Let’s set the scene.
You’ve had a hard day at work, you plan to get home, rustle up something delicious and enjoy a glass of wine. Everything is going to plan and you're anticipating a delicious glass of wine. When you pull the cork you notice small crystals on the base and similar fragments in your glass.
You begin to wonder if this means there’s something wrong with the wine - is it safe to drink?
The short answer is that nothing is wrong and you can absolutely still consume that wine.
However, if you'd like to know why those crystals are there, here's a simple explanation.
Three main acids exist in wine grapes: malic acid, citric acid and tartaric acid.
Of the three, it’s tartaric acid that’s responsible for the freshness/tartness we get in wine and it's also the acid that creates those lovely crystals. While malic acid is often converted to lactic acid during malolactic fermentation, tartaric acid maintains its chemical consistency.
During fermentation and barrel ageing, a percentage of tartaric acid will settle out and bind with lees and pulp, as well as any precipitated tannins and pigments. The rest remains soluble in the juice throughout the production process.
So, that soluble component is the primary acid you taste and is essential to the final balance of a wine, impacting mouthfeel and freshness. It also plays a role in the wine's colour and stability.
What's the snag
So, what's the snag? Well, tartaric doesn’t always like to stay dissolved inside the wine - it is susceptible to temperature fluctuations. Chill a wine down too far and the acid will actually solidify and fall out of the solution.
If a wine is chilled to temperatures below 4.5 degrees, the some tartaric acid can bind with the potassium in the wine (this potassium is naturally occurring). This process results in the formation of crystal-like deposits known as potassium bitartrates or tartrates (a.k.a wine diamonds).
On occasion, this can sometimes occur after you’ve chilled a wine in the fridge, but also happens to red wines.
That said, the higher quality your bottle of wine is, the more likely you are to see tartrates. This is because many entry level wines often undergo cold stabilisation, a process to precipitate the tartrates prior to bottling. It is down simply by chilling the wine down to just above freezing for a number of days.
While this process will ensure a wine’s clarity, it can also result in lowering a wine’s acid profile and, potentially, impact flavour and ageing potential.
So, if you find yourself coming across tartrates, either stuck to the bottom of your cork or in the bottom of your glass, consider it a good thing. This means the wine wasn’t over-processed.