Why do crystals form in jelly?
A. Crystals throughout the jelly may be caused by too much sugar in the jelly mixture, or cooking the mixture too little, too slowly, or too long. Crystals that form at the top of jelly that has been opened and allowed to stand are caused by evaporation of liquid. Crystals in grape jelly may be tartrate crystals. SOURCE:How To Make Jellies, Jams, and Preserves At Home. United States Department of Agriculture. Extension Service. Superintendent of Documents, Washington, D.C. Home and Garden Bulletin Number 56.