Potassium bitartrate crystallizes in wine casks
during the fermentation of grape juice, and can precipitate out of
wine in bottles. The crystals (wine diamonds) will often form on the
underside of a cork in wine-filled bottles that have been stored at
temperatures below 10 °C (50 °F), and will seldom, if
ever, dissolve naturally into the wine.
These crystals also precipitate out of fresh grape juice that has been chilled or allowed to stand for some time. To prevent crystals forming in homemade grape jam or jelly, fresh grape juice should be chilled overnight to promote crystallisation. The potassium bitartrate crystals are removed by filtering through two layers of cheesecloth; the filtered juice may then be made into jam or jelly. In some cases they adhere to the side of the chilled container, making filtering unnecessary.
