Tartrates - wine crystals on the cork and bottle bottom
You open a bottle of wine and see fine, transparent crystals on the underside of the cork or at the bottom of the bottle. The first thought is often worry: is the wine spoiled, is it glass, is it sugar? Relax, these are tartrates, sometimes called wine diamonds, and they are completely harmless. They form naturally from tartaric acid, the main acid of grapes, at low temperature. What is more, their presence can even be a sign of quality, indicating the wine was not over-processed. Many producers deliberately remove them by cold stabilization, but their appearance is no defect at all. Here is where tartrates come from, why they are harmless, why they are more often found in white wines, what cold stabilization is and why the crystals can be a signal of good winemaking.
What tartrates are
Tartrates are crystals of potassium bitartrate, a chemical compound forming naturally in wine. They form when tartaric acid, the main acid present in grapes, reacts with the potassium ions naturally present in wine at low temperature. The result is fine, transparent or slightly coloured crystals resembling shards of glass or sugar. They settle at the bottom of the bottle, on the underside of the cork, and in red wines they can be coloured dark by the pigments. They are colloquially called wine diamonds, which captures their crystalline look well. It is a purely natural phenomenon, stemming from the composition of the wine itself, not from contamination or spoilage. Understanding that tartrates are simply a crystallised natural component of wine dispels most fears. It is not a foreign body but part of the wine itself.
Where they come from
The mechanism of tartrate formation is simple and completely natural. Grapes contain tartaric acid, which passes into the wine, and potassium. Under the right conditions these two components combine to form potassium bitartrate, which is poorly soluble. The key factor is temperature: the solubility of tartaric acid compounds increases with heat, so cold is the primary trigger of crystallisation. When wine is chilled, some of the potassium bitartrate precipitates out of solution as crystals. That is why tartrates appear especially after storing or chilling wine. There is nothing unusual or worrying about it, it is simply chemistry reacting to temperature. Understanding the role of cold in the formation of the crystals explains why they appear precisely in certain conditions. Tartrates are the wine’s natural reaction to cold.
Why more often in whites
Tartrates are more often noticed in white wines, and there is a concrete explanation. White wines are usually chilled before serving, often to temperatures below four or five degrees Celsius. It is precisely at such low temperatures that crystallisation occurs most easily, sometimes within just a few days. Red wines, served at a higher temperature, less often reach conditions favourable to crystal formation. That is why consumers more often come across tartrates in whites, especially after storage in the fridge. In red wines the crystals can be coloured and settled on the cork, but they are less visible. This does not mean whites are more prone by nature, but that the way they are served more often triggers crystallisation. Understanding this relationship explains why wine diamonds are associated mainly with white and chilled wines.
Completely harmless
The most important thing to remember is that tartrates are completely harmless. Crystals on the cork or at the bottom of the bottle are no cause for alarm or a sign of spoilage. They do not affect the aroma, taste or quality of the wine, because they are simply a crystallised natural component that precipitated out of solution. You can calmly ignore them, and if they bother you visually, they are easy to remove by gentle decanting or pouring the wine off the sediment. They are not glass, not sugar and not mould. Consuming them does no harm, though they usually stay at the bottom. This knowledge helps avoid needlessly returning good wine as supposedly faulty. Tartrates are a cosmetic detail, not a fault. Understanding their harmlessness is the key to calmly enjoying a wine in which they have appeared.
Cold stabilization
Because many consumers fear the crystals, producers often apply a treatment called cold stabilization to prevent them. It involves chilling the wine to a temperature between minus five and minus ten degrees Celsius for several days before bottling. At such a low temperature the potassium bitartrate precipitates while still in the tank, as crystals that are then filtered out of the wine. As a result there is no longer an excess of this compound in the bottle and crystals will not form later at the consumer’s. It is a common and effective method, motivated mainly by aesthetics and market expectations, not by improving quality. Cold stabilization is an extra step that deliberately removes what would otherwise appear naturally. Understanding this process explains why some wines have crystals and others do not. It is a matter of the producer’s decision, not the wine’s quality.
A sign of quality
Interestingly, the presence of tartrates is sometimes seen as a sign of quality, not a fault. Many winemakers, sommeliers and academics regard the crystals as a signal that the wine was not over-processed. Cold stabilization and filtration, though effective, are extra treatments that some producers deliberately skip to preserve the wine’s naturalness. In this view, the appearance of tartrates testifies to a less invasive, more respectful approach to the wine. It is a reversal of the common intuition: the crystals a consumer fears can be proof of good winemaking. Of course their absence does not mean poor quality, because it is simply a different production decision. But their presence should not cause worry, but rather respect. Tartrates are an example of how something seemingly worrying is in reality a good sign.
Tartrates versus sediment in red wine
It is worth distinguishing tartrates from another kind of sediment found in red wines. In mature red wines, a sediment of pigments and tannins gathers at the bottom of the bottle, precipitating over time during ageing. It is a different kind of sediment from crystalline tartrates, though the two are sometimes confused and both are harmless. The tannin and pigment sediment is softer, more powdery or flaky, while tartrates are hard, crystalline grains. Both are removed by decanting, that is carefully pouring the wine off the sediment. Understanding this difference helps correctly identify what you are dealing with. You can read more about decanting wine and when it makes sense in the post on decanting wine. Both tartrates and sediment are natural phenomena, not faults requiring a complaint.
Tartrates versus wine faults
Since tartrates are sometimes confused with a defect, it is worth clearly distinguishing them from real wine faults. Tartrates are natural, harmless crystals that do not affect taste or aroma. Real wine faults, like cork taint, oxidation or excessive volatile acidity, change the smell and taste of the wine for the worse and are a real problem. Confusing these two things leads to needlessly returning good wine or, conversely, ignoring a genuine fault. So it is worth knowing that the crystals are not a defect, while also being able to recognise the signs of real spoilage. You can read more about real wine faults and how to recognise them in the post on wine faults. Understanding the difference between a harmless tartrate and a real fault is an important skill for anyone who drinks wine. Crystals are not grounds for a complaint.
What it means in the glass
For the drinker, tartrates mean above all no cause for worry. If you see crystals on the cork or at the bottom of the bottle, you can calmly pour the wine, optionally leaving the sediment at the bottom or gently decanting it. The wine tastes the same regardless of the presence of crystals, because they do not affect aroma or quality. What is more, you can treat them as a pleasant signal that the wine was made with less intervention. There is no need to return such a bottle or fear it. If you want to deliberately observe how wines behave and what their appearance reveals, record your observations in the app and note the details. Tartrates are one of those elements of wine that look mysterious but are in reality a simple, natural and harmless phenomenon. Wine diamonds are a reason for calm, not concern.
The key points
Tartrates are crystals of potassium bitartrate, forming naturally when tartaric acid from grapes reacts with potassium at low temperature. They are called wine diamonds and settle on the cork and at the bottom of the bottle. They are more often noticed in white wines, because these are chilled before serving, and cold is the primary trigger of crystallisation. They are completely harmless, do not affect taste or aroma and can be ignored or removed by decanting. Producers often prevent them through cold stabilization, that is chilling the wine before bottling and filtering out the crystals. Importantly, the presence of tartrates can be a sign of quality, indicating a less processed wine. They should not be confused with real wine faults or with tannin sediment in red wines. The crystals are a reason for calm, not a complaint.