Michigan State University Extension
Preserving Food Safely - 01600630
Crystals throughout jelly or jam may be caused by:
-Too much sugar in the jelly mixture.
-Cooking the mixture too long or too slowly. (Long slow cooking results in too much evaporation of water).
-Cooking the mixture too little. (When sucrose is boiled in acid fruit juice to make jelly, some of it is "inverted", or broken down, to dextrose (glucose) and levulose (fructose). Sugar would probably crystallize from most jellies if it weren't partially inverted during the cooking process. Too little cooking results in insufficient sucrose inversion. This is rarely a problem in jellies made without pectin since they require fairly long cooking. How- ever, sugar may crystallize from jelly made with added pectin because the boiling time-one minute- is too short to bring about much inversion.
Crystals that form at the top of jelly that has been opened and allowed to stand are caused by evaporation of water.
Needle-like crystals in grape jelly may be tartaric acid, the natural substance in grapes from which cream of tartar is made. To prevent formation of tartrate crystals in the jelly, let extracted grape juice stand in a cool place (refrigerator) overnight, then strain through two thicknesses of damp cheesecloth to remove crystals.