Tartrate crystals in wine
Have you ever opened a wine to find small crystals lining the bottom of the cork, glinting at the base of the bottle, or even floating in the wine itself?
They’re known as tartrate crystals but they have also been referred to as ‘wine diamonds’.
They’re not considered harmful to your health, but they can form naturally in wine under certain conditions.
‘Many people think these clear crystals floating in the wine or stuck to the cork are salt, sugar, sediment or even broken glass,’ said the late, great Gerard Basset OBE MW MS in response to a Decanter reader query in 2016.
‘In fact they are harmless by-products of wine, and some equate their presence as a mark of quality, in that the wine has not been overly manipulated in the cellar.’
If you find crystals in your wine at home, Basset said, ‘Once formed they’ll never disappear, but they won’t affect the aroma, taste or quality of the wine.’
He added, ‘If you find them unsightly, you can either decant the wine before serving or filter it through a muslin cloth.’
Where do wine crystals come from?
Tartrate crystals in wine trace their roots back to tartaric acid, one of the main acids that occurs in wine grapes.
Wine diamonds can be present in either red or white wines, and their formation is linked to temperature.
‘When exposed to cold temperatures, the tartaric acid naturally found in grapes binds with potassium to form a compound called potassium bitartrate,’ said Basset. ‘It’s the same as cream of tartar used in cooking.’
Basset added, ‘Many white wines often undergo a cold stabilisation process to remove these tartrates before bottling, but often some are left, ready to crystallise in your cold cellar or fridge.’
Cold stabilisation generally involves cooling the wine down close to its freezing point ‘to force the precipitation of tartaric salts prior to wine bottling’, as researchers writing in the peer-reviewed Foods journal explained last year.
Yet this process can also lead to a loss of colour and aroma, said the researchers, who examined other ways of achieving the same result.
Some wineries have also begun using or exploring alternatives to cold stabilisation, such as electrodialysis, as English sparkling wine estate Rathfinny noted in a previous blog.
This article was originally written by Gerard Basset OBE MW MS in response to a Decanter magazine reader query, and published online in 2016. It has been updated with new copy in August 2021.