar is separated and what remains is the free form of quercetin, known technically as the aglycone.

And free quercetin isn't nearly so happy about remaining dissolved in wine.

If enough of it accumulates, the wine can become supersaturated.

At that point, the quercetin essentially says:

Right. I'm coming out.

And it precipitates.

In experiments with Sangiovese, researchers found that visible deposits could develop at quercetin concentrations as low as around 3 mg/L, although the behaviour depends greatly on the composition of the wine and on time. Wine is an incredibly complicated chemical mixture, so there isn't one magic number at which every wine suddenly starts growing crystals.

What does quercetin in wine actually look like?

This is perhaps the most interesting part.

Because if quercetin formed enormous sparkling crystals like diamonds at the bottom of the bottle, we'd probably all find it rather charming.

Unfortunately, it isn't necessarily so pretty.

Researchers examining deposits from Sangiovese wines described them visually as a yellow-grey-brown residue. Under a microscope, however, something much more distinctive appeared: tiny needle-shaped crystals.

In one study, these were approximately 50 micrometres long. The researchers then used NMR analysis to confirm that the crystals were indeed quercetin.

To somebody opening a bottle at home, of course, none of this comes with a helpful little label saying:

"Hello! I'm a naturally occurring flavonol."

You simply see a strange deposit.

It might be floating in the wine. It might create a slight haze. It might settle somewhere in the bottle. And material caught around the cork can understandably cause alarm.

So is it mold?

Not necessarily, and this is exactly why appearances in wine can be misleading.

A crystalline or flaky deposit should not automatically be interpreted as mold or microbial spoilage.

Wine can produce several perfectly natural deposits during its life.

The one most wine drinkers know is tartrate crystals, sometimes affectionately called "wine diamonds." These are formed from tartaric acid and potassium naturally present in wine and are especially common in wines that have not undergone extensive cold stabilisation.

Then there is ordinary sediment in aged red wine, formed as tannins, pigments and other compounds gradually combine into larger molecules and fall out of solution.

And then, particularly with Sangiovese, we can add another possibility to the list:

quercetin crystals.

Without laboratory analysis, of course, you cannot identify every strange deposit in every bottle simply by looking at it.

But the important point is that a deposit is not, in itself, an indicator that a wine is spoiled.

Sometimes wine simply contains things that were once dissolved and have subsequently decided not to be.

But why does Sangiovese have so much quercetin?

Now we get back into the vineyard.

Quercetin isn't there by accident.

One of the roles of flavonols in grape skins is protection against sunlight and UV radiation.

Which leads to a fascinating relationship:

More sunlight on the grape skins can mean more flavonols.

Research on Sangiovese has specifically investigated what happens when leaves are removed around the bunches which is a common viticultural practice used for several reasons, including improving airflow and changing the amount of sunlight reaching the grapes.

Researchers found that increased bunch exposure was associated with higher concentrations of glycosidic flavonols. In other words, grapes receiving greater light exposure could potentially produce wines with a greater predisposition toward quercetin deposits.

But, and this is important, sunlight isn't the whole story.

Not every sunny vineyard produces wines full of quercetin crystals. Vineyard site, grape biotype, rootstock, ripeness, winemaking techniques and, crucially, the chemical composition of the finished wine can all affect what ultimately happens.

Wine chemistry almost never gives us the satisfaction of a simple answer.

And then there's climate change

There is another reason quercetin has become an increasingly interesting subject for winemakers.

The 2026 scientific review of quercetin instability in red wine highlights the relationship between increasing UV-B exposure associated with changing climatic conditions and flavonol biosynthesis in grapes. Greater production of quercetin and its glycosides potentially increases the risk of instability later in the wine's life.

That doesn't mean "climate change causes crystals in your Chianti."

Viticulture is much too complicated for that.

But it does mean that changing vineyard conditions may influence the concentration of compounds that eventually affect how stable a wine is in the bottle.

And suddenly the funny little deposit on the cork connects to a much bigger story happening out in the vineyard.

Can't the winemaker just get rid of it?

Ah.

This is where wine becomes interesting.

Yes, there are things winemakers can do.

Research has investigated several ways of reducing the risk of quercetin precipitation. Controlled oxygen exposure can reduce quercetin concentrations, and this may help explain why practices such as micro-oxygenation and ageing in wood can influence its stability.

