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Canning -- Jams and Jellies
Why does freezer jelly crystalize on the surface?
Occasionally, frozen jams or jellies will develop a top layer of a white, hard crystalline sugar. Some might think that it is mold. If the jelly is not sealed airtight and loses moisture during storage, the concentration of sugar in the top layer increases and some of the sugar comes out of solution to form this visible mass. To prevent its formation, some recipes call for a small amount of corn syrup, which introduces glucose to the recipe. Crystallization is less likely to occur if there is a mixture of sugar types rather than just sucrose. Another way to prevent crystallization is to seal the jars airtight so there is no loss of moisture during storage.