Many wine drinkers think there is a problem when they see tartrate crystals in the bottom of their wine glass. But there is absolutely nothing to worry about, apart from visually. The potassium tartrate crystals or ‘wine diamonds’, as they are sometimes called, are formed naturally during the course of the winemaking process and are small, clear or white.
There are many naturally occurring grape acids, the main one being tartaric, others include malic, lactic, acetic and citric acid. If grapes are harvested early with a fairly high acidity content then there will be a greater possibility of tartrates settling out later on. It occurs predominantly with white wines grown in cooler climates or higher altitudes that would be less ripe than those grown in warmer locations.
As grapes ripen, their natural sugar content rises while at the same time their acids fall, so it is very important to pick them at the optimum of both the sugar content and acid level. Sometimes this is not as easy to do due to poor weather which slows down ripening, keeping the acid levels high. It is usually from these grapes, when made into wine, that tartrate crystals are likely to form.
After fermentation finishes and the wines have been racked, there is little or no natural sugar in the wine. It is at this stage that the wine tastes its worst, that is very astringent with acids to the fore and no balancing sweetness. What is needed now is to try to reduce the acid level without additives, so by chilling the wine down, usually by cold stabilisation, the tartrate crystals will form and settle out sinking to the bottom of the vessel. The wine is then racked again before the wine warms up.
Although most of the tartrate crystals will be removed, some will not have been. They will have dissolved back into the wine, and therefore be ready to settle out in your glass after you have had your bottle in the fridge, or cool cellar, prior to opening it.
Why are ‘Wine Diamonds’ or tartrate crystals a sign of quality?
Acids are very much a winemakers friend, as long as they are not excessive, as a high acid wine is going to keep very much better, and longer, than one with a lower acid level which will tend to be lacklustre and flabby.
With more and more great white wines being produced at high altitude, it is these acids that are giving them a seal of quality. Of course the wine has to have good natural sugars to create that vital sugar/acid balance that all winemakers strive to achieve with their wines. But without enough tartaric acid the wine would have no structure or backbone at all.
The crystals themselves are totally harmless, it is just a matter of aesthetics, so next time you come across what looks like sugar granules in the bottom of your wine glass, don’t worry, just think ‘quality’.