What Are Tartrate Crystals? A Beginner’s Guide

Quick answer: what are tartrate crystals?

Tartrate crystals are hard, crystalline deposits formed when natural tartaric acid salts in wine come out of solution. The most common type is potassium hydrogen tartrate, also called potassium bitartrate or KHT.

You may hear them called “wine diamonds”. They can appear as clear, white or colourless crystals in white wine, or pink, red, purple or darker deposits in red wine when wine pigments become associated with the crystals.

Tartrate crystals are normally harmless. They do not make wine unsafe, and their presence does not automatically indicate poor winemaking, a premium wine, natural wine or unfiltered wine. They are mainly a presentation issue because people can mistake them for glass or other unwanted material.

Crystals may appear on the cork, around the bottle shoulder, on the sides of the bottle or settled at the bottom. Chilling can encourage them to form, so a bottle kept in a cool cellar, refrigerated transport or a household fridge may show them more clearly.

Why do tartrate crystals form?

Grapes naturally contain tartaric acid and potassium. During winemaking, these substances remain dissolved in the wine, but the balance can be close to the point where crystals form.

Temperature is one important factor. As wine becomes colder, potassium bitartrate becomes less soluble. If the wine is unstable, cooling can encourage small crystals to form and grow. The risk also depends on the wine’s composition, including its acidity, pH, potassium, alcohol and other natural compounds.

This is why crystals can appear after bottling or during storage. A wine may look clear when it leaves the winery but develop deposits later after exposure to cold conditions.

The chemistry is a little more complicated than simply “cold wine creates crystals”. Time, wine composition and changes during ageing or blending can also affect stability.

What do they look like?

In pale wines, tartrate crystals are often clear, colourless or white. They may look like tiny shards, grains or angular flakes.

In red wine, the deposits may appear pink, red, purple, reddish-brown or dark. This does not necessarily mean the basic crystal is different. Red-wine pigments and other phenolic material can become associated with the crystals and colour them.

Appearance alone cannot identify every deposit with certainty. Tartrates may be common, but not every hard-looking particle is automatically potassium bitartrate.

Are tartrate crystals dangerous?

No. Tartrate crystals are considered harmless and do not make a wine poisonous or spoiled. They normally have no meaningful effect on the wine’s aroma or flavour.

However, you do not need to handle or taste an unfamiliar sharp-looking particle. Tartrates can resemble broken glass, especially when they form larger angular deposits. If the bottle appears damaged or the material genuinely looks like glass, do not assume it is a tartrate. Set the bottle aside and seek advice from the retailer or producer.

Crystals are also different from cork taint, which is a smell and flavour problem rather than a crystalline deposit. Protein haze usually looks cloudy, while yeast or other sediment may look powdery or cloudy. Red wines can also contain colour, tannin and yeast sediment that is not tartrate.

The safest wording is: clear, hard, angular deposits are often tartrates, but you cannot diagnose every wine deposit just by looking at it.

What is cold stabilisation?

Cold stabilisation is a winemaking process used mainly to reduce the chance of potassium bitartrate crystals forming after bottling.

In broad terms, the wine is chilled so that unstable tartrates crystallise and settle. The wine is then separated from the crystals, often through racking or filtration, and checked for stability. Some wineries use seed crystals or other technical approaches to encourage the process.

Cold stabilisation reduces the risk; it does not guarantee that a wine can never form a deposit. It is also primarily aimed at potassium bitartrate. Calcium tartrate is a separate, less common deposit that forms differently and is not reliably controlled by ordinary cold stabilisation.

For a fuller explanation of the process, see What Is Cold Stabilisation?. The broader winemaking sequence is covered in How Wine Is Made.

How should you serve a wine with crystals?

Tartrate crystals are not a reason to discard an otherwise sound bottle. A simple approach is:

1. Keep the bottle upright before serving so any sediment can settle.
2. Open it carefully and inspect the bottle and cork.
3. Pour slowly, particularly towards the end of the bottle.
4. Stop pouring when the deposit approaches the neck or shoulder.
5. Decant only if there is enough sediment to make it useful.

Decanting is optional, not mandatory for a few crystals. The aim is simply to keep the deposit out of the glass.

If the wine also has unusual smells, leakage, unexpected fizz or obvious signs of spoilage, treat that as a separate question. Tartrate crystals alone are harmless, but not every wine problem is a tartrate problem.

The takeaway

Tartrate crystals are a natural wine-stability phenomenon. They are most commonly potassium bitartrate deposits, encouraged by cold conditions and influenced by the wine’s chemistry. They can be unattractive or surprising, but they are not automatically a safety problem or a sign of poor winemaking.

If you see them, settle the bottle and pour carefully. Leave the crystals behind, and enjoy the wine.

Frequently asked questions

Are tartrate crystals the same as wine diamonds?

“Wine diamonds” is an informal name for tartrate crystals, especially clear crystals found in wine. It is not a formal quality category or a guarantee that the deposit is a particular compound.

Do tartrate crystals mean a wine is bad?

No. Tartrate crystals do not normally affect safety, flavour or aroma. They can be a presentation concern, but they are not automatically evidence of defective wine.

Can tartrate crystals form in the fridge?

Yes. Cold conditions can reduce the solubility of potassium bitartrate and encourage crystals to form or become more noticeable. Do not treat one exact household temperature as a universal rule.

Are all wine crystals potassium bitartrate?

No. Potassium bitartrate is the most common crystalline deposit, but calcium tartrate and other types of sediment can occur. Appearance alone is not a definitive chemical test.