As a wine student sooner or later you will encounter a fascinating phenomenon known as “wine diamonds”. These tiny, sparkling particles may be found at the bottom of wine glasses, as particles in the bottle, or most likely to be encountered on the ‘mirror’ of the cork (the part of the cork that has direct contact with the wine). While they may resemble actual diamonds, wine diamonds have a different origin and significance in winemaking. We will delve into the mysterious world of wine diamonds, exploring their formation, impact on wine quality, and how they can be managed.
What are Wine Diamonds?
These crystals are primarily composed of tartaric acid which naturally occurs in grapes. Tartaric acid keeps the pH low which can prevent unwanted spoilage bacteria from flourishing. During fermentation and ageing, tartaric acid can combine with potassium ions to form potassium bitartrate which with time arranges into the crystallized form also known as ‘tartrates’. The potassium bitartrate, now formed as a wine diamond is an organic acid and when present in this crystalized form does not indicate any flaws or deterioration in the wine’s quality. Potassium bitartrate is also commonly found in kitchens where you know it as cream of tartar. Indeed, some wineries have a side-hustle selling off these crystals.
While all wines contain tartaric acid, the amount and rate of crystal formation can be influenced by various factors, including the grape variety, climate conditions, winemaking techniques, and ageing process. Certain grape varieties, such as Chardonnay and Sauvignon Blanc, have higher levels of tartaric acid and are more prone to developing wine diamonds. Wine diamonds typically form when wine is exposed to cold temperatures and can even form naturally during barrel ageing.
Here are some ways that tartrates are managed by winemakers.
Cold stabilization
Most white wine making includes cold stabilization. Stainless steel tanks hold super-chilled wine as cold usually between -2 and 2 degrees Celsius. At this frigid temperature potassium bitartrate crystal solid is large enough that it cannot stay suspended. The crystals are either filtered from the wine or the wine is carefully racked away from the precipitate before bottling. This ensures a visually appealing product for the consumer, but it does come with significant cost and time (sometimes several weeks) and is not considered completely reliable.
Red wine is not as likely to be chilled prior to serving and so do not usually see cold stabilization in the winery. Wine diamonds can also form in red wine if the bottle is exposed to cold temperatures or it rests for a significant period of time in a cool environment.
Addition of metatartaric acid
Where cold stabilization encourages tartrates to form, the addiction of metatartaric acid (a dehydrated form of natural tartaric acid) slows down crystallization as it decomposes over time. It is a gentle process for wine applied about five days prior to bottling after fining and before final filtration. Unlike cold stabilization, using a metatartaric acid is not a lengthy time commitment. This process is only used for wines intended to be consumed within 2 years.
Electrodialysis
Electrodialysis involves using an electric current to attract the potassium and bitartrate ions which once bound are then filtered through an ion-selective semipermeable membrane. As you might have guessed this requires significant investment in equipment and testing instruments.
| Tartaric acid is a white crystalline organic acid that occurs naturally in many plants, most notably in grapes [1] |
On the Vine
Grapes that hang for a longer period of time on the vine are somewhat less likely to develop wine diamonds. It stands to reason that acid levels drop (malic declines faster than tartaric acid) as ripening (sugar production) increases, so less tartaric acid is present in the grapes. This method is reliable only for wines consumed soon after release.
Despite the appearance of sediment wine, wine diamonds are harmless and do not affect the taste or safety of the wine. In fact, their presence can be seen as a positive sign, indicating that the wine has not been subjected to excessive chemical treatments or filtration processes that may strip away natural compounds.
Winemakers often aim for a balance between wine stability and preserving natural components, including tartaric acid, as it contributes to the wine’s structure, and ageing potential. While wine diamonds may be visually unappealing to some, they are easily separated by decanting or filtering the wine, ensuring a clear and pristine appearance.
So, the next time you spot those sparkling crystals at the bottom of your wine glass or clinging to the cork, appreciate the beauty of wine diamonds and the complex processes that contribute to this crystalline magic.
For information on wine glasses take a look at Wine Glasses: Why They Matter and How to Choose the Right One
