Sometimes when opening a jar of home canned grape juice or grape jelly you will find small sharp crystals inside the jar or in the actual food product. These crystals are tartrate crystals and are formed by the naturally occurring tartaric acid in the grape juice. The crystals are perfectly harmless and in no way affect the safety of the food product.
Tartrate crystals in grape products are formed by sediment in grape juice or other grape based products like wine or jelly. Many canners and jelly makers don’t like them and sometimes novice canners are upset to see them sitting in the bottom of a jar of grape juice.
The way that tartrate crystals can be prevented in home canning is by allowing the grape juice to rest or sit overnight in a refrigerator so all the sediment can collect at the bottom of the container.
In the morning carefully pour off only the clear juice and try not to disturb the sediment. By keeping the grape sediment out of the juice before it is canned, clear grape juice without little crunchy things floating around is almost guaranteed, and a beautiful and sparkling blue ribbon jelly is within reach.
In nature, grapes are the richest source of tartaric acid and the kitchen helper known as Cream of Tartar is obtained from grape sediment and is made from byproducts that are leftover from wine making. All grapes contain tartaric acid but some have more than others. Foods and wines from Concord grapes are notorious for forming crystals.
Cream of Tartar is a very old substance and in fact traces of calcium tartrate have been found in ancient pottery in northern Iran at the site of Hajji Firuz Tepe , which suggests the art of wine making is at least 7,000 years old.
Because it is acidic Cream of Tartar will clean brass or copper the same way lemon juice or vinegar will. Cream of Tarter is used in the kitchen to help stabilize and give more volume to beaten egg whites and helps to produce a creamier texture to certain candies and frosting because it retards the formation of sugar crystals. Often Cream of Tartar is used in the place of lemon juice or vinegar by to produce a lighter textured cake with a finer grain.
So if you should ever happen upon crystals in your home canned grape jelly or grape juice – don’t be alarmed they’re natural.