One characteristic of fine wines frequently captures the attention of wine enthusiasts is the presence of tartrate crystals, sometimes referred to as ‘wine diamonds’. These small, harmless crystals sometimes form in bottles or on corks, leaving wine lovers curious about their significance. They can appear in both red and white wines, but most commonly you see them in white wines, partly due to the fact that white wines are chilled in the fridges, precipitating the crystals (more on this below).
Tartrate crystals are a naturally occurring phenomenon in wines, composed of potassium bitartrate, commonly referred to as cream of tartar, appearing as colourless or white sediment particles. Tartrates are present in grape juice as an organic acid, and during fermentation, some of this acid precipitates as tartrate crystals.
Tartrate crystals develop due to a combination of low temperatures and the saturation of potassium bitartrate in the wine. As wines cool during storage or transportation, the solubility of tartrate decreases, leading to the precipitation of crystals. This process is more likely to occur in wines with higher levels of acidity and potassium.
The presence of tartrate crystals in fine wines is often viewed as a positive attribute. It indicates that the winemaker has avoided excessive stabilization techniques, such as cold stabilization or filtration, which can strip wines of their natural flavors and textures. Tartrate crystals suggest minimal intervention during production, allowing the wine to maintain its integrity and character. This approach goes hand in hand with taking great care over the quality of fruit, removing any diseased or damaged grapes or ‘MOG’, as it is known (‘Matter Other than Grapes’) at the sorting table.
Winemakers have various methods to manage tartrate crystals. Cold stabilization involves placing wines in a cold environment, typically below freezing point, for an extended period. This process prompts the precipitation of tartrate crystals before bottling. Another method is filtration, where wines are passed through a membrane to remove suspended particles, including tartrates.
The impact is easily noticeable, even by non-experts. We have taken part in tasting experiments in winery laboratories, tasting samples of the same wine, run through different forms of fining and filtration (kieselguhr, plate and membrane filters of decreasing size etc). The effect is obvious, with flavour becoming obviously more muted as the filtration becomes more severe.
All the commercial wines you might find on a supermarket shelf tend to have gone through the harsher levels of filtration, as stability is the key requirement for wine brands selling millions of cases per year. The care and attention taken by fine wine makers over the quality of their fruit and processes means they can focus more on retaining flavour and character in the bottle.
If you find crystals in your white or red wine, you can decant the wine out of the bottle, or through a piece of muslin to remove them (but we tend not to bother and just crunch them up on the palate!).