- Harmless tartrate crystals from tartaric acid that precipitate out of finished wine, especially when chilled. Can be a good sign, showing that the wine has not been over processed.
Acids are an important component of all wines. They provide structure (backbone), contribute to a wine’s impression of freshness, and help make wine food friendly. There are roughly a half dozen different acids to be found in wine, but tartaric acid is by far the most plentiful.
Winemakers put a lot of effort into managing acid, beginning in the vineyard where getting the sugar/acid balance just right is a primary goal. Depending on local rules, the winemaker may ‘adjust’ acid before starting fermentation. All through its stay at the winery, the wine’s acid will be monitored. One trick winemakers use to lower acidity is to ‘cold stabilize’ the wine prior to bottling. This entails refrigerating the wine to just above freezing for up to a week, which forces the tartaric acid to form crystals, softening the wine. (Incidentally, the crystals are later scraped from the tank and sold as a basic ingredient in baking powder.)
You may have seen these tartrate crystals in a bottle or glass, or on the cork. (These can look like bits of glass.) This actually is a good sign. It means the winemaker has not processed the dickens out of the wine and that it had ample acidity when bottled. Diamonds, like all sediment, is a good thing, although getting a mouthful of it is rather off-putting. If you see a lot of crystals in the bottle, pour very gently or -- better -- decant the wine off the sediment and enjoy!