And it's not limited to homemade wine. This phenomenon happens to wines that cost even hundreds of dollars per bottle, much to the chagrin of sommeliers and servers in wine shops and restaurants. The first time you see this you'll wonder if your wine is spoiled, if those are somehow bits of glass from the bottle, and if it's still safe to drink.
The good news is that this is a well-known and well-understood natural phenomenon, and one that's been around for as long as people have made wine. As a matter of fact, they have found this residue on wine vessels over ten thousand years old.
Not only is the wine safe to drink, it's going to be delicious, and the crystals, far from being dangerous or worrisome are something that you may have paid good money for under a different name (more on that below).
The crystals in question are sometimes called 'wine diamonds', and they're formed from one of the naturally occurring acids in grapes. There are three main acid types found in winemaking grapes, tartaric, malic, and citric. Citric is quite stable and occurs in tiny quantities, malic has a sort-of green apple quality that's not desirable in reds, so it's sometimes reduced through a process called 'malolactic fermentation’, while tartaric is the workhorse, balancing flavor and keeping the pH low so the wine doesn't oxidize easily or spoil.
But tartaric acid is kind of touchy where temperature or long storage is concerned. It can go through a chemical reaction where it combines with potassium in the wine to form a new compound, potassium bitartrate. properly known as potassium hydrogen tartrate, KC4H5O6. Because these crystals aggregate into little lumps that look like big grains of sugar they've been given the name wine diamonds. That also sounds a lot sexier than ' potassium hydrogen tartrate' when you're trying to explain them to a customer in your fine dining restaurant.
All wine develops these crystals, eventually. It's not usually a huge deal for commercial producers because in the one to two years it takes to finish a bottle of wine it settles out naturally and is disposed of. It can be hurried along by lowering the temperature of the wine, as tartrate crystals form most rapidly at lower temperatures kit wines are a bit different, given the short production period the instructions dictate. To get around this we put the juices for our Master Vintner kits through low temperature tartrate stabilization before blending them. We lower the temperature to just above freezing, 36F, the lowest temperature you can go before ice crystals form. We also 'seed' the juice with a dose of tartrate crystals to encourage more to form.
This can eliminate a lot of wine diamonds, but don't always get it out completely, partly because some of the tartaric acid is tightly bound in unfermented grape juice and partly because we can only hold them for a month or so before we need to put them into our kits. Sometimes you'll see a tiny bit of crystals in the juice bag, or perhaps your fermenter, but the most likely place they'll crop up is in a finished wine that's more than a year old.
But that's nothing to worry about! There's another name that tartrate crystals are known by: Cream of Tartar. A white, colorless and odorless powder, it's found in the baking section of the supermarket. In food preparation cream of tartar is used to stabilize egg whites (for whipped meringues) and to prevent sugar solutions from crystallizing. It's one of the components used to make baking powder, finely ground and mixed with baking soda.
As far as any health concerns go, tartrate crystals are completely harmless. If you consume them in very large quantities, they do have a lot of potassium in them, about 500 mg per teaspoon (normal dietary requirements are 3500 mg/day). Excess potassium consumption can cause stress on the kidneys and in extremely large quantities can cause stomach upset. But let's face it: nobody is going to eat a spoonful of tasteless, dry tartrate crystals, and a medium-sized banana (the go-to for potassium seekers) has over 400 mg and potatoes have more than double that.
Wine diamonds don't cause harm, don't taste like anything, and don't change the character of your wine. And they rinse away easily from equipment and bottles with no further effect. So, if you see diamonds, you needn't worry, just pour carefully and they'll stay in the bottle. If you're serving wine to your friends and don't want to stress them you could decant the wine into a serving carafe--but then you'd miss the opportunity to share the story of tartrate crystals, and isn't sharing the story of your wine as much fun as drinking it?