Fining is another possibility.

A 2023 study tested several fining products on Sangiovese, including PVPP, vegetable proteins and yeast-derived products. Some treatments significantly reduced quercetin and its glycosides and therefore the likelihood of deposits forming later.

The latest research also discusses approaches including micro-oxygenation, encouraging controlled crystal formation before bottling, PVPP and activated charcoal.

So yes, a winemaker has tools.

But every intervention presents another question:

Do you actually want to use it?

Welcome to the eternal wine dilemma

Consumers generally expect wine to be perfectly clear.

We don't particularly like bits.

We don't like haze.

We certainly don't like opening an expensive bottle of wine and discovering something strange stuck to the cork.

And therefore wineries have very good commercial reasons for making their wines visually stable.

But achieving perfect stability can involve intervention: Fining, filtration, cold stabilisation or other treatments depending on what instability the winemaker is trying to prevent.

And every winemaker has to decide where they sit on that spectrum.

How much intervention is necessary?

How much is desirable?

At what point are you improving the wine, and at what point are you simply making it more cosmetically perfect?

There isn't one correct answer.

But it's worth remembering that a completely natural agricultural product isn't necessarily visually immaculate.

Sometimes the slightly strange-looking bottle is simply showing us some chemistry.

Is quercetin dangerous?

The presence of quercetin crystals is principally considered a stability and appearance problem, rather than a sign that the wine has become dangerous or that something strange has been added to it.

Quercetin was already naturally present in the grapes.

The problem for the winery is a much more practical one: consumers understandably associate haze, floating material and unexpected deposits with faulty wine. Nobody particularly wants to pour a glass of red wine and discover mysterious yellowish crystals floating around in it, even if those crystals are naturally occurring.

That said, I wouldn’t recommend diagnosing every mysterious substance you find in a bottle as quercetin.

If a wine smells obviously wrong, is leaking, has an abnormal cork, shows signs of contamination or simply makes you suspicious, contact the producer or retailer.

But if you find a crystalline deposit in a bottle of Sangiovese, either in the wine or clinging to the cork, quercetin is one possible explanation. And while it may not look particularly appetising, its presence is primarily a wine-stability problem, not a sign that somebody has put something nasty into your wine, or that the wine has turned bad for drinking.

Mold, tartrates, sediment or quercetin?

Here's the very simplified wine-drinker's version:

Tartrates tend to look distinctly crystalline, sometimes like tiny shards, grains or pieces of glass. They are harmless natural crystals associated with tartaric acid.

Normal red-wine sediment is usually darker and can appear powdery, grainy or muddy. It becomes increasingly common as red wine ages.

Quercetin deposits can appear yellowish, greyish or brownish and may cause haze or very fine deposits. Under magnification, their distinctive form is tiny needle-like crystals. Sangiovese is particularly susceptible.

Mold or microbial spoilage is an entirely different phenomenon and cannot reliably be diagnosed simply because you've spotted an unfamiliar deposit on a cork.

And there's another useful diagnostic tool that requires absolutely no microscope:

smell the wine and taste it.

Wine generally tells you quite quickly when something has seriously gone wrong.

So should I drink my Sangiovese?

If what you're seeing is simply a natural quercetin precipitate, the strange appearance does not mean that the wine itself has suddenly gone bad.

Decanting can help leave unwanted sediment or deposits behind, just as we routinely do with older red wines.

And perhaps knowing what's going on makes the whole thing slightly less alarming.

Because wine isn't a perfectly static substance.

We tend to imagine that once the cork goes into the bottle, everything stops.

It doesn't.

Wine continues to evolve. Molecules react with one another. Pigments change. Tannins polymerise. Aromas develop. Compounds that were once soluble can become insoluble.

And occasionally, something that spent years quietly dissolved in your Sangiovese decides to crystallise and attach itself somewhere very visible.

Like the bottom of your cork.

Which is precisely when somebody opens the bottle and thinks:

“Oh no. Is that mold?”

It might not be.

Those strange crystals could simply be another reminder that wine is still changing long after it has left the winery.

So decant the wine, pour yourself a glass, and have a look at what’s on the cork.

Because sometimes the oddest-looking things in wine turn out to have the most interesting explanations.

And in this case, its name might just be quercetin.

Yes, if the visual doesn't bother you, you'll find the wine is fine!